213
AGUS74 KWCO 181512
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT WED SEP 18 2024

.Synopsis...
Isolated flooding impacts possible through tonight across Central
Montana... Coastal flooding and isolated flash flooding across the
Mid-Atlantic... Ongoing river flooding and coastal flooding across the
Carolinas... Rainfall for the Central Plains this weekend...

.Discussion...

.Central Montana...
Slow-moving showers and storms with locally heavy rainfall rates could
produce isolated flash and small stream flooding across portions of central
MT through tonight (mostly below 4,500 ft elevation). Low soil moisture
content is in place throughout the entire soil column (0 - 100 cm relative
soil moisture of 5 - 35%, NASA SPoRT) across central MT, likely mitigating
mainstem river flooding and widespread small stream flooding. Nonetheless,
isolated out of bank rises will be possible on smaller quick-to-respond
streams in complex terrain, with any flooding impacts likely being
restricted to areas in the immediate vicinity of the waterways. Debris
flows near recently burned areas and isolated urban/poor-drainage flooding
is possible as well.

The HRRR-forced National Water Model (NWM) Short Range Forecast (SRF) is
indicating an increasing probability (25 - 50%) for isolated rapid-onset
flooding (ROF) on small streams in basins encompassing the region between
Billings, Lewistown, Great Falls, and Havre. The greatest potential for
small stream flooding impacts looks to be just north of Lewistown across
portions of Judith, Fort Peck, Box Elder, and Bullwhacker-Dog basins where
forecast flows on headwaters have annual exceedance probabilities (AEP) of
4 - 10%. Peak flows in this area are not expected to begin until this
afternoon into the late evening hours.

.Mid-Atlantic...
Minor-to-moderate coastal flooding at high tide is possible through day 3
(Fri) from the Chesapeake Bay to Long Island Sound. This could continue
into the weekend. Check with your local NWS office for more information
related to coastal flooding impacts. Additionally, isolated flash and urban
flooding will be possible today into tonight across eastern VA, MD, DE, and
southern NJ as slow moving convection associated with lingering tropical
moisture continues across the region.

.Carolinas...
Isolated river flooding will continue across the Coastal Plain of the
Carolinas (Pee Dee, Cape Fear/NE Cape Fear, Neuse, Contentnea) into this
weekend in response to the recent tropical rainfall. Additionally,
minor-to-moderate coastal flooding at high tide is possible through day 3
(Fri) from Savannah, GA to Duck, NC.

.Central Plains...
Potentially heavy rainfall (totals up to 3") on days 4 - 5 (Sat - Sun) may
bring an isolated flooding threat to the region. Ahead of this rainfall,
antecedent conditions are dry, where abnormally dry to moderate drought
conditions are in place (USDM) and streamflows are below the climatological
normals (USGS). Most of this rainfall is expected to be beneficial, with
little to no hydrologic responses expected. However, isolated areas of
lowland flooding or urban flash flooding cannot be ruled out in areas of
heavier rainfall.

//JEC

$$