FOUS30 KWBC 280903
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
502 AM EDT Fri Aug 28 2020

Day 1
Valid 12Z Fri Aug 28 2020 - 12Z Sat Aug 29 2020

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MIDWEST TO EASTERN GREAT LAKES AND CENTRAL
APPALACHIANS...THE MID-SOUTH...TN VALLEY AND SOUTHERN
APPALACHIANS...AND SOUTHERN LOUISIANA...

...Wisconsin and Michigan to the upper Ohio Valley and eastern
Great Lakes...
A series of impulses in an active northern stream ahead of a
shortwave trough over the eastern Dakotas early this morning is
bringing ongoing heavy rain threats to southern MI, central WI,
and there is a bow echo crossing southern MN well ahead of the
trough axis early this morning. These areas of heavy thunderstorms
will generally progress east-southeast today. This focuses the
threat area on north-central WI/MI and across Lake Erie into
northeast OH/western PA and down into WV through this evening.
1.75 to 2 inch PWs across this area are 2 to 2.5 standard
deviations above normal and along with ample instability will
continue to provide a heavy rain threat. Much of this area had
been recently dry up until last night, but rates should be high
enough to exceed flash flood guidance. Particularly heavy rain in
central WI has made them rather sensitive and may require a
Moderate Risk, but the next approaching wave is rather
progressive, so a large Slight Risk was maintained. Of particular
note is the Slight was trimmed across the UP of MI and for the
northern LP of MI which is north of the low track associated with
the approaching shortwave.

...Laura remnants track from northeast Arkansas across KY
tonight...
Rain rates associated with the remnant circulation of Laura
greatly diminished overnight, but are expected to increase again
as activity shifts from eastern AR into western TN and northern MS
this morning. However, there is a great lack of instability west
from Memphis, so rain rates may remain suppressed well into this
morning. However, instability farther east over middle TN and
northern AL should allow redevelopment with locally heavy rain
rates, warranting the Slight Risk that extends east to the
southern Appalachians where southwesterly flow aides orographic
enhancement with a very moist environment (1.75 to 2 inch PWs are
2 to 2.5 standard deviations above normal over the southern
Appalachians).

...Southern Louisiana...
Returning moisture and instability focus across southern LA today
ahead of a mid-level trough developing on the south side of Laura
as it continues its extratropical transition. There is uncertainty
with this area, but general convection perhaps aided by a surface
trough with 2.25 inch PWs and ample instability across the swath
where Laura made landfall is rather sensitive to flash flooding at
this time, so the Slight Risk was maintained.

...Colorado/New Mexico/OK and TX Panhandles/Southwest Kansas...
A weak shortwave trough topping the mid-level ridge over the
Southern Rockies interacts with deep moisture and marginal
instability to produce convection this afternoon capable of heavy
to locally excessive rainfall over portions of CO and northern NM
that then spreads east onto the southern High Plains overnight.
The deepest moisture is expected over eastern portions of CO/NM,
as 1.25 to 1.5 inch precipitable water air spills west from the
south-central Plains. Mean flow is light and the storms track
east-southeast at 10 knots or less, so short term training and
cell mergers could result in hourly rainfall rates of 0.50 inches,
especially this evening.

There is a regional/global model signal for local 1 to 2 inch
rainfall. Given three hour flash flood guidance of 2 to 3 inches,
localized flash flooding is possible for cell mergers and
repeating activity. The Marginal Risk was maintained.

Jackson


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