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FOUS30 KWBC 112326
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
726 PM EDT Sat May 11 2024

Day 1
Valid 01Z Sun May 12 2024 - 12Z Sun May 12 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN AND NEAR
PORTIONS OF TEXAS...

A vigorous upper level low moves eastward, with an influx of deep-
layer moisture into the southern Plains/TX elevating over a
returning front. Upper diffluence will increase as the upper level
trough/closed low aloft approach TX. Thunderstorms near the front
in eastern NM have been trying to move eastward, which is
advertised by the 18z HREF guidance. Forward motion will remain
progressive as 850 hPa inflow veers through the period, but
sufficient MU CAPE (up to ~1000 J/kg north and 2000+ J/kg south)
and precipitable water values (>90th percentile... ~1" north and
towards 2" south) will favor hourly rain totals up to 2.5" beneath
stronger cores over TX, particularly where any short bouts of
training or mesocyclones manage to form. Farther north, storms
with heavy rainfall shift eastward from the High Plains while
weakening as they move into areas of lesser instability in OK.
Isolated risk of flash flooding summarizes the threat overall.
Farther east and later in the period (06-12Z Sun), advancing warm
front into southeastern TX will promote heavy rainfall into the
region with some occurring over urban areas, and some over more
sensitive areas that have seen 10-25"+ rainfall over the past 30
days, with much of the total occurring over the past two weeks.
A greater flash flood threat will exist after 12z, but is non-
zero before 12Z as HREF probabilities show a weak, but existent,
signal for 3"+ by 12z, which led to an eastward extension of the
Marginal Risk area from continuity.

Roth/Fracasso/Hurley


Day 2
Valid 12Z Sun May 12 2024 - 12Z Mon May 13 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
EAST-CENTRAL TEXAS...

...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. Upgraded a targeted area over east-central Texas to a
Moderate Risk of excessive rainfall for the Sunday and Sunday night
period within a broader Slight Risk outline that covers the
general convective threat.

...Central-Southern Plains and Portions of the Lower Mississippi
Valley to the Mid-Upper TX and western 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.
Convection may be ongoing around 12Z along/ahead of a warm front
over southeastern TX and smaller vort maxes streaming northeastward
out of northern Mexico/Rio Grande Valley which will be part of the
impetus for afternoon/evening convection into east-central TX
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. 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 central-eastern TX which had received quite a bit of rain
over the past week. Over this region eastward through western LA,
believe the ERO risk will be more of a "high end" Slight (i.e. at
least 25% risk of rainfall exceeding FFG within 25 miles of a
point).

Within the Slight Risk area, 12Z models show pockets of 3-5+
inches of rain during day 2 period within this area, but with
continued latitudinal differences (GEFS/GFS/NAM generally north,
Canadian south, ECMWF and many of the CAM guidance in the middle).
With a bit more agreement on the heavier QPF axis over more
sensitive areas (including where rivers are running high -- see the
NWC FHO for more info), and in coordination with the FWD/SHV/HGX
offices, felt the upgrade to Moderate was justified at this point.


Fracasso/Hurley


Day 3
Valid 12Z Mon May 13 2024 - 12Z Tue May 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE GULF
COAST REGION TO SOUTHERN PORTIONS OF THE MID-SOUTH FROM EASTERN
TEXAS TO THE FLORIDA PANHANDLE...

...Summary...
The aforementioned mid-upper level trough and corresponding
surface low/frontal systems are expected to pivot slowly into the
Mid Mississippi Valley by Monday night will continue to foster a
heavy rainfall -- which by Day 3 (Mon-Mon night) will shift a
little farther east to the central Gulf Coast region. Slight Risk
outline is centered on southeastern LA eastward across southern
MS/AL where there are higher-end probabilities. A broader Marginal
Risk areas extends farther north to the Midwest.

...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 while the anomalies and percentiles have come up
per the global guidance, forecast confidence for a prolonged period
of strong low-level southerly inflow/moisture transport off the
Gulf, which would help justify a Day 3 Moderate Risk ERO, remains
low. Latest CSU ML first guess fields support that thinking as well
-- showing higher probabilities within the Slight Risk outline that
 do not yet meet the criteria for a Moderate Risk. Could certainly
 see at least 2 rounds of more organized convection; one more
 elevated ahead of the advancing warm front, with the next perhaps
 a QLCS ahead of the cold front. However in either case given the
 shear profiles, the storms should be moving relatively swiftly
 given the degree of forward propagation inferred with westerly
 Corfidi Vectors averaging 20-30 kts through the Day 3 period.
 Model guidance 1 and 3 hour QPF progs Mon-Mon night all show this.
 Also, there`s a legitimate chance that the second round of
 heavier rain (ahead of the cold front) may be farther south than
 the first round -- hinted at by the north-south displacement in
 the global models and CMCreg/NAM. ECMWF EFI shows values around
 0.8-0.9 and a Shift of Tails of 1, but displaced farther north
 than the GFS/Canadian/UKMET and especially the ECMWF-AIFS which
 showed its QPF max along the coast (where FFG values are highest).
 Suspect ultimately there could be a Moderate Risk hoisted over a
 relatively small/targeted area, but given the aforementioned
 considerations along with the areal spread in the guidance QPFs,
 the forecast confidence to hoist a Moderate Risk at this point
 (Day 3 forecast) is too low.

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

Fracasso/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