361
FOUS30 KWBC 220057
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Sat Sep 21 2024

Day 1
Valid 01Z Sun Sep 22 2024 - 12Z Sun Sep 22 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN HIGH PLAINS...

...New Mexico into Texas/Oklahoma Panhandles...
...0100 UTC Update...
Few changes made to the Slight and Marginal Risk areas based on the
latest observational and mesoanalysis trends, along with the recent
HRRR and HREF guidance.

The large- scale flow across the southern Great Basin will
continue to draw moisture northward at the low levels while
becoming increasingly diffluent aloft in response to the approach
of an upper low moving across the Four Corners this morning.
Precipitable water values are forecast to exceed 1.5 inches across
parts of West Texas by late afternoon while the right entrance
region of an upper level jet rounds the base of an upper trough and
tracks over the region. This combination should help support
multi-cell storms capable of producing isolated rainfall rates of
an inch or more and storm total rainfall amounts in excess of 2
inches over portions of northeastern New Mexico into the Texas
Panhandle during the afternoon or evening hours with an associated
risk of flash flooding. 12Z CAMs have honed in on northeastern NM
into the OK/TX Panhandles while decreasing any heavier rainfall
threat north of the KS border, and have thus trimmed the northern
part of the MRGL and SLGT risk areas a bit.

...Mid-Atlantic...
Non-zero threat of isolated/urban flash flood threat for northern
and central PA, along with central-southern VA this evening with
clusters of convection moving slowly to the south. Convective
trends should be on the downswing between 01-03Z given the negative
MUCAPE trends. Nevertheless, MLCAPEs around 1000+ J/Kg early, owing
partially to mid level lapse rates ~6.5 C/KM, along with transient
upper shortwave energy/left exit region upper jet forcing will
maintain a non-zero flash flood threat through midnight. Given the
latest guidance, progression of the convection, and mesoanalysis
trends, opted to not hoist a Marginal Risk for what will be a
short term, non-zero (widely localized) flash flood risk.

Hurley/Fracasso/Bann

Day 2
Valid 12Z Sun Sep 22 2024 - 12Z Mon Sep 23 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID MISSISSIPPI VALLEY TO THE SOUTHERN HIGH PLAINS...

20Z update...Nudged the Slight Risk area over MO/IL southward per
recent 12Z guidance (and typical north bias in convective axes).
Otherwise, CAM guidance supports maintaining the SLGT over TX and
the larger MRGL outline encompassing both areas, within the broad
SW flow aloft and near/ahead of the frontal boundary.

Fracasso

The focus for areas of heavy rainfall will continue to be along a
plume of deeper moisture...precipitable water values generally at
or above 1,75 inches...from the western portion of Texas
northeastward to where it begins to encounter a cold front moving
in from the northern United States. An upper level shortwave trough
over the central and southern Rockies on Sunday morning will eject
eastward during the day...aiding the formation of storms over parts
of western Texas capable of heavy downpours around the time of
maximum heating. Farther to the east...model guidance tends to
generate at least some convection capable of producing isolated 2
to 3 inch rainfall amounts over portions of Missouri as moisture
flux convergence occurs ahead of a cold front pushing southward.
Mesoscale guidance...available through the first 12 hours of the
Day 2 period for the overnight EROs...does show low-end potential
for 2 inch per hour rates embedded within a broader area where
probabilities of 1 inch per hour rates are on the order of 10 to 25
pct. There was already a Slight Risk introduced here and only
needed a bit of realignment to fit the latest guidance.

Bann

Day 3
Valid 12Z Mon Sep 23 2024 - 12Z Tue Sep 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A PORTION OF
THE MISSISSIPPI VALLEY EASTWARD INTO THE OHIO VALLEY...

20Z update...Maintained the MRGL outline for the MS to Ohio Valley
region, with a shift a bit southwestward per trends in the guidance
favoring a bit slower progression. ML first guess fields still show
just a MRGL for now, with more clarity (hopefully) with some CAM
guidance in the next update or two.

Fracasso

As the large scale upper trough continues to move eastward on
Monday...the plume of available moisture gets elongated and the
moisture flux along the front becomes less well defined. There will
be some broader upper support along the front which will have taken
on a more east-west orientation...so there is at least some
potential for problems from run off with 1 to 2 inches or rainfall
over portions of Illinois into Indiana. But with convection
expected to become increasingly elevated and with flash flood
guidance tending to be 2.5 inches per hour or greater...will
maintain a Marginal Risk area for now and evaluate the need for an
upgrade if rates/amounts pick up in later model cycles.

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