Quantitative Precipitation Forecast
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000
FOUS30 KWBC 161957
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 PM EDT Sat Mar 16 2024

Day 1

Valid 01Z Sun Mar 17 2024 - 12Z Sun Mar 17 2024


...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH-CENTRAL AND SOUTHEAST TEXAS...

...16Z Update...

No changes were made to the inherited Slight and Marginal risk
areas across Texas with this update. Ongoing slow moving
convection may pose an isolated flash flooding risk into the early
afternoon, with a waning of storms through the rest of the day.
The main area of convective activity will be during the overnight
hours. Multiple rounds of storms through the late evening and early
morning hours will evolve into a more organized MCS through early
Sunday morning. Unlike ongoing convection, by tonight the storms
will be much faster moving, but more widespread. Corfidi Vectors
are parallel to the flow, which could support isolated training
convection, especially given the moisture-rich atmosphere with
PWATs above 1.75 inches. This is as much as 2.5 sigma above normal
for this time of year.

Guidance remains highly variable as to how the convection will
evolve tonight. The Fv3, GEM, and CONEST all suggest there may be
more training of fast-moving storms in various streaks across the
Slight Risk area, particularly just inland from the coast.
Meanwhile the historically more reliable HRRR and ARW members are
much lighter in the overall rainfall footprint, alternatively
suggesting that one more organized MCS will race across south
Texas and into the Gulf, perhaps clipping far southwestern
Louisiana before 12Z. Should this scenario occur, then the flash
flooding risk is much lower. Given this inherent uncertainty, have
opted to leave things as status quo. The average look in the
guidance suggests this to be a lower end Slight, largely boosted by
earlier heavy rains from last night across portions of the Slight
Risk area. FFGs remain rather high despite the recent rains, so it
appears it will require training convection to get the widely
scattered flash flooding consistent with the Slight. A fast-moving
MCS/squall line scenario would keep any flash flooding to a minimum
and would require a Marginal risk downgrade.

Wegman

...Previous Discussion...

Difluence aloft ahead of an upper level low moving slowly across
Arizona teams up with a quasi-stationary front to continue the
potential for heavy rainfall in the front`s vicinity. The guidance
has shifted northward when compared to this time yesterday, which
led to a similar shift in the Slight Risk area. Slight warming in
the temperatures at 700 hPa imply that the heavy rain threat
should edge northward, which appears to be beginning per recent
radar reflectivity trends in southeast TX, but how far north
remains the question. Inflow at 850 hPa from the Gulf should pulse
up to 25-30 kts at times, importing ML/MU CAPE of 2000 J/kg or so.
Effective bulk shear should be sufficient for organized
convection, capable of producing hourly rain totals to 2" and
local amounts to 4". The guidance indicates a couple rounds of
convection this period, early and late. Dispersion continues
concerning where within southern and eastern TX the heavy rainfall
is most likely to occur, but the amounts still appear high enough
to maintain the Slight Risk threat in the outlook. There was some
accounting for the heavy rainfall that fell on Friday near the
southern border of LA/TX in helping to define the eastern edge of
the Slight Risk area.

Roth


Day 2

Valid 12Z Sun Mar 17 2024 - 12Z Mon Mar 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF
SOUTHERN LOUISIANA AND COASTAL MISSISSIPPI...

...2030Z Update...

A low level jet with abundant Gulf moisture will run into an
advancing cold front moving down the Mississippi Valley Sunday. The
clash of these 2 air masses will provide a focus and lifting
mechanism for the moisture, supporting strong storms capable of
heavy rainfall rates to 2 inches per hour. The front will allow for
the storms both to form and to track east along the frontal
interface, which will extend along all of the northern Gulf Coast.

In coordination with LIX/Slidell, LA forecast office, the Slight
Risk has been expanded well to the east to account for the convection
that will be ongoing Sunday morning. The 2 inch per hour rainfall
rates will be capable of rapid-onset flash flooding, particularly
in urban areas such as Baton Rouge and New Orleans. Recent heavy
rainfall over and upstream of these areas have made local streams
and rivers high, which will lower the threshold needed for flash
flooding to occur. Once the cold front moves through, conditions
should rapidly improve, to include rapid drying of the atmosphere.
This will end the flash flooding threat caused by rainfall.

Wegman

...Previous Discussion...

Organized convection in the vicinity of a front, which should be
somewhere the southern border of TX/LA Sunday morning, is expected
to lead to heavy rainfall for portions of the northern Gulf coast.
Sufficient 850 hPa inflow off of the Gulf of Mexico, up to 40 kts,
will lure precipitable water values of 1.5-1.75" into the region
and MU CAPE up to 2000 J/kg. Difluence aloft should be fostered by
increasing troughing across the Great Lakes, Midwest, and
Mississippi Valley. There is some signal in the guidance for
cooling temperatures at 700 hPa, which is of concern as the
1000-500 hPa thickness diffluence is now very close to the LA
coast. This means that there is some chance that organized
convection could shift slightly offshore LA at some point in its
evolution, leading to decreasing confidence in heavy rainfall the
farther east you go in LA. Both the 00z NAM and 12z-00z ECMWF are
on the north side of the guidance envelope (which usually isn`t
good news for the ECMWF solution) while the remainder of the
guidance lies closer to the Gulf coast, including the normally
northward- biased 00z UKMET. The guidance shows a bit of
progression to the convective pattern, as an upper trough sharpens
across the Ohio and Mississippi Valleys and helps act to shift the
moisture plume/best 850 hPa inflow east with time. Hourly rain
totals to 2.5" with local amounts to 4" are possible where limited
cell training occurs and/or mesocyclones happen to develop.
Because
of the heavy rainfall that occurred on Friday near the southern
TX/LA border, and per coordination with the LCH/Lake Charles LA
forecast office, added a targeted Slight Risk area to areas where
soils recently became more sensitive.

Roth


Day 3

Valid 12Z Mon Mar 18 2024 - 12Z Tue Mar 19 2024

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

Wegman


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


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