406
FOUS30 KWBC 110043
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
843 PM EDT Fri May 10 2024

Day 1
Valid 01Z Sat May 11 2024 - 12Z Sat May 11 2024

The probability of exceeding flash flood guidance is less than 5
percent.

Chenard

Day 2
Valid 12Z Sat May 11 2024 - 12Z Sun May 12 2024

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

...20Z Update...

Primary pattern for the D2 ERO remains consistent with little
deviation in the overall QPF footprint and relevant synoptic
evolution. The initial HREF probabilities for the event across
eastern NM into west TX are sufficient for the current MRGL risk
and were expanded on the eastern and western flank to account for
the propagation bias being a bit too slow for expected convective
regime over TX. They were also brought back further west towards
the terrain in southwest TX to account for model bias discounting
the initiation off the Davis Mountains which has been a
climatologically favored region within this type of synoptic scale
evolution. We have seen more of a signal in a few hi-res
deterministic for totals breaching 1.5" over a few regions
confined within the MRGL risk bounds. The first is over northeast
NM in conjunction with the greatest upper forcing on the lee-side
of the approaching closed 5H low ejecting out of the Four Corners.
A secondary max will likely be obtained within the central and
eastern Permian Basin where the strongest mid-level vorticity
maxima will advect overhead out of the base of the mean trough in
correlation with the sub-tropical jet alignment.

HREF EAS probabilities for at least 1" of rainfall in both of
these regions are between 40-60% with neighborhood probabilities
for 1"/hr rates generally between 20-30% with some bullseyes of 35%
beginning to show up within the Permian Basin. The quick mean
storm motions will limit the higher potential for flash flooding
concerns across the Permian Basin, but the area is littered with
low-water crossings and smaller urbanization factors that will
bring at least a 5-10% chance of flooding during the afternoon and
evening hours Saturday with an exit expected by early Sunday
morning. This is right within the low-end of the Marginal Risk
threshold giving credence to continuity from the previous
iteration of the forecast.

Of note, a tertiary QPF max is plausible across the Edwards Plateau
down towards the southern end of Val Verde county with a few pieces
of guidance, mainly the ECMWF, continuing to depict a separate,
strong vorticity max ejecting out of Coahuila and developing a
cluster of heavier thunderstorms over portions of the Big Bend and
adjacent locales. The signal is there within ML output of the ECMWF
Graphcast as well, so the MRGL over the southern end was
maintained given some uncertainty, but environmentally favorable
for the period in question.

Kleebauer


...Previous Discussion...

...Summary...
Vigorous upper level shortwave trough moving slowly across the
Four Corners region and southern Rockies will lead to a quick
influx of deep-layer forcing and moisture across the southern High
Plains in eastern NM, much of the TX Cap Rock-Rolling Plains, and
into the TX Hill Country/South-Central TX. The Marginal Risk of
excessive rainfall from yesterday`s Day 3 ERO was expanded across
these areas based on the latest guidance trends, where the
potential exists for isolated instances of flash flooding.

...Eastern NM into portions of Western and Central TX...
Ahead of the approaching upper shortwave trough, a gradual uptick
in difluence aloft will ensue across the outlook area.
Strengthening low-level moist S-SE inflow will also take place Sat-
Sat night along and east of the dryline, as 850 mb moisture flux
anomalies peak between 2-3 standard deviations above normal.
Convection forming off the terrain will obtain quite a bit of
forward propagation given the strong deep-layer shear profile,
therefore cell training is not a concern per the simulated
reflectivity guidance form the models. However, available deep-
layer instability (MUCAPEs at least ~1000 J/Kg over the northern
portion of the Marginal area...1500-2500+ J/Kg south) along with
PWs averaging 2 standard deviations above normal will favor maximum
rain rates of 1-1.5"/hr and 1.5-2.0"+/3hr underneath the strongest
cores. Supported by the latest CSU ERO first guess guidance,
runoff from these stronger storms will pose an isolated or
localized risk for flash flooding, even with the antecedent dry
soils over parts of the outlook area.

Hurley


Day 3
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 MUCH OF LOUISIANA AND SOUTHWEST
MISSISSIPPI...

...20Z Update...

Only modest changes were made to the previous forecast with the
expected pattern continuing to favor locally heavy rainfall within
the confines of east TX through the western half of LA. The biggest
change was the trimming of the eastern flank of the SLGT/MRGL risk
areas to reflect the trends in the heavier QPF distribution
favoring areas further west near the TX/LA border. Ensemble bias
corrected QPF footprint was skewed a bit back into eastern TX with
the QPF maximum generally between 2.5-3" located within the same
regions that have seen prolific rainfall and significant flash
flooding in the past few weeks. Considering the expected alignment
of a stalled frontal boundary bisecting southeast to northwest over
the eastern reaches of TX up through the central plains, there is
concern for heavy rain emphasis to be fixated within the confines
of the boundary. Recent ML data sets also remained fixed within the
bounds of the Lower Mississippi Valley out across eastern TX with a
proxy of heavier precip located upstream from the immediate Gulf
coast. PWAT anomaly forecasts also remain steadfast from previous
runs with a solid 2-3 deviation above normal forecast in the
aforementioned areas of concern. This threat has potential for
further upgrades given the antecedent soil moisture and elevated
river/stream flows that are well-documented over the recent
streamflow forecasts and observed crest heights in eastern TX.

The upper low progression out of NM will translate eastward into
the southern plains around OK with increasing ascent focused within
the diffluent pattern to the north and east of the primary upper
low/trough. A secondary maximum across KS is forecast with some
localized amounts between 1.5-2" expected if the expected upper
level evolution maintains continuity. Rates will be capped
considering the better PWAT anomalies and theta-E pattern more
favorable to the south of the Red River, but enough upper level
ascent could very well provide instances of heavy rainfall during
portions of Sunday into early Monday. The MRGL risk up north
remains with little change from the previous forecast.

Kleebauer


...Previous Discussion...

...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...
For now at least, just a modest kinematic response is noted from
the guidance during day 3 ahead of the the approaching shortwave.
850 mb wind anomalies for instance are currently modeled to be
around 1-1.5 standard deviations above normal (while at the same
time 850-700 mb moisture flux anomalies top out between +2-2.5).
The absence of a a more robust subtropical jet streak is the likely
culprit; however, the presence of the 90-100 kt upper jet streak
traversing the Southern Plains and TN Valley will be felt by way of
subtle right-entrance region forcing (focused area of upper
divergence/deep-layer ascent and low-level frontogenesis). This
will enhance the fgen along the leading edge of the surface warm
front, while also retarding it`s east-northeastward motion. 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 central LA and southwest MS, feel the
ERO risk will be more of a "high end" Slight (i.e. at least 25%
risk of rainfall exceeding FFG within 25mi of a point). 00Z models
show pockets of 3-5+ inches of rain during day 3 within this area,
with the past 2 ECMWF and UKMET runs consistently wetter than the
other models.

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