Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
435 AM 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...

Late night convection developed along and near the Upper Texas
coast overnight bringing additional rainfall to areas that have
become water-logged recently. SPC mesoanalysis showed the on-going
convection forming along a tight gradient of instability hugging
the coastline that was capped. Without much CAPE inland...will keep
the Marginal area focused there.

The focus of attention then expands across a broader area of 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
recently 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.

...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 SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHERN
MISSOURI SOUTHWARD TO THE UPPER TEXAS COAST...

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 SOUTHERN PLAINS ON MONDAY AND MONDAY NIGHT...

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