Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Thu Sep 5 2024

Day 1
Valid 16Z Thu Sep 05 2024 - 12Z Fri Sep 06 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ALONG THE
CENTRAL GULF COAST...

16Z Update...
Introduced a Slight risk area over portions of coastal Florida into
adjacent coastal Georgia where convection based on a combination of
short term radar imagery showing convection lingering off-shore
from an area where heavy to excessive rainfall fell on
Wednesday...plus at least some hint that convection could still
build westward during the day. With high precipitable water values,
high freezing levels and weak steering level flow...felt a Slight
Risk was warranted where the ingredients tended to overlap. On the
other hand, deterministic QPF from any given global model and a
number of the higher resolution CAMs are fairly light, the
occurrence of excessive rainfall is far from certain.

Along the Gulf coast...the previously issued Moderate and
surrounding Slight risk areas appear to be in pretty good shape and
only minor adjustments were made. Satellite/radar imagery during
the morning continue to keep a majority of the coldest cloud
tops/greatest rainfall rates off-shore and along the immediate
coast where 2 to 3 inch per hour rainfall rates appear to be most
likely. However...signals persist for those kind of rates to spread
northward in association with deep-layer moisture flux convergence
as suggested by the HRRR. If that were to happen...concern is for impactful
flooding in more populated/urbanized regions with more widespread
impervious land coverage.

No change made in New Mexico.

Bann

...Previous Excessive Rainfall Discussion...

...Gulf Coast, Lower Mississippi Valley and Southeast...

In coordination with local forecast offices near the Mississippi
Delta region, a Moderate Risk area was hoisted for this period to
include portions of southeast Louisiana and southern Mississippi. There
is consensus for heavier rainfall along the central Gulf Coast
given the bulk of PWs >99th climatological percentile residing
along the Gulf Coast. The region is also closer to the
surface-850mb southeasterly flow that will sustain a steadfast
regime of low level theta-e advection into the nearby stationary
frontal boundary. Additionally, the latest guidance shows a
potential for locally higher amounts upwards of 10" per NBM 95th
percentile to focus in the vicinity of the New Orleans metro and
points east and south. A Slight Risk spans from the southeast Texas
coast to southern Mississippi.

Deep-layer instability across portions of the Southeast may be
lacking, however PWs of 2.2-2.4" will make for highly-efficient
rainfall rates as the quasi- stationary front shifts a bit
northward by tonight as the upper shortwave trough dips across the
western Gulf Coast. A Marginal Risk spans from extreme southern
South Carolina to central Florida and westward to the central Texas
coast.

...New Mexico and Colorado...

Convection is expected to fire up ahead of an approaching cold
front thanks to PWs topping 0.75" and up to 500 J/kg of MUCAPE.
Soils across this part of the region are sensitive given recent
moisture and burn scars. The Marginal Risk area was maintained for
the localized flash flood potential for the afternoon and evening.

Campbell/Hurley

Day 2
Valid 12Z Fri Sep 06 2024 - 12Z Sat Sep 07 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL MAINLY ACROSS
SOUTHEAST LOUISIANA...

...Gulf states and Southeast...
Introduced a Moderate risk that was largely confined to southeast
Louisiana where low pressure riding along a quasi- stationary front
will continue to focus and support areas of heavy to potentially
heavy rainfall on top of widespread moderate to heavy rainfall in
the Day 1 period. A mid and upper level trough will be digging
across the western Gulf Coast region which increases low- level
frontogenesis along the central-eastern Gulf Coast to the Georgia
Coast. Heaviest short-term rainfall rates should be confined to a
corridor where precipitable water values of 2.2 to 2.5 inches,
tall/skinny CAPE profile and greatest low-level FGEN along the
quasi- stationary surface boundary align.

While there continues to be some spread on exactly where the
highest QPF will focus...two camps of thought are developing with
the broader/coarser guidance being slower to move the heaviest
rainfall off shore while the higher resolution guidance favored a
faster solution that brings more drying in from the north with
correspondingly lower QPF. Given the probabilistic nature of the
Excessive Rainfall Outlook, opted to introduce the Moderate risk
area to highlight the potential for additional heavy rainfall with
some overlap into the lower QPF shown by the higher res guidance
favored by the WPC QPF desk. Either way...there is a general
consensus for 1 to 3 inch amounts in the rain area and several
pieces of guidance that indicate isolated maxes of 4 to 8 inches,
especially along the coastline.

Bann


Day 3
Valid 12Z Sat Sep 07 2024 - 12Z Sun Sep 08 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE EASTERN GULF COAST AND SOUTHEAST ALONG WITH PORTIONS OF THE
NORTHEAST...

2030 UTC Update...
Trimmed away some areal coverage of the Marginal Risk area in the
Northeast U.S. given an overall lack of instability to help support
rainfall rates (an idea supported by pretty meager model QPF).
Otherwise, few changes needed.

Bann

Previous Excessive Rainfall Discussion...

...Gulf Coast and Southeast...

Slow moving convection will continue to focus along the draped
frontal system along the Gulf Coast and Southeast U.S. Abundant
Gulf moisture will continue to feed into the region leading to
higher rainfall rates. Multi-day rains will have increased soil
saturation and lowered flash flood guidance by this period thus
there will be a continued threat for excessive rainfall and local
flooding concerns. A Marginal Risk is in effect across northern
Florida and southern Georgia.

...Northeast...

An upper trough/low will pivot across the region during this
period sending showers and thunderstorms across the Northeast where
multiple recent rains have increased soil sensitivity. A Marginal
Risk area is in effect for eastern New York, northeast
Pennsylvania, northern New Jersey and northwest Vermont.

Campbell


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