Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
444 AM EDT Fri Sep 29 2023

Day 1
Valid 12Z Fri Sep 29 2023 - 12Z Sat Sep 30 2023

...A MODERATE RISK OF EXCESSIVE RAINFALL IS IN EFFECT OVER
PORTIONS OF THE NORTHEAST...

...Mid-Atlantic into Northeast...

An upper-level trough will support the amplification of an
inverted surface trough and low pressure system off the
Mid-Atlantic/Northeast Coasts today. There will be thunderstorms
with heavy rainfall and serious flash flooding concerns over parts
of the northern Mid-Atlantic and Northeast. Rounds of showers and
thunderstorms, which began overnight, will continue and pickup in
intensity through the morning rush hour. Some of the 00z hi-res
guidance didn't capture the 01z-02z storms that moved into the New
York Metro so there is a concern that this under performance may
continue into the day 1 period. This antecedent rainfall has also
primed surfaces for runoff of subsequent rainfall. A favorable low
level jet, shallow instability and substantial surface
frontogenesis will combine to produce efficient rainfall rates on
the order of 0.25-0.5"/hr from northeastern New Jersey to coastal
Connecticut where a Moderate Risk of Flash Flooding is in effect.
Anywhere from 2-6" of rainfall are expected to accumulate by later
this evening when the greatest threat of flash flooding will
subside. Areas that are prone to flash flooding across the New
York Metro Area will likely experience flash flooding today, so
turn around don't drown!

...Upper Midwest/Great Lakes...

Shortwave energy propagating south-north into Canada will drive
showers and scattered to isolated thunderstorm activity over the
Upper Midwest today. Another inverted surface trough will direct
and enhance anomalous moisture plume with PWATs of 1-2" into much
of Minnesota and northwestern Wisconsin. Rainfall totals of 1-2
inches are possible over portions the Upper Midwest by tomorrow
morning.

...Florida...

The back end of a slow moving cold front will act as a focus for
scattered to isolated showers and thunderstorms today. A Marginal
Risk area was continued to account for any localized flash
flooding concerns that may arise.

Kebede


Day 2
Valid 12Z Sat Sep 30 2023 - 12Z Sun Oct 01 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE GREAT BASIN AND FLORIDA...

...Great Basin/Nevada...

A deep upper-trough will transport a modest plume of moisture into
the West on Saturday. A cold front associated with a deepening low
pressure system will act as a focus for convection over the Great
Basin in particular. Some parts of central Nevada, southeastern
Oregon and southwestern Idaho could see rainfall rates in excess
of 0.25"/hr. Ample forcing over elevated terrain could lead to
more efficient/enhanced rainfall. Between 0.5-1" total areal
rainfall is forecast over 3-4 hour period therefore, localized
flash flooding is our primary concern.

...New Mexico/west Texas...

Convection supported by an upper trough to the West and shortwave
energy downstream from it will support another localized flash
flood threat over parts of eastern New Mexico and into west Texas.
Decent instability, modest moisture flux and low level jet may
produce bursts of heavy rainfall over our marginal risk area.
Urban areas and dry washes are most vulnerable of flash flooding.

...Florida...

A slow moving shortwave trough will allow for the continuation of
scattered to isolated thunderstorms across the central/southern
peninsula on Saturday. Some cells may produce localized heavy
rainfall within the marginal risk area. Antecedent conditions will
also contribute to the sensitivity to any isolated heavy rainfall
threats.


Kebede

Day 3
Valid 12Z Sun Oct 01 2023 - 12Z Mon Oct 02 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
FLORIDA...

...Florida...

Shortwave energy present across the state of Florida will support
isolated flash flooding concerns on Sunday. Scattered to isolated
thunderstorms will continue to develop along a quasi-stationary
front draped across the southern peninsula. There'll be plenty of
instability in place so there's always the risk of some cells over
performing with efficient rainfall rates exceeding flash flood
guidance.


Kebede


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt