990
FOUS30 KWBC 111929
QPFERD

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

Day 1
Valid 16Z Sat May 11 2024 - 12Z Sun May 12 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHERN HIGH PLAINS INTO PARTS OF CENTRAL AND SOUTH TEXAS...

...Eastern NM/southeastern CO into portions of Western and Central
TX...

A vigorous upper level low over the Four Corners this morning will
continue to move eastward this afternoon and early evening, with
an influx of deep-layer forcing and moisture across the southern
High Plains into much of the TX Cap Rock-Rolling Plains/Hill
Country into parts of South Texas. Upper diffluence will increase
as height falls approach the area east of the Rockies in tandem
with an increase in moisture via S/SE flow along/east of the
dryline. 850mb moisture flux anomalies peak around +2 to +3 sigma
this afternoon/evening from southeast CO southward and eastward to
around 100W, which forms the bulk of the outline for the Marginal
Risk area. Forward motion will remain progressive, but sufficient
CAPE/ML-CAPE (up to ~1000J/kg north and ~2000J/kg south) and
precipitable water values (>90th percentile... ~1" north and 1.5"
south) will favor max rain rates of around 1-1.5"/hr and 1.5-2"/3hr
beneath stronger cores esp over TX where FFG values are higher.
Farther north, lower FFG values along with lower rates (generally
<1"/hr) extend into southeastern CO where the Marginal outline was
tugged northward. Isolated risk of flash flooding summarizes the
threat over the area. Farther east and later in the period (06-12Z
Sun), advancing warm front into southeastern TX will promote
rainfall into the region with potential for some modest rainfall
rates over some sensitive areas that have seen much more rainfall
than normal (>200%) north of Houston toward Dallas. Greater threat
will carry into the D2 period, but is non-zero before 12Z.

Fracasso/Hurley


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