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000
FOUS30 KWBC 242029
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 PM EDT Sat Oct 24 2020

Day 1
Valid 2018Z Sat Oct 24 2020 - 12Z Sun Oct 25 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST
FLORIDA AND FROM PORTIONS OF THE DEEP SOUTH INTO THE SOUTHERN
APPALACHIANS...


...South Florida and the Keys...
Tropical Depression #28 well to the south of the region will
continue to shed rain bands north of Cuba and into south Florida
through tonight. There is fairly good consensus among the 00Z
models with respect to the placement and degree/depth of low-mid
layer moisture transport into the Keys and South Florida, with the
axis of stronger theta-e advection/transport extending a little
farther northward along the east coast of the peninsula. Per the
guidance, the highest areal-average rainfall totals (2+ inches)
are more likely across the Upper Keys and the far southern tip of
the FL peninsula; however, given the favorable thermodynamic
profiles (MUCAPEs averaging 1000-1500 j/kg, PWs increasing to
2-2.25"), the high-res CAMs show pockets of 3-6+ inch totals
across the outlook area. Largely because of the high degree of
spread with these maximum totals, and considering the latest HREF
probabilities of 2+ inch/hr rates (less than 30%) and of 2+ inches
in 3 hours (less than 50%), for now have opted to keep with the
Marginal or isolated flash flood risk.

...Portions of the Deep South into the Southern Appalachians...
The northward pooling of subtropical moisture ahead of a compact
southern stream trough (bolstered by a vigorous MCV) and
associated slow-moving frontal system should continue the heavy
rainfall threat tonight from the FL Panhandle-GA northeastward
into the southern Appalachians and adjacent Piedmont region.  The
QG dynamics along the southern Appalachians and Piedmont region of
western NC and Upstate SC will be bolstered by right-entrance
region forcing of a 70-90 kt upper level jet streak. PW values of
1.75-2.00" near the Gulf Coast and around 1.5" along the western
Carolinas will be around 2 standard deviations above normal for
late October. Deep-layer instability will be modest at best --
mainly over the southern half of the outlook area (500-1000 j/kg),
with values closer to 500 j/kg or less over the higher terrain of
the western Carolinas. The slow moving MCV and surface front will
enhance the excessive rainfall potential, especially over GA where
the latest HREF probabilities of 2+ inches of rain in 3 hours are
above 50%.  Areal-average totals are expected to range between
1.5-3.0 inches over this area, while between 1-2" farther north
across the mountains in western NC. While the short term rainfall
rates (1-3 hourly) will be somewhat constrained by the lack of
instability, recent heavy rainfall (200-400% of normal over the
past couple of weeks) will nevertheless likely lead to at least
isolated runoff issues across the southern Appalachians from far
northern GA into western NC.

Roth/Oravec/Hurley


Day 2
Valid 12Z Sun Oct 25 2020 - 12Z Mon Oct 26 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF
SOUTHEAST FLORIDA...

The models indicate potential for isolated excessive rainfall over
parts of far southern Florida as moist onshore flow
persists with high values of precipitable water, peaking near 2.25
inches during Sunday and Sunday night.
Modest coastal convergence maxima due to speed convergence should
aid in shower formation.
Locally heavy showers are possible given the high available
moisture. Most guidance continues forecasting 1-2 inches, with
heavier amounts outliers.

NHC Guidance takes the center of the tropical depression northwest
into central Gulf of Mexico and develops the system in a tropical
storm, which results in the core of heavier rainfall likely
remaining south of Fl/Keys.

Consequently, we will continue with the Marginal Risk for the time
being and keep an eye on any trends which may develop.

Petersen/Bann


Day 3
Valid 12Z Mon Oct 26 2020 - 12Z Tue Oct 27 2020

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

...Southern Plains...
There will be the potential for some localized heavy rainfall over
parts of the southern Plains Monday and Monday night.  The region
will be located between a broad positively tilted trough over the
western United States and a mid-level ridge positioned over
northern Florida.  There should be increasing coverage of showers
and thunderstorms as a surface cold front approaches from the west
during the day but gradually slows its forward speed.  The
combination of the surface front and upper level support suggest
that there will be good coverage of showers and some thunderstorms
from parts of the Red River Valley of Texas across southeast
Oklahoma into southern Missouri and northwestern Arkansas.  The
24/12Z suite of numerical guidance shows maximum rainfall amounts
in the 1.5 to 2.5 inch range.

The limiting factors with regard to flooding include the lack of
any substantial instability to help drive the rainfall rates in a
region which has largely experienced 25 percent of normal rainfall
during the previous 14 days.  Precipitable water values peak close
to 1.5 inches according to the GFS/12z ECMWF. A Marginal Risk is
indicated given the slow moving frontal boundary and persistent
convergence near the front.

Petersen/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


$$