590
AGUS74 KWCO 071526
HMDNWC

National Hydrologic Discussion - EXPERIMENTAL
NWS National Water Center - Tuscaloosa AL
1015 AM CDT SUN JUL 7 2024

.Synopsis...
Heavy tropical rainfall to begin Sunday afternoon and will bring locally
considerable flooding impacts across Texas into the Lower Mississippi
Valley... and potential flash and urban flooding to the Middle Mississippi
Valley into the Ohio River Valley by midweek... Ongoing and forecast heavy
rainfall may cause isolated flooding responses in Southern Kansas and
Oklahoma... River flooding continues for the Upper Mississippi Valley...

.Discussion...

.Texas into the Lower Mississippi Valley...
Locally considerable flash and urban flooding are likely to begin this
evening and continue into day 3 (Tue) for portions of the TX Gulf Coast and
East TX and into the southern Ozarks (AR) due to heavy tropical rainfall
associated with Tropical Cyclone Beryl. Rainfall totals of 5 - 10" (locally
15") is forecast and continues to trend up as the track and speed of the
system adjusted to new solutions. Antecedent conditions are variable and
their influence on potential impacts is highly dependent on where TC Beryl
makes landfall. Antecedent conditions along the TX Gulf Coast and inland
into East TX, are favorable for hydrologic responses and rapid rises. While
this area has had their surface 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 forecast rainfall
totals diminish slightly (1 - 2") across the southern portion of the Lower
Mississippi Valley as the gradient for QPF tightens. As mentioned above,
this analysis is highly dependent on track and the system has been
continuously shifting around making the exact location and timing of
impacts difficult to nail down.

The National Water Model (NWM) Medium Range Forecast (MRF) is signaling
rises on reaches generally at 50% annual exceedance probabilities (AEPs)
but as low as 4% on the smaller streams and tributaries begin day 2 (Mon)
in southern and eastern TX. The GFS-forced rapid-onset flooding service
continues to suggest some streams may see some rapid rises. Additionally,
the NWM has increasing signal density of basin responses along with a
slight increase in general to AEPs overall. The basins around the
Mississippi Delta that are being highlighted do seem to still be bouncing
around somewhat so confidence remains low on how those may materialize.
Once TC Beryl moves out of the area the hydrologic focus shifts to the
lingering river and small stream flooding. River forecasts in East TX are
showing minor to isolated major river flooding may occur. River and small
stream 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.

.Southern Kansas and Oklahoma...
Heavy rainfall (1 - 3"+) is ongoing and forecast to continue through this
afternoon and may cause isolated flash and urban flooding, along with new
rises on small streams, for southwestern KS into much of central OK. Soils
are beginning to show initial wetting responses in the soil column (0 -
10cm RSM, NASA SPoRT) due to the ongoing rainfall across the area. This
rainfall may serve to prime some of the surface soils but deeper layer
soils are still very dry which would suggest that there is plenty of
available storage and may assist in the drawdown of any ponding or pulling,
particularly if there is a break between the rounds of expected rainfall
today. Additionally, what runoff does occur should be captured by the
streams that still have plenty of in-channel storage available to route
downstream. However, intense rainfall can still overwhelm these conditions
quickly and cause some hydrologic responses, especially isolated flash and
urban flooding in typically flooding prone areas or areas with poor
drainage.

The NWM Short Range Forecast (SRF) continues to show isolated AEPs of 10 -
4% on small streams and creeks which would indicate that some streams may
experience high rises even if they are near low flows for this time of
year. The SRF is also signaling rapid-onset flooding in southwestern OK
further hinting that isolated rises are a possibility with this system. The
larger extents are likely to adequately handle the ingest of water and keep
it within their banks.

By day 2 (Mon), TC Beryl may bring heavy rainfall to basins in far eastern
OK. However, these basins have a similar setup regarding current antecedent
conditions discussed above, and should not be impacted by the rainfall
today. Then NWM MRF is not highlighting much in the way of rapid rises or
high AEPs except for some smaller reaches. The flooding threats are similar
as well and will predominantly be flash and urban flooding, particularly in
the more complex terrain at the edges of Springfield Plateau and Boston
Mountains.

.Middle Mississippi Valley and Ohio River Valley...
Lingering rainfall is forecast to continue today for portions of far
southern IA and northern MO which may lead to delayed recessions on
elevated streams and rivers. Hydrologic conditions remain primed with wet
soils and swollen streams (0 - 10 cm RSM NASA SPoRT, USGS). While the
rainfall forecast today may not be very much (0.5 - 1.5") the ground
conditions are not able to handle very much input. As such, isolated flash
and urban flooding is unlikely, yet some nuisance flooding may present
itself in urban areas.

By day 3 (Tue), heavy rainfall associated with TC Beryl enters the area and
with it comes the threat of flash and urban flooding, some of which could
be significant, along with new and renewed rises on rivers and creeks.
Antecedent soil conditions are generally on the drier end of the spectrum
with surface soils near 35 - 65% (0 - 10 cm RSM, NASA SPoRT). Streamflows
on the other hand start out above normal across much of the Middle
Mississippi Valley and as it gets into the eastern extent of the Ohio River
Valley flows back down a bit to normal, some areas below normal, for this
time of year.

The GFS-forced NWM is starting to signal highwater flows across the area
starting out on day 3 (Tue) for much of MO and extending to the northeast
as far as Detroit, MI. The High Flow Magnitude forecast along those streams
is showing that most reaches will be near the 50% AEP or highwater. The
largest cluster of streams that are suggested to be lower AEPs are from
streams near the St. Louis, MO metropolitan area and expands northeast to
Peoria, IL with AEPs as low as 2%. These signals would suggest that some of
the flashier basins in this region may react quickly and efficiently to the
heavy rainfall, but generally within bank responses more likely.

Some concern exists with the placement of the heaviest QPF (2 - 4") being
located out ahead of the flood wave moving downstream on the Mississippi
River. This portion of the Mississippi River is already in moderate flood
and this rainfall may create new rises on streams and delay recessions due
to the routed flow coming out of the tributaries. However, the initial
concern with the rainfall will be the flash flooding and flooding in flood
prone urban areas. While it is currently unlikely that considerable impacts
will be observed, the dynamics of the speed and track of the system will
dictate the overall threat in the coming days.

Upper Mississippi Valley...
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
Lansing, IA. By day 3 (Mon) dry conditions settle across the region helping
to improve the hydrologic conditions, and allowing for unimpeded recessions
to resume.

//Capp




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