269
AGUS74 KWCO 061751
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT SAT JUL 6 2024

.Synopsis...
Heavy tropical rainfall to begin Sunday afternoon across Texas into the
Lower Mississippi Valley...Isolated flash and small stream flooding remains
possible across the Upper and Middle Mississippi Valleys... Isolated
flooding possible for portions of Kansas and Oklahoma...

.Discussion...

.Texas into the Lower Mississippi Valley...
Flash and urban flooding impacts, some of which may be locally
considerable, are likely day 2 - 4 (Sun - Tue) for portions of the TX Gulf
Coast and East TX due to tropical rainfall associated with Tropical Cyclone
Beryl. Heavy rainfall (5 - 10", locally 15") is forecast and has been
trending up as the track continues to shift. Antecedent conditions are
variable and their influence on potential impacts is highly dependent on
where TC Beryl makes landfall. Antecedent conditions in South TX are
generally dry, as relative soil moisture is low (0 - 30%, 0 - 10 cm RSM
NASA SPoRT) and streamflows are generally below normal (USGS) for this time
of year. These conditions would suggest that at least initially, there is
ample infiltration capacity and in-channel storage in the streams and
rivers. The further north and east you go, conditions become more favorable
for hydrologic responses in East TX and LA. While this area has had their
soils dry out somewhat (35 - 60% RSM) the streams are still generally
normal to above normal. Furthermore, the complex terrain of the southern
extent of the Ozarks in AR may further enhance the flooding characteristics
by increasing flows downslope causing areas of flash flooding and rapid
rises in streams.

The National Water Model (NWM) Medium Range Forecast (MRF) is signaling
some rises to high water and AEPs as low as 4% on the smaller streams and
tributaries begin on day 3 (Mon) in southern and eastern TX. The GFS-forced
rapid-onset flooding service is beginning to suggest some streams may see
some rapid rises. The basins around the Mississippi Delta that are being
highlighted do seem to still be bouncing around somewhat so confidence is
lower on how those may materialize. River ensembles (HEFS and PQPF) are
suggesting some isolated riverine responses of minor to isolated moderate
flooding may be possible in coastal TX and eastern TX. Initially, the
primary hydrologic impacts associated with this tropical cyclone will be
flash and urban flooding. However, small stream and river flooding could
become particularly problematic as this event unfolds due to the impacts
the spring flood season has had on the basins in East TX. The timing and
magnitude of any flooding impacts will be highly dependent on the track and
intensity as the models are continuing to resolve their solutions.

.Upper and Middle Mississippi Valleys...
Lingering rainfall is forecast to continue through the weekend for portions
of MN, IA, WI, northern MO, and western IL which may continue to cause
isolated flooding, along with delayed recessions on elevated streams and
rivers. Hydrologic conditions remain primed with wet soils and swollen
streams (0 - 10 cm RSM NASA SPoRT, USGS). The NWM suggests scattered to
widespread small stream responses in the northern extent of this region and
decreases further to the south in MO. Generally, peak flows are ongoing and
expected to continue as runoff routes into the stream network.
Corresponding annual exceedance probabilities (AEPs) from the NWM MRF High
Flow Magnitude forecast continues to show that most rises will be modest
and the lingering rainfall will likely prolong elevated flows. Furthermore,
these tributaries are resolving the high flows into rivers that are in
flood. Moderate to major river flooding continues across portions of
southern MN, and much of IA, with moderate to major flooding forecast along
portions of the mainstem Missouri and Mississippi rivers through early next
week. The broad flood wave on the Mississippi River is currently just south
of La Crosse, WI. By day 4 (Tue) dry conditions settle across the region
helping to improve the hydrologic conditions, and allowing for unimpeded
recessions to resume.

.Kansas and Oklahoma...
Forecast heavy rainfall (1 - 3"+) may cause isolated flash and urban
flooding, along with new rises on small streams, beginning on day 2 (Sun)
for south-central KS into much of central OK. Surface and shallow soils (0
-10 cm and 0 -100 cm) are very dry (0 - 30% RSM, NASA SPoRT) with variable
climatological streamflow percentiles for this time of year (USGS). Given
the dry soils, runoff may still be a factor if intense rainfall rates
exceed infiltration rate/capacity. However, streams look to be in rather
good shape overall and will likely have enough storage capacity available
to mitigate more widespread flooding responses.

The NWM MRF forcings continue to signal magnitudes of 10 - 4% AEPs on the
smaller streams and creeks with peak flow arrival times by day 2 - 3 (Sun -
Mon). These signals would suggest that smaller order streams may experience
some isolated higher flows if these totals were to materialize. However,
small stream flooding is the secondary threat for this event with primary
concern being pluvial flooding in typically flood prone areas and urban
areas with poor drainage. Some lingering rainfall will be possible by day 4
(Tue) before dry conditions settle across the region.

//Capp




$$