249
FOUS30 KWBC 250903
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
503 AM EDT Wed Sep 25 2024

Day 1
Valid 12Z Wed Sep 25 2024 - 12Z Thu Sep 26 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST UNITED STATES...

Even though Helene should still be well off-shore during the
period...abundant moisture should be streaming northward ahead of
the storm. The models have been increasingly aggressive with
rainfall amounts coming off the Gulf into the Florida panhandle
with locally excessive amounts of rainfall along the coastline. Of
greater concern is the moisture that continues to stream northward
into portions of northern Georgia and upstate South Carolina where
a surface cold front acts to focus a predecessor rainfall event.
Some pieces of guidance...including the HAFS A and B parent grids
...drop 5 or 6 inches or rainfall within a broader area of 2 to 4
inches of rainfall. The overall agreement on placement of the axis
is really very good with only subtle east/west variations...which
necessitated an expansion of the moderate risk area and now
includes some of the complex terrain of the southern Appalachians.
Slight and Marginal risk areas surrounding covered the risk of
convection forming along broad synoptic scale flow getting drawn
into the environment ahead of Helene.

Bann

Day 2
Valid 12Z Thu Sep 26 2024 - 12Z Fri Sep 27 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL IN PORTIONS OF THE
SOUTHERN APPALACHIANS...

Given the increasing confidence about the overlap of the axis of
a predecessor rainfall event on Day 1 period and the amount of
rainfall expected as the center of Helene approaches the southern
Appalachians during the latter part of the Day 2 period...there has
been a growing number of models generating close to 10 or 11
inches of rainfall for the 2-days combined in the complex terrain.
That raises concerns about impactful flooding becoming widespread
with considerable and potentially life-threatening flash and urban
flooding as well as areas of significant river flooding over
portions of Florida, the Southeast U.S. and the southern
Appalachians. Widespread minor to moderate river flooding is likely
and major river flooding is possible. Landslides are possible in
areas of steep terrain in the southern Appalachians given these
rainfall amounts. This prompted the upgrade to a high risk over
portions of northwestern Georgia into far western South Carolina
that was mainly confined to the terrain. What effectively amounts
to a high-end moderate risk surrounds the high that extends along
much of the track as Helene makes landfall along the Florida
panhandle coastline around 27/00Z (give or take a couple of hours)
based on the latest NHC guidance. In terms of model guidance...the
tracks tended to be clustered well resulting in NHC guidance being
close to most model runs. For the Day 2 period...that resulted in
the highest rainfall totals being close to each other where the
orographic forcing was consistent. A second axis of excessive
rainfall branched off to the northwest from Helene`s track towards
a deep-layered low developing over the southern portion of the
Mississippi Valley as moisture gets entrained into that system
leading to scattered convection that could produce locally heavy
amounts. Mainly confined the Slight risk to the Tennessee Valley
and only a Marginal risk extending back towards the core of the
upper system.


Bann

Day 3
Valid 12Z Fri Sep 27 2024 - 12Z Sat Sep 28 2024

...INTRODUCED A MODERATE RISK OF EXCESSIVE RAINFALL OVER A PORTION
OF THE UPSLOPE REGION INTO THE SOUTHERN APPALACHIANS...

Based on latest NHC track guidance...Helene should be in northern
Georgia and tracking on a more northwestward course during the day.
This suggests the flow of deep moisture should be on-going to the
terrain of western Carolinas into southwest Virginia. Latest model
guidance shows several inches of rainfall in addition to what falls
in the predecessor 48 hours...felt a focused Moderate risk placed
where there was best model consensus was warranted. It was also a
multi-day rainfall total farther south along the NHC track in the
eastern Tennessee Valley that was the main reason to keep a Slight
Risk where model QPF be a little too light to support one. That
portion of the Slight Risk area extends westward to the deep-
layered low where steepening low level lapse rate and the
additional moisture streaming off of Helene could lead to some
showers and thunderstorms capable of producing locally heavy
downpours.


Bann


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