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FOUS30 KWBC 121555
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1155 AM EDT Sun May 12 2024

Day 1
Valid 12Z Sun May 12 2024 - 12Z Mon May 13 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
EASTERN TEXAS INTO CENTRAL LOUISIANA...

...Summary...
Upper low evident on WV imagery this morning moving across eastern
CO will deepen as it traverses to the east, closing off over
Kansas by tonight, and producing height anomalies as ow as
-2 sigma between 500-700mb according to the NAEFS ensemble tables.
This evolution will result in a downstream increase in low-level
wind speeds emerging from the Gulf of Mexico, noted by 850mb winds
surging out of the S/SW as high as 20-40 kts by this evening. This
manifests as increasing moisture and instability advection, as more
favorable thermodynamics are drawn northward, reflected by a
ribbon of PW anomalies of +1 to +3 sigma via NAEFS (1.25" into KS,
as high as 2" in TX), collocated with MUCAPE reaching as high as
2000-3000 J/kg south of a surface warm front across TX/LA, and as
high as 1000 J/kg farther north into the Central Plains. These
thermodynamics become increasingly acted upon by strengthening
ascent as spokes of vorticity/shortwaves lift northeast around the
upper low, the upper jet streak positions favorably to produce
diffluence, and the warm front gradually pivots northeast. This
will result in two primary areas of heavy rain and areas of
potential flash flooding.

...Texas, Lower Mississippi Valley, and Central Gulf Coast...
The most significant rainfall and highest probability of flash
flooding will occur today from eastern TX into central LA. Here,
the most anomalous moisture (PWs to 2". above the 90th percentile
for mid- May according to the SPC sounding climatology) will
combine with MUCAPE approaching 3000 J/kg to provide robust
thermodynamics for heavy rainfall. This will occur in the vicinity
of the surface warm front which will drive low-level convergence to
serve as a focus for convective initiation in the presence of the
aforementioned impressive synoptic lift. There is high confidence
that multiple rounds of heavy rainfall will occur, with rainfall
rates of 1-3"/hr likely as progged by HREF neighborhood
probabilities and the HRRR 15-min rainfall rate fields. The
convection associated with these intense rates will likely expand
through the day, with training expected in the vicinity of the warm
front as reflected by mean cloud-layer winds generally parallel to
the warm front, so despite progressive Corfidi vectors this will
enhance the rainfall amounts today, and this will fall atop soils
that are saturated from recent heavy rainfall, especially in
eastern TX, noted by NASA SpoRT soil moisture reaching the 90-95th
percentile.

Instances of flash flooding today still appear likely, and the
inherited moderate risk remains. However, some adjustments were
made due to newer CAM guidance, and there is still uncertainty in
the placement of this heavy rain axis. The NSSL MPAS models appear
to be initializing quite well, and they have shifted a bit south,
which is pretty common in these setups, but they remain on the
southern edge of the unfortunately large ensemble envelope. There
has also been a bit of a pivot east in the max axis among the
various models. This necessitates a small expansion south and east
of the MDT risk, which is preferred over a shift due to the
likelihood of regeneration of cells to the west along the warm
front and in the area of greatest MUCAPE. Where the most pronounced
training occurs, rainfall of 3-5" is probable, with maximum
amounts exceeding 6" possible near the TX/LA border where moisture
convergence is maximized.

...Southern and Central Plains...
Farther to the north, a secondary axis of heavy rain is likely
across OK and into southern KS in the vicinity of the upper low.
Here, a narrow corridor of overlapped moisture and instability will
be drawn northward, resulting in an expansion of convection with
heavy rain rates exceeding 1"/hr ahead of a surface low and
occluded front. The CAMs indicate there could be multiple rounds of
thunderstorms this aftn/eve lifting northeast, and mean 0-6km
winds will likely be S/SW, parallel to the eastward advancing front
which could permit some training of echoes. There is some concern
that strong convection over TX could "steal" some of the
moisture/instability lifting north, capping the intensity of this
activity, resulting in a lowered flash flood risk, but the
latitudinal difference between the convection south and
regeneration north may be enough to reduce this impact. This area
of OK/KS has been generally dry in the past week except for
portions of north central OK into southeast KS, and after
coordination with OUN/ICT, added a small SLGT risk area where the
highest HREF and ECENS probabilities exist for 3"/24hrs, which
overlaps the lowest FFG due to recent rainfall. Still some
uncertainty in placement of heaviest rainfall, but any training of
cells across these more sensitive soils could result in scattered
instances of flash flooding, which is additionally suggested by
steadily increasing NWM RoFS probabilities and recent CSU first
guess fields.


Day 2
Valid 12Z Mon May 13 2024 - 12Z Tue May 14 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST REGION FROM EASTERN LOUISIANA TO THE
WESTERN FLORIDA PANHANDLE...

...Summary...
The mid-upper level trough and corresponding surface low/frontal
systems are expected to pivot slowly into the Mid Mississippi
Valley by Monday night. Favorable dynamical forcing along with the
anomalous TPW and deep-layer moisture transport ahead of the trough
will continue to foster an excessive rainfall threat --which by
Day 2 (Mon-Mon night) will shift a little farther east to the
central Gulf Coast region. Per collaboration with WFOs MOB, LIX,
and Jan, we have hoisted a Moderate Risk area across parts of
southern MS-AL, far eastern LA, and far western portions of the FL
Panhandle. This is where the thermodynamical profile and highest
likelihood of multiple rounds of convection (and potential for cell
training) Monday night will be highest comparatively.

...Lower-Mid Mississippi Valley, Parts of the Midwest, Much of the
Gulf Coast Region, Parts of the Southeast...
Buckling upper flow ahead of the trough will help to bolster the
low-level frontogenesis for a bit ahead of the advancing cold
front, and while the TPW and low-level moisture flux anomalies
remain solid into Day 2 (+2 to +3 standard deviations above normal
per the ensembles), the forecast confidence in terms of pinpointing
the area of greatest concern in terms of an excessive rainfall threat
is average at best for a Day 2 outlook. Expect to see at least 2
rounds of more organized convection; one more elevated and affecting
a larger area on Monday ahead of the advancing warm front. Monday
night is when the forecast becomes tricky. Within the warm sector,
do expect an effective front/boundary setting up along or just
inland from the coast, essentially along the strongest low-level
theta-e and surface-based CAPE gradient. Expect the additional
rounds of convection Monday night to propagate toward/along this
boundary, which would put areas closer to the coast at a greater
risk for flash flooding despite the initially higher FFGs.
Meanwhile, as the 1000-850mb flow increases to 30-40 kts and veers
more westerly overnight (more closely aligning with the mean
850-300 mb flow), cell training will become more likely.

Much of the guidance (not just the high-res CAMs) show swaths of
3-7" of rainfall, mostly centered within and around the Moderate
Risk area.

Farther north, a broad Marginal Risk is outlined in advance of the
upper low with less instability but lower FFG values.

Hurley


Day 3
Valid 12Z Tue May 14 2024 - 12Z Wed May 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
SOUTHEAST AND LOWER MID ATLANTIC REGION...

...Summary...
The aforementioned mid-upper level trough and corresponding
surface low/frontal systems will continue eastward across the TN
Valley and Lower OH Valley and southern Appalachians Tue-Tue
night. Deep-layer forcing and moisture will remain anomalous,
however the northern stream upper trough north of the Great Lakes
region will help enhance the southern stream trough`s eastward
progression somewhat. Therefore, with the more organized/widespread
convection expected to be more forward propagating than otherwise,
especially given the strong deep-layer shear (more limited cell
training potential), for now anticipate the excessive rainfall risk
to remain below the Moderate threshold.

...Tennessee Valley, Lower Ohio Valley, Mid Atlantic, and
Southeast...
While fairly transient, the models show a healthy dose of upper
divergence/deep-layer ascent ahead of the trough, aided by the
frontogenetic right-entrance region forcing from the upper jet
streak lifting through the northern Mid Atlantic region. Moisture
transport, IVT, and PW anomalies/percentiles remain rather robust
during Day 3 per the ensembles/ESAT. In fact, 850-700 mb moisture
flux sigmas climb to +4 to +5 eastern AL, southern 2/3 of GA, and
into the Carolinas. Plenty of deep-layer instability will be
present as well across the Southeast and Lower Mid Atlantic
(southern NC), particularly behind the warm front, as surface-based
CAPEs climb to 1500-2500+ J/Kg per the 00Z ECMWF. While the
ingredients are in play for heavy, potentially excessive rainfall
(both dynamical and thermodynamical), the uptick in westerly MBE/Corfidi
Vectors with time Tuesday, coinciding with a more progressive
frontal progression (and ensuing negative TPW advection) over much
of the region appears to be sufficient to maintain the Slight Risk
of excessive rainfall that was inherited from yesterday`s Day 4 ERO
and at least for now forgo any upgrades to a Moderate Risk. This
idea matches the UFVS-verified, GEFS ERO first-guess field from
CSU. Based on the latest guidance trends, we did expand the Slight
Risk a bit northward to cover more of the Carolinas (into far
southern NC).

The one area that will need to be monitored for a potential upgrade
to a Moderate Risk in subsequent outlooks will be across southern
GA into northern FL, where the front and the 2.00+ TPW plume may
get hung up a bit Tuesday night as the southwesterly low-level flow
veers more westerly and aligns more parallel to the mean 850-300
mb flow later Tuesday afternoon into Tuesday night. At this point,

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