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FOUS30 KWBC 070820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
320 AM EST Thu Nov 7 2024

Day 1
Valid 12Z Thu Nov 07 2024 - 12Z Fri Nov 08 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHEAST AS WELL AS PORTIONS OF WEST TEXAS AND WESTERN OKLAHOMA...

...Southeast...

Heavy rain continues across Georgia and South Carolina this
morning. A nearly stationary front across the region is acting as a
focus for that rainfall, which due to an influx of tropical
moisture moving into the front from the Atlantic remains stuck. The
moisture running into the front is allowing for convective
development along the front in Florida and Georgia, which then
advect northeastward into South Carolina. While convective coverage
has been slowly diminishing, along with the storms with the
heaviest rainfall, it`s likely that the rain will continue into the
day today. The inherited Slight and Marginal risks were largely
left unchanged with this update, as the rainfall is generally
behaving as expected. Since there will be a very slow drift of the
heaviest rainfall towards the South Carolina coast, which due to
the swampy terrain is generally much better equipped to handle
heavy rainfall rates, a Slight Risk is sufficient to cover the
flash flooding threat, rather than a continuation of the Moderate.

...West Texas and Western Oklahoma...

A strong upper level low over the Southwest will interact with an
LLJ of Gulf moisture and a developing surface low forming ahead of
the upper level low. A large area of rain will develop from the
Texas and Oklahoma Panhandles south and east over portions of the
Cap Rock of Texas and much of Oklahoma west of Oklahoma City.
Instability will be a significant limiting factor on short-term
rainfall rates and the strength of any convection, but the fact
that all of those synoptic systems just described will be very slow
moving should increase the duration of steady rainfall. Since much
of this region was hit hard with heavy rain a few days ago, FFGs
remain low enough that they should be overcome in widely scattered
instances in the Slight Risk area, and in isolated instances in the
Marginal Risk region.

Wegman

Day 2
Valid 12Z Fri Nov 08 2024 - 12Z Sat Nov 09 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHERN PLAINS...

The upper level low over the Southwest and its attendant surface
low will both continue moving northeast across the Plains on
Friday. With continued influx of Gulf moisture, the typical comma
shaped precipitation footprint will develop from Texas to Oklahoma.
Out west in Colorado and New Mexico the precipitation will be
mostly in the form of snow. Where it remains all rain, the heaviest
rain will be from north Texas across Oklahoma and into south
central Kansas. As today, instability will remain the primary
limiting factor for heavy rain in most areas, as MUCAPE values
generally stay under 1,000 J/kg. For the Slight Risk area, there
has been a general eastward/faster shift in the guidance, but that
will at least be somewhat offset by the rainfall expected today
over the Panhandles and western Oklahoma.

The rationale for the Slight is two-fold. While somewhat lesser
amounts of rain are expected overall for southern Oklahoma and
Texas...greater instability there will favor at least some storms
capable of heavy rainfall in that area. Meanwhile, north of
Oklahoma City, expect a much longer duration rainfall as the
surface low takes its time getting its act together. So while
instantaneous rates will likely be lower than further south, it
will be made up for by the longer duration of the rain.

Wegman

Day 3
Valid 12Z Sat Nov 09 2024 - 12Z Sun Nov 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
LOWER MISSISSIPPI VALLEY...

The upper level low that was driving the heavy rainfall the
previous two days will move faster towards the northeast as it
continues its return to the jet stream over the northern tier of
states. The area will still have plenty of moisture from the Gulf
to work with as Rafael (or what`s left of it) remains over the
central Gulf but the plume of moisture its embedded in continues to
be drawn north ahead of the surface low`s strong cold front. The
front will likely be the dominant forcing for storms in this
region. As on Friday, the southern areas of Louisiana and
Mississippi will have the greater moisture and instability, while
further north FFGs are much lower due to recent heavy rainfall.
Thus, once again the Slight Risk is a two-fold combination of
factors that individually increase the flooding risk. The front
should still be slow enough moving that training storms tracking
north along the front could still cause flooding problems,
especially in Louisiana.

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