FOUS30 KWBC 040808
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
308 AM EST Mon Nov 4 2024

Day 1
Valid 12Z Mon Nov 04 2024 - 12Z Tue Nov 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM EASTERN TEXAS
THROUGH WESTERN LOWER MICHIGAN...

...Changes...

Only a few modest changes were made to the inherited forecast.
There was a small southward nudge to the axis of heaviest rainfall
around Lake Michigan, so the Slight risk area was trimmed some in
Wisconsin and northern Lower Michigan in favor of a slightly
expanded Slight in eastern Illinois and far northwestern Indiana.
With some rain in portions of southeast Texas around Houston today,
the expectation of feeder cells into the main plume of moisture,
showers and storms further north warranted a small southward
extension of the Slight to near Houston.
A small Marginal risk was added to portions of east central New
Mexico.

...Texas to the Great Lakes...

A progressive trough over the Plains will continue tracking
eastward towards the Mississippi Valley today. Plentiful shortwave
energy and a strengthening southerly jet streak out ahead of the
trough will both work to increase forcing across the entire Slight
Risk area. The greatest lift and heaviest rainfall is expected from
far southeastern Oklahoma northeast to around St. Louis. Since this
area crosses the heart of the Ozarks, any upslope enhancement will
locally increase rainfall rates. The topography of the area may
also focus the flash flood potential in the valleys, further
increasing the risk of impacts. Plentiful Gulf moisture moving
north on a 40 kt LLJ will run into a developing surface low over
the Plains. As this moisture runs ahead of the NE-moving low the
combination of forcing and topography will enhance rainfall rates.
While there has been some rain in this area from eastern Oklahoma,
Arkansas, and southern Missouri, the heaviest rainfall amounts have
generally been west of where the heaviest rains are expected today.
This fact should mitigate the flood potential and impact somewhat.
Nonetheless, 2 days of heavy rain with rates to 3 inches per hour
in the strongest cells shows no signs of abating for today with all
the aforementioned ingredients still in place. The progressive
nature of the trough and resultant storms should also lessen the
flood risk just a little. Thus, the Slight Risk inherited for this
region continues to look good and no upgrades were considered. This
corridor is highlighted in an internal higher-end Slight.

Further south across Texas, there will be plentiful moisture but
the forcing will both be transient and largely missing this area.
Some rain fell today around Houston, so the threat is in the lower
end Slight range as any storms that form, while isolated to widely
scattered, will be capable of those higher end 3 inch per hour
rates, which especially over any flood prone urban areas could
cause flooding problems quickly. 

Meanwhile further north towards Lake Michigan, plentiful forcing
will be in place but both instability and moisture amounts will be
decreasing with latitude. Thus, expect more stratiform rain across
this region...with much lower chances of the higher end rates. That
said, the rain should be longer duration than the heavier rain
further south, and for the Chicagoland area here too there will be
urban considerations which bump the risk level into the Slight
range.

...New Mexico...

A leeside low currently over south central New Mexico will track
northeast as several mesolows work to consolidate into a dominant
single low over Texas and Oklahoma late today. With peak heating,
some wraparound moisture and modest instability over an area with
modestly depressed FFG values, steady rain with some convective
cells ongoing over the area by this afternoon could result in an
isolated flash flooding threat, particularly over any burn scars or
other flood sensitive areas. Thus, a small Marginal Risk area was
added with this update. Precipitation will be ongoing further north
and into the mountains of southern Colorado but temperatures there
will be cold enough for most if not all of the precip to fall as
snow, precluding any flash flooding risk.

Wegman


Day 2
Valid 12Z Tue Nov 05 2024 - 12Z Wed Nov 06 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MISSISSIPPI AND OHIO VALLEYS...

No changes were made to the inherited Marginal Risk area.

The upper level trough responsible for the past few days of heavy
rain across the southern and central Plains will finally retreat
back into the broader jet into Ontario and Quebec. The upper level
energy supporting the corridor of storms will continue supporting
storms across the Mississippi and Ohio Valleys Tuesday and into
Tuesday night respectively. As the energy lifts north, it will mark
the end of persistent heavy rain and storms from south to north.
Thus, the flooding threat will be more in the morning in Texas, and
more towards the night for the Ohio Valley. Training lines of
storms will move across the Mississippi Valley for much of the day.
These storms will be capable of heavy rains which may cause
isolated flash flooding, especially in any urban areas such as
Memphis. By Tuesday night, weakening of the storms and the shift
well away from any prior days' rain footprint will effectively end
the flooding threat.

Wegman

Day 3
Valid 12Z Wed Nov 06 2024 - 12Z Thu Nov 07 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST...

Very few changes were needed to the inherited forecast.

Deep tropical moisture will have overspread all of Florida and will
be advancing into GA and SC at the start of the period Wednesday
morning. A lingering front over the Southeast may act as a focus
for convection across GA and SC as the deep tropical moisture moves
north into the front. Potential Tropical Cyclone Eighteen (PTC18)
is expected to move into the Gulf from Cuba and the northwest
Caribbean as well. For the Florida Peninsula, much of the flooding
threat will be determined by the eventual track of PTC18. The
latest guidance has kept the center of PTC18 far enough west over
the Gulf that despite shear pushing much of the rain associated
therewith to the eastern side of the circulation, it will still
largely remain offshore. However, with PWATs above 2.25 inches, it
won't take much for daily convection to produce heavy rainfall
almost anywhere in the state. 

A bullseye of heavier rain is currently forecast across central GA
into SC. While the focus for the heavier rain will be the stalled
out front, there is considerable uncertainty as to instability,
which will greatly determine how strong any storms can get which
could cause flooding. For now it appears much of the rain in GA/SC
will be spread out through Wednesday and into Wednesday night,
which should easily be handled by the antecedent dry conditions.
Thus, the Marginal for this area remains in place with only minor
tweaks made. The area will continue to be monitored for a potential
Slight upgrade, which may not take much given the abundance of
tropical moisture any storms would have to work with.

Wegman

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