906
FOUS30 KWBC 111557
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1157 AM 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 bulf 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 Sun May 12 2024 - 12Z Mon May 13 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
CENTRAL AND EASTERN TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

...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, where a high-end Slight Risk of excessive rainfall is
anticipated Sunday and Sunday night.

...Central-Southern Plains and Portions of the Lower Mississippi
Valley to the Mid-Upper TX and western LA Gulf Coast...
The latest guidance (00Z 5/11 model cycle) showed a bit of an
uptick in the exit region forcing ahead of the approaching mid-
upper level trough, at least in terms of the low-level ageostrophic
response across central-eastern TX into the Lower MS. Valley. More
specifically, the ensembles now show 850 mb southerly flow
anomalies getting to 2-2.5 standard deviations above normal (a
little more robust compared to yesterday`s guidance). Meanwhile,
at the same time 850-700 mb moisture flux anomalies top out between
+2.5-3.0, which is also an uptick from yesterday`s model progs.
The duration of the best moisture transport/anomalous moisture flux
within the lower-mid levels remains rather transient however, while
the degree of kinematic and thermodynamic response east of the
upper trough does seem to be mitigated by the absence of a more
robust subtropical jet streak. An uptick in low-level
frontogenesis along the warm front is still anticipated however
via the right- entrance region of a 90-100 kt 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, 00Z models show pockets of 3-5+
inches of rain during day 2 within this area, with the 00Z RRFS
noting a narrow band of 6-8" totals just south-southwest of the
ArkLaTex, including Longview TX and Shreveport LA. For now, given
the areal spread in the guidance in terms of the heavy rainfall
footprints, have opted to not hoist a Moderate Risk. Later shifts
can re-evaluate the need for a targeted Moderate Risk based on
the most recent observational and model trends.

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. Compared to
yesterday`s Day 4 ERO, only minor changes were made to the Slight
and Marginal Risk areas.

...Lower-Mid Mississippi Valley, Parts of the Midwest, Much of the
Gulf Coast Region, Parts of the Southeast...
As with the Day 2 ERO, anticipate at least a high-end Slight Risk,
which for Day 3 would be across the Gulf Coast region from eastern
LA and southern MS-AL to the western FL Panhandle. 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
helps justify a Day 3 Moderate Risk ERO, remains low. Latest CSU
ML first guess fields support that thinking as well. 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. 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.

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