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.
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 conditions are favorable for hazardous weather to develop: be prepared. A Warning means hazardous weather is imminent or occurring: take action immediately. Warnings are more urgent than watches.
CAPE
Convective Available Potential Energy, a measure of atmospheric instability and thunderstorm potential. CAPE values above 1000 J/kg indicate instability, 2500 to 3500 J/kg is very unstable, and extreme instability starts above roughly 3500 to 4000 J/kg.
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. NCEP retires the NAM at 12 UTC on October 6, 2026, along with SREF, HREF, HiresW, and NAM MOS; the Rapid Refresh Forecast System (RRFS) and its ensemble REFS take over that day.
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.
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.
Explore Other Terms
Tornado
A violently rotating column of air extending from a thunderstorm to the ground. Tornadoes can produce winds exceeding 300 mph and cause catastrophic damage along their path. They are rated on the Enhanced Fujita (EF) scale from EF0 to EF5.
Supercell
A severe thunderstorm characterized by a deep, persistently rotating updraft called a mesocyclone. Supercells are the most dangerous type of thunderstorm and can produce tornadoes, large hail, damaging winds, and flash flooding.
Freezing Rain
Rain that falls as liquid but freezes on contact with surfaces at or below 32°F (0°C), creating a coating of ice called glaze. Freezing rain is extremely dangerous as it can make roads impassable and cause widespread power outages from ice-coated power lines.
Sleet
Frozen raindrops that bounce when hitting the ground. Sleet forms when snowflakes partially melt falling through a warm layer, then refreeze in a cold layer near the surface. Unlike freezing rain, sleet is frozen before it hits the ground.
Barometric Pressure
The weight of the atmosphere pressing down on a given point, measured in inches of mercury (inHg) or millibars (mb). Rising pressure typically indicates improving weather, while falling pressure suggests approaching storms or precipitation.
Dew Point
The temperature at which air becomes saturated and water vapor begins to condense into liquid. A higher dew point indicates more moisture in the air. Dew points above 65°F feel humid, while above 70°F is considered oppressive.
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Learn Ensemble Forecast in Context
These explainers and product surfaces connect this definition to real-world forecasting and decision-making.
Weather Education
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Weather Education
GFS vs HRRR vs NAM: Which Weather Model Should You Trust?
Learn the differences between GFS, HRRR, NAM, and the Euro model. Understand which forecast model to trust for different situations and how weather enthusiasts can interpret model data.
Weather Education
What Is the Polar Vortex? Understanding Extreme Winter Cold
Learn what the polar vortex is, how it affects winter weather across North America, why it sometimes 'breaks down' to deliver extreme cold, and what to expect during polar vortex events.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Feature
Conversational AI
Ask about rain timing, alerts, and what the forecast means in plain language.
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.
Guide
How to Read Weather Radar
Learn the basics of radar interpretation so you can tell where storms are headed and how intense they may become.
See Ensemble Forecast in Action
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