AWUS01 KWNH 240708
FFGMPD
LAZ000-TXZ000-241230-
Mesoscale Precipitation Discussion 0715
NWS Weather Prediction Center College Park MD
308 AM EDT Wed Jul 24 2024
Areas affected...Middle Texas Coast through Southwest Louisiana
Concerning...Heavy rainfall...Flash flooding likely
Valid 240707Z - 241230Z
Summary...Showers and thunderstorms blossoming over the Northwest
Gulf of Mexico will spread onshore through the morning.
Backbuilding of 2-4"/hr rates will result in 3-5" of rain with
locally higher amounts. Flash flooding is likely.
Discussion...The GOES-E WV imagery early this morning clearly
portrays the extreme environment setting up across the Texas and
Louisiana coasts this morning. An upper low is positioned over
Tamaulipas, Mexico, while a secondary vorticity impulse spins
across the NW Gulf of Mexico (GoM). This is helping to advect
copious low-level moisture, noted via the SFC-850mb ALPW,
northward into the western Gulf Coast. At the same time, a stream
of high level moisture emerging from the eastern Pacific is
traversing northeast in the 500-300mb ALPW fields to intersect
this same region. This is manifesting as deeply saturated 00Z U/A
soundings at KCRP and KLCH with measured PWs of 2.35 and 2.27
inches, respectively, nearing the daily record at both sites. This
is combining with slowly increasing MLCAPE of 1000 J/kg (more than
2000 J/kg over the source region in the GoM) to yield extremely
favorable thermodynamics for heavy rainfall. Into these
thermodynamics, ascent is slowly increasing through the approach
of the broad mid-level trough axis to the NW, a surface trough
approaching from the GoM, weak mid-level impulses shedding through
the flow, and strengthening frictional/moisture convergence.
The regional radar mosaic shows a few convective cells beginning
to propagate onshore as even more widespread activity blossoms
over the GoM. Regional VWPs indicate that 850mb inflow out of the
S/SW has increased to 15-20 kts, drawing the more robust
thermodynamics northward, and as this persists thunderstorm
development onshore should rapidly increase. This is progged by
nearly all of the recent high-res CAMs simulated reflectivity,
leading to higher confidence in the evolution the next few hours.
As thunderstorms expand and intensify, rainfall rates will almost
certainly (>70% chance on the HREF and REFS) exceed 2"/hr, and at
times may reach 4"/hr as shown by HRRR 15-min accumulations of 1".
Most concerning about these rain rates is that the Corfidi vectors
will become increasingly aligned anti-parallel to the mean 0-6km
wind and collapse to just 5 kts, suggesting storms will backbuild
into the higher instability over the GoM and may exhibit near-zero
net motion at times along the frictional convergence boundary.
Where this occurs, rainfall of 3-5" is likely, and the HREF has a
40-60% chance for more than 5 inches from Galveston Bay to Lake
Charles, LA. This area is also favored for the heaviest rain by
the REFS PMM of more than 5 inches, and the experimental OU-CAPS
SAM which aligns closely with the HREF EAS probabilities, further
increasing the confidence that this will be the hardest hit region
through the morning. Farther southwest, training of cells could
still produce 2-3" of rain with locally higher amounts as far as
Oso Bay, but confidence is somewhat less as to whether the
heaviest rain will stay offshore in this area of the Middle Texas
Coast.
14-day rainfall across the Gulf Coast has generally been above
normal, although a subtle axis of below normal rain does exist
along the Upper TX Coast into LA. However, recent rainfall has
saturated the the top soils as noted by NASA SPoRT 0-10cm RSM that
is 70-90% across the entire area. Regardless of soil conditions,
slow movement and repeating rounds of these intense rain rates
will likely cause flash flooding across much of the region this
morning.
Weiss
...Please see
https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!7ZIo7tgK6mkZLua1FK-pAv29wfcj6iGEv9ldJ_qvTJXye5prHzLD7pHit028SMXCWBJu= dcG_mBKjgBjxMPiVA_LgCf0$ for graphic product...
ATTN...WFO...CRP...HGX...LCH...
ATTN...RFC...LMRFC...WGRFC...NWC...
LAT...LON 30559378 30549312 30349272 29989258 29729273=20
29659315 29419384 29359428 29169462 28899513=20
28659565 28439617 28149664 27849693 27579715=20
27409739 27499756 27759765 28209726 28789667=20
29249599 29789545 30229506 30519436=20
=3D =3D =3D
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