TROPICAL STORM IAN
U.S. Watch/Warning Local Products
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WPC Excessive Rainfall Outlooks:   Day 1   Day 2   Day 3
About this product:
In the Excessive Rainfall Outlooks, the Weather Prediction Center (WPC) forecasts the probability that rainfall will exceed flash flood guidance within 40 km (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. The ERO is a highly collaborative product and benefits from the input of meteorologists and hydrologists among the WFOs, RFCs, and National Water Center.
The risk of excessive rainfall is expressed both probabilistically and categorically according to the table below.
|Risk Category||Probability of Rainfall Exceeding Flash Flood Guidance Within 40 km (25 mi) of a Point|
|Marginal (MRGL)||At Least 5%|
|Slight (SLGT)||At Least 15%|
|Moderate (MDT)||At Least 40%|
|High (HIGH)||At Least 70%|
For more information about this product, please see the documentation on the WPC website.