498
FOUS30 KWBC 040056
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
856 PM EDT Fri May 3 2024

Day 1
Valid 01Z Sat May 04 2024 - 12Z Sat May 04 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN AND CENTRAL PLAINS, SOUTHEAST, AND IN THE VICINITY OF
WEST VIRGINIA...

...Southeast Texas...
Convection across central TX, or any outflows emanating from such
which could spur new convection, is likely to track/propagate
east- southeast to southeast across portions of south-central TX
towards the Middle TX Coast to the south of Houston/Galveston.
This area has been much drier than areas farther north and the 18z
HREF is not emphatic about the potential. This update drops the
risk level across portions of TX to a Marginal Risk, as a
precaution, should another convection complex form and develop some
isolated to widely scattered flash flood potential not seen
through the whole of the mesoscale guidance suite, whose
performance has been okay but not stellar lately.


...Central Plains...
The Marginal Risk area in Kansas has been shifted south in
deference to the 18z HREF guidance, which in itself seems a bit far
to the north based on radar reflectivity trends. A squall line is
moving across the state and there`s some evidence in the mesoscale
guidance and radar reflectivity that some cell mergers with
activity that forms in advance of the feature would raise some
flash flood potential. There`s always a chance that the southwest
flank of the line could slow and allow for cell training, so the
Marginal Risk accounts for such potential.


...Southeast...
An MCV from earlier activity across TX and LA is making a turn
east-northeast and gaining some convection while doing so. In case
a warm advection convective band or cell mergers with less
organized convection to its northeast occurs, extended the Marginal
Risk to the southwest to account for such potential.


...In and near West Virginia...
Active convection with heavy rainfall is expected to shift somewhat
eastward into the Blue Ridge mountains of western VA before fading
or forward propagating thereafter. The previous Marginal Risk has
been shifted somewhat eastward as a result.

Roth



Day 2
Valid 12Z Sat May 04 2024 - 12Z Sun May 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF
CENTRAL AND NORTH TEXAS...

...2030Z Update...

...Southern Plains...

Model guidance has shifted south quite a bit with the most recent
updates. The general consensus now is for storms to form along the
dry line in West Texas, then develop upscale and towards the east
in central Texas. As the storms develop, there will be cell mergers
and other interactions, as well as possible training that could
lead to local rainfall totals to 5 inches. Further, soils in this
area have been saturated due to rainfall from thunderstorms as
recently as yesterday. Thus, the Slight Risk area was expanded
south to include much of central Texas. The surrounding Marginal
was expanded further south to account for likely guidance changes
prior to the event. Most of the flash flood producing rainfall is
expected overnight Saturday night, continuing into Sunday morning.

On the northern side of the Slight, what happens further south will
be critical as to how much rain falls along the Red River. Since
this area too has nearly saturated soils, the northern side of the
Slight was trimmed less on the north side than it was expanded on
the south. Nonetheless, the northern side of the Slight has more
uncertainty.

...Sacramento Valley of California...

The inherited Marginal was shifted a few miles west to move the
area off of the higher elevations of the Sierras. Unseasonably cold
air associated with an upper level low will keep most of the
precipitation at the higher elevations and passes through the
Sierras as snow. Any rainfall at the lower elevations will be
confined to the valley. The event is a strong one for this time of
year, and so abundance of moisture still keeps the area as a low
end flash flooding threat.

Wegman

...Previous Discussion...

Height falls aloft ejecting eastward from the Southern Rockies and
the presence of a dryline over West Texas will foster an
environment for thunderstorms capable of producing heavy rainfall
by later in the day on Saturday that persists into Saturday
night/early Sunday morning. Precipitable water values will have
increased to nearly 1,5 inches...some 2 to 2.5 standardized
anomalies greater than climatology...over portions of central and
west central Texas by 00Z Sunday in response to a period of
southeasterly flow off of the Gulf of Mexico. Consensus of model
guidance QPF in the range of 1 to 3 inches seems
reasonable...although the UKMET was a notable outlier.

...Northern Sierra Nevada Foothills...

A fairly robust IVT pulse will slide down the west coast with
southwesterly flow aimed orthogonally to the northern and central
Sierra`s by Saturday. Higher elevations will be greeted with more
snow, but the adjacent foothills will remain warm enough to benefit
from all rainfall. Rates will be borderline overall, but the
prospects for over 1" of precip has grown in the past succession
of runs with the ensemble bias corrected guidance now upwards of 2"
now forecast within the elevations below 8000` MSL, falling mainly
in a span of 8-12 hours. The threat is on the lower end of MRGL. Maintained
continuity from previous forecast and will assess whether it needs
adjusting, or removal pending future trends in guidance.

Bann

Day 3
Valid 12Z Sun May 05 2024 - 12Z Mon May 06 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHERN
MISSOURI TO NEAR THE UPPER TEXAS COAST...

...2030Z Update...

As the guidance has shifted southward on Saturday, the same has
occurred on Sunday. Storms will likely be ongoing in portions of
east Texas at the start of the period Sunday morning, and will
continue pushing east into the morning. It appears likely that
there will be an east-west-oriented boundary along which storms
will be training on the south end of a more progressive line of
storms. There is inherent and typical uncertainty with where that
line will set up, especially with recent large southward shifts in
the guidance. Thus, to cover the large changes in the guidance, as
well as very recent and ongoing heavy rain across southeast Texas,
the Slight was expanded well south of inherited, and likely will be
upped at least to a higher-end Slight in Texas if the current
shifts in the guidance hold, with a distinct possibility for a
Moderate upgrade with hopefully agreeable CAMs guidance.

For the Ozarks region, significant uncertainty also exists, as the
area will be on the northern side of the strong shortwave trough
driving all of the convective activity. Much like much of the lower
Mississippi and points west, the area also has average to above
average soil moisture, which in turn will support the development
of flash flooding given enough rainfall. The northern side of the
storm complexes often get cut off from the Gulf moisture supporting
them in favor of storms further south and closer to the Gulf. This
increases the uncertainty in the forecast for this area. For now,
guidance spread has led to an expansion of the Slight into
south-central Missouri and north-central Arkansas...as well as the
surrounding Marginal as far east as Alabama. These are likely to
change due to the highly variable forecasts of convection for this
area.

Wegman

...Previous Discussion...

The potential for excessive rainfall makes its way eastward on
Sunday in association with a closed low aloft and a cold front at
the surface. Model QPF was not as heavy as forecast on Day 2 as
shifting low level flow results in weaker low level moisture flux
convergence along the front. Despite model QPF generally being 2
inches or less...opted for a Slight Risk given the wet antecedent
conditions over portions of eastern Texas and eastern Oklahoma.

Bann


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt