FOUS30 KWBC 190849
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
449 AM EDT Wed Jun 19 2024

Day 1
Valid 12Z Wed Jun 19 2024 - 12Z Thu Jun 20 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTH TEXAS...

...South Texas...

Potential Tropical Cyclone One (PTC1) remains on track to move
inland into Mexico late tonight based on the latest forecast out of
the National Hurricane Center. For more details on the storm,
please refer to their page http://www.hurricanes.gov .  Very impressive
moisture advection out of the Caribbean continues to the north of 
the developing low. Despite this, the storm has had the most
difficult time developing convection around the center. There is an
obvious swirl in the cloud formation on IR satellite, but
relatively little convection. All of this to say, the storm has
been struggling to organize. There is still time for it to do so,
but the clock is ticking.

The result of this lack of organization has been a disorganized
area of showers and storms that is moving into the Texas Coast as
of the time of this writing. Rainfall rates have significantly
underperformed thusfar, and storm total rainfall amounts continue 
to come down and shift southward (closer to the center). Given that
trend and in coordination with HGX/Houston, TX forecast office, the
Moderate Risk was canceled northeast of Matagorda Bay, and the
Slight Risk area was shifted southwest of Houston and south of San
Antonio.

Assuming the storm gets its act together soon...expect the still
very impressive moisture advection to the north of the center of
circulation on the order of 1,250 to 1,500 kg/m/s IVT values to 
move into the Lower Texas Coast today, resulting in storms that can
produce prodigious rainfall rates as high as 2.5 inches per hour.
Still expect isolated maximum rainfall amounts to exceed a foot by
the time all is said and done Thursday. The heaviest rain will push
westward with time, moving up the Rio Grande Valley tonight. The
highest totals remain closer to the coast as that is closer to the
moisture source (the Gulf), but rainfall totals of 4-6 inches are
likely well up the Rio Grande through Thursday morning. Given the
heavy rainfall rates, this will likely result in numerous, if
localized instances of rapid onset flash flooding as the dry soils
struggle to handle such a large amount of water all at once. 

...Northeastern New Mexico, Texas and Oklahoma Panhandles, and
southwestern Kansas...

A snail's pace moving front is crawling south across the Texas and
Oklahoma Panhandles this morning. A robust LLJ is funneling
plentiful Gulf moisture north up West Texas this morning. A strong 
ridge of high pressure over the western Dakotas is funneling 
slightly cooler and drier air southward through Kansas and into the
front from the north. These clashing air masses have resulted in 
the front moving as slow as it is. Rainfall amounts have been 
nothing short of impressive, with several areas of 7 inch + totals 
just in the last few hours in the Oklahoma Panhandle and far 
southwest Kansas. By the start of the period, the storms now into 
the Texas Panhandle and northwest Oklahoma should be much weaker 
than they are currently, as diurnal heating begins to disrupt the 
moisture advection from the south. Nonetheless, outflow will 
continue rainfall into the morning, and with such large amounts of 
rain into the Panhandles currently and over the last few hours, 
FFGs are very low, so the additional rainfall will support 
continued flash flooding. The inherited Slight was expanded south 
and west to highlight these areas for the morning...though it's 
possible isolated convection may re-impact these areas later this 
afternoon into this evening.

The Slight continues to be expanded southwest to cover much of
northeastern New Mexico. This is largely for expected dry line
convection that will break out late this afternoon through this
evening. The dry line has retreated well west into NM this morning,
thanks in part to moisture advection from the easterly flow of
PTC1. The typical eastward advance of the dry line this afternoon
with unusual amounts of atmospheric moisture will make for
scattered showers and thunderstorms capable of up to 2 inch per
hour rates. Given NM is usually a desert and very dry, these high
rainfall rates are likely to result in rapid-onset flash flooding
in the impacted areas.

...Midwest and Great Lakes...

Ridge running convection tracking westward around the western
periphery of the heat dome bubble of high pressure over the
southern Appalachians will cause storms to race eastward across the
area today and into tonight. While the storms will be fast-moving,
unidirectional flow may result in some localized training which may
cause isolated flash flooding.

Wegman

Day 2
Valid 12Z Thu Jun 20 2024 - 12Z Fri Jun 21 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE RIO GRANDE
VALLEY AND SOUTH TEXAS AS WELL AS FOR PORTIONS OF THE NORTHERN 
PLAINS AND UPPER MIDWEST...

...Rio Grande Valley and South Texas...

Remnant rainfall from what will be rapidly weakening Potential
Tropical Cyclone One (PTC1) Thursday morning will continue to push
west with the full force of the open Gulf supporting it. Easterly 
flow will be well established across the entire Gulf. The rainfall 
will be moving into increasingly dry areas of west Texas and New 
Mexico Thursday. Thus...while the heaviest rainfall rates will be 
diminishing with longitude...what heavy rainfall does occur will be
moving over areas less and less used to heavy rain. Thus, the 
flooding threat will continue into the Rio Grande Valley and south 
Texas. Further, increasing interaction with the terrain will 
locally enhance rainfall rates, especially on east-facing slopes. 

The storm is expected to be moving a little bit faster than
inherited forecasts. This has resulted in a bit lower max totals,
while spreading the heaviest rains further west. Most of the
increases were in Mexico. For the Texas side of the Rio Grande,
this has resulted in a drier forecast for much of central Texas.
Thus...the Slight, has been trimmed from the north and east, now
almost to the Rio Grande near Del Rio. 

For South Texas, the aforementioned broad easterly flow over the
entirety of the Gulf will keep isolated to widely scattered showers
and thunderstorms that develop over the Gulf moving westward across
the area...with drier conditions expected to the north where areas
to the east are not the abnormally hot Gulf, but rather unusually
hot land. Thus...the threat through the day really is focused on
areas due west of the Gulf...i.e. south Texas.

The combination of upper level ridging and a slow moving trough
over the Intermountain West will turn the prevailing flow more
southerly, generally following up the Rio Grande Valley...Thus, the
rainfall threat will expand northward into west Texas and much of
eastern New Mexico through Thursday night. Terrain impacts will
locally enhance rainfall rates resulting in isolated flash
flooding, as well as any overlap from Day 1 rainfall in
northeastern NM.

...Northern Plains and Upper Midwest...

A stalled out front will be over eastern South Dakota, southern
Minnesota and northwestern Iowa today. Convection may impact some
of these areas to start the day Thursday morning from overnight
convection tonight, but the bulk of the expected rainfall in this
area will occur Thursday night. Developing troughing over the area
and a departing area of high pressure will turn the front into a
warm front that moves north through the night. Heavy rainfall rates
are likely from storms as moisture convergence over the Slight Risk
area interacts with the front. Much of this area from far northern
NE, southeastern SD, and all of southern Minnesota have been hard
hit with heavy rains in recent days. Thus, a repeat of the heavy
rain over the area will likely result in widely scattered instances
of flash flooding. Should the daily rainfall maximum continue to
shift south as has been the trend with much of the nation's
forecast convection, increasingly hard hit areas will be
increasingly impacted. Thus, a targeted upgrade is certainly not
out of the question.

...4 Corners Region...

Moisture from the remnants of PTC1 will track northwestward into
the 4 Corners Region Thursday and Thursday night. Upslope flow,
especially into the San Juans of southwest Colorado may result in
isolated flash flooding, especially around burn scars. Increasingly
anomalous moisture advecting into the area will result in
increasing risk for flash flooding.

...Finger Lakes of NY and northwest PA...

Convection capable of heavy rainfall rates will develop over this
region today. Soils in the area are wetter than normal, so any
training or stationary storms from the terrain may cause heavy rain
over a concentrated area, resulting in potential isolated flash
flooding.

Wegman
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