FOUS30 KWBC 081545
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1145 AM EDT Sun Sep 8 2024

Day 1
Valid 16Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, DEEP SOUTH TEXAS, AND SOUTHEAST...

...16Z Update...

Very little changes were made to the overall D1 outlook with the
consensus remaining steady for low to medium end MRGL impacts
expected across portions of the West, Southeast, and Deep South
Texas. A small shift was made across South TX where the MRGL risk 
was tightened even further to the coast to confine mainly 
Brownsville and South Padre Island as the two main areas of focus 
with the northern periphery of any banding associated with the 
tropical wave to the south. The latest 12z HREF mean QPF output 
depicts a sharp western gradient for precip through the period with
the best potential aligning with the overnight time frame as the 
wave scoots more to the northwest, bringing some of the deeper 
tropical moisture north into the tip of South TX. There's some 
discrepancy on the timing of any banding with a 50/50 split amongst
CAMs and some global deterministic. There's enough merit to 
warrant the continued MRGL to maintain continuity, especially with 
a tropical disturbance positioned to the south. There's a chance 
this materializes after the period, but as a precaution for the 
low-end threat, decided against removal. 

Over the Southeast, the main focus will reside along the coasts of
FL and perhaps over the coast of SC as convection will align near
the coastline during peak diurnal destabilization. The best
opportunity for heavy rain will likely occur with the sea breeze
across Northeast FL between Jacksonville to Melbourne with the HREF
neighborhood probabilities and EAS both fairly aggressive in the
latest update for >3" of rainfall in the vicinity. The airmass in
place is sufficient for these higher hourly and intra-hour rates to
produce some localized flash flood concerns, especially within the
urbanized areas along the coast. This was enough maintain the MRGL
risk with no real changes to the risk positioning from previous
forecast.

Out West, the only change occurred over the northern extent of the
Peninsular range where 12z CAMs indicate a few cells developing 
just north of where the risk area was forecast. HREF mean SBCAPE 
values lie within the 500-1000 J/kg with a few CAMs a bit more 
robust in the depiction. Considering a marginally more favorable 
airmass overhead and the complex terrain additions over the area, 
thought the expansion was favorable and was agreed upon in 
coordination with the San Diego WFO. Otherwise, no other changes 
were necessary from the previous issuance across the west. 

Kleebauer

..Previous Discussions..

...Southwest...

A bit more widespread convection is expected with the monsoon this
afternoon across the Southwest. Much of the convection will once
again be focused along the various mountain ranges from the
Peninsular Ranges of southern California through the Sangre de
Cristos of Colorado. As usual in the desert Southwest, somewhat
limited moisture will be the limiting factor, even though areas
could be 2 sigma above normal for atmospheric moisture for this
time of year. Thus, the primary flood threat is for urban areas,
slot canyons, and arroyos where slow moving convection in chaotic
steering flow may result in long-duration localized heavy rain from
the most persistent convection. Any resultant flash flooding will
be isolated.

...Deep South Texas...

The Marginal Risk area was trimmed closer to the coast with this
update, now primarily for the urban area of Brownsville over to the
South Padre Island beaches. A stalled out front will begin to focus
an area of heavy rainfall associated with a slowly developing
tropical low over the western Gulf. Guidance has shifted east a
bit, resulting in lower forecasted rainfall totals for all of Deep
South Texas, with little if any rain expected west of Brownsville
through tonight. However...given the abundance of atmospheric
moisture available any stronger showers and storms will be capable
of very heavy rainfall rates which could cause isolated flash
flooding.

...Southeast...

The stalled out front that has been haunting the Southeast for well
over a week now remains in place from far southern South Carolina
through the northern Florida Peninsula. Besides the front itself
being strong...indicative of a tight gradient between the extremely
humid air mass over the Gulf and an outright autumnal air mass to
its north...instability remains very lacking since this front
hasn't moved in days, so diurnal heating is limited under the
extensive low cloud cover. Thus, almost all of the rain will be
light across this region. This is countered by the aforementioned
abundant amounts of atmospheric moisture available for any
convection that can form. Thus, a low-end Marginal remains in
place, with the greatest threat for flooding over the Florida
Peninsula well south of the front where diurnal heating can
actually make a difference.

Wegman


Day 2
Valid 12Z Mon Sep 09 2024 - 12Z Tue Sep 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP
SOUTH TEXAS...

As on Day 1, very few changes were needed to the inherited ERO risk
areas.

...Coastal Deep South Texas...

Weak easterly flow north of a developing tropical low over the
southwestern Gulf will advect increasingly deep tropical moisture
into coastal Deep South Texas on Monday. This is highly likely to
result in much more rainfall for South Padre Island and over to
Brownsville as compared with Day 1/Sunday. Given PWATs that could
exceed 2.5 inches, or 3 sigma above normal for this time of year,
any convection that moves west off the Gulf into the coast will be
capable of very heavy rain rates. Thus, the real question regarding
flood coverage will be how frequent storms producing those rates
move over urban areas. Think right along the coast widely scattered
instances of flash flooding can be reasonably expected. Any
training could quickly result in significant flash flooding. That
said, the system will only really just be getting organized, so
overall rains will likely be more intermittent in most areas. Urban
areas such as Brownsville will be at a higher likelihood for flash
flooding.

...Four Corners Region...

Convective coverage should shift northeastward a bit on Monday.
Thus, the flooding threat shifts into the Four Corners region and
away from the West Coast. Flooding concerns will be greatly
focused in area slot canyons and arroyos of southern Utah and
southwest Colorado Monday afternoon and evening with peak heating.

...Rest of the Gulf Coast...

Rinse and repeat weather pattern will continue, albeit shifted just
a bit south of Sunday's focus. Thus, most of the north Gulf Coast
will get a relative break from the potential for steady heavy rain,
as the heaviest rains shift south a bit away from the coast. The
exception will be into the Florida Peninsula, where a repeat day of
sea breeze showers and thunderstorms can be expected to the south,
with more widespread lighter rain expected further north.

Wegman
--- SBBSecho 3.20-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)