434
AGUS74 KWCO 101518
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT THU JUL 10 2025

.Synopsis...
Isolated flash and urban flooding possible in the Mid-Atlantic and Central
Plains through the Upper Great Lakes... Additional rainfall expected in the
Southern Plains...

.Mid-Atlantic...
Isolated flash and urban flooding remains possible across portions of NC
and the Delmarva region as additional periods of locally heavy rainfall
develop over already hydrologically sensitive areas. Of particular concern
are the Southside VA and NC Piedmont regions, where 7-day rainfall totals
of 2 to over 10"+ (RFC QPE) have left soils saturated and streamflows
elevated, with little to no recovery time. While confidence remains low in
the exact placement and magnitude of flooding impacts due to the convective
nature of the rainfall, locally intense rates will likely generate some
flooding, potentially significant in some areas. The HRRR-forced National
Water Model (NWM) Short-Range Forecast (SRF) is highlighting potential for
rapid-onset flooding across the Piedmont, however, streamflow responses
remain modest, and the NWM may be underestimating probabilities. Meanwhile,
the NWM is signaling isolated pockets of annual exceedance probabilities
(AEPs) in the 10 - 2% range across the same area, supporting the potential
for locally significant flooding. Nonetheless, flooding impacts will be
possible anywhere training rainfall or intense rainfall rates develop,
especially in urban areas.

.Central Plains through the Upper Great Lakes...
Isolated flash and urban is possible through day 3 (Sat) across much of the
region, but particularly across portions of IA, northwest IL, and southern
WI, where the axis of heaviest 3-day rainfall totals (3 - 5", with locally
higher amounts possible) is forecast. Leading into this event, soils are
generally dry to near normal, except across portions of IA, MN, and WI,
where 0 - 40 cm soil moisture levels are above the 80th percentile (NASA
SPoRT). In combination with above to much above normal streamflows (USGS),
this suggests a hydrologic environment more favorable for flooding
responses in those areas. However, localized high rainfall rates may
overwhelm soils and urban or poor drainage areas regardless of antecedent
conditions, and the potential for back-building/training convection will
further increase flash flooding potential anywhere that sees repeated
rounds over the same area. Both the NWM SRF and Medium Range Forecast (MRF)
are signaling only modest streamflow responses across the region, with
rapid-onset flooding probabilities below 50% and AEPs generally at or below
bankfull. While the GFS-forced MRF does indicate some higher streamflows
and lower AEPs in portions of western IA, this is likely due to differences
in model QPF, as the deterministic GFS continues to run wetter than the
NBM. Nonetheless, rapid rises on area streams and creeks will still be
possible, primarily in smaller, more responsive basins.

.Southern Plains...
Additional rounds of rainfall are expected across the region beginning on
day 3 (Sat) and continuing through the forecast period, bringing a renewed
threat of flooding impacts, especially from central TX into northeast OK,
where wet antecedent conditions remain in place following recent heavy
rainfall. While daily rainfall totals are generally expected to remain
below 2", each round will further increase hydrologic vulnerability,
especially in areas that experience repeated rainfall. Low water crossings,
arroyos, and areas of complex terrain will be most susceptible, and debris
flows remain possible near recently burned areas in NM. For now, confidence
in the exact placement of flooding impacts remains low given the extended
timeframe.

//Freeman



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