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Message   Mike Powell    All   HVYRAIN: Excessive Rainfa   September 11, 2024
 8:34 PM *  

FOUS30 KWBC 120053
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Wed Sep 11 2024

Day 1
Valid 01Z Thu Sep 12 2024 - 12Z Thu Sep 12 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
LOUISIANA AND SOUTHERN MISSISSIPPI...

...Southeast...

Francine has come inland over southern Louisiana but continues to
show a healthy circulation approaching the greater New Orleans
areas. Though some dry air has disrupted part of the core, intense
rainfall rates of 2-3.5"/hr are shown per MRMS, with storm totals
over 7 inches in south central LA. Rainfall tapers off notably to
the southwest of the center. Will maintain the Moderate Risk area
near the path of the center overnight given the likelihood of high
rainfall rates to continue as additional moisture is drawn in off
the Gulf across into southeastern LA and southern MS. The New 
Orleans metro area will bear the brunt of the rainfall this evening
before translating northward, with the inflow band off to the east
responsible for the eastward bump in the outline. Francine will
continue inland overnight where the threat for excessive rainfall
extends northward trough much of Mississippi. Removed much of the
rest of Florida that was in a Marginal risk as lingering activity 
should be diminishing over the next couple of hours. 

...Intermountain West...

A deep layer cyclone moving across the northern Continental Divide
has brought afternoon convection and some flash flooding to parts 
of Idaho into western Montana. Precipitable water values are 
sufficient for heavy rain-related issues (0.75";), and the Slight 
Risk area remains for portions of western MT and just into west- 
central ID overnight. Hourly rainfall rates of 0.50-1.00" have been
observed and/or shown via MRMS which have been enough to reach 
generally low FFG values in areas of complex terrain. With earlier 
rainfall soaked in, additional rainfall may still exceed FFG values
overnight, but did trim off the western side of the risk area as 
the focus should be mostly over western MT tonight closer to the 
frontal boundary.

Fracasso


Day 2
Valid 12Z Thu Sep 12 2024 - 12Z Fri Sep 13 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
MID-SOUTH & SOUTHEAST...

...Mid-South/Southeast...

21Z update... The latest guidance favored a general
south/southeast trend with the QPF footprint associated with
Francine from the previous cycle. As such, this resulted in WPC
increasing amounts across eastern Tennessee, northern and central
Mississippi, and northern and central Alabama. This uptick
increased the threat for excessive rainfall and flooding concerns
for both rural and some major metropolitan areas. The Slight Risk
area was trimmed a little eastward across eastern Arkansas but also
expanded to the south and east to encompass more of central
Mississippi, central Tennessee, and west-central and eastern
Alabama. The latest guidance also suggested an increase in feeder
band convection across the Florida Panhandle thus the Marginal Risk
area was extended 1-2 counties to the south.

Campbell

Francine is expected to be fully extratropical, with a well-
developed comma head and trailing front. Guidance suggests that MU
CAPE will be modest, 250-500 J/kg. Effective bulk shear should
continue to be sufficient for convective organization where enough
instability develops/exists.  Precipitable water values remain
near 2", so hourly rain totals up to 1-2" should be possible, which
would be most problematic in urban areas and initially beneficial
elsewhere. This region has been quite dry lately. With the volume
of rain expected, and the concept of this being on the cusp of a
flash flood and longer duration flood event, kept the Slight Risk
area which showed minimal change from continuity. Across the
Southeast, there is a risk of training convection along Francine's
cold front which shows a greater signal for excessive rainfall
overnight. Broadened the risk area to encompass the possible
training rain band, as enough moisture, instability, and possible
stalling to be concerned about hourly rain totals to 3".

...Western MT, Idaho Stovepipe & Vicinity...

21Z update... Multiple impulses within the flow will trigger
showers and thunderstorms that will track across the Northern
Rockies region. PW values around 0.50-0.75 inches will be pooled
over the region throughout this period and will bolster rainfall
efficiency wit each passing impulse. Per the CAM guidance any of
these storms will be capable of producing periods of moderate to
heavy rainfall as they pass over the favored upslope areas and then
exit into the adjacent High Plains. There was a trend for
increasing QPF coverage and amounts across portions of the Hi-
Line. The Slight Risk area was expanded eastward to now include
more of north-central and northeast Montana and southwestward to
extend into portions western Montana and into Idaho.

Campbell

One or two surface lows are expected to lift northward this period
through MT, drawing in sufficient upslope flow, moisture, and
instability from the east to be concerned about the potential for
organized convection. The Slight Risk raised by continuity has been
maintained across portions of western MT. Hourly rainfall rates may
intensify up to 2 inches/hour during this period, with a few
isolated locations possibly accumulating up to 4 inches by the end
of the day.

Roth/Campbell
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