The Weather Prediction Center (WPC), a division of the National Oceanic and Atmospheric Administration (NOAA), issues an excessive rainfall outlook daily, forecasting the potential for flash flooding up to three days out in time.
The WPC describes this service here:
“In the Excessive Rainfall Outlooks, the Weather Prediction Center (WPC) forecasts the probability that rainfall will exceed flash flood guidance (FFG) within 40 kilometers (25 miles) of a point. Gridded FFG is provided by the twelve NWS River Forecast Centers (RFCs) whose service areas cover the lower 48 states. WPC creates a national mosaic of FFG, whose 1, 3, and 6-hour values represent the amount of rainfall over those short durations which it is estimated would bring rivers and streams up to bankfull conditions. WPC estimates the likelihood that FFG will be exceeded by assessing environmental conditions (e.g. moisture content and steering winds), recognizing weather patterns commonly associated with heavy rainfall, and using a variety of deterministic and ensemble-based numerical model tools that get at both the meteorological and hydrologic factors associated with flash flooding.”
This Wednesday into Wednesday night, portions of central Pennsylvania have been placed in a High Risk for excessive rainfall, the most severe level possible. That indicates at least a 50 percent probability of rainfall exceeding flash flood guidance within 25 miles of a point.
Alex Lamers, a meteorologist at the WPC, shared on Twitter earlier this year what it means when a high risk is issued, describing that “over half (54%) of WPC High Risk days have at least 1 flood-related fatality or injury, compared to 22% of Moderate Risk Days, and 17% of days in general.”
Over half (54%) of WPC High Risk days have at least 1 flood-related fatality or injury, compared to 22% of Moderate Risk Days, and 17% of days in general.
— Alex Lamers (@AlexJLamers) January 29, 2018
The Storm Prediction Center also issues outlooks indicating the likelihood for severe weather. Lamers notes, “just like a High Risk from SPC — a High Risk from WPC is a clear signal of increased likelihood of a serious and very impactful weather event, in this case flooding.”
High Risk days are unusual, and when the WPC issues this probability in their outlook, it often guarantees flooding in some areas. Lamars says that between January 2015 and July 2017—a 31 month period—”WPC had a High Risk valid on 41 days, or ~4% of all days in that time period. But this small subset of days accounted for about 35% of all flood-related fatalities and nearly 80% of flood related damages in the U.S.”
Behind heat, flooding is the most deadly weather-related event in the US over the past 30 years. It’s important to monitor the forecast for the potential for flash flooding from outlets like WeatherOptics to help you stay safe.