AWUS01 KWNH 230901
FFGMPD
LAZ000-ARZ000-TXZ000-OKZ000-231500-

Mesoscale Precipitation Discussion 0306
NWS Weather Prediction Center College Park MD
500 AM EDT Thu May 23 2024

Areas affected...southeast OK into the Ark-La-Tex

Concerning...Heavy rainfall...Flash flooding possible

Valid 230900Z - 231500Z

Summary...A compact MCS will produce heavy rainfall across
recently soaked portions of southeast OK into the Ark-La-Tex,
possibly resulting in isolated to scattered instances of flash
flooding.

Discussion...A cluster of thunderstorms that formed overnight in
southern OK has gradually organized into a compact MCS over the
past several hours, largely being missed by the recent 00z HREF
suite. While hi-res CAMs entirely failed to initiate convection in
a timely matter in this region, the mesoscale environment does
appear favorable for continued MCS maintenance and growth into the
early morning hours (which the 06z and 07z HRRR did finally pick
up on after a run or two of the model that failed to properly
assimilate the WSR-88d data). Looking downstream into the
Ark-La-Tex region, SPC mesoscale analysis indicates PWATs of
1.5-1.8 inches (above the 90th percentile, per SHV sounding
climatology), ML CAPE of 1000-1500 J/kg, and effective bulk shear
of 40-50 kts. Additionally, the storms appear positioned near a quasi-stationary surface front with a diurnally strengthening
20-30 kt LLJ bringing ample low-level moisture transport. These
meteorological factors and parameter space suggest high
favorability for maintaining an MCS into the early morning hours
(as indicated by an MCS maintenance parameter of 90-100).

While the hi-res CAMs are not particularly useful in this case,
the latest HRRR runs do provide a decent depiction of what is
expected (though perhaps still a bit underdone in the overall
coverage and intensity of heavy rainfall compared to observational
trends). The expectation is for storms to become increasingly
organized and cold pool dominate, which would favor a more
southerly component to storm motion (following the upwind
propagation vector towards the southeast, rather than the mostly
easterly dominate motion thus far). Concerns for localized flash
flooding are highest along the southern flank of the developing
MCS, where 1-3"/hr rainfall rates may have their greatest
residence time, resulting in localized totals of 2-4". Given that
the bulk of the region has seen at least moderate to heavy
rainfall over the past 6-12 hours (with localized totals of 2-4"
from the prior storms as well), isolated to scattered instances of
flash flooding are possible (with 3-hr FFGs still generally
ranging from 2-4"). Should the MCS make it all the way into
northwest LA (and particularly into the Shreveport metro area)
where FFGs are generally 2.5" or less, flash flooding may be
locally significant.

Churchill

ATTN...WFO...FWD...LZK...OUN...SHV...TSA...

ATTN...RFC...ABRFC...LMRFC...WGRFC...NWC...

LAT...LON 35489564 35269400 34039241 32749265 32479418
33189612 33789729 34509652 35169627

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AWUS01 KWNH 151332
FFGMPD
VAZ000-WVZ000-TNZ000-KYZ000-INZ000-MSZ000-MOZ000-ARZ000-151930-

Mesoscale Precipitation Discussion 0038
NWS Weather Prediction Center College Park MD
830 AM EST Sat Feb 15 2025

Areas affected...Portions of the OH/TN Valleys and Mid-South

Concerning...Heavy rainfall...Flash flooding likely

Valid 151330Z - 151930Z

SUMMARY...An expansive axis of heavy showers and thunderstorms
with strong cell-training concerns will be promoting a widespread
flash flood threat over the next several hours across portions of
the OH/TN Valley region and Mid-South.

DISCUSSION...The latest GOES-E IR satellite imagery shows an
expansive areas of heavy shower and thunderstorm activity
impacting areas of eastern AR up across central and western TN and
much of central and southern KY. Light to moderate stratiform
rains are noted north of here closer to the OH River and also into
the central Appalachians.

The convection that is organizing is associated with strong warm
air advection which is being aided by a powerful southwest
low-level jet of 50 to 60+ kts overrunning a strong frontal zone.
This is driving very strong moisture transport which is showing up
in an impressive fashion with the experimental CIRA-LVT (Layered
Vapor Transport) imagery as it is depicting SFC/850 layer LVT
magnitudes of 300+ kg/m/s. Meanwhile, a nose of MUCAPE values on
the order of 500 to 1000 J/kg is now oriented up across central to
northeast AR through western TN and western KY and this is helping
to facilitate the broader axis of elevated convection.

Over the next several hours, very strong warm air advection and
moisture transport will continue to support organizing bands of
convection that will be aligned with the deeper layer mean flow
and thus will be conducive for a substantial level of
cell-training. Increasing CAPE values with time along with
strengthening shear profiles will support stronger convective
elements that will yield increasingly heavy rainfall rates.

Rainfall rates with the stronger storms may reach as high as 1 to
1.5 inches/hour at least going through midday, but with
significant cell-training concerns, this may result in some
rainfall amounts by early afternoon alone of 2 to 4 inches. The
latest CAM guidance suggests areas of western/northern TN through
southern KY will have the heaviest totals going through the 18Z to
20Z time frame, with somewhat lesser amounts farther north.

Given the extremely sensitive/saturated soil conditions and
already elevated streamflows for many locations, these rainfall
totals over the next several hours are likely to begin resulting
in widespread flash flooding. Additional MPDs will be issued this
afternoon to update what should evolve into a high-impact and
life-threatening flash flood event in time.

Orrison

$$
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