898 
AGUS74 KWCO 272259
AHDNWC

Area Hydrological Discussion #120 - EXPERIMENTAL
NWS National Water Center - Tuscaloosa, AL
5:54 PM CDT Sat May 27 2023

WHAT: Localized flash and small stream flooding
WHERE: Portions of western SD, northeast WY, and southeast MT
WHEN: Through late Saturday evening

DISCUSSION...
Heavy rain from scattered thunderstorms could lead to flash and small
stream flooding impacts in the area of concern through this evening.
Antecedent conditions are favorable for runoff as soils are
relatively moist throughout the area (NWM). Snowmelt and previous
rainfall has streamflows running remarkably high, over the 90th
annual percentile (USGS), indicating little additional runoff is
needed before flood impacts will begin. Any areas that receive heavy
or persistent rainfall are especially at risk for flash and small
stream flooding.

The NWM SRF is signaling rapid-onset flooding (ROF) probabilities of
25 - 50% with scattered pockets of 50 - 75%. The High Flow Magnitude
(HFM) forecast is signaling widespread annual exceedance
probabilities (AEPs) of 50% and isolated stream reaches with AEPs of
20 - 10%. The highest ROF signals and the lowest (most extreme) AEP
values are found between the SD/WY border and Rapid City, SD. These
signals indicate the threat for isolated significant flash and small
stream flooding impacts within the area of concern.

GRAPHICAL REPRESENTATION weather.gov/owp/operations-ahd

Additional National Water Center products are available at
weather.gov/owp/operations


//Bliss

ATTN...WFO...UNR...BYZ...CYS
ATTN...RFC...KRF...WPC
