Quantitative Precipitation Forecast
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FOUS30 KWBC 212255

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
654 PM EDT Mon Jun 21 2021

Day 1
Valid 2222Z Mon Jun 21 2021 - 12Z Tue Jun 22 2021


...Lower MS Valley/TN and OH Valleys...
2200 UTC Update -- Adjusted the back edges of both the Slight and
Moderate Risk areas, based on the latest observational and
mesoanalysis trends.

...Previous Discussion...
A trough moving across the Mid-West will amplify through the
period as it advances toward the east coast.  In response, strong
southerly flow will usher in rich Gulf moisture into the region.
Precipitable water values will climb above 2 inches in some
locations (which is around 2 standard deviations above the mean)
aided by 20 to 30 knot southwesterly low level flow.  Aloft, as
the trough sharpens, the right entrance region of the upper jet
will align over portions the TN/OH Valleys with mid-level
shortwaves moving atop the instability gradient/surface trough.
Therefore, plenty of synoptic scale ascent will be present to
produce scattered showers and thunderstorms.  MUCAPE values will
surge to over 3000 J/kg ahead of the associated cold front.  Given
the warm rain process, hourly rain rates could climb above 1.5
inches. And with multiple rounds of precipitation advancing from
southwest to northeast, areal average precipitation will range
between 1-3+ inches with locally higher amounts expected.

Fairly dry antecedent conditions exist across much of the Lower MS
Valley region should limit the overall flood threat. However,
heavier precipitation and training may occur within this region to
support isolated to scattered flash flooding. This is also evident
by higher and prolonged HREF probabilities of 1+/2+ inch hourly
rain rates. One adjustment in the morning update was to fill in
the gap between slight risk areas in east Texas and central LA,
where merging outflows could results in redevelopment of
showers/storms/locally heavy rain similar to surrounding areas
supported by the HREF blended mean QPF.
In FL, showers and storms in the panhandle have developed and are
moving east further than the prior outlook, so the slight risk was
expanded east in conjunction with both radar trends and higher QPF
in the HREF members/mean.

Across portions of the Deep South into the TN Valley and
Appalachians, FFG values are lower with higher soil saturation.
Therefore, it will take less rainfall to saturate the basins.

...Gulf Coast...
A weak surface trough positioned either along or just off the
coast which will act as the focus for convection through the
forecast period.  As the upper level trough approaches from the
north/west, increased divergence aloft and mid-level shortwaves
will provide ample large scale lift.  Precipitable water values
will climb above 2 inches aided by 20 to 30 knot low level
southerly flow off the Gulf. This combined with instability of
2000+ J/kg and very high freezing levels should support very
efficient rain rates of over 2 inches/hour.  Areal average
precipitation will range between 1-3+ inches with locally heavier
amounts expected. Given portions of southeast LA and southern MS
observed well above average precipitation from Claudette, any
additional heavy rain could result in scattered flash flooding.

...Portions of South-Central Texas...
Convection growing upscale along and south of the TX Big Bend into
northeast Mexico will eventually lead to an MCS per a multi-model
consensus. The activity is being bolstered by robust deep-layer
instability south of the quasi w-e oriented surface front, with
mixed-layer CAPEs of 3000-4000+ j/kg per the latest SPC
mesoanalysis. Later this evening, as 850 mb e/ese flow increases
to 20-30 kts, the resultant uptick in low level moisture flux and
theta-e transport (PWATs climbing above 2 inches) will favor areas
of very heavy to potentially excessive rain under the strongest
cells, though the activity should be fairly progressive. Localized
2 to 3 inch/hr rainfall rates may pose an isolated flash flood
threat over portions of south-central TX, which has prompted a
Marginal Risk over this area in the Day 1 ERO.


Day 2
Valid 12Z Tue Jun 22 2021 - 12Z Wed Jun 23 2021


An amplified trough and associated surface cold front will
continue to advance across the southeast U.S. through the forecast
period.  Strong return flow will provide sufficient deep layer
moisture and usher in warm air advection and instability into the
region. Meanwhile, a shortwave moving atop the surface boundary
will also be a focus for convection across eastern VA and NC.

...Southeast to the Florida Panhandle...
Convection is expected across areas that have recently received
rains from Claudette. With rich deep layer moisture and
instability climbing above 2000 J/kg, anticipate training
convection over saturated soils could result in scattered flash
flooding.  With the 12z NAM showing the moisture advection
resulting in precipitable water values increasing to 2-2.25 inches
ahead of the approaching cold front, a slight risk was extended
across central to eastern South Carolina and North Carolina.
Localized flash flooding is possible where cell mergers and brief
training result in locally heavy rain.

...Eastern VA...
Precipitable water values will surge close to 2 inches, aided by
25 to 35 knot southwesterly flow which is nearly 2 standard
deviations above the mean. This combined with instability above
1000 J/kg and mid-level shortwaves aloft should result in an area
of convection that moves through the region Tuesday/Tuesday
afternoon.  While rain rates could exceed 1.5 inches/hour, the
progression of the activity may help to limit overall amounts.
However, some guidance is suggesting that the mean propagation
vectors will align with the approaching front allowing for
training to occur. With this in mind, areal average precipitation
should range between 1 to 3+ inches with locally higher amounts.
Given the very dry antecedent conditions, especially across
eastern VA, the potential for flash flooding may be limited.
However, if model guidance continues to highlight this region with
HREF probabilities increasing, an upgrade to a Slight Risk may be


Day 3
Valid 12Z Wed Jun 23 2021 - 12Z Thu Jun 24 2021

...Southeast/Florida panhandle...
A cold front will advance off the Carolina Coast Wednesday
morning.  However, the front is expected to stall and linger
across southern SC into southern portions of GA/AL and into the
Florida Panhandle.  As a result, convection will likely focus
within this region with activity training from west to east.  With
precipitable water values above 2 inches and sufficient
instability, hourly rain rates may climb above 1.5 inches/hour.
Areal average precipitation is around 1-3+ inches.
Flash flood guidance is mostly in the 3-4 inch range in 3 hours,
so  only isolated locations are expected to exceed flash flood
guidance. Consequently, the risk is only depicted as marginal.


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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