FOUS30 KWBC 150829
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Wed May 15 2024

Day 1
Valid 12Z Wed May 15 2024 - 12Z Thu May 16 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE RED RIVER
VALLEY OF THE NORTH REGION, PORTIONS OF THE CENTRAL PLAINS, THE
WESTERN APPALACHIANS, AND THE CENTRAL AND NORTHERN FLORIDA
PENINSULA...

...Red River Valley of the North...

A deepening shortwave trough in the polar jet will dip south into
the Dakotas and Minnesota later today into tonight. This will
support some modest cyclogenesis of a small surface low. The low
will focus an area of persistent rain in the comma-head region of
north and west of the center. During the day, diurnal heating will
support some modest instability, allowing convection to develop in
the area. Once the sun goes down and the convection largely
dissipates, steady stratiform rain will continue into northwestern
Minnesota. Soils in this area are about average for this time of
year, but the persistent rains some of which may be briefly heavy
will support an isolated flash flooding threat. A Marginal Risk
area was introduced with this update.

...Central Plains...

A developing front between 2 opposing air masses across the Central
Plains will be the focus for convection today and especially into
tonight across northeastern Oklahoma. To the north, drier air
traveling southbound down the Plains will collide with a weak LLJ
of warmer and moister air. Showers and thunderstorms will develop
along this boundary, tracking east with time. Soils are at or above
normal in this area for this time of year, with the wettest soils
into northeastern Oklahoma. The inherited Marginal was adjusted,
largely to remove most of western Kansas, which will both be on the
dry side of the boundary and also have very dry soils for this time
of year. Easterly flow into the mountains of Colorado will upslope
along the Front Range, so areas south of Denver, including Colorado
Springs remain in the Marginal Risk area.

...Western Appalachians...

The inherited Marginal across West Virginia was expanded both north
into eastern OH and western PA, and also south into southwest VA
and NC. Almost all of the resultant convection will be diurnally
driven and largely disorganized. However, much of the Marginal Risk
area has low FFGs so any stronger storms will be slow moving and
capable of isolated flash flooding. The storms will be most focused
in the Marginal Risk area along a weak deformation zone in the
upper atmosphere, with largely chaotic or no movement. 

...Central and Northern Florida Peninsula...

Ongoing convection in the Marginal Risk area will continue growing
upscale into a larger line of storms that will track east across
the Peninsula through the morning. Training storms will be possible
in the fast westerly flow across the Peninsula as they tap
additional moisture ahead of a trough associated with an upper low
over the Ohio Valley. Abundant moisture available for the storms to
feed on will support higher rainfall rates, but accelerating storm
motion, dry antecedent soil conditions, and weakening of the line
by midday should keep any flash flooding isolated, particularly in
urban and poor-drainage areas.

Wegman


Day 2
Valid 12Z Thu May 16 2024 - 12Z Fri May 17 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM THE PINEY
WOODS OF TEXAS TO SOUTHWESTERN MISSISSIPPI...

...Southern Plains and Lower Mississippi Valley...

Little has changed in the convective evolution and overall setup
across eastern Texas over to southwestern Mississippi Thursday. The
nose of a 100 kt jet will push a strong shortwave trough eastward
across the region, increasing divergence aloft ahead of the
shortwave. At the surface, a strong and very moisture-laden air
mass originating in the Gulf will surge northward into a front that
will quickly become a warm front as the LLJ pushes northeastward.
The warm front will support continuous convection as the moisture-
laden air mass rides up the frontal interface. A cold front
following behind it associated with the progressive shortwave
trough will enhance lift on the back/west side of the warm sector.
This will support development of an MCS, which will it will have a
decently quick forward speed...will support backbuilding and
training strong thunderstorms capable of very heavy rain,
particularly across Texas during the day, then shifting into
Louisiana and Mississippi overnight Thursday night.

Soils across the Piney Woods and into western Louisiana are very
saturated from multiple weeks of heavy rain. The Piney Woods have
the bullseye, picking up as much as 20 inches of rain in the last 
2 weeks alone. Multiple rounds of strong and training convection
capable of rainfall rates as high as 3 inches per hour over
especially this hard-hit area...but really much of the
South...spells a nightmare scenario for the area. Significant and
considerable flash flooding is likely as a result, as multiple
inches of rain fall in a matter of a few hours. The area is
considered in a higher-end Moderate risk, where the probability of
flash flooding is between 55 and 70 percent. The one piece of good
news is that guidance is little changed on the magnitude of the 
event, generally expecting between 3 and 5 inches of storm total 
rainfall, albeit in a short time. 

By Thursday night, the MCS will pick up some forward speed as it 
moves across Louisiana and eventually Mississippi. While this will 
modestly decrease expected storm total rainfall in this area...the 
abundance of moisture in the LLJ will continue to support
convection capable of very heavy rainfall, and while most of
northern Louisiana and southwest Mississippi hasn't seen quite as
much rain in recent weeks as east Texas, there has still been 300
to 400 percent of normal rainfall in the area the past few weeks,
so soils are similarly fully saturated, so all rainfall will
convert to runoff. Thus, a lower-end Moderate continues for most of
northeast Louisiana and with model guidance now suggesting the rain
extends into Mississippi by Friday morning. The Moderate Risk was
expanded eastward to account for this change, in coordination with
LIX/Slidell, LA and JAN/Jackson, MS forecast offices.

There remains some uncertainty in the guidance as to where exactly
this axis of heaviest rainfall along the warm front will set up.
The American models remain big outliers, but have been very slowly
"catching up" to the rest of the model suite. As such, once again
they have been largely tossed from the forecast. The 06Z HRRR
suggests the axis of heaviest rainfall may be a bit south and west
of the current Moderate Risk area, so additional changes to the
forecast are likely in coming days. Unfortunately even the 06Z HRRR
scenario does not spare the Piney Woods from multiple inches of 
rain. 

Further north into Oklahoma, the boundary that will serve as a
focus for convection on Day 1/today will still be draped across the
area and largely stationary on Day 2/Thursday. Guidance remains
highly variable in how much convection develops again here,
particularly Thursday night, but given expected rainfall in the
area on Day 1/today, it's likely that soils will be more saturated
than now by this second round Thursday night. Thus, the area was 
upgraded to a Slight with this update.

Wegman

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