339
AGUS74 KWCO 281506
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1006 AM CDT TUE MAY 28 2024

.Synopsis...
Locally considerable flooding impacts possible across portions of the
Southern Plains... Continued rainfall and flood threat across Puerto Rico
and USVI... Rain and snowmelt across Idaho and western Montana... Snowmelt
induced flooding impacts continue across Alaska... Ongoing flooding and
additional rainfall for portions of the Middle Mississippi Valley...

.Discussion...

.Southern Plains...
Multiple rounds of heavy rainfall are expected across southern OK,
north-central TX, and southeast TX through Tuesday night, introducing a
threat for locally considerable flash, urban, and small stream flooding
impacts. Antecedent conditions vary across north-central TX, where the
heaviest rainfall totals are expected (2 - 4"+). Areas west of DFW have
seen some rainfall recently, but soils are not saturated and streams have
capacity to hold some additional runoff, however antecedent conditions
south and east of DFW are conducive for hydrologic responses, as near
surface soils are generally wet and flows across headwaters of east TX
basins remain elevated. Rainfall has generally been greater than 200% of
normal over the past 14 days with some areas over 400% of normal south of
Dallas (RFC QPE). Soil moisture ranges from 50 - 70% (0 - 10 cm RSM NASA
SPoRT) with values closer to 70% between Dallas and Waco and along the far
eastern TX/OK state line. The greatest threat will be urban flash flooding
with the initial rounds of rainfall this morning, with the potential for
small stream rapid onset flooding increasing late today and tonight with
potential subsequent rounds of heavy rainfall. Larger rivers such as the
Trinity, Neches, and Lower Sabine are still in at least minor flood in some
locations adding credence to the vulnerability of the region. Additional
minor-to-moderate river flooding impacts (PQPF ensembles, HEFS) cannot be
ruled out across central, north-central, and potentially east TX by the end
of this week as rainfall totals through day 6 (Sun) add up into the 3 - 7"+
range.

Both the GFS and NBM-forced National Water Model (NWM) Medium Range
Forecasts (MRF) continue to show potential for more significant responses
including rapid-onset flooding, particularly across the Upper Trinity
basin, Middle Brazos basin, Little basin, Red River tributary basins, and
southeast KS basins (particularly the Neosho Basin). Peak flows on many
rivers look to be in the days 4 - 5 (Fri - Sat) timeframe. River ensemble
guidance (HEFS) continues to show the potential for scattered minor river
flooding at the 30% chance exceedance probability. Additionally, the SR
PQPF ensembles are also showing more robust responses across the region
than in previous days. The potential of more significant river flooding
impacts will be contingent on if the same basins receive the heaviest
rainfall on consecutive days, which it is still uncertain if this scenario
will play out.

.Puerto Rico and USVI...
A wet weather pattern and threat for flash and urban flooding continues
across the islands through the weekend. Soils are wet and streamflows are
above annual means given repeated rounds of heavy rainfall over the past
couple of weeks, increasing vulnerability to rapid stream rises and
subsequent flooding impacts.

.Idaho and western Montana...
Rain (0.5 - 1.25" totals) through day 2 (Wed) and snowmelt (1 - 3",
SNODAS) in basins draining the Rocky Mountains through day 3 (Thu) may
combine to produce enough runoff to elicit isolated small stream flooding
impacts through day 3 (Thu), as soils are already wet (NASA SPoRT, 0 - 10cm
RSM) and streams are already elevated (USGS). Low lying areas along
snowmelt fed creeks and streams will see the greatest potential for
flooding through the end of this week.

.Alaska...
Across the Yukon Flats, snowmelt induced flooding impacts remain possible
along the Porcupine River and near Ft. Yukon. Flooding in these areas will
continue through the week.

.Middle Mississippi Valley...
River flooding is ongoing across the region, with elevated flows continuing
through the week. The Wapsipinicon River near Dewitt, IA is forecast to
remain in major flood through the end of the week. The Iowa River at
Marengo, IA and the Green River at Paradise, KY are currently in moderate
flood and forecast to remain in moderate also through the end of the week.
The mainstem Mississippi River from New Boston, IL to Cape Girardeau, MO
has ongoing minor river flooding or is forecast to reach minor flood later
this week. Additional rainfall is expected across the region late this week
through the weekend, which could bring a new and renewed flooding threat to
the wet region, but confidence is low at this point on specifics.

//JEC

$$