Weather Glossary

Baroclinic Zone

Also: baroclinic

A region with a strong horizontal temperature contrast, usually where cold polar air meets warm subtropical air in the mid-latitudes. That contrast is the energy source for mid-latitude storms, and it also powers the jet stream overhead. In winter, the difference between the pole and the tropics is at its strongest, so the baroclinic zone is sharper and sits farther south, putting the Sierra right in the path of the storm track.

Synoptic Scale

Also: synoptic

The scale of weather systems roughly 600 to 3,000 miles across that last a few days: the troughs, ridges, highs, lows, and fronts you see on a weather map. When models agree on the "synoptic pattern," the big features are in the same place, even if details like exact precipitation totals still differ. Smaller features such as thunderstorms, terrain-driven winds, and convergence zones are called mesoscale.

Jet Stream

Also: jetstream, polar jet, Pacific jet, 200 mb wind, 250 mb wind

A narrow band of strong wind near the top of the troposphere, roughly 30,000 to 40,000 ft up (the 300 to 200 mb levels), where speeds often reach 100 to 200 mph. It forms along the boundary between cold and warm air masses. Think of it as a storm highway: it steers storms, and areas where air spreads out beneath it help storms strengthen. There are two main branches: the polar jet, which drives most of our winter weather, and the subtropical jet, which sits farther south and can carry tropical moisture into California. Forecasters usually look at 250 mb or 200 mb wind charts to see it.

Subtropical Jet

Also: subtropical jet stream, subtropical jetstream, southern branch, southern jet, southern stream, split flow

The southern of the two main jet streams, found around 25 to 30° latitude near the 200 mb level (about 40,000 ft). It is fed by the tropics: air rising in tropical thunderstorms flows poleward aloft and speeds up as it moves away from the equator. During El Niño, extra tropical heating in the central and eastern Pacific strengthens the subtropical jet and extends it east toward California and the southern US, often as a distinct southern branch beneath a weaker northern branch (split flow). A strong southern branch can carry subtropical moisture into California and undercut ridges that would otherwise block storms.

Jet Streak

Also: jet streaks

A core of especially fast wind within the jet stream, often a few hundred miles long. Air speeds up as it enters a jet streak and slows as it leaves, which sets up areas of rising and sinking motion around it. The left exit and right entrance regions favor rising air, so storms and heavier precipitation often line up with those quadrants. Watching where jet streaks are headed helps pinpoint which storms will strengthen.

Rossby Wave

Also: Rossby waves, Rossby wave breaking, planetary waves, longwave trough, longwave pattern

A large-scale meander in the jet stream, thousands of miles long, that exists because the effect of the Earth's rotation changes with latitude. The big troughs and ridges on a 500 mb map are Rossby waves. They move slowly, sometimes even backward against the flow, and carry energy downstream to distant regions, which is the physical basis of teleconnections. When they grow very amplified they can break, producing blocking ridges and cutoff lows.

Trough

Also: troughs, troughing

An elongated area of low pressure aloft, seen as a dip or southward bulge in the jet stream. Rising air ahead of (east of) a trough brings clouds, precipitation, and cooler, unsettled weather. The deeper the trough, the greater the storm potential. On model charts, troughs show up as lower-than-normal 500 mb heights (blue areas). When a trough digs into the West Coast, that's when things get interesting for the Sierra.

Ridge

Also: ridging, ridges

An elongated area of high pressure aloft, seen as a northward bulge in the jet stream. Sinking air under a ridge brings dry, warm, settled weather, and the ridge deflects storms around it. On model charts, ridges show up as higher-than-normal 500 mb heights (red/orange areas). A persistent ridge off the West Coast is every Sierra skier's worst nightmare: it's the drought maker.

High Pressure System

Also: high pressure, high-pressure

An area where surface air pressure is higher than its surroundings. Air sinks and warms as it descends, which suppresses clouds and brings clear, dry conditions. In the Northern Hemisphere, air circulates clockwise (anticyclonically) around a high. Surface highs usually sit beneath or just downstream of an upper-level ridge, which shows up on model charts as positive 500 mb height anomalies (red/orange areas). In winter, a high parked over the Great Basin traps cold air in the valleys and drives offshore winds toward the coast.

Low Pressure System

Also: low pressure, low-pressure

An area where surface air pressure is lower than its surroundings. Air converges toward the center and rises, cooling as it goes, which produces clouds and precipitation. In the Northern Hemisphere, air circulates counterclockwise (cyclonically) around a low. Surface lows usually develop ahead of an upper-level trough, where divergence near the jet stream removes air from the column and lets surface pressure fall. A mature low carries a warm front and a cold front, and the moist warm sector ahead of the cold front is often where an atmospheric river sits.

500mb Height Anomaly

Also: 500mb, 500 mb, 500 millibar, 500 mb height, 500 mb heights

The 500 millibar pressure surface sits around 18,000 ft (about 5,500 m), roughly halfway up the atmosphere by mass. Its height is lower in cold air and higher in warm air, so a map of 500 mb height shows the troughs and ridges that steer storms. The anomaly is how far that height departs from its average for the date. Blue areas are lower than normal (troughs, cooler air, storminess) and red areas are higher than normal (ridges, warm and settled weather). It's the single most useful chart for seeing the big-picture pattern at a glance.

Front

Also: cold front, cold-front, warm front

The boundary between two air masses of different temperature and density. At a cold front, advancing cold air undercuts warmer air and forces it upward quickly, which often produces intense but shorter-lived precipitation. At a warm front, warm air glides up and over retreating cold air, producing more gradual, widespread precipitation. Most Sierra storms are driven by cold fronts sweeping in from the Pacific.

Pacific Trough

Also: mid-latitude cyclone, mid latitude cyclone

The classic Sierra storm. A wave in the Pacific jet stream spins up a low-pressure system with a cold front and a warm front, which then moves inland with the jet. These storms are relatively well organized and predictable because the jet steers them. Most of our winter storms are some version of this mid-latitude cyclone, varying in strength with the jet and the moisture available. Cutoff lows and inside sliders are the main exceptions.

Shortwave Trough

Also: shortwave, shortwaves, shortwave troughs

A smaller, faster-moving trough embedded in the larger jet stream pattern, typically a few hundred to about a thousand miles across. Shortwaves ride through the bigger longwave troughs and ridges, passing over in a day or two. On their own they often bring wind, cooler temps, and a few showers or thunderstorms, but a shortwave that digs far enough south to tap moisture, or phases with another system, can produce a real storm.

Upper-Level Low

Also: upper level low, upper-level lows, upper level lows, closed low, closed lows

A low-pressure center in the middle and upper atmosphere, drawn as a closed circle of height contours on a 500 mb chart (a "closed low"). It forms when a trough deepens enough to close off. An upper-level low that stays connected to the jet keeps moving; one that separates from the jet becomes a cutoff low. The cold air aloft makes the atmosphere unstable, so these systems often bring showers, thunderstorms, and low snow levels.

Cutoff Low

Also: cutoff, cut-off low, cutoff lows, cut-off lows

An upper-level low that has pinched off from the jet stream and is isolated from the main flow. Without the jet to steer it, it can stall for days, wobble around erratically, and bring prolonged unsettled weather, which makes these systems notoriously hard to forecast. Cutoff lows are most common in fall and spring, when the jet is weaker and wavier; in midwinter, strong westerlies usually keep lows moving. Key rule: don't trust any single model run when a cutoff low is involved.

Inside Slider

Also: inside sliders

A low-pressure system that drops south from western Canada on an inland track, east of the Sierra crest, rather than coming in off the Pacific. Inside sliders are typically cold but moisture-starved, since they spend little time over the ocean. On the Eastside they usually bring gusty north winds and a few snow showers, and the offshore pressure pattern they create often kicks off Santa Ana winds in Southern California. If one tracks far enough west to pick up Pacific moisture, it can be a powder machine. A classic boom-or-bust setup: a small shift east or west makes or breaks the storm.

Phasing

When two separate disturbances in the jet stream, often one in the northern branch and one in the southern branch, line up and merge into a single, stronger system. Phasing can turn two modest features into a significant storm, but it depends on precise timing: if one arrives a few hours early or tracks a little too far east, they stay separate and the storm underperforms. That's why forecasts with two moving parts tend to swing from run to run.

Teleconnection

Also: teleconnections

A link between weather in places thousands of miles apart, usually through a recurring large-scale pattern in pressure or the jet stream. ENSO, the PNA, the EPO, the MJO, and the NAO are all teleconnection patterns, each tracked with an index. They tilt the odds toward certain weather for the Sierra without guaranteeing it. Slow ones like ENSO help with seasonal outlooks; fast ones like the PNA and NAO change week to week and mostly describe the current pattern rather than predict a season.

El Niño–Southern Oscillation (ENSO)

Also: ENSO, El Nino, El Niño, La Nina, La Niña, Southern Oscillation

A recurring cycle of ocean temperatures and atmospheric circulation in the tropical Pacific. During El Niño, surface waters in the central and eastern equatorial Pacific run warmer than normal; during La Niña, cooler. An event is declared when the Niño 3.4 anomaly stays beyond about ±0.5°C for several months, and a strong event exceeds about ±1.5°C. Events recur every two to seven years and usually peak in early winter. Strong El Niños tend to energize the subtropical jet and the Gulf of Alaska low, tilting the odds toward a wet California winter, with the wettest response often backloaded toward January and February. But ENSO isn't destiny: some strong El Niño winters have disappointed and some La Niña winters have been huge, because the PNA, EPO, MJO, and polar vortex all shape how a winter plays out.

Niño 3.4 Index

Also: Niño 3.4, Nino 3.4, Niño3.4, Nino3.4, ONI, Oceanic Niño Index, Oceanic Nino Index, RONI, Relative Oceanic Niño Index

The standard measure of ENSO strength: the sea surface temperature anomaly averaged over the Niño 3.4 region of the equatorial Pacific (5°N to 5°S, 170°W to 120°W). Its three-month running mean is the Oceanic Niño Index (ONI), which defines El Niño (+0.5°C or warmer) and La Niña (−0.5°C or cooler). NOAA's Climate Prediction Center also publishes a Relative ONI (RONI), which subtracts the warming of the tropics as a whole so that a warmer ocean doesn't make every year look like El Niño. The Climate Lab on this site classifies each winter's ENSO state by its December–February RONI.

East-Based El Niño

Also: east-based El Nino, east-based, central Pacific El Niño, El Niño Modoki, Modoki

An El Niño whose warmest water sits in the eastern equatorial Pacific toward South America, as opposed to a central Pacific (Modoki) El Niño, where the warmth peaks near the dateline. East-based events pull the tropical thunderstorms and the warm pool farther east, which tends to strengthen the subtropical jet into California. 1982-83 and 1997-98 are the textbook east-based events, and both brought very wet winters to California. Central Pacific events have a much less reliable California signal.

Indo-Pacific Warm Pool (IPWP)

Also: IPWP, Indo-Pacific Warm Pool, warm pool, ENSO Longitude Index

The largest body of very warm ocean water on Earth (typically above about 28°C), stretching from the Indian Ocean across Indonesia into the western Pacific. Tropical thunderstorms cluster over it, and their heating drives much of the atmosphere's large-scale circulation. During El Niño, the eastern edge of the warm pool slides east along the equator and the thunderstorms follow. The ENSO Longitude Index tracks the average longitude of that thunderstorm activity, estimated from where the water is warm enough to support it: the farther east it reaches, the farther east the tropical heating and the stronger the response in the North Pacific jet.

Westerly Wind Burst (WWB)

Also: westerly wind burst, westerly wind bursts, WWB, WWBs

A spell of unusually strong westerly winds near the surface over the western or central equatorial Pacific, lasting several days to a few weeks. Normally the trade winds blow from east to west; a WWB reverses them locally. That pushes warm water eastward and launches oceanic Kelvin waves that deepen the warm layer beneath the eastern Pacific. WWBs are one of the main ways El Niño starts and strengthens, and the MJO passing through the Pacific often triggers them.

Walker Circulation

Also: Walker cell, standing wave, Maritime Continent, velocity potential

The east-west loop of air over the tropical Pacific. Air rises over the warm western Pacific and the Maritime Continent (Indonesia and the surrounding islands), flows east aloft, sinks over the cooler eastern Pacific, and returns west near the surface as the trade winds. During El Niño the loop weakens and shifts: rising air and thunderstorms park over the central and eastern Pacific as a quasi-stationary "standing wave," with sinking air over the Maritime Continent. Upper-level velocity potential maps show this, usually with rising air in green and sinking air in brown. When the atmosphere settles into that configuration, El Niño is said to be coupled.

Pacific-North American Pattern (PNA)

Also: PNA, Pacific-North American, Pacific North American

A recurring wave train in the jet stream that arcs from the central Pacific near Hawaii, over the Gulf of Alaska and western North America, and down into the southeastern US. In the positive phase, a deep low sits over the North Pacific and a ridge builds over western Canada and the Rockies, usually meaning milder, drier weather for the Pacific Northwest and often the Sierra, though a strong southern jet can still undercut that ridge. In the negative phase, troughing over the West opens the door to storms and colder air. The PNA shifts on a timescale of a few weeks; El Niño winters tend to lean positive.

Eastern Pacific Oscillation (EPO)

Also: EPO, Eastern Pacific Oscillation, East Pacific Oscillation

Describes the pattern of 500 mb heights over Alaska and the eastern North Pacific. In the negative phase, a ridge builds over Alaska and the Gulf of Alaska and sends cold air south into western Canada and the US. For the Sierra, that usually means storms dropping in from the north: cold, but moisture-starved and dependent on the exact track. In the positive phase, a low sits over the Gulf of Alaska and a strong Pacific jet floods the West with milder Pacific air and storms, often aimed at the Pacific Northwest first.

North Atlantic Oscillation (NAO)

Also: NAO, North Atlantic Oscillation

The leading pattern of pressure variability over the North Atlantic, measured by the sea-level pressure difference between the Azores High and the Icelandic Low. In the positive phase, both are stronger than normal, driving a strong jet across the Atlantic into Europe; the negative phase is the reverse and favors blocking and cold outbreaks in the eastern US and Europe. The NAO is closely related to the Arctic Oscillation (AO). It strongly affects eastern US temperatures, but its influence on California precipitation is weak and indirect, and it changes on a timescale of weeks, so it describes the current pattern more than it predicts a season.

Pacific Decadal Oscillation (PDO)

Also: PDO, Pacific Decadal Oscillation

A long-lived pattern of sea surface temperatures in the North Pacific. In the positive (warm) phase, water along the North American coast runs warm and the central North Pacific runs cool; the negative (cool) phase is the reverse. The PDO can stay in one phase for years, and much of its variation is driven by ENSO and the Aleutian Low rather than being an independent cycle. On its own, it has a surprisingly small effect on California winter precipitation, and a strong El Niño typically outweighs it.

Quasi-Biennial Oscillation (QBO)

Also: QBO, Quasi-Biennial Oscillation, EQBO, WQBO

A slow reversal of the winds high above the equator in the stratosphere, alternating between easterly and westerly about every 28 months on average. Each new phase starts high in the stratosphere and works its way down over many months, so different levels can show opposite phases at the same time; this site uses the 50 mb wind (about 67,000 ft). On its own, the QBO has only a small effect on California precipitation, but it modulates other players: an easterly QBO makes sudden stratospheric warmings more likely, and the QBO changes how the MJO reaches the mid-latitudes, so the same MJO phase can bring opposite results in easterly and westerly QBO winters.

Madden-Julian Oscillation (MJO)

Also: MJO, Madden Julian Oscillation, Madden-Julian Oscillation, RMM

A large cluster of tropical thunderstorms, and the circulation that goes with it, that travels east from the Indian Ocean across the Maritime Continent and into the Pacific, circling the globe in roughly 30 to 60 days. Its position is described by eight phases on an RMM diagram: phases 2 and 3 over the Indian Ocean, 4 and 5 over the Maritime Continent, 6 and 7 over the western Pacific, and 8 and 1 over the Western Hemisphere and Africa. Distance from the center of the diagram is its strength; inside the circle, the MJO is considered weak. Its tropical heating sets off waves in the jet stream that reach the mid-latitudes in about one to three weeks, making it one of the main sources of predictability beyond 10 days. The effect on California depends heavily on the season, ENSO, and the QBO: this site's composites show little signal from the MJO phase alone, but clearer ones once ENSO or the QBO is taken into account.

East Asian Mountain Torque (EAMT)

Also: EAMT, East Asian Mountain Torque, mountain torque

The push of the atmosphere against the mountains and high plateaus of East Asia, including the Tibetan Plateau. When strong high pressure builds over Mongolia and China, the pressure difference across these mountains transfers momentum between the solid Earth and the atmosphere. A positive EAMT tends to extend the Pacific jet stream eastward over the following week or two, which can feed troughing downstream in the Gulf of Alaska and an active pattern for the West Coast.

Weather Model

Also: model run, model runs, dynamical model, dynamical models

A computer simulation of the atmosphere. Traditional (dynamical) models start from a snapshot of current conditions (temperature, wind, moisture, and pressure at every level, blended from observations through data assimilation) and step the laws of physics forward in time. AI models learn the same task from decades of past weather. Either way, a single model run is one possible outcome, not a promise. Always compare multiple models and watch the trends from run to run.

Operational Model

Also: deterministic model, deterministic models, operational models, deterministic run

A model's main, single "best guess" run, also called a deterministic run. It starts from the best available analysis of current conditions and usually runs at higher resolution than that model's ensemble members. Global operational models run out to about 10 to 16 days, with output typically every 1 to 6 hours. They're most useful within about 5 days; beyond that, treat a single run as one plausible scenario rather than a forecast, and lean on ensembles instead.

Ensemble Model

Also: ensemble, ensembles, EPS, GEFS, ensemble models, ensemble mean, ensemble members

A set of many runs of the same model (typically 20 to 50 or more "members"), each started from slightly different initial conditions and often with small tweaks to the model physics, to capture the uncertainty in a forecast. When most members agree, the signal is far more trustworthy than any single run; when they fan out, the forecast is genuinely uncertain. The ensemble mean smooths out noise but also washes out extremes, so look at the spread too. Beyond about 5 days, ensembles are the best tool available. Examples include the ECMWF ENS (51 members), AIFS ENS (51), GEFS (31), Canadian GEPS (21), and ICON-EPS (40).

Ensemble Percentiles

Also: percentile, percentiles

A way of summarizing an ensemble by ranking its members. The 50th percentile (the median) is the middle member: half are wetter, half drier. The 10th percentile is near the dry end and the 90th near the wet end, so the range between them covers most plausible outcomes, while the 25th to 75th percentile range covers the middle half. Percentiles show how wide the uncertainty is in a way a single mean or deterministic run can't. Precipitation amounts are skewed, since a few very wet members can pull the mean well above what most members show, which is why the precipitation lines on this site's model plumes are medians.

Ensemble Plume

Also: ensemble plumes, plume diagram, plume chart, model plumes, precipitation plumes, snowfall plumes

A chart showing a forecast variable (precipitation, snowfall, snow level, temperature) over time at one location, with a line for each ensemble member. When the lines stay bundled together, the forecast is confident; when they fan out like a plume of smoke, it isn't. Plumes are one of the best ways to see the full range of outcomes for a specific place, like Mammoth's Main Lodge, rather than relying on a single number.

Global Forecast System (GFS)

Also: GFS, Global Forecast System

The US global model, run by NOAA's National Centers for Environmental Prediction (NCEP) four times a day out to 16 days. It's free and widely available, which is why so many weather apps use it. It has improved a lot, but it still tends to be jumpier from run to run than the ECMWF, showing a big storm one run and dropping it the next. Its ensemble is the GEFS.

European Centre for Medium-Range Weather Forecasts (ECMWF)

Also: ECMWF, Euro, European model, the euro, IFS, IFS ENS, IFS HRES, HRES, Euro ensemble

An intergovernmental forecasting center based in Reading, UK, supported by more than 30 countries. Its physics model, the IFS, is widely considered the most skillful global model. ECMWF runs a single high-resolution forecast (HRES, about 9 km, out to 15 days), a 51-member ensemble (ENS), extended-range weeklies out to 46 days, and seasonal forecasts out to 7 months, plus its AI model, the AIFS. Often referred to simply as "the Euro."

AIFS (ECMWF AI Model)

Also: AIFS, AIFS ENS, AIFS Single, Euro AI, Euro AI model

The Artificial Intelligence Forecasting System, ECMWF's machine-learning model. It starts from the same analysis as ECMWF's physics model (the IFS) and comes in two versions, both operational since 2025: AIFS Single, a deterministic run, and AIFS ENS, a 51-member ensemble. It's often called the "Euro AI" to tell it apart from the IFS. Its scores rival or beat the IFS for many large-scale fields, which makes the two a useful pair to compare.

ECMWF Weeklies

Also: Euro Weeklies, weeklies, extended range, extended-range, subseasonal

ECMWF's extended-range ensemble, which runs every day out to 46 days. Instead of day-by-day detail, it's read as weekly or monthly averages: is a given week likely to be wetter, drier, warmer, or colder than normal? Skill drops off a lot after week two, but a signal that holds consistently from run to run in weeks three and four is worth paying attention to.

GDPS (Canadian Model)

Also: GDPS, Canadian model, GEPS, Canadian ensemble

The Global Deterministic Prediction System, run by Environment and Climate Change Canada (ECCC). Its ensemble counterpart is the GEPS, and ECCC's seasonal system is CanSIPS. Together they offer an independent perspective alongside the American and European models.

ICON (German Model)

Also: ICON, ICON-EPS, ICON EPS

The Icosahedral Nonhydrostatic model from the German Weather Service (DWD), named for the grid of triangles it uses to cover the globe. The global version runs at about 13 km, and its ensemble, ICON-EPS, has 40 members. It provides another independent take on the atmosphere.

AI Weather Models

Also: AI model, AI models, machine learning model, machine learning models

Weather models that use machine learning in place of traditional physics. Trained on decades of past weather, mostly the ERA5 reanalysis, they learn how the atmosphere evolves and can produce a forecast in minutes on far less computing power. They still start from the same observation-based analysis as physics models. Leading examples include ECMWF's AIFS, Google's GraphCast, GenCast, and WeatherNext, and Huawei's Pangu-Weather. They now match or beat physics models for large-scale patterns in the medium range, though they can smooth out extremes and fine detail.

WeatherNext 3

Also: WeatherNext, WeatherNext3, WN3

An experimental AI weather model from Google, run as a 64-member ensemble. Google releases its forecasts as experimental data intended for research, not for real-world decisions, which is why blog posts that show it include a citation and disclaimer.

Seasonal Models

Also: seasonal model, seasonal guidance, seasonal forecasts, Copernicus, C3S, multi-model mean, CanSIPS, SEAS5

Coupled ocean-atmosphere models run once a month out to six months or more, forecasting how a whole month or season is likely to compare with normal. They can't predict individual storms, but they capture slow-moving drivers like ENSO. The Copernicus Climate Change Service (C3S) gathers seasonal forecasts from several centers, including ECMWF (SEAS5), the UK Met Office, Météo-France, DWD, and NCEP, so they can be averaged into a multi-model mean, and Canada's CanSIPS adds another. Individual systems often miss, so agreement across many models matters more than any one.

Hi-Res Model

Also: hi-res, high resolution model, RRFS, HREF, REFS

Models with fine spatial resolution (about 3 km or less) that cover a limited area and run frequently, some every hour. They're best at 0 to 48 hours because they resolve the terrain and can simulate individual thunderstorms and precipitation bands directly instead of approximating them. Examples include the HRRR, the NAM 3 km, and the newer RRFS. Short-range ensembles such as HREF, and its successor REFS, combine several hi-res runs.

HRRR

Also: High Resolution Rapid Refresh

The High Resolution Rapid Refresh, a 3 km NOAA model that runs every hour out to 18 hours, extending to 48 hours four times a day (00, 06, 12, and 18 UTC). The go-to model for short-range detail: precipitation bands, snow levels, thunderstorm cells, and timing. If a storm is within two days, this is where you look.

National Blend of Models (NBM)

Also: NBM, National Blend, National Blend of Models

A statistically post-processed blend of dozens of models and ensembles, produced by the National Weather Service. The NBM corrects each model for its known biases and weights them by recent performance, giving NWS forecasters a consistent starting point for many of their forecasts. Blends like this often beat any individual model on average, because individual errors partly cancel out, though they can also smooth away the extremes of a big storm.

Atmospheric River

Also: atmospheric rivers, Pineapple Express

A long, narrow corridor of concentrated water vapor moving through the lower atmosphere, usually ahead of a cold front, carrying moisture from the tropics and subtropics into the mid-latitudes. ARs deliver roughly 30 to 50% of California's annual precipitation and most of its biggest floods. The "Pineapple Express" is a type of AR whose moisture traces back to near Hawaii. ARs are measured by integrated vapor transport (IVT) and ranked from AR1 (weak) to AR5 (exceptional) by strength and duration. Warm ARs bring high snow levels and rain at ski-area elevations, while colder ones can bring huge dumps of snow. When an AR runs into the Sierra, orographic lift wrings out the moisture, and that's where the big totals come from.

Gulf of Alaska Trough

Also: Gulf of Alaska low, Gulf of Alaska troughing, storm door

A deep trough or low parked over the Gulf of Alaska and the northeast Pacific. It strengthens and extends the Pacific jet stream and sends a train of storms toward the West Coast, which is why it's called an open storm door. Where those storms land depends on how far south it sits: centered off British Columbia it favors the Pacific Northwest, and storms that do reach California can pull up mild air and high snow levels; when it sags farther south, the storm track shifts toward California. A persistent Gulf of Alaska trough is a classic El Niño winter pattern.

Bomb Cyclone

Also: bombogenesis, bomb cyclones

A storm that undergoes explosive intensification (bombogenesis): its central pressure drops by at least 24 millibars in 24 hours at 60° latitude, with a lower threshold farther south (about 18 mb at 40°N). These intense storms can bring extreme winds and heavy precipitation. When one targets the Sierra, it's usually a memorable event.

Narrow Cold Frontal Rainband (NCFR)

Also: NCFR, narrow cold frontal rainband

A tight, intense band of precipitation that forms along particularly sharp cold fronts. It's easy to spot on radar as a narrow line of heavy precipitation. NCFRs typically bring strong, gusty winds and brief but intense rain or snow. When one sweeps through, you'll know it: conditions can go from calm to wild in minutes.

Aleutian Ridge

Also: Aleutian ridge

A ridge of high pressure over the Aleutian Islands and the central North Pacific. It diverts the storm track north around the ridge and then sends it back south along the ridge's eastern side, toward the West Coast. With the ridge well offshore, that can mean cold storms dropping down the coast into California; with the ridge too close to the coast, it means dry north flow instead. The farther west the ridge, the better the odds for Sierra snow.

Hudson Low

Also: Hudson low

A low or deep trough centered over Hudson Bay in Canada. It usually comes with a wavy (meridional) pattern: deep troughing in the east and, often, a ridge upstream over western North America. That can keep California dry while the northern tier and eastern US turn cold and active.

Meridional Flow

Also: meridional

A flow pattern with a strong north-south component, following lines of longitude. The jet stream makes big, wavy dips and peaks rather than flowing straight west to east. Meridional flow leads to more extreme weather: deeper troughs, stronger ridges, slower-moving systems, and bigger temperature swings.

Zonal Flow

Also: zonal

A flow pattern running mainly west to east along lines of latitude. When the jet stream is zonal, it's relatively flat and fast, producing a steady parade of quicker, usually weaker storms rather than big, dramatic events. The strong westerlies of a zonal winter jet also keep lows moving, which is one reason cutoff lows are less common in midwinter. Good for consistent but unspectacular skiing.

Mono Lake Convergence Zone

Also: Mono Lake convergence, convergence zone

A local phenomenon where two flows collide in the Mono Basin during storms: cold easterly flow from the Great Basin meets the moist flow spilling over the Sierra from the Pacific. That collision of air masses, plus extra moisture from the salty, rarely frozen Mono Lake, can produce surprising, localized snow and precipitation on the east side.

Santa Ana Winds

Also: Santa Ana, Santa Anas

Very dry, gusty winds from the northeast or east that blow through the passes and canyons of Southern California, most often from fall through winter. They develop when high pressure builds over the Great Basin with lower pressure off the coast. Air flowing out of the high descends toward the coast, compressing, warming (adiabatic warming), and drying out along the way. Santa Anas are a major wildfire concern. Inside sliders and cutoff lows that track over the Great Basin often set them up, which is why a storm that brings gusty north winds to the Eastside can bring Santa Anas to Southern California.

Orographic Lift

Also: orographic, orographics, orographically

Moist air being forced upward as it runs into terrain like a mountain range. As the air rises, it cools and condenses, producing clouds and enhanced precipitation. This is why the western slope of the Sierra gets far more precipitation than the east side. Mammoth is orographically favored because it sits at a low gap in the crest at the head of the Middle Fork of the San Joaquin, a southwest-facing canyon that funnels moisture uphill. Most atmospheric rivers carry moisture from the southwest, so this alignment is ideal. With southerly flow, however, the high terrain of the southern Sierra blocks Mammoth, and it tends to be rain shadowed relative to nearby peaks.

Rain Shadow

Also: rain shadow, rainshadow, rain shadowed, rainshadowed

The dry zone on the downwind (lee) side of a mountain range. As moist air rises over the mountains, it drops most of its precipitation on the windward side. By the time it descends the other side, it has been wrung dry and warms as it sinks. The Owens Valley is one of the most dramatic rain shadows in North America: Bishop averages only about 5 inches of precipitation a year, while the crest just to its west gets several times that. This is why the east side is high desert. Rain shadows also depend on wind direction, so a peak that's favored in one storm can be shadowed in the next.

Polar Vortex

Also: polar vortex, stratospheric polar vortex, stretched polar vortex

A large ring of strong westerly winds circling the Arctic in the stratosphere, roughly 10 to 30 miles up, that forms each fall and breaks down in spring. A strong, round vortex keeps cold air bottled up near the pole. A weaker vortex can be stretched into an elongated shape, displaced off the pole, or split by a sudden stratospheric warming. A stretched vortex tends to favor a ridge over western North America and a trough over the east, and a displaced or split vortex can send cold outbreaks south in the weeks that follow. It's one of the main wildcards for winter patterns beyond two weeks.

Sudden Stratospheric Warming

Also: sudden stratospheric warming, stratospheric warming, SSW

A rapid warming of the polar stratosphere, sometimes by 30 to 50°C in a few days, that happens when large Rossby waves travel up from the lower atmosphere and break against the polar vortex. In a major SSW, the vortex's westerly winds reverse to easterly at about 100,000 ft (10 mb, 60°N), and the vortex is knocked off the pole or split in two. The effects can work down to the surface over the following weeks, shifting the jet stream and storm tracks and sending cold air into the mid-latitudes. Models can often see an SSW coming one to two weeks ahead, but how, and whether, it affects surface weather is much harder to predict.

Temperature Inversion

Also: inversion, inversions

A layer of the atmosphere where temperature increases with altitude instead of the normal decrease. In the Owens Valley, inversions are common in winter: cold air pools on the valley floor while warmer air sits above. This can mean freezing fog and sub-zero temps in Bishop while it's 20°F warmer at Mammoth's base lodge. Inversions also trap smoke and pollutants and limit visibility. For skiers, a strong inversion means you might drive through a fog layer into bluebird skies above.

Lenticular Cloud

Also: lenticular, lenticulars, wave cloud, wave clouds

Lens- or saucer-shaped clouds that form downwind of mountain ranges when stable, moist air flows over the terrain and sets up standing waves. They're extremely common over the Eastern Sierra and a hallmark of the landscape. They look dramatic (people mistake them for UFOs), but mostly they indicate strong winds aloft. For skiers, lenticulars often mean wind holds on upper lifts.

Radar

Shows real-time storm structure: where rain or snow is intensifying or weakening, how much of a storm is left to come, the melting layer near the snow level, and individual thunderstorm cells and their tracks. It's the earliest indicator of boom or bust from a storm. In the Sierra, mountains block much of the radar beam, so coverage is patchy and radar often misses or underestimates precipitation, especially on the east side. RadarScope is great for detailed analysis, and the free NWS radar works well too.

North American Monsoon

Also: monsoon, monsoonal

Unlike winter storms, which are driven by the jet stream, summer thunderstorms get their energy from the sun. Daytime heating combined with moisture over the mountains creates localized rising motion; with enough moisture and lift, you get storms. In summer, a "Four Corners high" sets up over the desert Southwest, and its clockwise circulation draws moisture north from the Gulf of California and Gulf of Mexico, fueling thunderstorms. The monsoon delivers a large share of the desert Southwest's annual precipitation, and surges of monsoonal moisture periodically reach the Sierra.

CAPE (Convective Available Potential Energy)

Also: CAPE, Convective Available Potential Energy

A measure of how much energy is available for thunderstorm updrafts. Over the mountains, as little as 300 to 500 J/kg of CAPE can be enough for storms when there's moisture and upslope flow, much less than the 1,000+ J/kg typically needed over flat terrain. CAPE, combined with mid-level humidity (at 700 mb and 500 mb), is one of the key parameters for forecasting summer thunderstorms. CAPE + moisture + daytime heating over mountains = recipe for storms.

Climatology

Also: climo, climatological

The long-term average weather for a place and time of year, usually based on 30 years of data. Forecasts and anomalies are measured against it: "150% of normal" means 1.5 times the climatological average. Climatology also describes what's typical for the season, like the storm track sliding south from the Pacific Northwest in November toward California by January and February.

Snow to Water Ratio

Also: snow-to-water, snow ratio, Sierra Cement

The ratio of snowfall depth to its liquid water equivalent. The classic rule of thumb is 10:1, meaning 10 inches of snow per inch of water. Sierra Cement, the heavy, dense snow the Sierra is famous for, often comes in at 6 to 8:1 in warm, wet storms, while cold storms can reach 15:1 or more. Utah's famously dry snow commonly runs 13 to 15:1, and occasionally 20:1 or higher. Lower ratios mean heavier, wetter snow that's great for the snowpack but tough to ski; higher ratios mean lighter, drier powder.

Snow Level

Also: snow line, snow levels

The elevation where precipitation changes from rain to snow during a storm. It's typically about 1,000 ft below the freezing level, because snowflakes take time to melt as they fall through above-freezing air, and it can drop even lower in heavy precipitation or dry air as melting and evaporation cool the air. This is critical for the Eastern Sierra: a storm with a 6,000 ft snow level dumps snow on all of Mammoth, while the same storm with a 9,000 ft snow level means rain at Main Lodge. Warm atmospheric rivers push snow levels up; cold storms bring them down.

Freezing Level

Also: freezing level, 0°C level

The altitude where the air temperature drops to 0°C (32°F). It's not the same as the snow level, which is typically about 1,000 ft lower because snow takes time to melt as it falls through slightly-above-freezing air. The freezing level determines what kind of precipitation falls at different elevations and is one of the first things to check when a storm approaches.

Sea Surface Temperature Anomalies

Also: SST, sea surface temperature, SST anomalies, SSTs

How far sea surface temperatures depart from their average for the date. Ocean temperatures shape the large-scale pattern of a winter by influencing where tropical thunderstorms form, the strength and position of the jet stream, and how much moisture is available. ENSO is the best-known SST anomaly pattern, but the PDO and regional SSTs off the West Coast also matter for California storms.

QPF (Quantitative Precipitation Forecast)

Also: QPF, liquid equivalent

The forecast amount of liquid precipitation (rain plus melted snow) over a period, in inches. Maps show where it falls; an ensemble mean QPF averages all the members, while percentiles show the range. QPF is one of the most practical forecast fields for the mountains: multiply it by the snow-to-water ratio to estimate snowfall.

Integrated Vapor Transport (IVT)

Also: IVT, integrated vapor transport, integrated water vapor transport

How much water vapor is moving horizontally through a column of the atmosphere, combining how much moisture is there with how fast the wind carries it. It's measured in kilograms per meter per second (kg/m/s). Values above 250 define an atmospheric river; the AR scale then bins strength into 250 to 500 (weak), 500 to 750 (moderate), 750 to 1,000 (strong), 1,000 to 1,250 (extreme), and above 1,250 (exceptional), adjusted for duration. IVT is the best single number for spotting an AR: a moist airmass with light winds won't deliver much, while strong winds pushing moisture into the mountains will.

Precipitable Water (PWAT)

Also: precipitable water, PWAT, TPW

The total water vapor in a column of atmosphere from the surface to the top, expressed as inches of liquid. If you condensed every drop of moisture in the air above you, that's the PWAT value. Values above about 1 inch are moist for the Sierra region, and atmospheric rivers often carry 1 to 1.5 inches or more as they approach the coast. PWAT shows how much moisture is there; IVT shows how much of it is moving.

Snow Water Equivalent (SWE)

Also: SWE, snow water equivalent

The amount of liquid water contained in the snowpack: if you melted the entire snowpack, how deep would the water be? SWE is the most important measure of water supply and snowpack health. A deep snowpack with low SWE (light, fluffy snow) holds less water than a shallower pack with high SWE (dense, wet snow). California's water supply depends heavily on Sierra SWE, which is measured by snow pillows and snow courses reported through CDEC, plus NRCS SNOTEL stations.

Isothermal Snowpack

Also: isothermal

When the entire snowpack has warmed to 32°F (0°C) from top to bottom. This happens during warm storms or prolonged warm weather, especially with rain on snow. An isothermal pack has no cold left to refreeze meltwater internally, so water percolates through and the snow becomes heavy and structurally weak. For skiers, it means wet, heavy conditions and increased wet-slide avalanche risk until sustained cold refreezes it. In spring, once the pack has ripened, daily melt-freeze cycles at the surface produce corn snow.

Corn Snow

Also: corn, spring corn, corn snow

The holy grail of spring skiing. Corn forms through a daily melt-freeze cycle: the snow surface melts during the day and refreezes overnight into a hard crust. The next morning, as the sun warms the surface, the top layer softens into smooth, granular snow that's a blast to ski. Timing is everything: too early and you're on bulletproof ice, too late and you're in slush. Follow the sun: east-facing slopes soften first, then south-facing, then west-facing in the afternoon.

Wind Loading

Also: wind loaded, wind-loaded, wind loading

Wind moving snow from exposed (windward) terrain and depositing it on sheltered (leeward) terrain, building slabs and pillows. Wind loading can add significant depth to lee slopes, often creating the best skiing but also the most dangerous avalanche conditions. On the Sierra crest, prevailing westerly winds load east-facing bowls and chutes. If the wind is howling from the west, east-facing terrain is likely getting loaded and should be approached with caution.

Hovmöller Diagram

Also: Hovmöller, Hovmoller

A chart with longitude (or latitude) on one axis and time on the other, so you can watch features move. In tropical forecasting it's used to track westerly wind bursts, the MJO, and ocean Kelvin waves as they travel east along the equator: a feature moving steadily shows up as a slanted streak.

Winter Weather Advisory

Also: Winter Weather Advisories, Winter Storm Warning, Winter Storm Watch

Winter alerts are issued by local National Weather Service offices (NWS Reno for Mono County, NWS Las Vegas for Inyo County). A Winter Storm Watch means a significant winter storm is possible in the next few days. A Winter Storm Warning means one is expected or occurring, with heavy snow or ice that will make travel very difficult. A Winter Weather Advisory is a step below: snow, blowing snow, or ice that will be a hassle and could make travel hazardous, but below warning levels. Snowfall thresholds vary by elevation and region.