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FOUS30 KWBC 160052
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Wed May 15 2024

Day 1
Valid 01Z Thu May 16 2024 - 12Z Thu May 16 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS, THE WESTERN APPALACHIANS, THE CENTRAL AND COASTAL
SOUTHERN NEW ENGLAND...

Confidence has grown enough to remove the risk of excessive
rainfall areas from the Northern Plains and from the Florida
peninsula while only minor adjustments were made elsewhere.

...North Florida/Northern Peninsula...
Between the loss of daytime heating across the Florida peninsula
and the cold front having moved south of the area...am expecting
little if any additional rainfall.

...The Northern Plains...
Rain will accompany a compact area of low pressure northward
towards the international border overnight.. low pressure will
continue to move northward. With surface stability confined well to
the south, the rates look to fall short of the Flash Flood
Guidance across the region except for very short duration. With the
HRRR and HREF neighborhood probability values generally below 5
percent...removed the Marginal risk area.

...Central Plains...
Convection over the Texas panhadle will be spreading eastward this
evening and overnight...with the potential for flash flooding
increasing over portions of Kansas and northeast Oklahoma later
tonight. The 18Z run of the HREF continued to depict 12 hour
rainfall amounts in the 1.75 to 2.5 inch range as convection
encounters a thetae ridge in the low levels.  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 present and within the lower-end of the MRGL risk
threshold. The 18Z HREF showed probability of rainfall amounts
exceeding 1 inch per hour building with the development of the low
level jet through late evening before beginning to wane in the
early morning hours.

...Central Appalachians...
Upper level circulation over West Virginia will translate eastward
through the period helping to focus additional scattered
thunderstorms across 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 nearly with gradually diminishing potential for those amounts
by late tonight. Steering pattern aloft will be lackluster, so any
storms will tend to be slow movers...leading to the localized
concerns of flooding, especially within the complex terrain of WV,
southwest VA, and western NC.

...Southern New England...
A low pressure system will meander northward slowly overnight just
offshore. Moisture will continue to be wrapped around the
system...putting southern New England in a persistent fetch of
moisture and rainfall. Thinking is that there should be a period of
moderate rain as the low pivots up the east coast and eventually
occludes. Model guidance showed some 2-3+" ending at 16/12Z aided
by elevated instability. Still think some isolated thunder will be
plausible between 06-12Z...so the previously issued Marginal Riks
area largely remains unchanged to cover the possibility for a low-
end flash flood threat.

Bann

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...

...20Z Update...

The overall synoptic pattern has maintained continuity in the
expected evolution and subsequent convective development through
the course of the D2 window. The biggest change is a slow move to
an agreement on where the heaviest precip will transpire with the
discrepancy in the position of the QPF max now mainly within a
span of 50-75 miles latitudinally. Recent ensemble blends and bias
corrected QPF distribution highlights the area from the Piney Woods
of TX through central LA as the primary focal point of heaviest
precip during the time frame beginning 18z Thursday through early
Friday morning as the shortwave and surface low responsible
translates eastward with the best axis of convection likely
following the fringe a prominent theta-E gradient located from the
Gulf coast up through central and eastern TX. Cell initiation
upstream over north TX will quickly grow upscale and propagate to
the east-southeast with heavy rainfall falling in its wake as it
moves through central TX. A higher-end SLGT risk is expected within
the I-35 corridor, including the DFW metroplex and surrounding
locales as 1-2"/hr rates within the organized convective cluster
are increasingly likely with the latest HREF neighborhood
probabilities signaling upwards of 50% for rates over 1"/hr and
low-end probabilities for up to 2"/hr with the best threat south of
the metro.

From there, sufficient instability and increasing sheared
profiles from local forecast soundings over southeastern TX
indicate an environment favorable for updraft maintenance and
moisture convergence ahead of the propagating cold pool generated
by the upstream convection. Totals of 3-5" with locally higher
amounts are forecast within a zone extending from 20 miles east of
Austin all the way through central LA to the LA/MS border with the
northern and southern bounds extending ~50-75 miles on either side.
This is reflective of the minor uncertainty in the latitudinal push
of the organized area of thunderstorms as it moves southeastward,
but well within the prospects based on the latest probabilities for
at least 2" and 3". The area of greatest concern continues to be
the stretch from Bryan/College Station over through Huntsville and
the Lake Livingston area north of Houston where local FFGs are
hovering around an abysmal 1" for hourly rate exceedance with
current probabilities for at least 1"/hr are now within the 60-80%
range, and 2"/hr and 3"/hr probabilities locally above 25% over
parts of east TX through western LA. As a result, this stretch is
within a higher-end MDT risk with a chance at a future High Risk
update if the forecast reflects higher confidence in the
positioning of the heaviest QPF footprint, and/or the upper
quartile of potential QPF increases further over the most prone
area(s).

A few other changes reflected in the current update include a
tighter confined area of the SLGT risk across the southern plains
over into the Lower Mississippi Valley, and an expansion of the
MRGL risk into NM to reflect some convective development under the
guide of the mean shortwave trough traversing out of the
Southwestern U.S. Totals are generally around 0.5-1" over the
higher terrain of central and southeast NM, but enough large scale
ascent and modest instability focused under the general circulation
could produce a few "over-performing" cells that could expel 1-2"
of precip in a short period of time. Mean storm motions will be
slow, so the prospects increase slightly with meandering convective
development, and is plausible under the propagating mid-level
trough.

Kleebauer


...Previous Discussion...

...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...

...20Z Update...

The D3 forecast had only minor changes to the overall risk areas
due to lingering uncertainty carrying out of the D2 period with
what occurs upstream. A complicated pattern relies heavily on what
transpires across TX and the Lower Mississippi Valley before
shifting eastward into the Deep South. The current forecast has
some deviation in how the convective complex upstream holds
together as it enters the Southeast, as well as some discrepancies
on the handling of a secondary shortwave ejection out of the sub-
tropical jet later Friday that could re-introduce more convective
chances across the Gulf coast from southeastern LA through southern
MS/AL. Current ML output is fixated on the southern periphery of
guidance, likely due to the bias reflected in the expected axis of
instability. Ensemble means are pretty prolific between 2.5-4" with
the bias corrected QPF upwards of 5-6" located over the southern
portions of MS/AL which is very close to where the AI QPF footprint
is located. The pattern screams very heavy rainfall somewhere
between the I-20 corridor to the immediate coast given the
forecasted progression of the frontal boundary and correlated
instability axis along and south of the front in question. Given
the latest ensemble orientation and magnitude of expected rainfall,
there was plenty justification for maintaining continuity with some
minor adjustments based on the latest QPF trends.

Kleebauer


...Previous Discussion...

...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