Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
335 AM EST Sun Dec 04 2022

Day 1
Valid 12Z Sun Dec 04 2022 - 12Z Mon Dec 05 2022

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST ARIZONA AND SOUTHWEST NEW MEXICO...

Most of Arizona and New Mexico will remain beneath the
right-entrance region of a ~100 knot 250mb jet streak while,
simultaneously, an upper low off the Pacific Northwest coast
continues to funnel rich subtropical moisture overhead. NAEFS
climatological percentiles remain exceptionally high over
southeast Arizona and much of New Mexico this morning and
afternoon with values above the 99th climatological percentiles in
some cases. The IVT also remains quite robust as the upper low off
the Pacific Northwest coast and the ridge over the Gulf of Mexico
work in tandem to generate IVT values of 400-500 kg/m/s, which is
also above the 99th climatological percentile. This remarkable
amount of moisture is more than 5 standard deviations above
normal, a true anomaly for this time of year. Similar to Saturday,
however, instability will continue to be hard to come by, keeping
a lid on rainfall rates from becoming too heavy. This will be what
largely saves this area from having much more serious flooding
concerns, given how incredibly anomalous this amount of
atmospheric moisture is for this region. The region has been
picking up steady rainfall from Saturday and soil moisture is more
saturated than it was entering the weekend. The Marginal Risk
remains largely unchanged, save a small northward expansion to
account for recent trends in the rainfall noted on radar. While no
reports of flash flooding have yet been received, the expectation
of continued rainfall in the Marginal Risk area through much of
the day today before weakening and shifting south should be enough
time for isolated flash flooding to remain a concern.

Wegman/Mullinax



Day 2
Valid 12Z Mon Dec 05 2022 - 12Z Tue Dec 06 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR FAR SOUTHERN
TENNESSEE AND NORTHERN ALABAMA AND GEORGIA...

A strong cold front pushing through the Midwest on Monday will
slowly sink south as a warm front lifts north into the Mid-South.
High pressure over the East coast will work in tandem with lower
pressure in lee of the southern Rockies to generate a
southwesterly 850mb moisture moisture fetch out of the western
Gulf of Mexico underneath an expansive 120 kt southwesterly jet
over much of the country. This slug of moisture will join the
subtropical jet emerging from the Southwest and result in a robust
IVT across the Southern Plains and into the Mid-South. 00Z NAEFS
suggests IVT values of 400-750 kg/m/s are possible across the
South Central U.S. on Monday, which is >97.5% climatological
percentile in many cases. As the upper low off the California
coast and deep upper low in south-central Canada continue to help
generate a strong jet streak oriented WSW to ENE across the U.S.,
diffluent flow will be positioned over the South, setting the
stage for a heavy precipitation event along the warm front in the
Mid-South on Monday.

The latest trends in the guidance have generally been falling as
far as total forecasted rainfall across the Slight risk area, and
that axis of heaviest rainfall has nudged a bit towards the east
as well. Should this trend continue, the Slight Risk may need to
be abandoned in favor of the larger Marginal in place, but since
there should still be a sizable swath of more than 2 inches of
rain for far southern Tennessee and northern Alabama primarily,
think despite no more explicit 3 inch or more daily totals being
forecast, that there certainly is potential for isolated to widely
scattered amounts over 3 inches, which certainly could pose a
flooding threat, especially should that occur over downtown
Huntsville or Chattanooga. The eastward shift in the guidance
resulted in all but the northeastern-most corner of MS being
removed from the Slight, and essentially all of AR as well. While
there remains some uncertainty where the heaviest rain will be
seen, that uncertainty has diminished quite a bit from 24 hours
ago. However across Georgia, in coordination with the FFC/Atlanta,
GA forecast office, the Slight Risk was expanded to include metro
Atlanta and points north and west. While Atlanta isn't yet
expected to get the brunt of the rainfall, even the 1-2 inches
expected could cause flash flooding concerns given the inherently
lower FFG values due to urbanization. Any additional small
southward shift in the guidance could bring downtown Birmingham,
AL also into the Slight risk.

Wegman/Mullinax


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 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