473
AGUS74 KWCO 031527
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1025 AM CDT WED JUL 3 2024

.Synopsis...
Renewed flooding, isolated flash flooding possible this week for the Upper
and Middle Mississippi Valley... Heavy rainfall possible this weekend
across South Texas... Daily convection continues across the Four Corners
Region... Isolated heavy rainfall through Thursday across portions of
Puerto Rico... Increased snowmelt and glacier runoff, heavy rainfall
expected in Alaska...

.Discussion...

.Upper and Middle Mississippi Valley...
Moderate to major river flooding continues across portions of extreme
southeastern SD, southern MN, and much of IA, with moderate to major
flooding forecast along portions of the mainstem Missouri and Mississippi
rivers through the end of the week. Scattered thunderstorms expected
through at least day 4 (Sat) may contribute to new and renewed flooding
impacts.

For today, ongoing convection across portions of eastern MO and western IL
will continue through midday, potentially generating isolated to scattered
instances of flash and urban flooding. Soil moisture across this region is
variable, with drier soils (25 - 50% RSM, 0 - 10 cm NASA SPoRT) across
eastern MO and west-central IL and wetter soils (RSM greater than 70%)
across north-central and northeastern MO, where this convection has
generated rapid river rises to minor and moderate flood stage. Efficient
rainfall rates will be the primary driver for impacts and will be most
likely in urban areas (particularly the St. Louis metro) where flooding of
typical flood prone areas is possible. Previous runs of the HRRR-forced
National Water Model (NWM) were underestimating the coverage of small
stream responses in this area as the HRRR struggles to resolve current
radar trends. However, the High Flow Magnitude Forecast does indicate
isolated to scattered annual exceedance probabilities (AEPs) as low as 2%
across east-central MO, suggesting there is some increased likelihood of
rapid stream rises and subsequent flooding. The threat of flooding will
shift further south into KS, southern MO/IL this afternoon, and similar to
the ongoing convection further north, the intensity of the rainfall rates
will be the primary driver for impacts.

Elsewhere, additional rainfall (2 - 4") across MN, IA, and WI may cause
delayed recessions and new/renewed river flooding through the weekend
(HEFS, PQPF ensemble). Both the GFS and NBM-forced NWM is currently
indicating peak flows through day 5 (Sun) in response to this rainfall.
Corresponding AEPs are also the potential for new significant stream and
river rises, with values as low as 4% across portions of southwestern WI
and IA. Given the ongoing flooding and saturated antecedent conditions
across this area, isolated significant flooding impacts are possible.

South Texas...
Heavy rainfall associated with Hurricane Beryl may impact portions of Deep
South TX and the Rio Grande Valley on days 5 - 7 (Sun - Tue). Ahead of this
rainfall, antecedent conditions are very dry (RSM less than 30%, 0 - 10 cm
NASA SPoRT), which suggests that rainfall from this system will at least
initially be beneficial. However, confidence in the potential for flooding
impacts remains low given the high uncertainty in the strength and
intensity of this system when it makes landfall early next week.
Nevertheless, this region will be closely monitored over the next several
days for potential flooding impacts from Beryl.

.Four Corners Region...
Daily convection will continue to bring the potential for isolated
significant flash flooding in arroyos, slot canyons, dry washes, urban
areas, and recently burned areas. This potential will persist through the
end of the week, with the highest chances occurring through today,
particularly in NM, southern AZ, and eastern CO. Locations that have
received multiple rounds of rainfall over the past week are most
susceptible to new flooding impacts.

.Puerto Rico...
A tropical wave arriving late tonight will bring the potential for flash,
urban, and small stream flooding through tomorrow. Streamflows across the
island are generally normal to above normal in comparison to both the daily
and annual climatological normals (USGS), and soils range from dry to wet
but not saturated (NASA SPoRT). Considering these antecedent conditions and
the convective nature of the rainfall, flooding impacts should remain
isolated.

.Alaska...
Elevated flows and isolated minor flooding from increased snowmelt and
glacial melt continues along the Chilkat River near Klukwan, Klutina River
near Copper Center, and along the Nuyakuk River northeast of Dillingham.

Widespread moderate to heavy rainfall is expected along the western Brooks
Range (2 - 4") and across much of the Interior (including Fairbanks, 1 -
2") beginning today and continuing through day 3 (Fri). This rainfall may
elicit rapid rises on small streams and rivers and potentially lead to
flooding in these areas.

//Pritchard/Wood



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