GFS
Global Forecast System, the primary American global weather model run by NOAA. The GFS provides forecasts out to 16 days and is widely used for medium-range forecasting. It runs four times daily with global coverage.
More Forecasting & Models Terms
SPC Outlook
Severe weather forecasts issued by the Storm Prediction Center categorizing risk levels from Marginal (1) to High (5). Outlooks cover tornadoes, damaging winds, and large hail for 1-3 days ahead and highlight where organized severe weather is most likely.
Watch vs. Warning
A watch means the risk of hazardous weather has risen but its timing, location, or occurrence is still uncertain: be prepared. A warning means hazardous weather is occurring, imminent, or likely and threatens life or property: take action now. An advisory covers events that are occurring or likely but less serious than a warning.
CAPE
CAPE (Convective Available Potential Energy) is the amount of energy a rising parcel of air can draw from the atmosphere, measured in joules per kilogram (J/kg). The higher the CAPE, the faster a storm's updraft can rise, and the stronger thunderstorms can become once something sets them off.
Lifted Index
A stability index comparing the temperature of a rising air parcel to the environment at 500mb (about 18,000 feet). Negative values indicate instability. Values below -6 suggest strong instability and severe thunderstorm potential.
NAM
North American Mesoscale Model, a regional weather model covering North America at 3-12 km resolution and NCEP's short-range workhorse for mesoscale features like fronts, lake-effect snow, and sea breezes. Its successor is the Rapid Refresh Forecast System (RRFS) with its ensemble REFS, and the same changeover ends SREF, HREF, HiresW, and NAM MOS. The National Weather Service notice of October 2, 2026 set the changeover date as November 3, 2026, moved from October 14.
HRRR
High-Resolution Rapid Refresh, a real-time 3km resolution weather model updated hourly. The HRRR excels at short-range (0-18 hour) forecasts of convection, precipitation, and aviation weather across the contiguous United States.
Ensemble Forecast
A collection of multiple model runs with slightly different initial conditions, showing the range of possible outcomes. Ensemble forecasts help quantify uncertainty and are especially valuable for medium-range predictions.
Probability of Precipitation
Probability of precipitation (PoP) is the chance that a given point receives measurable precipitation, at least 0.01 inch, during the forecast period. A 40% chance of rain means a 4 in 10 chance of at least 0.01 inch at your location. It does not mean rain over 40% of the area or for 40% of the time.
ECMWF
ECMWF is the European Centre for Medium-Range Weather Forecasts, an intergovernmental organization supported by 35 states, and the name commonly given to its global weather model, often called the European model or the Euro. Its physics-based Integrated Forecasting System (IFS) runs on a global grid of about 9 km and forecasts up to 15 days ahead.
Zulu Time
Zulu time, written with a Z, is the name used in aviation and meteorology for Coordinated Universal Time (UTC), the time at the Greenwich prime meridian. Weather observations, model runs, and aviation reports all use it so that a time means the same thing everywhere. 12Z is 8 in the morning Eastern Daylight Time and 7 in the morning Eastern Standard Time.
GFS: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Weather Education
Spaghetti Models Explained: What Those Tangled Hurricane Lines Actually Mean
Spaghetti models are a tangle of individual forecast tracks, not a forecast. Here is what each line is, what the labels like AVNO and EMX stand for, and how to read the spread instead of the scariest strand.
Weather Education
What Is an Invest in Weather? What 90L, 91L, and 92L Actually Mean
An Invest is a data-collection label, not a forecast. Here is what the National Hurricane Center means by Invest 90L, how the numbering and basin codes work, and how an Invest becomes a named storm.
Weather Education
El Niño 2026: How Strong It Is, and What It Actually Means
El Niño is strengthening in 2026, with NOAA calling a strong-to-very-strong event likely through winter and 83% odds of a historic peak that exceeds every event since 1950. What that means for hurricane season and winter.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Guide
GFS vs HRRR vs NAM
Learn which forecast model to trust for different situations and how to interpret model data.
Tool
Pressure Tendency Calculator
Calculate 3-hour barometric pressure tendency from rising or falling readings.
Guide
Understanding Snow Totals
Why snow forecasts give ranges instead of exact numbers: snow ratios, banding, and elevation effects.
Feature
Conversational AI
Ask about rain timing, alerts, and what the forecast means in plain language.
Explore Other Terms
Snow Squall
An intense, short-lived burst of heavy snow accompanied by strong winds and often causing a sudden drop in visibility to near zero. Snow squalls can create extremely dangerous driving conditions despite lasting only 30-60 minutes.
Heat Index
The heat index is how hot it feels when relative humidity is combined with the air temperature. Humid air slows the evaporation of sweat, so the body cools less efficiently and feels hotter than the thermometer reads. At 100°F with 55% relative humidity, the heat index is 124°F.
Velocity
Doppler radar measurement of how fast precipitation is moving toward or away from the radar. Velocity data reveals wind patterns, rotation in thunderstorms, and potential tornado signatures that reflectivity alone cannot show.
IFR
Instrument Flight Rules govern flights conducted under instrument procedures and can be used in clear weather too. Separately, the IFR weather category means a ceiling from 500 to less than 1,000 feet and/or visibility from 1 to less than 3 statute miles, with lower values classified as LIFR. Legal operating requirements depend on airspace, regulations, equipment, and pilot qualifications.
Post-Tropical Cyclone
A post-tropical cyclone is a former tropical cyclone that no longer has enough tropical characteristics to be classified as one. It can still bring heavy rain and strong winds. The two kinds are extratropical cyclones, which draw their energy from temperature contrasts between air masses, and remnant lows, which have winds below 34 knots (39 mph).
Alberta Clipper
A fast-moving low-pressure system that originates near Alberta, Canada, and tracks southeast across the northern United States. Clippers typically bring light snow (1-4 inches) followed by bitter cold and gusty winds.
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