325
AGUS74 KWCO 201518
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT SAT JUL 20 2024

.Synopsis...
Flash and urban flooding in the Eastern Carolinas and Southeast Virginia,
Texas, Lower Mississippi Valley, ArkLaTex and Southeast... Monsoon activity
increases this week in the Southwest... Ongoing river flooding in the
Midwest and Great Lakes...

.Discussion...

.Eastern Carolinas and Southeast Virginia...
The potential for flash and urban flooding will continue through at least
early next week as periods of showers and storms continue to bring locally
heavy rainfall across the region. The most vulnerable areas for flooding
impacts are across portions of eastern NC and far southeastern VA, where
soils are wettest from recent rainfall (60 - 80% RSM, 0 -10 cm NASA SPoRT)
and additional rainfall will generate immediate runoff and lead to isolated
flooding impacts. Flooding impacts will primarily occur in urban areas and
poorly-drained locations, with small stream responses more dependent with
rainfall intensity and slow/training convection. The National Water Model
(NWM) supports this idea, as the GFS, HRRR, and NBM-forcings of the NWM are
only indicating isolated small stream responses within the next 24 hours.

.Southwest...
Monsoonal showers and storms could lead to flash flooding along arroyos and
complex terrain, along with urban flooding through mid next week,
particularly in NM, western AZ, and southern CO where the heaviest rainfall
is forecast. Slow moving convection may cause flash flooding issues,
especially over vulnerable areas like burn scars, dry washes, and slot
canyons. Locally significant impacts cannot be ruled out across recently
burned areas where soils are hydrophobic and in areas that receive multiple
days of heavy rainfall.

.Lower Mississippi Valley, ArkLaTex and Southeast...
Showers and storms with locally enhanced rainfall rates remain possible
through at least mid-week, bringing the potential for isolated instances of
flash and urban flooding. Top soil moisture content has become increasingly
moist (70 - 90% RSM, NASA SPoRT) over much of MS, AL, and northern GA
following repeated rounds of moderate to locally heavy rainfall over the
last several days, which indicates diminished infiltration capacity. As
such, additional periods of moderate to heavy rainfall today may
immediately runoff into nearby creeks/streams and poorly drained areas,
thus leading to flash flooding. The HRRR-forced NWM is indicating isolated
small stream responses across the region, with the most concentrated
responses across portions of AL, GA, and the FL Panhandle for later today.
Corresponding AEPs, per the High Flow Magnitude Forecast, suggests that
most stream rises should remain within bank (AEPs 50% or greater), although
rapid stream rises remain possible in areas that receive more robust
rainfall rates. Impacts will be dependent on rainfall rates and
slow/training storm motions.

.Texas
Flash and urban flooding is possible beginning on day 3 (Mon) across
portions of northeast, coastal, and south-central TX in response to periods
of showers and storms impacting these regions. Outside of urban areas,
antecedent conditions are generally dry, with the exception of east TX,
where soils are moderately wet (50 - 70% RSM, NASA SPoRT) and streamflows
remain elevated (USGS) following recent heavy rainfall. This area is the
most vulnerable to flooding impacts, although regardless of antecedent
conditions, areas that receive multiple days of rainfall will possibly face
hydrologic responses, as slow moving and efficient rainfall rates diminish
infiltration rates in soils However, impacts are not expected to be
widespread in nature at this time. The NWM supports this, as both the GFS
and NBM-forced NWM guidance continues to indicate small steam responses
across south-central TX. River flooding impacts are not likely, as both
HEFS and PQPF guidance indicates generally rises to action stage across the
region, suggesting that flash and urban flooding will be the primary
impacts.

.Midwest and Great Lakes...
Recessions of minor to major flooding will continue across portions of the
Rock (WI/IL) and Upper Illinois (IL/IN) river basins, and along the
mainstem Mississippi River, into next week.

//Pritchard



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