What the radar thinks it saw

QPEquantitative precipitation estimateHow much rain the radar thinks fell. Radar-only QPE is fast; multi-sensor QPE is adjusted toward the rain gauges that reported inside it and is better and later. Neither is a measurement of rainfall. is an estimate and the E is load-bearing

MRMSMulti-Radar Multi-SensorThe system that merges every radar in the network, plus gauges, satellite and model data, onto one grid and derives products from it including rainfall rate and accumulated precipitation estimates every two minutes. merges every radar in the network onto a common grid and produces rainfall rate estimates every two minutes, plus accumulated QPE in two flavours: radar-only, and multi-sensor, which is the radar field adjusted toward the rain gauges that reported inside it. The second is better and it is also later, because it has to wait for gauges.

Neither is a measurement of rainfall. The radar measures reflectivityHow much of the radar pulse bounced back from whatever the beam was passing through on that tilt. It is the familiar green-to-red precipitation picture, and it describes what is up in the beam, not what is reaching the ground. and a relationship converts it to a rate, and that relationship is a statistical fit to a drop size distribution that varies with the storm. Dual-polA radar upgrade that sends pulses both horizontally and vertically, so the returns say something about the shape of what they hit. It is what lets radar recognise lofted debris. has improved this considerably by letting the radar infer something about drop shape and size. It has not made it a measurement.

Flash floodFlooding that begins within about six hours of whatever caused it, counted from the causative event rather than from when anyone noticed the water. The cause is usually intense rainfall, but a dam failure, a levee failure or an ice jam qualifies and can do it on a clear day. guidance, and the ratio

Rainfall on its own does not tell a forecaster whether anything will flood, because the same three inches behaves differently on saturated ground and on dry. Flash flood guidance, issued by the river forecast centres, closes that gap: it is the amount of rain over a given area and duration needed to bring small streams up to bankfullThe stage at which a channel is full to the top of its banks, with the next increment of water going overland. What flash flood guidance is calculated to reach., given what the ground is currently like. It is recalculated as conditions change, so it falls through a wet week.

QPE to FFG ratio

Observed rainfall divided by the guidance for the same duration, computed for 1, 3 and 6 hour windows. A ratio of 1 or more means the rain that fell has met or beaten what it takes to flood small streams there. It is the single most direct corroboration a flood report can get.

ARI

Average recurrence interval: how unusual the observed rainfall is at that place, expressed as a return period. A 100 year ARIaverage recurrence intervalHow unusual an observed rainfall is at that place, expressed as a return period. A rare rainfall is useful context and is not the same statement as "this will flood". says the rainfall is rare, which is useful context and is not the same statement as "this will flood".

FLASH

Flooded Locations and Simulated Hydrographs. An NSSLNational Severe Storms LaboratoryA research laboratory, not an operational office. It develops the techniques forecasters use, but issues no products and takes no reports. system that runs hydrologic models on the MRMS rainfall field over a grid of roughly one kilometre, turning a rainfall estimate into a streamflow estimate.

CREST

The distributed hydrologic model inside FLASHFlooded Locations and Simulated HydrographsAn NSSL system that runs hydrologic models on the MRMS rainfall field over a grid of roughly one kilometre, turning a rainfall estimate into a streamflow estimate.. Its headline output is maximum unit streamflow, the flow per unit of drainage area, which is comparable between a large basin and a small one.

For CRESTCoupled Routing and Excess StorageThe distributed hydrologic model inside FLASH. Its headline output is maximum unit streamflow, the flow per unit of drainage area, which is comparable between a large basin and a small one. unit streamflow, the reference figures in the literature are around 2 cubic metres per second per square kilometre as a warning-guidance reference, and around 8 as indicative of a potentially significant flash flood. Treat both as order-of-magnitude signposts rather than thresholds; they are model output, and the model inherits every QPE error in the table above.

Forecasting the chance of exceeding guidance

WPCWeather Prediction CenterThe national office for precipitation and excessive-rainfall forecasting. issues the product that everything else hangs off, the excessive rainfall outlookExcessive Rainfall OutlookThe WPC product forecasting the probability that rainfall will exceed flash flood guidance within 40 km of a point. Marginal at least 5%, Slight 15%, Moderate 40%, High 70%, and High is issued on days 1 to 3 only., and its definition is worth knowing precisely because it is not what most people assume. It forecasts the probability that rainfall will exceed flash flood guidance within 40 kilometres, about 25 miles, of a point. It is a probability about a neighbourhood, not about your address.

The practical reading for a spotter: a Marginal is not "nothing will happen". It is a one-in-twenty chance of guidance being exceeded somewhere within a 25 mile circle of any given point, across a large area, which over a season is a great deal of flooding. And a Moderate or High over your county is the day to have the gauge emptied and the low-water crossings in mind before dark.