Titan U glossary

Quantitative Precipitation ForecastQPF

A Quantitative Precipitation Forecast (QPF) is a forecast of how much precipitation will fall over a given area during a set period of time. Instead of only the chance that rain or snow occurs, it states an amount, which in the United States is usually given in inches.

Storm Forecasting Rainfall

How WPC issues it

The Weather Prediction Center (WPC) issues the national QPF. For Days 1, 2 and 3, forecasters manually create amounts for 6-hour periods plus a 24-hour total, and for Days 4 to 7 they create 24-hour amounts.

WPC also publishes multi-day totals for Days 1 to 3, Days 1 to 5 and Days 1 to 7. Those longer totals are useful for planning but hide the timing of individual bursts of rain.

What a value means

WPC defines its QPF as the expected areal average, in inches, on a 20 by 20 km grid, about 12 by 12 miles. A number on the map describes that box, not the rain gauge in one backyard.

One number or a range

A standard QPF map is deterministic, giving one best-estimate amount for each place. WPC's probabilistic QPF has added uncertainty by building a probability distribution around its own 6-hour QPF, using the spread of an ensemble of model forecasts.

Those products show either the chance that 6- or 24-hour totals will equal or exceed a threshold, or the amount tied to a given percentile. They cover Days 1 through 3. A September 2026 National Weather Service (NWS) notice set the Probabilistic Precipitation Portal, built on National Blend of Models ensemble information, to become their operational home on or about 10/01/26, so the method behind them may change.

Where it goes and where it struggles

QPF is built into river forecasts, where it helps predict flood potential and rises to critical river levels. After the event, forecasters compare it with quantitative precipitation estimates of what actually fell.

Thunderstorm rain is the hardest to pin down, because the heaviest totals often fall in narrow swaths. A modest area average can include a much wetter spot under slow or repeating storms, so read QPF together with flood outlooks rather than as a cap.

Keep going

How Quantitative Precipitation Forecast connects

Builds on 2

Rainfall Accumulation
A QPF states an expected amount over a set area and time window, so it builds on the idea of a rainfall total tied to its period.
Ensemble Forecast
Probabilistic QPF adds uncertainty to a single amount by using the spread of an ensemble of model forecasts.

Leads to 2

River Crest
Crest forecasts lean on forecast rain, so the expected height and timing can change as new rain is added to the river models.
Hydrograph
A forecast hydrograph starts from the rainfall forecast, so if more or less rain falls than expected, the real curve departs from it.

Part of 1

Weather Prediction Center
The national QPF is issued by the Weather Prediction Center, whose forecasters build amounts for set periods out to several days.

Contrasts with 1

Quantitative Precipitation Estimate
A QPF forecasts how much precipitation will fall, while a QPE estimates how much already fell, and forecasters compare the two after an event.

Appears here 1

Liquid Equivalent
Liquid equivalent is the unit a precipitation forecast speaks in, since the QPF includes the melted water of snow and ice before any ratio turns it into snow depth.