FOUS30 KWBC 200821
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
421 AM EDT Fri Sep 20 2024

Day 1
Valid 12Z Fri Sep 20 2024 - 12Z Sat Sep 21 2024

......THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF 
THE SOUTHERN UNITED STATES......

...Southern California/Western Arizona Deserts...

An upper low moving down the coast early today will move inland over 
the Desert Southwest later today.  Increasing coverage of showers 
and thunderstorms are expected as moisture will accompany the low 
into the usually dry areas of southeast California, southern Nevada, 
and northwestern Arizona..with greatest coverage tied to the 
steepest low- to mid-level lapse rates at the center of the upper 
low where upper level temperatures will be coldest. That is also 
where steering flow becomes weak...leading to slow moving storms. 
With any more persistent showers and storms, expect isolated flash 
flooding with any burn scars, dry washes, and other flood-prone 
areas.PWATs between 0.5 and 0.75 inches in these areas will peak at 
about 1.5 sigma above normal for this time of year.

...New Mexico...

Broad upper level diffluence ahead of the positively tilted low
over the Desert Southwest will cause a somewhat narrow 50-100 mile
wide plume of training showers and possibly embedded thunderstorms
to develop over eastern New Mexico and into portions of the far
western Texas and Oklahoma Panhandles. Given the sensitivities of
some of these areas to prolonged rainfall, the Marginal risk
remains in place, especially for any burn scars or flood prone
areas.

...South Florida...

Today looks to be the final day of heavy rainfall potential is 
expected in South Florida...especially in the urban corridor from 
West Palm Beach south through Homestead. Highly efficient rainfall 
producing thunderstorms should be slow-moving on the leading edge of 
an advancing cold front.  The front will clear South Florida by 
early Saturday turning the predominant flow more northeasterly. This 
will cut down on the precipitable water values some, while also 
greatly limiting, though not eliminating the coverage of storms by 
Saturday. Thus, the Marginal flash flooding threat should end once 
the convection simmers down late this afternoon or this evening. 

Bann


Day 2
Valid 12Z Sat Sep 21 2024 - 12Z Sun Sep 22 2024

......THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE CENTRAL ROCKIES AND CENTRAL PLAINS......

A broad upper trough with evolution of a closed low over the West 
will translate eastward on Saturday with an evolving setup capable
of showers and thunderstorms across the Rockies through the 
Central Plains, especially once the trough axis picks up forward 
speed. A combination of favorable ascent and increased moisture 
advection thanks to a developing LLJ pattern east of the Front 
Range will lead to thunderstorms capable of producing several 
heavy cores with locally enhanced rainfall. The 20/00Z HREF 
neighborhood probabilities...available only during the first half 
of the day 2 outlook period on the overnight shift...focuses brief 
periods of 30 percent or greater probabilities of exceeding 1 inch 
per hour developing out over the plains of eastern New Mexico that 
ease into West Texas by early evening...with the threat continuing
into Saturday night in response to the development of a low level
jet. Farther north...the signals for locally heavy rainfall also 
appear across portions of the Plains states as thunderstorms form 
along and ahead of a cold front tied to an amplifying upper trough 
building across the northern US. As mentioned in a previous
discussion...antecedent conditions and the fact that much of the 
area is rural will help mitigate some of the flash flooding concern
due to amount of infiltration possible.

Bann


Day 3
Valid 12Z Sun Sep 22 2024 - 12Z Mon Sep 23 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHERN
GREAT LAKES SOUTHWESTWARD ACROSS PORTIONS OF THE CENTRAL PLAINS...

An upper trough initially over the central and southern Rockies on
Sunday morning will move eastward on Sunday into early Monday
morning...with showers and thunderstorms developing ahead of the
front in an airmass characterized by precipitable water values
generally in the 1.6 to 1.8 inch range. Given the moisture in place
and enough mid- and upper-level shear to support some convective
organization...the possibility exists for excessive rainfall due to
some intense downpours. Tended to favor the cluster of models
guidance led by the ECMWF/GFS/CMC with respect to the potential for
heaviest amounts somewhere from southeast Kansas into
Missouri...but did not go as far east as suggested by the
aggressive UKMET. Suspect there is a non-zero threat of excessive
rainfall farther to the southwest where storms may also be able to
organize but for now favored the better moisture and the region of
better upper difluence. 

Bann
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