020
FOUS30 KWBC 212022
QPFERD

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

Day 1
Valid 16Z Sat Sep 21 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...
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.

...Northern AZ into southern UT...
Added a small Marginal Risk as the upper low moves across this
region, with a continued threat of isolated convective cells
capable of producing ~0.50"/hr rates for a time. Main threat would
be over more sensitive areas near steep terrain (e.g., around Zion
NP).

...Mid-Atlantic...
Non-zero threat of isolated/urban flash flood threat for the
DC/Baltimore metro area this afternoon with developing convection.
Antecedent conditions have been fairly dry, and the guidance still
shows a bit of spread, so opted to keep out the region with the
16Z update.

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