189
FOUS30 KWBC 200104
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
904 PM EDT Wed Jun 19 2024

Day 1
Valid 01Z Thu Jun 20 2024 - 12Z Thu Jun 20 2024

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

01Z Update...
Across South Texas, maintained but adjusted south the Moderate and
Slight Risks. Bands of convection associated with Tropical Storm
Alberto will continue to move across the region, producing
additional areas of locally heavy rainfall through the evening.
HREF and recent runs of the HRRR indicate that additional amounts
of 3 inches or more are likely within the Moderate Risk area.

Further to the north, maintained a Slight Risk centered over
eastern New Mexico where slow-moving storms, producing locally
heavy rainfall, fueled by moist, low-level, easterly winds may
continue for the next few hours. Additional flash flooding is a
concern, especially where heavy rains fall across sensitive burn
scar areas.

Trimmed away the Slight Risk area from areas further to the
northeast into southern Kansas where the signal for any widespread
additional heavy rain and flash flooding concerns has waned.
However,cannot rule out some isolated concerns for storms to
develop and train along a slow-moving front extending northeast
into the Great Lakes. Some guidance, including recent runs of the
HRRR, show isolated heavy amounts from the lower Missouri Valley
northeastward.

Pereira

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

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PARTS OF
EASTERN SOUTH DAKOTA INTO SOUTHEASTERN MINNESOTA...

2030z Update

Moderate Risk introduced over parts of South Dakota and Minnesota
due to increased QPF trends and over performing antecedent
rainfall/storms. The Rockies and Southern High Plains Marginal Risk
areas were merged and a Slight Risk area was introduced over parts
of the Four Corners/Mesa Verde regions and San Juan Mountains to
account for increased QPF trends over elevated terrain.

...Northern/Central Plains and Upper Midwest...

Strong diffluence over the North-Central Plains paired with a
robust heat dome over the Eastern half of the country will promote
active weather over the Central U.S. on Thursday. A stalled out
surface front oriented diagonally northeast to southwest across
the Plains and Upper Midwest will act as a focus for convection.
Impressive instability (1000J/Kg+ MLCAPE) paired with substantial
atmospheric moisture content (1.5-2" PWATs) will produce very
efficient rain rates within MCS activity that propagates through
parts of the Northern/Central Plains and Upper Midwest. The
greatest threat of Excessive Rainfall will be over parts of
southern South Dakota and southern Minnesota where anywhere
between 3-5" of rain could fall by Friday morning.

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

Remnant rainfall from what will be rapidly weakening Tropical Storm
Alberto 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 move a little bit faster than inherited
forecasts. This resulted in a bit lower max totals, while
spreading the heaviest rains farther 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.

...4 Corners Region...

Moisture from the remnants of Tropical Storm Alberto will track
northwestward into the 4 Corners Region Thursday and Thursday
night. Upslope flow of anomalous moisture, especially into the San
Juans of southwest Colorado, may result in flash flooding. Burn
scars in the area are particularly susceptible to flash flooding
as well.

...Interior Northeast/Lower Great Lakes...

Convection capable of heavy rainfall rates will develop over this
region on Thursday. Soils in the area are wetter than normal, so
any training or stationary storms that occur, especially over
the terrain of the Adirondacks or White/Green Mountains, may lead
to isolated but dangerous runoff.

Kebede/Wegman

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

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM SOUTH-
CENTRAL MINNESOTA TO WESTERN WISCONSIN...

2030z Update

A Moderate Risk of Excessive Rainfall was introduced with this
afternoon`s update. This was done, in part, because of the strong
signal for heavy rainfall over much of the same areas on Day 2.
FFGs are likely to be low and soil moisture high at the start of
day 3 (12z Friday) so another round of strong [rainfall efficient]
MCSs paired with the aforementioned antecedent conditions support
the upgrade. Minor changes were made to the Four Corners/Southern
High Plains areas and the Marginals in south Texas and the
Northeast.


...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 Tropical Storm Alberto makes its way into
the Northern Plains. This tropical moisture and advection,
strengthening low, and very favorable antecedent conditions over
much of the Moderate 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.

...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.

Kebede/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