602
FOUS30 KWBC 151553
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1153 AM EDT Wed May 15 2024

Day 1
Valid 16Z 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...

...16Z Update...

Only minor adjustments made to the bounds of the MRGL risk areas
across the CONUS, but an additional small MRGL was included across
portions of the Northeastern U.S (More on this in a separate sub-
heading below). Recent trends in guidance support the maintenance
of each area with the lowest threat residing over the Florida
Peninsula as conditions for flash flood prospects deteriorate with
the next 2-4 hours the primary time frame of interest. Shortwave
pulse over the Gulf will move into the western shores of FL with
locally heavy rainfall possible within the urban corridor spanning
Clearwater down through Sarasota and points inland. Area FFG
indices are still very high for all markers with the hourly FFG
intervals still between 3.5-4" which will be the upper quartile of
potential with the current setup. Expect the next update to be a
removal of a the MRGL as conditions improve and the nocturnal
period trends quieter for the Peninsula.

Across the northern plains, a shortwave trough analyzed over
northern SD will continue to progress eastward, becoming more
defined as we step forward in time leading to focused ascent under
the primary upper circulation. A tongue of elevated instability
will situate itself over the ND/MN border with afternoon
destabilization providing the initiation point for convection
under the aforementioned trough. PWAT indices are slightly above
normal with NAEFS anomalies pinning ~+1 deviation which would
promote some locally enhanced rainfall concerns under the focused
ascent. Relative QPF mean around 1-1.5" is forecast across the
outlined area within the MRGL with a few hi-res deterministic
peaking at 2-2.25" for the event, much of it falling within a span
of 4-8 hrs. FFG`s are right on the cusp of the current forecast
hourly rates and 3 to 6-hourly totals with the 12z HREF 5 and
10-year ARI exceedance probabilities nestled around 10% across the
southeast ND into western MN. This is sufficient for a
continuation of the MRGL risk area with the threat overall
remaining isolated and more likely over any urbanized areas along
the Red River.

Marginal Risk area over the central and southern plains will
maintain continuity with basically no change from the previous
forecast package. Convection will initiate out over the TX
Panhandle up through KS and translate east over the course of the
late-afternoon and evening time frame. The primary location for
flash flood potential is likely over southeastern KS and northeast
OK where the HREF blended mean QPF signals between 1.75-2.5" of
precip over the area with a bullseye to the northwest of Tulsa
where the organized area of convection is forecast to ride along a
tongue of higher theta-E`s, likely outlining the remnants of a weak
warm front ahead of the primary surface reflection. More convection
will also fire across the lee of the terrain in CO with an isolated
risk of heavy rainfall within the corridor extending south of
Denver through Pueblo. QPF forecast is modest within this zone, but
5 to 10-year ARI exceedance probabilities are between 15-20% within
that zone, so the threat is certainly non-zero and within the
lower-end of the MRGL risk threshold.

Upper level circulation over northern KY will translate eastward
through the period with scattered thunderstorms likely over
portions of the Ohio Valley into the central Appalachians given the
favorable upper ascent pattern and lingering moisture/instability
presence over the interior. Scattered totals of 1-2" are plausible
in this setup with pulse convection driving a bulk of the mean
precip field. Steering pattern aloft will be lackluster, so any
storms will be rooted in the same location over their lifespan
leading to localized concerns of flooding, especially within the
complex terrain of WV, southwest VA, and western NC. 10-year ARI
exceedance probs are generally 10-15% within the terrain, which is
plenty to maintain the current MRGL risk in place.

...Eastern Long Island into Coastal Southern New England...

Visible satellite and radar composite indicate a maturing surface
cyclone along the eastern seaboard with the general circulation
positioned over the southern Delmarva with an expected track to the
northeast through the period. Cyclone will produce a period of
moderate rain as it pivots up the east coast, eventually occluding
as it stalls south of LI as the primary upper trough over the Ohio
Valley finally catches up with the surface reflection and becomes
vertically stacked just offshore. A persistent east to northeast
fetch off the Atlantic will spur a period of moderate to heavy rain
anchored over the eastern half of LI up through southern CT/RI/SE
MA. Model guidance has really ramped up the precip totals in the
current suite with some 2-3+" totals now being reflected within the
individual outputs, and around 1.5-2" within the HREF means. A
tongue of elevated instability will wrap into the aforementioned
areas this evening as the storm occludes and the warm conveyor gets
oriented to the coastal plain as the storm begins its occlusion
phase. Some isolated thunder will be plausible during the very back
end of the forecast period between 06-12z tomorrow morning with
rates peaking at 0.5-1"/hr before finally settling just after the
D1 time frame. With collaboration from the local WFO`s in the
region, a MRGL risk was introduced for a low-end flash flood
threat overnight into early tomorrow morning.

Kleebauer


...Previous Discussion...

...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

Day 3 Valid 12Z Fri May 17 2024 - 12Z Sat May 18 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN MISSISSIPPI AND SOUTHWESTERN ALABAMA...

...Southern Mississippi and Southwestern Alabama...

By the start of the period 12Z Friday morning, a strong LLJ
originating out of the Gulf and advecting air with PWATs above 2
inches into the Gulf Coast will be oriented southwest to northeast.
The strong southwesterly flow will not abate much, keeping a
steady supply of fuel into the ongoing thunderstorms moving across
Mississippi and into Alabama. The MCS will weaken with daytime
heating, as typically occurs, so expected rainfall will drop off
into southeast Alabama and most of the Florida Panhandle. The
dissipation of the MCS with the first round of storms will mean the
LLJ will stall out and stop progressing east.

On Friday night, a trailing upper level shortwave, which will be
deeper than the first, stronger, and slower-moving will drag a cold
front east, which should allow for renewed eastward movement of
the moisture-plume for a time, but will be the animus for a second
round of strong showers and thunderstorms across southern
Mississippi and Alabama as the moisture plume is initially drawn
back to the north and west as the cold front and associated storms
approach. As the second round will quite probably produce more rain
than the first, any flash flooding started with the first round of
storms will be greatly worsened by the second.

Soils across this area are very wet from storms as recently as
yesterday, so all of the rain that falls from both rounds of storms
will convert to runoff. Flash flooding is likely and significant
flash flooding is possible. One positive is that we are in the
heart of growing season and the soils are generally flood-
resistant. However, this will be offset by the 2-3 inch per hour
rainfall rates that will be quite possible with the strongest
storms. Urban and poor drainage areas are at greatest risk for
flash flooding.

In coordination with MOB/Mobile, AL and JAN/Jackson, MS forecast
offices, a Moderate Risk upgrade was introduced with this update.
It`s quite possible heavy rain may spread more into central
Alabama, but since these kinds of convective events tend to trend
southward with time and central Alabama is on the northern fringe
of the heaviest rainfall, held those portions of Alabama including
Montgomery, in a Slight for this update. Should trends indeed
evolve southward with time, then the hardest hit areas from today,
including Gulfport, Mobile, and Pensacola could all be in line for
a second round of flash flooding. Fortunately, most of the Moderate
Risk area is in relatively low population areas, which should
limit overall impacts somewhat.

...Mississippi and Ohio Rivers Confluence Region...

In coordination with SGF/Springfield, MO, PAH/Paducah, KY and
LSX/St. Louis, MO forecast offices, a Slight Risk upgrade was
introduced with this update for the Mississippi and Ohio Rivers
Confluence region. Guidance has been somewhat consistent for a few
days now that a secondary area of showers and storms associated
with a shortwave trough tracking across the area will be capable of
widely scattered instances of flash flooding. The area has seen
wetter soils from rainfall both yesterday and expected over the
coming days, such that by Day 3/Friday, flash flooding will be more
likely as the storms track along with an advancing cold front
across the region.

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