Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
807 PM EDT Fri Jul 26 2024
Day 1
Valid 01Z Sat Jul 27 2024 - 12Z Sat Jul 27 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MIDDLE/UPPER TEXAS COAST, THE COASTAL CAROLINAS, AS WELL AS COASTAL
SOUTHEAST LOUISIANA AND MISSISSIPPI...

...01z Update...

Diurnal convection is waning across much of the country, but a
Slight Risk has been maintained where convection remains active
across coastal portions of LA/MS where convection (see MPD #740).
While this activity is poised to weaken and diminish with the loss
of daytime heating, there should be reinitiation of convection
late tonight over portion of the Middle and Upper TX Coast (where a
Slight Risk has been maintained and expanded). The 18z HREF
indicates 40-50% odds for localized 3" exceedance (per a 40-km
neighborhood method), mostly after 06z. Broader Marginal Risk areas
were maintained across the Southeast, Mid Atlantic, Southwest, and
Intermountain West (where localized flash flooding will continue
for at least a few more hours). 

Churchill


...16Z Update...

Several changes to note with today's midday update:

...Southeast Texas & The Mississippi Delta...

The Slight across southeast Texas has been expanded to include
much of Louisiana and the Mississippi Gulf Coast. Ongoing heavy
rainfall has resulted in numerous Flash Flood Warnings across east
Texas and northwestern Louisiana. A plume of deep tropical moisture
is surging northward across the Slight Risk area. CAMs guidance
shows with peak heating this afternoon, numerous showers and
thunderstorms will form over the Gulf and move inland into southern
Louisiana and the Mississippi Gulf Coast. With unidirectional flow,
it's probable there will be training storms embedded within the
broader field of storms. Meanwhile, the plume of moisture over
northern Louisiana now should decrease in intensity as peak heating
results in universal increases in instability, resulting in
dissipating storms reliant on a coherent plume of moisture.

PWATs are near their climatological maximum across Louisiana as
they approach 2.25 inches this afternoon. This will allow the
expected afternoon convection plenty of moisture to translate into
efficient rainfall rates. Urban areas including New Orleans,
Gulfport, and Baton Rouge are at a higher threat for flash
flooding.

...Carolinas...

In coordination with RAH/Raleigh, NC; CAE/Columbia, SC; and
ILM/Wilmington, NC forecast offices, the Slight for the coast was
expanded westward to cover the ongoing convection along the NC/SC
border and aligned with MPD 734. While the convection has been
shallow, very efficient warm rain processes and training has
resulted in multiple inches of rain in the impacted areas. The
focus going into this afternoon will be much closer to the coast,
but any disorganized afternoon convection that impacts this same
area...with sufficient clearing of the cloud cover between now and
then, could result in widely scattered instances of flash flooding.

...New Mexico...

In coordination with ABQ/Albuquerque, NM forecast office, a Slight
Risk upgrade was introduced with this update for the afternoon
convection in the Sacramento Mountains. Increasing instability this
morning and continued above normal atmospheric moisture should
result in a renewed round of afternoon storms. The Sacramento
Mountains have been both hard hit in recent weeks with heavy rain,
and have very sensitive and vulnerable burn scars around Ruidoso
that would enhance the impacts of any potential flash flooding in
that area.

Wegman

...Previous Discussion...

...Portions of Southeast Texas...
Radar was showing increasing coverage of showers in a region of
confluent flown off the Gulf of Mexico in the early morning
hours...and the expectation is that convection will be persisting
beyond 12Z. Given some overlap with a region of lower Flash Flood
Guidance values and potential for 1 to 2 inch per hour rates
redeveloping later in the day as shown by the HREF 40 km
neighborhood probability guidance, maintained a Slight Risk from
coastal Texas northeastward along the axis of highest precipitable
water/instability. There is some upper support in the form of a
trof axis between 200 mb and 300 mb with a 50 kt speed max rotating
around the east side of the trof axis that looks help draw the
moisture towards the Southern Mississippi valley late in the
period,

...Portions of the Southeast US Coast...
Surface low pressure will linger along the Carolina coastline for
much of the period with an associated frontal boundary helping
provide some focus for convection as mid-level height falls
approach from the north and west. Much of the operational guidance
showed the heaviest rainfall along or immediately off the Carolina
coastline...although the NAM maintained some threat of 2 to 5 inch
amounts falling inland. Between that and the fact that the 26/00Z
HREF showed 40 to 60 pct neighborhood probabilities of 1- and
3-hour QPF exceeding flash flood guidance along the immediate
coastline at time of maximum daytime heating...maintained the
Slight risk but continued to shrink the area somewhat from the placement
of the inherited Slight Risk area.

Surrounding the two Slight Risk areas was an expansive Marginals
Risk area. Models depict abundant moisture and instability in this
area but weak forcing mechanism and meager steering flow. Thus
cells that form will be slow moving and efficient rainfall
producers.

...Southwest...

Enough moisture and instability will still be in place over
portions of the Southwest and the Intermountain basin to support
the development of convection in the afternoon and which persists
into the evening...although the best focus will have shifted
eastward since Thursday. There is still concern that any storm
which forms will have the potential for 1+ inch per hour rates and
isolated storm total amounts in the 1 to 2 inch range...enough to
result in flooding and run off problems with the greatest risk
being over recent burn scars and in normally dry washes. Some minor
adjustments were made to the previous MRGL risk area but overall
there was a fair degree of continuity.

Bann

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt