465
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 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

Day 3
Valid 12Z Fri Jun 21 2024 - 12Z Sat Jun 22 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN SOUTH
DAKOTA THROUGH MUCH OF WISCONSIN AS WELL AS FOR MUCH OF THE
NORTHERN COLORADO PLATEAU...

...Eastern South Dakota through Wisconsin...

Multiple sources of forcing will support multiple rounds of heavy
rain across the northern Plains into the Upper Great Lakes on
Friday and Friday night. During the day on Friday, the warm front
over the Slight Risk area will slow to a stall during the morning
as the typical weakening of the LLJ occurs due to diurnal heating.
This too should result in diminishing coverage and intensity of
rainfall over this area through the morning.

Then Friday afternoon and evening, a strengthening low that will
develop as a shortwave ejects into the northern Plains will
increase moisture advection ahead of the low and attendant cold
front into the Slight Risk area. PWATs will increase with the
storms into Friday night perhaps above 2 inches as the bulk of the
moisture that was once Potential Tropical Cyclone One (PTC1) makes
its way into the northern Plains. This tropical moisture and
advection, strengthening low, and very favorable antecedent
conditions over much of the Slight Risk area are all ingredients
contributing to increasing concern for numerous and more widespread
flash flooding in this region. The center of the low will move to
central Minnesota by Saturday morning. Expect strong storms with
both the warm front to the east of the low and the cold front south
of it due to plentiful moisture.

Uncertainty remains high as all of the aforementioned ingredients
need to all come together at the same time. There have been
distinct southward shifts with time in previous days in the
placement of heavy rain from convection with all the convection
impacting the nation. Thus, the primary point of uncertainty is
where the heaviest rain will occur. For now, a higher-end Slight is
considered in effect for the area of far eastern SD, nearly all of
the southern half of Minnesota, and western Wisconsin. Should the
southern portion of this area remain in the forecast for the
heaviest rain of similar magnitude as the model consensus suggests,
a Moderate Risk will likely be needed with future updates. Stay
tuned.

...Northern Colorado Plateau...

In coordination with SLC/Salt Lake City, UT and GJT/Grand Junction,
CO forecast offices, a Slight Risk upgrade was introduced with this
update. A potent shortwave moving into the West Coast will interact
with some of the remnant moisture from PTC1, which will continue to
be advected into the area from the Gulf. Atmospheric moisture will
be a much as 4 sigma above normal. Interaction with the terrain,
the shortwave increasing forcing, and local burn scars in both CO
and UT will all increase the potential for flash flooding in this
area, particularly in the south facing slopes of the San Juan
Mountains.

...South Texas...

Continued easterly flow over the Gulf will push additional
convection developing over the Gulf into south Texas Friday and
Friday night. The convection is likely to be mostly isolated to
widely scattered with little in the way of organization. The
potential for local training and the likely very favorable
antecedent conditions/low FFGs from the recent passage of PTC1 in
the area continues to support a Marginal Risk for this area.

...New England and Hudson Valley...

A Marginal Risk area was introduced in this region for the third
consecutive day of afternoon convection occurring as waves continue
to run the top of the ridge that remains over the Southeast. A
small area of high pressure will increase northwesterly flow into
northern New England. Meanwhile the plume of moisture originating
from the Gulf, moving north up the Plains, then turning west across
the Great Lakes and into New England will be ever present. The
front pushing south with drier air will provide a source of forcing
for more concentrated convection, particularly over southern New
England Friday afternoon. The signal remains weak, but isolated
flash flooding is possible in flood-prone areas.

Wegman


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt