Quantitative Precipitation Forecast
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000
FOUS30 KWBC 200853
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 AM EST Wed Feb 20 2019

Day 1
Valid 12Z Wed Feb 20 2019 - 12Z Thu Feb 21 2019

...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS NEAR THE NORTHERN
ALABAMA/MISSISSIPPI BORDER...

Southeast/Mid-South/Southern Appalachians...
A heavy rainfall event should be on-going Wednesday morning as
thunderstorms currently in the Mississippi Valley and rain in the
Ohio Valley shifts eastward, aided by a well defined moisture
plume coming off the Gulf of Mexico, a strong mid-level shortwave
and a strong upper level jet.  Inflow at 850 hPa is forecast to be
40-65 knots with precipitable water values expected to be in the
1.25-1.75" range.  The flow should be unidirectional with height
in this region, supporting the idea of cell training.  MU CAPE in
the 500-1000 J/kg range is forecast by the most recent GFS
guidance, with the greatest values close to the Gulf Coast.

The moderate risk area was shifted westward to an area that
received heavy rainfall with low flash flood guidance values near
the northern AL/MS border after coordination with BMX (Birmingham,
AL) and JAN (Jackson, MS) WFOs, where thunderstorms are expected
to cause heavy rainfall later this morning into early this
afternoon.  Overall, the guidance indicates that maximum hourly
rain totals of 0.5-1" are expected, but with the instability and
moisture present, 1.75" an hour totals are possible should long
enough training bands or mesocyclones -- which there is enough
effective bulk shear to support -- develop.  Rainfall amounts are
forecast to be less than on Tuesday/Tuesday night, with most areas
expecting under 2" areal average amounts.  The 00z mesoscale
guidance generally indicates local amounts in the 3" range.

The risk areas reflect the higher ground moisture content and
reduced flash flood guidance values in the central to southern
Appalachians and adjacent valley areas of eastern Kentucky and
Tennessee.  An increase in flash flood guidance values further
south in both MS/AL plus some implied mid level capping (700 hPa
temperatures of 6C+ until late Wednesday night along the immediate
Gulf Coast) leads to a lower risk being depicted despite the
higher rainfall totals forecast.  Overall, the 00z HREF
probabilities of 0.5"+ totals per hour fade by 06z, which should
end the flash flood threat overall.  Areas of frozen precipitation
were used to hem in the northern and eastern periphery of the risk
areas.

Roth


Day 2
Valid 12Z Thu Feb 21 2019 - 12Z Fri Feb 22 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHERN U.S. ON THURSDAY INTO EARLY FRIDAY...

...Gulf Coastal States to the Southeast U.S....
Maintained a Slight Risk area for excessive rainfall close to the
previous Day 3 ERO position as it becomes today`s Day 2 ERO.

There was a consensus among the 20/00Z ECMF and 20/00Z GFS that
the tail end of a boundary will still be present in this
area...with a sprawling area of high pressure to the north and
deeper (albeit fairly modest) moisture located to the south of the
boundary.  The models still suggest there may be a lull in
rainfall coverage and rainfall intensity early-on in the forecast
period, but all depict a push of moisture from the Gulf of Mexico
making its way on-shore over parts of Texas/Louisiana late in the
day.  There is a large degree of model spread with respect to how
much interaction there is between the moisture and a lingering
boundary...which in turn led to a broad range of QPF solutions.

One of the more consistent signals to focus precipitation...and
the potential for additional flooding...was along the boundary
over portions of Arkansas and Mississippi with decreasing
agreement farther east.  Flash Flood Guidance values have lowered
considerably there in response to rainfall over the past couple of
days, including Tuesday and Tuesday night.  So only made minor
adjustments to the on-going Slight Risk area.  As such, it still
captures the position of the modest inflow of moisture and the
placement of the front/moisture interaction.

Bann


Day 3
Valid 12Z Fri Feb 22 2019 - 12Z Sat Feb 23 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTH CENTRAL U.S. TO THE TENNESSEE VALLEY...

...Southern U.S. to parts of the Southeast U.S. and far southern
Appalachians...
The overall pattern changes little from Day 2 into Day 3...with
the Slight Risk area in roughly the same place given similar
placement of a quasi-stationary boundary at the surface, broad
southwest to northeast flow aloft at the jet-stream level and
channeled vorticity in the mid-levels.  That configuration is not
terribly well suited to focus heavy rainfall and the potential for
excessive rainfall.  As was the case on Day 2, the models
struggled quite a bit with placement and amounts over the area
until very late in the period (approaching 12Z Saturday). The
spread limits confidence in issuing any more than a Slight Risk of
excessive rainfall.  The other consideration is that some parts of
Mississippi and Alabama already have lower Flash Flood Guidance in
place which is not likely to recover much in the next couple of
days.  At this point, will maintain a Slight Risk and watch for
more definitive trends in the guidance and wait for a clearer
picture as to which areas will be more prone to flash flooding.

Bann





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


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