FOUS30 KWBC 130815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Mon May 13 2024

Day 1
Valid 12Z Mon May 13 2024 - 12Z Tue May 14 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GULF COAST...

...North Gulf Coast...

An incredibly low-confidence forecast panning out for the central
Gulf coast today. A low tracking over Kansas and Missouri will 
drag a cold front across the western Gulf Coast through tonight.
Ahead of that cold front, deep convection being supported by
mesoscale upper level shortwaves taking advantage of ample Gulf
moisture is making this a very difficult forecast. The storms
reacting to these upper level impulses are creating outflow
boundaries of their own, which in turn is providing the forcing for
additional convection. As the storms ongoing across Mississippi and
Louisiana now turn and redevelop towards the south and east, their
outflow will track along and reinforce a boundary along which
additional convection will try to form through the day and into
tonight. This boundary will be largely orthogonal to the mean
southwesterly flow, which will support training convection. 00Z
HREF 40-km neighborhood probabilities are up to 40% for 8 inches of
rain along the Mississippi Gulf Coast, and over 70% for 5 inches of
rain for the Mobile Metro Area through 12Z.

There remain several points of uncertainty in this unfolding
forecast scenario. First, how much convection will form along this
boundary. Second, will the boundary move. Third, does the boundary
move offshore. Fourth, how fast will the storms that form along the
boundary move. Let's address these in order:

First, how much convection will form along the boundary: CAMs are
in poor agreement as to whether the storms that form along the
boundary will be swift-moving segments that train towards the ESE,
or if in between these segments, additional weaker storms continue
forming. This will play a huge role as to whether communities in
the MDT risk area see a few rounds of heavy rain with long breaks
in between, or if it will be lightly raining with occasional
moderate bursts in between the heavy rain producing segments. Of
course, the latter would result in much more widespread and
significant flash flooding along the boundary. Should the former
occur, then it's probable the Moderate Risk is an overforecast.

Second, will the boundary move. While the CAMs are in reasonable
agreement that the boundary remains in place, the question remains
as to where that boundary sets up. The further north the boundary,
the more severe the resultant flash flooding. 

Third, does the boundary move offshore at any point. This of course
will play a huge role as to the severity of flash flooding. If the
boundary forms or quickly moves offshore, this will dramatically
reduce the flash flooding severity and coverage. The CAMs have been
trending southward quite quickly, hence the reduction in the
coverage of the Moderate, so any further southward shifts will need
to result in downgrades since all the heaviest rain, regardless of
training, falls offshore. This is a distinct, but still highly
uncertain possibility.

Fourth, how fast will the storms move. As always, the faster the
storm movement, the less severe the flash flooding as any one
location sees heavy rain for a shorter amount of time. There is
reasonably good agreement than any one cell or segment will be fast
enough moving to reduce the flooding threat any one cell or segment
may cause. It will take expected training of multiple
cells/segments to get the potentially significant flash flooding
indicative of a Moderate Risk.

All of the above to say, even a small shift in the forecast will
lead to large changes in the flooding severity. Stay tuned.

Outside of the Moderate Risk, the surrounding Slight risk was
expanded to the west to include more areas north and west of
Houston with this update. This area has been repeatedly hard hit
with multiple days of storms over the past week or so, resulting in
very saturated soils that take little additional rain to result in
flash flooding. While storm coverage won't be nearly as plentiful
along the Upper Texas Coast as further east, even the scattered
storms that do form will have significant potential to produce
additional flash flooding.

For reasons mentioned above, the surrounding Slight was also
greatly trimmed on the north side from inherited, as the southward
shift in the guidance means the greatest instability fueling the
strongest storms also stays south. Much of the rain in central 
MS/AL is ongoing now.

...Portions of the Midwest...

The Slight across portions of Missouri, Iowa, and Illinois was
adjusted south by about a row of counties with this update. This
follows the warm frontal interface ahead of the developing low over
Kansas and eventually southern Missouri. Convection in the warm
sector will collide into the warm front, which will remain largely
stationary. This in turn may result in localized training along
that boundary. The latest CAMs resolve the warm front to be a
little bit south of previous runs, and as such the Slight was
adjusted accordingly.

The cold frontal convection between the 2 Slight areas is highly
likely to have a fast forward speed across Arkansas and southern
Missouri. Should the convection move over a particularly sensitive
area, isolated flash flooding is possible, so the Marginal in this
area remains in effect and largely unchanged.

Wegman


Day 2
Valid 12Z Tue May 14 2024 - 12Z Wed May 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
FLORIDA PANHANDLE...

...Florida Panhandle...

Storms will be ongoing across the Florida Panhandle to start the
period Tuesday morning. They are likely to be progressive, moving
along to the east across north Florida through the morning. The
uncertainty is how much convection develops behind the first line
of storms. The broad unidirectional southwesterly flow would bring
any additional convection into the Florida Panhandle through the
day Tuesday. When added to any rainfall the area gets Monday night,
this could lead to widely scattered instances of flash flooding,
particularly between Pensacola and Tallahassee. The area has very
high FFGs, even though soils are wetter than normal. Due to the
unusually high PWATs associated with the Gulf moisture streaming
into the Panhandle ahead of a cold front which will clear out the
convection to the east by Tuesday night, any storms that form will
be capable of rates of 2 inches per hour.

...Portions of the Southeast, Mid-Atlantic, and Ohio and Tennessee
Valleys...

North of the Florida Panhandle, the cold front will be racing
eastward more quickly than previous model runs had indicated. Thus,
now most of the heaviest convection is expected to be largely
offshore of the Carolinas and Georgia, and therefore are much less
likely to cause flooding. The greatest threat of flooding from
these storms would be along the immediate coast, but given the
sandy soils, are unlikely to result in flash flooding. Thus, the
inherited Slight for Georgia and the Carolinas was downgraded with
this update.

Further north and west, the parent low and associated upper level
cold air will result in widely scattered convection Tuesday and
Tuesday night for the Ohio/Tennessee Valleys and over to the Mid-
Atlantic. Instability will be modest, but non-zero along with
modest atmospheric moisture. Thus, any storms that form, especially
those that undergo cell mergers or training, have the potential to
produce isolated instances of flash flooding. The inherited
Marginal risk was kept largely the same, but was expanded a bit to
include the DC and Baltimore metros due to low FFGs from recent
heavy rainfall. 

Wegman

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