FOUS30 KWBC 140828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Tue May 14 2024

Day 1
Valid 12Z Tue May 14 2024 - 12Z Wed May 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH FLORIDA AND FAR SOUTHERN GEORGIA...

...North Florida and Far Southern Georgia...

The combination of an MCS ongoing as of the time of this writing
over southeast Alabama, southwest Georgia, and the Florida
Panhandle, followed by a trailing cold front tonight will bring at
least 2 rounds of rain to northern Florida. In between, daytime
heating and abundant atmospheric moisture will support potential
isolated to widely scattered convection across north Florida this
afternoon. All these chances of rain will all occur in the Slight
Risk area of north Florida through tonight. 

Antecedent soil conditions from NASA Sport imagery have well above
normal soil moisture for portions of the eastern Florida Panhandle,
and hugging the Florida/Georgia Line up to the coast. For much of
the Florida Peninsula, even the northern Peninsula, soils are well
below normal for saturation. Thus, the Slight Risk was adjusted
northward to cover areas with more saturated soils, and were
conservative on the southern side in central Florida, which should
be able to handle all of the forecasted rain with only isolated
instances of flash flooding.

A bit of uncertainty persists as to how much rain will fall after
12Z, with the MCS rapidly traversing the Panhandle now. A faster
movement would reduce overall rainfall expected today. However,
given very favorable antecedent atmospheric conditions...namely
MUCAPE values between 1,000 and 2,000 J/kg and PWATs to 2 inches,
which is 2.5 sigma above normal for this time of year and above the
90th percentile climatologically, any storms that form will have no
trouble producing rates up to 2.5 inches per hour. Despite the
sandy soils and flat terrain of north Florida, these rates could
overwhelm local drainage quickly. The Slight is considered a
lower-end Slight, given the very high FFGs in place across most of
north Florida.

...Mid-Atlantic and Ohio/Tennessee Valleys...

For the Mid-Atlantic, the nose of the LLJ will advect abundant Gulf
moisture north up the Carolina Coast and into Virginia today. As a
low over the Ohio Valley approaches, convection is expected to
become commonplace across the Southeast and southern Mid-Atlantic.
For most of the day, ridging in place aloft should work to prevent
most organization of the storms, though cell mergers and localized
outflow boundaries could cause isolated organization that could
prolong heavy rainfall rates. The area most likely to see
convection today into tonight...eastern North Carolina and southern
Virginia have generally average soil moisture conditions, and so
should be able to handle most of the rainfall, even if briefly
heavy with only isolated instances of flash flooding possible. The
Marginal was trimmed south a bit from inherited to remove the DC
area and much of the central Appalachians, as easterly flow should
prevent most convection from getting to the west side of the Blue
Ridge.

For the Ohio/Tennessee Valleys, a vertically stacked low with
associated upper level cold air will take advantage of about normal
atmospheric moisture and abundant solar heating to result in some
organization to any convection that forms. Expect most of the
convection to be diurnally driven...so the most likely time
heavy rainfall will occur will be this afternoon...though due to
continued forcing, convection may persist into the overnight hours.
Since the vertically stacked low will be moving overhead of western
areas of the lower Ohio Valley...the convection may be able to
organize a bit better there. This will be generally in the vicinity
of southern Illinois and Indiana. Despite this, a relative lack of
moisture should limit any heavy rainfall, so the Marginal remains
largely unchanged.

Wegman

Day 2
Valid 12Z Wed May 15 2024 - 12Z Thu May 16 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
CENTRAL AND SOUTHERN PLAINS AS WELL AS FOR THE NORTHERN FLORIDA
PENINSULA...

...Central and Southern Plains...

Return flow developing in advance of an approaching positively
tilted longwave trough will begin to re-moisten the atmosphere
across the central and especially southern Plains Wednesday and
Wednesday night. Fortunately, all the ingredients for heavy rain
and storms will not be coincident with each other. Thus, the
signals for storms and heavy rain are very sparse and in poor
agreement. From inherited, the focus for more widespread convection
is favoring Oklahoma and into southeastern Kansas, while signals
across hard-hit areas of Texas have been decreasing. Since much of
Oklahoma and Kansas have had time to recover from heavy rains the
past couple weeks, soil moisture conditions are at or below 
average for this time of year. Thus, any organized convection,
besides not having a direct connection to deep Gulf moisture will
also be occurring over areas that would likely find the rainfall to
be beneficial. Thus, the Marginal Risk in place across the area
remains unchanged.

For hard hit Texas, the decreasing signal for storms indicates any
convection that develops will struggle to develop, and will remain
isolated. This should keep any resultant flash flooding as very
isolated. However, given how saturated the soils are over almost
the entire northeastern half of the state, even isolated convection
if it moves over a particularly sensitive area could cause flash
flooding. Increasingly abundant Gulf moisture returning north over
south Texas could support a storm or two capable of heavy rainfall
rates. Fortunately most of south Texas hasn't been as wet as areas
further north and east, so once again the flash flooding threat
should be isolated.

...Northern Florida Peninsula...

The cold front moving south down the Florida Peninsula Wednesday is
expected to stall out somewhere over the central part of the state,
generally near Tampa and Orlando. This front will act as a focal
point for convection...mostly Wednesday morning and early afternoon 
as the front makes its final push south before stalling out. Soil 
moisture levels in this area are well below normal for this time of
year, and given the resistance to flooding typical of this area 
represented by very high FFGs, it will take training heavy 
thunderstorms to cause isolated flash flooding. However, given the 
front will act as a "train track" of sorts in the fast westerly 
flow south of the front, training is possible. The Marginal risk 
remains in place albeit shifted a little bit south with the latest 
model trends.

Wegman

Day 3
Valid 12Z Thu May 16 2024 - 12Z Fri May 17 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EAST TEXAS AND NORTHERN AND WESTERN LOUISIANA...

...Portions of East Texas and Northern and Western Louisiana...

Yet another round of very heavy rainfall is expected Thursday and
Thursday night across the many of the same very hard hit areas of
east Texas through northern and western Louisiana that keep getting
hit with very heavy rain, including just yesterday in some areas.

A positively tilted longwave trough with attendant 100+ kt jet
streak rounding the southern/eastern sides of the trough and
attendant shortwave will all eject out of the southern Rockies and
across Texas Thursday and approach Louisiana Thursday night. Return
flow off the Gulf will tap into impressive amounts of atmospheric
moisture for any time of year. PWATs will climb above 2.25 inches
for the upper Texas Coast, southwestern Louisiana, and extending
well inland. This is above the 95th percentile for this time of
year compared with climatology, and more than 3 sigma above normal.
In addition to atmospheric moisture, abundant surface moisture will
be available as moderate to major flooding is already ongoing
across much of this area. The aforementioned trough, jet streak,
shortwave, and enhanced upper level diffluence supporting
additional lift, and will all combine forces over this already
exceptionally hard hit area. 

With weak surface forcing synoptically, outflow boundaries will 
likely be the primary forcing at the surface...but given all the 
other very favorable ingredients in place that should be all that 
it takes to result in MCS development. The abundant moisture will 
allow the MCS to persist as it moves across eastern Texas and into 
western Louisiana through Thursday night. The embedded convection 
will be capable of producing rainfall rates of 3 inches per hour, 
with the strongest storms capable of higher. Instability will not
be a problem either, as the abnormally moist atmosphere and strong
May sun will advect MUCAPE values over 3,000 J/kg into the
southern/western sides of the MCS. This will support backbuilding
convection, slowing the overall movement of the MCS. West-pointing Corfidi
Vectors will be orthogonal to the southerly flow of moisture and 
instability into the MCS. This will support convection developing
along the outflow boundaries that are part of the MCS, and training
with the MCS over the same areas, locally enhancing storm total
rainfall.

In coordination with SHV/Shreveport, LA; FWD/Dallas-Fort Worth,
TX; HGX/Houston, TX; LCH/Lake Charles, LA; and JAN/Jackson, MS
forecast offices, a Moderate Risk upgrade was introduced with this
update. Given the above synoptic setup, it seems probable that a
High Risk consideration may need to be given in future forecasts,
especially if the "bulls-eye" of heaviest rain remains over this
hard hit areas of Texas and Louisiana. Moderate to Major flooding
is already ongoing across much of east Texas even before the start
of this event, and drainage has been slow...such that the much
needed 2 day break is unlikely to alleviate potential flash
flooding concerns much. Considerable/significant flash flooding is
likely in the Moderate Risk area. That said, this event still 
being on forecast Day 3, there remains quite a bit of time for the
forecast to change, and almost any movement of the heaviest
rainfall would result in lessening flooding concerns. That said,
guidance has been in very good agreement on the placement of this
heaviest rain for several days already, which lends confidence to
this scenario unfolding for the Moderate Risk upgrade.

Despite very good agreement in most of the guidance for a Day 3
forecast, the American models (NAM/GFS) have been distinct 
outliers in suggesting this event will not occur at all, with any 
heavy convection staying well north and west into Oklahoma. These 
models have been discarded as part of the forecast given the very 
good agreement in all of the rest of the guidance. It should be 
noted however that as persistent as all of the foreign guidance has
been in forecasting this event, the NAM and GFS have been equally 
persistent that it will not happen.

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