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FOUS30 KWBC 120859
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
459 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 WESTERN LOUISIANA...

...Summary...
Mid-upper level trough traversing the Central-Southern Plains
north of the Red River will lead to a surge of deepening
instability and moisture from the western Gulf of Mexico as a warm
front pushes into eastern TX and southwest LA. Favorable dynamic
and thermodynamic profiles will result in widespread convection
across central-eastern portions of TX into parts of the Lower MS
Valley, areas of which have seen well above normal amounts of
rainfall. Based on the latest trends in the CAM guidance, have
expanded the Moderate Risk from yesterday`s Day 2 ERO a farther
south and east to include more of the Piney Woods in eastern TX and
western LA. Have also expanded the southern extent of the Slight
Risk to the Upper TX and southwest LA coast, including Houston and
Lake Charles.

...Central-Southern Plains and Portions of the Lower Mississippi
Valley to the Mid-Upper TX and Central LA Gulf Coast...
Upper low centered over CO Sun morning will continue eastward into
KS with a modest/strong ~100kt upper jet moving across Texas.
Scattered elevated convection Sunday morning along/ahead of a warm
front over southeastern TX and smaller vort maxes streaming
northeastward out of northern Mexico/Rio Grande Valley will be
part of the impetus for afternoon/evening convection into east-
central TX and areas eastward. Both moisture flux and precipitable
water value anomalies will rise to around +2 to +3 sigma as 850
winds increase to 25-40 kts. The duration of the best moisture
transport/anomalous moisture flux within the lower-mid levels
remains rather transient but could be enough to yield heavier rates
in excess of lower FFG values near/south of Dallas. An uptick in
low-level frontogenesis along the warm front is still anticipated
via the right-entrance region of the upper jet streak traversing
the Southern Plains and TN Valley. This will also help retard the
warm front`s east-northeastward progression -- and potentially the
more widespread area of convection along/near the boundary.
Largely elevated, convection will become more widespread downwind
of the warm front in a favorable deep-layer warm/moist advection
pattern off the western Gulf of Mexico. The lingering low-level
frontogenesis/slow progression of the surface front will allow for
some cell training, especially across parts of eastern TX and
western LA which had received quite a bit of rain over the past
couple of weeks (300-600% of normal per AHPS). Over this region
eastward through western LA, believe the ERO risk will be maximized
(Moderate), which is also supported by the 00Z UFVS-verified, GEFS
ERO first-guess field from CSU.

The 00Z CAMs continue to show spread with the heaviest rainfall,
though most show footprints of 3-6+ inches in the 24 hour period.
The Moderate and Slight Risk areas were bounded by the latest HREF
PMM, however it should be noted that the 00Z ARW was a notable
southern outlier with the axis of heaviest rainfall (5-8+ inches
down into Houston metro). Even the 00Z HRRR was trending south,
though not to the extent of the ARW. Such a trend will need to be
monitored with subsequent ERO updates today, especially with any
new convectively-induced outflow boundaries or effective fronts,
given the strong gradient of deep-layer instability and pool of
low-level 350+ K theta-e air currently just offshore early this
morning.


Day 2
Valid 12Z Tue May 14 2024 - 12Z Wed May 15 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