Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
509 PM EDT Wed May 1 2024
Day 2
Valid 12Z Thu May 02 2024 - 12Z Fri May 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
MISSISSIPPI VALLEY EXTENDING FROM THE NORTHERN MIDWEST DOWN TOWARDS
THE GULF COAST...

...2030Z Update...

...Midwest...

Trimmed down the far northern and western flanks of the Marginal
and Slight in the Midwest where probabilistic guidance in heavier
QPF totals >1" were decreasing and instability will be lower to the
north of the surface low track through the Upper Mississippi
Valley Thursday evening. The area most likely to see the heaviest
rainfall will be closer to the triple point low, which in this case
would favor an area from southern IA on north and east through
eastern IA and into southwest WI. NAEFS shows this designated area
sporting >90th climatological percentile PWs (1.3-1.5") between 18Z
Thurs and 00Z Fri, as well as IVTs above the 97.5 climatological
percentile (500-700 kg/m/s) in that same period. 12Z HREF shows
MUCAPE between 500-1,000 J/kg will also be present. Hourly rainfall
rates up to 2"/hr are possible in the strong est storms, which
would support the potential for slash flooding in these areas.
Farther south, a negatively tilted 500-700mb axis emerging out of
the Ozarks will still be capable of producing strong-to-severe
thunderstorms that could track across much of western MO and
eastern KS. Localized rainfall rates surpassing 2"/hr cannot be
ruled out given PWs topping 1.5", MUCAPE approaching 1,500 J/kg,
and mean 1000-500mb RH values as high as 90%.

...Southern Plains and Lower Mississippi Valley...

The expansive Slight Risk in the South Central U.S. was expanded a
little farther west more into central TX and central OK where
latest CAMs guidance has slowed down the cold front approaching
from the northwest. Farther south, there remains some uncertainty
on the position and duration of the ongoing cluster of storms in
the early-mid morning hours Thursday. Should storms stick around
longer into the morning hours along and north of I-10, there could
be the need for either an additional expansion of the Slight Risk
or an upgrade to Moderate Risk should flash flooding be more
extensive and linger beyond 12Z Thursday. Recent rainfall in
western Louisiana (2-5" above normal over the past 7 days) has left
soils in the area more sensitive to 2-3"/hr rainfall rates, which
are supported with PWs just short of 2.0" and skinny MLCAPE up to
1,000 J/kg present. Overall, the Slight Risk remains on track but
the setup in eastern TX and western LA will be closely monitored
overnight.


Mullinax


---Previous Discussion---

...Midwest...

Previous forecast still has merit with little deviation from past
issuance. Thus, maintained continuity from the last discussion with
some minor changes to details within the discussion below....

Overnight convection from this evening will continue to march
northeastward as the congealed cold pools from the initial cell
mergers will advance through the central Midwest with sights on
northern IA into WI and MN on Thursday morning. The complex will
eventually peter out as it moves northward into a less favorable
environment which will put an end to the flooding threat as we move
into the afternoon. Later in afternoon and evening, a cold front
will propagate to the east out of the central plains with a
redevelopment of convection over eastern KS, IA, and northwest MO.
This will allow for the secondary peak of flash flooding potential
as thunderstorms migrate through the mid and upper Mississippi
Valley, impacting some areas that were just affected in the last 24
hrs. With the lower FFGs expected, the threshold for flooding will
be lower than climatological norms for some areas within the
expected secondary convective regime. Totals are currently within
the 1.5-3" range on guidance which is significant enough to
warrant at least the current SLGT risk.

...Southern Plains and Lower Mississippi Valley...

As stated previously in the D1 outlook, our complex of
thunderstorms in TX will migrate eastward into the Lower
Mississippi Valley with heavy rainfall likely within the western
half of LA into southern AR. The setup has now trended towards a
"front loaded" forecast where the primary rainfall in the period
that would induce flash flood concerns will occur during Thursday
morning and early afternoon before dissipating due to the loss of
sufficient upper forcing. The back end of the latest CAMs and
associated HREF signal the opportunity for upwards of 2-3" of
rainfall during that short time frame Thursday AM which will be
enough to cause problems given the overlap from the previous
weekend event that dropped copious amounts of rainfall over
southeast TX into western LA. 00z HREF blended mean has an areal
extent of 1.5-2" with probability matched mean upwards of 3" along
the TX/LA border. HREF neighborhood probability for rates exceeding
2"/hr are also upwards of 30-40% within the same areas, correlating
with the forecasted rainfall in that 2-3" zone. The setup really
comes down to the timing of the complex in TX as to whether a
reintroduction of a MDT risk will be needed after it was downgraded
and shifted one period earlier. This will be assessed in future
forecast updates as we closely monitor radar and hi-res guidance
trends over the course of today and overnight into Thursday. For
now, the SLGT risk will remain with wording of it being on the
higher end of the risk threshold.

Further north across KS and OK, the cold front to the north will
move south with a push from high pressure nosing in through the
plains in wake of the low pressure moving to the north. An area of
convection will likely form over the central plains and move south
with the boundary leading to more locally heavy rainfall in
association with the area of convection. The threat is less
aggressive compared to what is expected further south, but the
chances are still prevalent. The SLGT risk was maintained from the
previous D3 connecting the two primary areas of interest to the
north and south.

Kleebauer