918
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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 PM EDT Tue May 14 2024

Day 1
Valid 16Z Tue May 14 2024 - 12Z Wed May 15 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR NORTHERN
FLORIDA, AND PORTIONS OF THE OHIO VALLEY AND MID ATLANTIC...

...16Z Update...

The biggest change for the period was the removal of the SLGT risk
across northern FL and southern GA given the recent trends in
convective precip output favoring less rainfall that would be
necessary for a higher end risk given areal FFG indices. Locally
heavy rainfall is currently moving through northern FL with a
small complex that originated off the Gulf of Mexico. The steady
forward propagation in the organized area of convection will limit
better flash flood potential with QPF maxes generally between
2-3", at best from the latest hi-res deterministic and correlated
HREF blended mean. Probabilities for 1"/hr have come down even
further from the previous HREF iteration which is still well-below
what would be sufficient for exceedance across the area proposed.
This allowed for a degradation of the risk to a MRGL with lower
end probability for occurring with the best opportunity residing
in more urban-centric locations (Jacksonville metro).

Further north, not as much has changed from the previous forecast,
but did make an expansion of the MRGL further north into the
central Mid-Atlantic with the I-270/70 corridor between the Blue
Ridge over to the DC/Balt metros seeing a low-end threat for flash
flooding early tomorrow morning given the expected surface
cyclogenesis and prevailing easterly fetch that will drop 0.5-1"
of rainfall across NoVA up through central MD between 06-12z
Wednesday as per the latest hi-res guidance and correlated
ensemble. This area already has fairly low FFG indices given the
recent rains, and the urbanization factor. Similar setups in the
past have accounted for localized flood concerns, so wanted to
cover for some of the higher end deterministic output and low-end
probabilities that fall within the MRGL risk threshold.

Convection will continue across the Ohio Valley and Midwest under
the presence of a prominent upper low situated over the
Mid-Mississippi Valley. Totals have not budged from previous
forecast with some modest instability and better upper forcing the
trigger to the potential. Best threat will reside across southern
IL/IN into northwest TN with northern bounds over central IN/OH.
The MRGL risk was maintained.

Kleebauer


...Previous Discussion...

...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 PORTIONS OF
THE CENTRAL AND SOUTHERN PLAINS, THE NORTHERN FLORIDA PENINSULA,
AND THE CENTRAL APPALACHIANS...

...20Z Update...

The biggest changes with regards to the D2 period was the removal
of the MRGL risk across the southern half of TX with the
maintenance of the previous MRGL across the Red River and points
north. Sufficient capping should preclude any convective
development that would be worthy of flash flooding consideration
with the central Rio Grande as one of the few areas that could see
favorable rainfall for isolated flash flooding. That area has not
been the benefactor of significant precip like much of the state to
the northeast and east, so the prospects for flooding are below the
Marginal Risk threshold. Areas further north into the Red River
basin through OK and KS maintain the best signal with regards to
flash flood concerns, although improvement in areal soil moisture
will curb the risk to a lower-end probability which is agreed upon
by the latest CSU First Guess Fields and relevant ML guidance.
Totals of 1-2" will be most common with a few 3+" amounts plausible
given the increasing low to mid-level moisture field with a general
1-1.5 deviations above normal PWAT anomaly forecast in-of the
outlined area from the Colorado Front Range over towards eastern
Kansas/Oklahoma. The MRGL risk was maintained for those areas given
the continuity in guidance.

The same goes for the northern FL Peninsula where most
deterministic indicate a period of heavier rainfall within the
confines of a stalling frontal boundary south of the Panhandle. The
best opportunity will occur during the front half of the forecast
time frame as a pretty stout low-level moisture flux will advect
out of the Gulf and move over the Peninsula Wednesday morning and
afternoon. Totals of 1-3" with locally up to 5" are possible within
the bounds of the MRGL risk in place with the best flash flooding
opportunity within any of the urban corridors along either coast.
Neighborhood probability for at least 3" is between 40-55% over
west-central FL just to the northeast of Tampa/St. Petersburg. This
is the area to monitor in the early stages of the event with areas
like Daytona down to Melbourne as the secondary area of focus as
very high FFGs between the coastal population centers will likely
reduce the flash flood risk, comparatively.

An additional MRGL risk was added across portions of the central
Appalachians across mostly WV and adjacent areas of Ohio and
Kentucky. More on that potential was added into it`s own sub-
heading, "Central Appalachians", below.

...Central Appalachians...

Our Upper low will translate eastward through the Ohio Valley,
becoming an open wave is it makes headway through southern OH into
WV. Flow aloft will become very weak, limiting shear and prospects
for better organized convection during peak diurnal destabilization
across portions of the central Appalachians, but there will be
sufficient low and mid-level moisture under the confines of the
upper low creating a period of slow-moving, heavy rainfall concerns
within the terrain focused across WV and adjacent high country of
eastern KY and southeast OH. Forecast soundings out of central WV
indicate a tall, skinny CAPE structure with no capping and a
saturated column up towards the tropopause. This is a textbook
environment for favorable heavy rain signatures within any
convective development that occurs under the influence of the
passing upper low, open mid-level trough. 12z HREF neighborhood
probabilities are reasonably high for at least 1"/hr rates with a
broad area of 30-50% probability located over southwest WV up
through Yew Mountains, the highest peaks within the state. Local
FFG`s for all 1/3/6 hour markers are all low with the 1"/hr FFG
hovering between 1-1.5", matching the current forecast potential
over the area. The combination of a favorable environment, slow
movement of cells, and terrain influences increases the likelihood
of some localized flood concerns in the above areas, leading to a
MRGL risk inclusion on this update.

Kleebauer


...Previous Discussion...

...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 northern third of the state, even isolated convection
if it moves over a particularly sensitive area could cause flash
flooding.

...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...

...20Z Update...

There were not many significant changes in the previous forecast
outside some trimming of the eastern edge of the MDT risk area to
account for the latest trends in guidance with the primary risk
being focused over east TX through northern and western LA.
Consistency amongst ML guidance and an agreement on all non-GFS
deterministic for the QPF max to be situated along that corridor
east of I-35 in-of central TX over into eastern TX through portions
of LA gives merit to the previous forecast and was thorough for
maintaining continuity over the above areas. The synoptic scale
pattern has wavered very little from run-to-run with the 5H delta
over the latest GEFS/ECENS showing only minor deviations in the
mean height field, indicative of a steady state mid-level
shortwave propagation between runs that aids with confidence in the
forecast. Small mesoscale features will be hashed over the next
series of forecast updates as we introduce the latest CAMs suite
and associated HREF output that will help deduce whether we will
need to upgrade the risk further from the MDT risk in place. With
totals pushing between 4-5" on the mean and locally as high as 7"
within certain deterministic output, there is the possibility for
an upgrade to a High Risk for a targeted location in east TX,
specifically just given the anticipated rainfall and antecedent
conditions in-place across areas north and northeast of Houston
proper. For perspective, 30-day totals out over portions of east TX
near Lake Livingston are between 20-30" with much of the rainfall
occurring the last few weeks. This area is expected to see another
3+" of rainfall over the course of Thursday evening into early
Friday morning. The prospects for significant flooding are already
elevated, so the additional rainfall will not take much to cause
major problems. Future updates will gauge the potential for
upgrades with the hi-res guidance coming into focus with more
specifics in the, "Where", and "How much" will fall.

Kleebauer


...Previous Discussion...


...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