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000
FOUS30 KWBC 090829
QPFERD

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

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

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHERN MISSISSIPPI VALLEY...

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 south of ArkLaTex into the
southern Piney Woods of eastern TX, though northern LA and into the
ArkLaMis.

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 between 3-4 standard deviations
above normal, the swift progression of the upper trough and surface
front will inhibit the prospects of flash flooding.

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