Quantitative Precipitation Forecast Issued by NWS
000
FOUS30 KWBC 161549
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1149 AM EDT Sat Mar 16 2024
Day 1
Valid 16Z Sat Mar 16 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
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS NEAR THE SOUTHERN
LOUISIANA/TEXAS BORDER...
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.
Roth
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