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000
FOUS30 KWBC 091551
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1151 AM EDT Tue Apr 9 2024

Day 1
Valid 16Z Tue Apr 09 2024 - 12Z Wed Apr 10 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EAST TEXAS, THROUGH THE SOUTHERN MISSISSIPPI VALLEY...

...16Z Update...

The general consensus this morning is for a greater westward
expanse of the heavy rain threat as convection blossoms in-of east
TX through northern LA along our stationary front currently
positioned south of the I-20 corridor from the TX Hill Country to
50-75 miles south of the interstate line in LA. Significant
thunderstorm development has occurred over the past hour with rates
hovering between 1-2"/hr with a few overachieving cells reaching
upwards of 2.5"/hr given the latest hourly obs out of central TX.
Environment is rich with moisture as of the latest 12z soundings
out of SHV indicating PWATs up around 1.7" which is a solid 2.5-3
deviations above normal climatologically. Large scale ascent ahead
of a deep upper low over the boot heel of NM will continue through
the period with multiple shortwaves ejecting out of the base of the
trough, interacting with the downstream environment and subsequent
boundary juxtaposed over the South. Latest 12z HREF blended mean is
much more robust compared to the last run with widespread 3-5"
totals now encompassing east-central TX along and east of the I-35
corridor near Waco and points east-northeast. As a result, HREF EAS
probabilities have soared for all three intervals (1"/2"/3") with
the 3"/24-hr contour now indicating up to 70-80% for that area near
Waco to the northeast. 30-60% stretches all the way out to
Shreveport with a large 20-30% area for at least 3" now expanding
south into central LA which is aggressive considering that area is
not anticipated to see the heavy rain threat until a little later
in the period. Neighborhood probabilities are more bullish given
the definition, but even those probabilities are very high
(80-90+") for at least 5" within that target zone between I-35 out
to Palestine, TX where 3+" fell last night and early this morning.

Rainfall rates are sufficient for significant flood potential,
especially for areas that have been hit overnight through the
morning as areal FFG indices for 1/3/6 hr(s) have all taken a
nosedive compared to where they were yesterday. HREF probabilities
for 2"/hr or greater are between 20-40% this afternoon across
portions of east TX and northern LA but ramp up further this
evening thanks to an expected complex of thunderstorms initiating
over south-central TX, growing upscale as it approaches the TX/LA
border and continues its propagation to the east. This is due to a
stronger mid-level vorticity maxima ejecting out of MX and helping
organize the areal thunderstorm development within a very favorable
environment.

Given all the above trends and forecast probabilities, have
expanded the Moderate Risk area further west into portions of
central TX and points to the east-northeast. This threat will be on
the higher end of the MDT threshold and some discussion of a High
risk was entertained. This will be monitored over the course of the
period, but have maintained the MDT for now. Further west over the
TX Panhandle, severe thunderstorms due to the favorable ascent
pattern downstream of the upper low has allowed for locally heavy
rainfall across the northern Caprock. This convection is scattered
in nature for a heavy rain threat as we are missing a primary focal
point to pin for training potential that is customary for
significant rainfall setups. Still, any smaller towns and low-water
crossings across the area between Lubbock and Amarillo will have
the threat for localized flooding concerns, thus maintained the
SLGT risk in place.

Kleebauer

...Previous Discussion...

BLUF: Compared to yesterday afternoon`s Day 2 ERO, there`s a slight
expansion of the Marginal Risk area across OK, northwest AR, and
southern MO. Otherwise, the Slight Risk area was largely unchanged,
while the Moderate Risk was shifted a tad south into the southern
Piney Woods of eastern TX, though northern LA and into the
ArklaMiss.

The trends in the guidance, particularly the high-res CAMs, has
resulted in a southward shift in the heavier QPF footprints,
largely owing to the deep-layer CAPE trends (greater farther south) and
thus the impact on short-term rainfall rates (highest farther
south). Deep-layer forcing ahead of the amplifying southern stream
trough pivoting into the southern Plains by Wednesday morning will
cover quite a bit of real estate, with the broad diffluent flow
aloft and coupled left exit/right entrance region forcing from the
southern stream/northern stream jet streaks respectively. For this
region the Marginal and even Slight Risk areas remain quite large.
However, within the Moderate Risk area will be an overlap with a
more favorable thermodynamic profile as 850 mb moisture
flux/transport anomalies get closer to +3 standard deviations above
normal, PWs to 1.75+ inches, and MUCAPEs closer to 1000 J/Kg.
Within the Moderate Risk area, the latest (00Z) HREF indicates the
highest probabilities...between 25-40+ percent...of 24hr QPF
exceeding 5 inches, with 15-25+ percent probs of exceeding 8
inches.

Hurley


Day 2
Valid 12Z Wed Apr 10 2024 - 12Z Thu Apr 11 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER A PORTION OF
THE MID-SOUTH TOWARDS THE FLORIDA PANHANDLE...

BLUF: No significant changes made from yesterday`s Day 3 ERO.

Once again per coordination with local WFO`s, given the continued
progressive nature of the convective pattern from the latest 00Z
guidance (including the 60hr FV3 and RRFS), have maintained a
rather targeted Moderate Risk area over the area (including central
AL that will most likely see significant rainfall during a larger
portion of the forecast period). This area continues to align well
with the highest areal-average QPF per WPC and the NBM (2.5 to 4+
inches). However this remains a low confidence ERO forecast,
especially given the progressive nature of the convection as with the
upper trough and surface front. In addition, some of the extended
CAMs, including the 00Z FV3 and RRFS, are now farther south with
the heavier rainfall footprint during the Day 2 period (12Z Wed-12Z
Thu), i.e. closer to the Gulf Coast. For now, again per
collaboration with the WFOs, have opted to maintain continuity with
only minor adjustments, with the anticipation of better clarity in
the guidance over the next couple of forecast cycles as the event
gets within all of the high-res windows.

Hurley


Day 3
Valid 12Z Thu Apr 11 2024 - 12Z Fri Apr 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE APPALACHIANS TO WEST OF THE I-95 CORRIDOR FROM THE MID ATLANTIC
TO NORTHEAST...

BLUF: Have scaled back on both the Slight and Marginal Risk areas
compared to yesterday`s Day 4 ERO.

Guidance has trended down with the QPF across the outlook areas,
despite the favorable deep-layer forcing ahead of the upper trough
which will become more negatively tilted with time. Despite 850-700
mb moisture flux anomalies peaking aoa 4 standard deviations above
normal, the swift progression of the upper trough and surface
front will inhibit the potential and areal extent of flash
flooding. By the same token, there is some deep-layer instability
noted per the guidance, especially within the Slight Risk area
(generally under 1000 J/Kg). This will allow for more intense
short-term rainfall rates, which again could pose a localized (or
slightly more enhanced) threat for short term runoff issues,
despite the faster eastward progression of pre-frontal quasi-
linear segments and thus a more limited risk for cell training
during the period.


Hurley

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