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FOUS30 KWBC 041954
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
354 PM EDT Sat May 4 2024

Day 1
Valid 12Z Sat May 04 2024 - 12Z Sun May 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL TODAY AND TONIGHT
ACROSS PORTIONS OF THE SOUTHERN PLAINS...

..Southern Plains and Gulf Coast...

16Z update... Very small adjustments were made to the Marginal`s
northern bounds and the southern bounds of the Slight Risk to
adjust for a very small southward trend with the QPF footprint.
Consensus still favors the highest amounts to focus near the Texas
Hill country and surrounding locations to the immediate east and
north. Hi-res guidance is showing several hours where the hourly
rainfall rates will be 2-2.5 inches/hr with a couple instances
where it surges to 3-4 inches/hr.

Campbell

The focus of attention becomes the Southern Plains as a mid- and
upper level trough emerges from the southern Rockies later today
and encounters an atmosphere becoming increasingly moist and
unstable over much of Texas given persistent flow off the Gulf of
Mexico. The general consensus of guidance opinion is that storms
will be forming along the dry line in West Texas and then develop
upscale and towards the east in central Texas. As the storms
develop, there will be cell mergers and other interactions, as well
as possible training that could lead to local rainfall totals to 5
inches. In addition...soils in this area have been saturated due
to rainfall from thunderstorms as recent as yesterday. Thus, the
Slight Risk area was expanded south to include much of central
Texas. The surrounding Marginal was expanded further south to
account for likely guidance changes prior to the event. Most of the
flash flood producing rainfall is expected overnight tonight/early
Sunday morning with a gradual northward and eastward expansion
across the Plains. Also made an expansion of the Slight Risk
southward towards southeast Texas in deference to signals from the
00Z HRRR of a complex that bows out and spreads yet more rain into
the area that does not need any more late tonight/early Sunday
morning.

Bann


...Eastern Ohio and Tennessee Valleys, Appalachians and Mid-
Atlantic...

Convection is firing up across portions of the Carolinas/Mid-
Atlantic region as confluent deep moisture streams interact with
the shortwave energy lifting out of the Southeast. Scattered areas
of heavy rainfall (1.5 to 2 inches/hr) and localized flash flooding
will be possible. There may be isolated max accumulations nearing
3 inches where storm motion is slow. Further north across the
eastern portions of the Tennessee and Ohio Valleys there will be
ample instability and mid-level forcing to support stronger cells
to move southward, especially into flashy prone areas. A Marginal
Risk area was hoisted for this period, spanning from central
Georgia north to eastern Ohio and southern Pennsylvania.

Campbell


...Sacramento Valley of California...

Maintained the Marginal Risk area with few changes as a deep and
cold low moves into the Pacific Northwest and spread rain into
the lower elevations of northern California. Latest guidance
continues to advertise 1 to 3 inches of rain possible in the
foothills of the Sacramento Valley and west aspect of the
mountains at lower elevations. This continues to be a low-end
Marginal...but excessive rainfall is possible in the event the
rainfall rates are enhanced by any thunderstorms.

Bann

Day 2
Valid 12Z Sun May 05 2024 - 12Z Mon May 06 2024

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

21Z update... Given the uptick in QPF trends over the water-logged
locations of southeast Texas, a Moderate Risk was raised for this
period. Areas of flooding are ongoing and additional rainfall will
only serve to aggravate or worsen conditions. Hi-res guidance is
depicting severals hours of rainfall rates pulsing from 1 to 3+
inch/hour as the cells cross the state. Over half of these
solutions show the heavy rains tracking across areas south and
east of of the Houston metro, so there may be the need for an
expansion of the Moderate Risk with future updates.

Campbell

They system over the Plains today will continue to move eastward on
Sunday. Storms will likely be ongoing in portions of
eastern/southeastern Texas at the start of the period Sunday and
will continue pushing eastward with training of cells appearing
likely along an east- west- oriented boundary on the south end of
a more progressive line of storms. There is inherent and typical
uncertainty with where that line will set up, especially with
recent large southward shifts in the guidance and a fairly large
spread within any suite of guidance. Thus, to cover the large
changes in the guidance, as well as very recent and ongoing heavy
rain across southeast Texas, maintained the large Slight Risk area.
For the Ozarks region, significant uncertainty also exists, as the
area will be on the northern side of the strong shortwave trough
driving all of the convective activity. The big concern is whether
or not the active convection farther south will disrupt the feed of
moisture and instability. For the moment...will not make too many
changes to the guidance that led to an expansion of the Slight
into parts of Missouri and Arkansas earlier.

Bann

Day 3
Valid 12Z Mon May 06 2024 - 12Z Tue May 07 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL AND NORTHERN PLAINS ON MONDAY AND MONDAY NIGHT...

21Z update... There is a growing consensus for much of eastern
Montana and surrounding areas to get a soaking rain for this
period. Much of this region has nearly saturated soils which has
reduced the 3-hr FFG to as low as 1 inch. Portions of the Bear Paw
Mountains in north-central Montana still have liquid equivalent of
1 to 2.5 inches in the snowpack. Rain over this snowpack will
likely speed up snowmelt while increasing the amount of water
available for runoff. Areas of local flash flooding will be
possible.

For the Central Plains, the latest guidance and WPC forecast
shifted the QPF to the east/northeast about 100-150 miles. This
resulted in trimming the back edge of the Marginal to central
South Dakota to northeast Oklahoma and expanding the eastern
boundary into southern Minnesota and eastern Iowa. The Slight Risk
was shifted eastward and now encompasses eastern Kansas/Nebraska,
southeast South Dakota, western Iowa and western Missouri.

Campbell

Heavy rainfall that results in flash flooding is possible from
convection that is expected to develop over the Plains on Sunday as
an upper trough ejects northeastward across the northern/central
Plains on Monday. Precipitable water values of 1.2 to 1.6 inches
will be drawn northward on 850 mb flow of 30 to 45 kts and modest
height falls ahead of the trough aloft and a surface dry line.
Rainfall rates over 1.5 inches per hour appear reasonable given the
amount of instability to aid updraft strength. The broadly
diffluent aloft that forms could result in repeated rounds of
heavy rainfall in some cases but broader/larger scale organization
may keep individual cell motion progressive enough to mitigate at
least some of the excessive rainfall potential.

Bann


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