Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Thu Feb 25 2021

Day 1
Valid 16Z Thu Feb 25 2021 - 12Z Fri Feb 26 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

However, a swath of moderate to heavy rainfall is expected around
the Arklatex, where amounts of ~2" are expected. Good coupling of
low level moisture convergence and upper level divergence within
the right entrance region of the 250mb jet, supports the expanding
coverage of rain into tonight. A forecast of 2" areal averaged
rain is often enough to consider at least a Marginal risk. However
with only weak instability in place, the rates just do not seem to
be there with this event. Localized swaths of 1" in an hour are
likely...but overall it looks like 1hr and 3hr rainfall should
peak at only ~50% of FFG. Also, current streamflows and soil
saturation over the region are around typical levels for the time
of year...thus not giving any reason to suggest flooding is likely
below FFG levels. Thus in general it looks like this heavy rain
later today into tonight will help saturate soil conditions and
result in some ponding of water in spots, but overall is not
expected to result in much of a flash flood impact...thus will
carry no risk area at this time.

Chenard


Day 2
Valid 12Z Fri Feb 26 2021 - 12Z Sat Feb 27 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE LOWER MISSISSIPPI VALLEY...TENNESSEE VALLEY...CUMBERLAND
PLATEAU...SOUTHERN APPALACHIAN MOUNTAINS...AND THE CAROLINA
PIEDMONT...

2030 UTC Update -- Have hoisted a Marginal Risk over the area,
based on the latest guidance trends, particularly late in the
period (00-12Z Sat) as the warm front lifts slowly northward
Fri-Sat. PWs Friday night climb to 1.25+ inches, which is about 2
standard deviations above normal. This as southwesterly low-level
inflow increases to 50-55 knots, becoming nearly parallel while
approaching the magnitude of the mean 850-300 mb low -- setting
the stage for upwind propagation and cell training with elevated
convective bands north of the surface warm front Friday night.
Instability looks to be on the marginal side, with MUCAPEs
generally averaging between 250-500 j/kg, which for the most part
will cap 3 hourly rainfall rates between 1-1.5 inches.

Previous discussion..

The latest guidance continues to show Gulf moisture being
transported northward within an increasing low level flow. The
best mid-upper level lift will be present with the best lift
occurring 00Z-12Z near northern Mississippi/western and central
Tennessee as the associated front advances. Periods of moderate to
locally heavy rainfall will be supported within this environment;
with a couple of the hi-res guidance showing rain rates of 0.50
inch/hour across parts of Arkansas into Mississippi and western
Tennessee during the 12-18Z period. This is also where the swath
of higher QPF is expected to occur, with WPC forecasting 1 to 2
inches. During this time isolated locations may experience ponding
or rapid runoff but the aerial coverage of flooding is not
expected to be widespread. It will however increase soil
saturation across the region and lower the FFG from the Southern
Plains/Lower Mississippi Valley to the Tennessee Valley.

Hurley/Campbell


Day 3
Valid 12Z Sat Feb 27 2021 - 12Z Sun Feb 28 2021

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE
SOUTHERN PLAINS INTO PORTIONS OF THE LOWER-MID MISSISSIPPI VALLEY,
THE NORTHERN TENNESSEE VALLEY, SOUTHERN OHIO VALLEY, AND WESTERN
SLOPES OF THE CENTRAL APPALACHIANS...

2030 UTC Update -- Nudged the bottom portion of the Marginal Risk
area a bit farther northward based on the latest trends in the
guidance, while expanding the outlook area slightly downstream to
include more of the lower OH Valley and western WV. Elevated
instability north of the warm front at this appears modest at
best, averaging ~250-500 j/kg over the outlook area, with pockets
closer to 1000 j/kg. This should be more than sufficient to
support 1.5+ inches of rainfall within a 3 hour period, especially
with some training likely Sat night given the uptick in
southwesterly 1000-850 mb flow into a strengthening low-level
frontogenetic zone within the right entrance region of an upper
level jet streak.

Previous discussion below...

Scattered to widespread rain will persist during the day 3 period
after spreading 1 to 2 inches across the region the day prior.
This antecedent moisture will have likely lowered flash flood
guidance across the Lower Mississippi and Tennessee Valleys where
an additional 1 to 3 inches are forecast. Although the hourly
rates across this region are expected to be less than 0.50
inch/hour, precipitation of this magnitude will lead to an
increased threat for flash flooding to develop. In coordination
with the local forecast offices from far northeast Texas to
eastern Kentucky/Tennessee a Marginal Risk area was hoisted. This
area will continue to be monitored as the threat of moderate to
heavy rainfall continues into the day 4 period.

Hurley/Campbell



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