650
FOUS30 KWBC 150056
QPFERD

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

Day 1
Valid 01Z Wed May 15 2024 - 12Z Wed May 15 2024

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

Maintained the Marginal Risk areas over portions of the Ohio and
Tennessee Valleys as well as the Mid-Atlantic and northern Florida.

...Northern Florida and Far Southern Georgia..
Latest runs of the HRRR continue to advertise convective
redevelopment later tonight in the predawn hours as a cold front
advances into a moisture laden atmosphere. Overall...recent trends
in convective precip output continue to favor less rainfall than
would be necessary for a higher end risk given areal FFG indices if
parts of the area had not had the passage of a complex earlier in
the day and with a second complex well offshore over the Gulf of
Mexico this evening. The steady forward propagation in the
organized area of convection will limit better flash flood
potential...but precipitable water values increasing to values at
or slightly above 2 inches...maximum rainfall amounts could still
be in the 2 to 3 inch range and the 18Z HREF showing a swath of 20
to 30 percent probability of 2 inches per 3 hours. Those rates are still
well- below what would be sufficient for exceedance across the
area proposed but precludes removal at this point.

...Mid Atlantic Region...
Maintained the Marginal Risk area as a compact coastal low moves
northward. The resulting easterly flow north of the low center
should result in upwards of an inch of rainfall across the area
with embedded higher amounts between 06-12z Wednesday as per the
latest hi-res guidance and correlated ensemble. Flash flood
guidance is already fairly low given the recent rains...and
urbanization will locally enhance the chances for flooding. One change
to the Marginal Risk area was to trim away at the southern
boundary where little or no additional rainfall is expected. The
other change was to limit the northeastern extent where air with
low thetae being fed in from the northeast seemed to be eroding the
northern side of the rain shield. Still a low end risk of moderate
to heavy rain being able to make some northward advancement later
tonight if the feed of low thetae air can be disrupted by low level
winds back to east or southeasterly...so did not make too drastic
of a change.

...Portions of the Ohio and Tennessee Valleys...
The flash flooding threat will continue to be focused around a
vertically stacked low with associated upper level cold air making
its way slowly south and east. Being cold core...the overall
convective trend should continue to be downward with the loss of
daytime heating...but some embedded shortwaves within the mean flow
and lingering instability in the Ohio and Tennessee Valleys keep
the threat of flooding/flash flooding in the realm of
possibilities overnight. Think the threat has been
minimized...for the most part...where the airmass has been turned
over. South of that...in parts of northern Alabama and the
northwest corner of Georgia there could be excessive rainfall where
convection had started to develop across a northeast to southwest
line that was beginning to re-orient parallel to cell motion. While
the latest HRRR tended to extinguish the convection
quickly...satellite imagery was showing cooling cloud tops. So the
Marginal Risk area was extended into the area.

Bann

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