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WPC Day 2 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 2001 UTC Sat Aug 18, 2018
Valid: 12 UTC Aug 19, 2018 - 12 UTC Aug 20, 2018
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 PM EDT Sat Aug 18 2018
Valid 12Z Sun Aug 19 2018 - 12Z Mon Aug 20 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE GREAT PLAINS INTO THE OZARKS...
...Great Plains into the Ozarks...
A shortwave trough shifts southeast across the central Great
Plains Sunday and Sunday night. A swath of 2.0 inch PW air along
the Red River will be drawn north in 20 to 30 kt 850mb flow ahead
of the surface low and warm front with 1.75 inch PW reaching SD
(1.5 to 2 standardized anomaly across the region) and the
deformation zone to the north of the wave. The combination of
multiple sources of lift, ample moisture, instability
(particularly south of the warm front) and wet antecedent
conditions warranted combining existing Slight Risks into one that
stretches from the ND/SD/MN border to OK/AR. Instability will be
limited north of the warm front, but dynamic forcing from the
trough (that may briefly close into a low should be sufficient in
the northern portion of the Slight Risk.
A stationary front lifts north as a warm front Sunday with 2.0
inch PW in the wake from the MS delta region to northern GA.
Little deep flow means slow moving airmass thunderstorms may cause
localized flash flooding. A Marginal Risk was maintained across
Maintained the Marginal Risk area along and ahead of a stationary
front over southern VA to the southern Appalachians. Forcing is
weak, but moisture is 1 to 1.5 standard deviations above normal.
Scattered slow moving heavy thunderstorms are likely over a broad
area from the Carolinas and Virginia.
...Greater New York City...
Maintained a small Marginal Risk for NYC given the 12Z ECMWF
return to a more amplified trough/mid-level low over the northern
Mid-Atlantic which may combine with the surface reflection
offshore and peak Sunday heating to maintain moisture/instability
and an area of locally heavy rain. This is also supported by the
12Z ARW2 and NMMB models. Wet antecedent conditions make for low
FFG. Other guidance like the GFS and other CAMs are much more
disperse with the pressure pattern in this area and push moist air
...Central Gulf Coast...
A Marginal Risk was maintained for the central Gulf Coast/greater
New Orleans with the continued threat of localized flash flooding
from lake breeze driven convection in ample moisture and limited
deep layer flow per consultation from WFO LIX.
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt