Quantitative Precipitation Forecast
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000
FOUS30 KWBC 230101
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Sun Apr 22 2018

VALID 01Z Mon Apr 23 2018 - 12Z Mon Apr 23 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 WNW ECP 10 NW OZR 20 NNW TOI 30 S TCL 20 SW 1M4 20 SSE MKL
35 SSW CIR 10 NNE CIR 15 ENE M30 35 E CKV 35 SSE EKQ OQT
30 ENE CHA 20 NE DNN 20 SW RHP 10 N 1A5 15 W AVL GYH 30 SSW AGS
20 S VLD 50 ESE AAF 25 S AAF 30 SSW PAM 15 WNW ECP.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 WNW BHM MDQ 10 SW 4A9 10 SSE RMG 10 SE 47A 15 ESE GVL
10 SE WDR 10 SSE OPN 15 ENE LSF 15 WNW LSF 15 WSW ALX 10 WNW BHM.


0100 UTC update

Changes made to the day 1 ERO were mainly to pare the
western-northwestern periphery of the SLIGHT and MARGINAL risk
areas based on the latest observational trends (composite
radar/satellite/mesoanalysis) along with the CAM output from the
more recent runs (including HRRR/experimental HRRR). Overall,
based on the current satellite and radar presentation, the compact
warm conveyor belt (wcb) remains rather healthy, with the difluent
upper flow and favorable dynamical exit region forcing east of the
mid-level circulation continuing to be bolstered by the
subtropical jet axis which is now nudging across southern MS.
Available instability continues to be modest at best (500-1000
j/kg tops, distributed through a deep layer), which continues to
put a cap on short-term rainfall rates. The slow but steady
eastward progression of the closed mid/upper level low and
pivoting comma region of multi-linear convective lines will
nevertheless continue to pose an enhanced risk this evening and
overnight, especially south of the TN border where the best
instability will lie. WPC continued to align the SLIGHT risk
within the area where the HREF probabilities of >2" in a 3 hour
period are highest (while the latest 3 hourly FFGs remain between
2.5-3.0").

Hurley


...Southeast U.S...

A closed low of moderate strength and maintaining itself along its
slow eastward march - characteristically a very April system -
will spread rain and thunderstorms throughout areas from the lower
Ohio Valley to the Gulf Coast.

Model guidance is in decent agreement, and the notion of
widespread 1 to 3 inch rainfall during the 24-hour period. The
main question is whether rain rates will be intense enough or
persistent enough over any given location to yield much in the way
of flash flooding. Generally the most persistent forcing and
better alignment of lower and upper level flow should occur during
the day from northern Mississippi into western Tennessee. This is
perhaps where training will be most prevalent, but instability
will be more marginal. Areas farther south should see somewhat
greater instability, but with mid level flow cutting across from
west to east, the mean 0-6 km winds are at a sharp angle to
Corfidi vectors, suggesting training will not be too common. Given
flash flood guidance values are also much higher, and will be
difficult to surpass with southward extent, WPC trimmed back a bit
on the inherited Slight Risk area for the 12z Sun- 12z Mon period.
Otherwise, Slight Risk is maintained in areas of lower FFG across
northern MS AL into southern TN, and including the Atlanta Metro
to the east, as well as the more favorable training environment in
western TN.

Throughout these areas areal average precipitation is forecast at
1.5 to around 3.0 inches for the 24-hour period, and judging by
moisture availability and hi-res model solutions, hourly rates
could peak around 1.50 inches, leading to at least some risk of
flash flooding, especially from the time of peak heating into the
evening hours.

Burke
$$




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