Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
335 AM EST Sun Dec 04 2022
 
Day 3
Valid 12Z Mon Dec 05 2022 - 12Z Tue Dec 06 2022 

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE 
TENNESSEE VALLEY...

In response to the trough digging across the West on Tuesday, the 
jet stream and surface fronts will begin drifting northward across 
TN & KY. Generally laminar west-southwesterly flow from the jet 
level to the surface, an unfavorable right exit region of the jet, 
and the warm front beginning to win out over the colder air over 
the Great Lakes will all combine to reduce the overall 
precipitation along the front, despite continued highly anomalous 
atmospheric moisture and PWATs in the Marginal risk area. As such, 
the Marginal Risk area is largely a continuation of the rainfall 
expected from Monday, with the northward shift accounting for any 
rainfall seen across northern Tennessee from Monday. Soils are 
very dry across KY, so despite the likelihood of widely scattered 
to scattered daily rainfall totals around an inch, the soils and 
below normal river flows in this region should easily be able to 
handle this moisture. The southern end of the Marginal risk area 
is not expected to get as much rainfall as areas further north 
across Tennessee, but given the wet conditions from Monday along 
with some uncertainty as to how quickly the bulk of the rainfall 
drifts north, decided it was prudent to keep far northern AL & GA 
in the Marginal Risk, which was agreed upon with the 
HUN/Huntsville, AL, FFC/Atlanta, GA, and OHX/Nashville, TN 
forecast offices. It's worth noting that this very strong 
baroclinic boundary over the OH River Valley is unlikely to move 
much at all over the coming week, which certainly is a pattern 
favorable to eventual flooding concerns in KY, AR/eastern OK, and 
maybe portions of the Mid-Atlantic later in the week, but it will 
take a few days of rainfall to bring soils and rivers in this area 
to a level where flooding may become a concern.

Changes from the inherited map included removing all of AR from 
the Marginal, considering the eastward shift on Monday of the bulk 
of the rainfall. This will keep all of AR at around 1" of total 
rainfall forecast for the combined Monday/Tuesday time 
frame...well shy of any flooding concern.

There was also a Marginal risk area inherited for portions of the 
Northeast from Philadelphia through New York City and including 
the Catskills, Berkshires, and Green and White Mountains of New 
England. In coordination with the PHI/Philadelphia, PA, OKX/New 
York City, NY, ALY/Albany, NY, and BGM/Binghamton, NY forecast 
offices, that Marginal risk area was removed entirely. Rainfall 
totals will struggle to reach an inch for the day in most of New 
England, save any potential very localized upsloping. This is 
quite simply not enough rainfall to generate any flooding concerns.

Wegman
 
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt