FOUS30 KWBC 120838
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Thu Sep 12 2024

Day 1
Valid 12Z Thu Sep 12 2024 - 12Z Fri Sep 13 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF CENTRAL AND NORTHERN ALABAMA...

...Mid-South into the Southeast...
As of 08z Tropical Storm Francine continues to move northward out 
of Louisiana and into Mississippi. The consolidated core of heavy 
rainfall to the north of Francine will lift north across MS today, 
and eventually into portions of far eastern AR and western TN by 
this afternoon. This is the corridor that will likely see the most 
widespread axis of 3-6" of rainfall. However rainfall rates along 
this axis will generally be on the decline through the day. At 12z 
we still may seeing some 1-2" per hour rainfall just north of the 
center, but by 18z we are probably below 1" per hour for the most 
part, and by 00z we may be struggling to even get 0.5" per hour. 
Thus even though this axis has the highest HREF EAS probabilities 
of 1,2 and 3", the flash flood risk is likely mitigated by the 
lower rainfall rates. In fact despite the high QPF in the HREF, the
probability of exceeding 3 or 6 hr FFG in the HREF is negligible 
over this area. Now 3-6" is still a lot of rain, and will probably 
see some urban and perhaps small stream flooding, however currently
not expecting to see more widespread higher end impacts. Thus the 
Slight risk should cover the threat.

The higher risk of flash flooding may actually end up well east of
Francine over portions of central and northern AL. Areal averaged
rainfall may be lower here than areas along the track of Francine,
but rainfall rates and upper bound rainfall potential is likely
higher. A narrow axis of enhanced convergence appears likely, with
south southwesterly flow east of Francine, and easterly flow from 
the building high over the western Atlantic. This axis may stay 
rather persistent though the period, with around 1000 j/kg of 
instability advecting northward behind Francine as well. This 
environment does appear conducive for possible training convective 
bands and several high res model solutions suggest we do see 
periodic training convective bands across this area by this 
afternoon and evening. HREF neighborhood probabilities of exceeding
5" are 50-70% and exceeding 8" are as high as 30-40%. Not 
necessarily expecting widespread coverage of these amounts, however
localized swaths of 6-10" of rain are a possibility today/tonight.
This could lead to locally considerable flash and urban flood 
impacts. For this reason we have upgraded to a small MDT risk where
the conditional threat of high end impacts appears greatest.

Slow moving/backbuilding convection also appears possible today
over portions of the FL Panhandle. Model agreement on the 
potential is not quite as strong as over AL, but certainly some 
potential for higher end impacts here as outer banding east and 
southeast of Francine could train/backbuild at times near the 
coast. Given a bit more uncertainty and higher FFG here, we will 
not upgrade at this time, but do consider this area as a higher end
Slight risk.

...Montana...
We did trim the Marginal and Slight risk areas some with this 
update. Overall not convinced we will see much in the way of true 
flash flooding today over MT. Instability is lacking over much of 
the state which will keep rainfall rates generally below flash 
flood thresholds. However with strong and persistent forcing the 
latest WPC deterministic rainfall forecast is 1-3" near and just 
south of Great Falls. If this occurs it would be an event that 
falls in the 5-25 year ARI range. So despite weak rates rainfall of
this magnitude could certainly cause some areal flood concerns. 
Over northeast MT there should be more instability and the 
potential for rainfall totals over 1"/hr. Current indications 
suggest this stronger convection would be progressive in nature 
likely limiting the flood risk...however given the high PWs and 
instability forecast could certainly see pretty impressive sub 
hourly rates which could cause some flood concerns in any more 
sensitive areas. Thus even though the Slight risk appears lower 
end...we opted to maintain it over areas that may either see a 
higher ARI longer duration rain or the potential for at least short
duration high rates.

Chenard


Day 2
Valid 12Z Fri Sep 13 2024 - 12Z Sat Sep 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TENNESSEE VALLEY INTO THE SOUTHEAST...

By Friday the remnant surface circulation of Francine will likely 
be somewhere near the AR/TN border. By this time the surface 
feature will not be a main driver of heavy rainfall. However the 
pattern will remain favorable for periodic training convective 
bands from portions of TN into AL/GA and the FL Panhandle. While 
the surface circulation of Francine will be dissipating, we will 
still have plenty of mid level energy left over. Meanwhile we will 
also have a trough of low pressure off the Southeast coast and a 
high pressure to the north over the Western Atlantic. The 
combination of these features will likely result in a corridor of 
persistent low level convergence over TN/AL/GA. We should have 
plenty of moisture and instability in place as well, and thus a 
notable flash flood risk could continue. We will carry a higher end
Slight risk across portions of TN/AL/GA with this update. 
Depending on what happens on day 1 (impacting soil and streamflow 
conditions) can not rule out needing a MDT risk over a portion of 
this area. However there is a bit more uncertainty on convective 
details by Friday compared to Thursday, and so would like to 
continue to evaluate future high res model runs, as well as see how
convection evolves today, before making any decision on an 
upgrade. Either way flash flooding appears possible, and a locally
considerable threat could evolve.

Uncertainty increases with northeastward extent. There is some 
flash flood risk over the western Carolinas, but unclear how far 
northeast the risk will get. Quite possible the better convergence
and convective threat stays further west and southwest on Friday 
as that appears to be the trend at the moment. Thus will only 
carry a Marginal risk here. There is also some risk over the 
coastal Carolinas where a low pressure may try to form offshore. 
Again at the moment the most likely scenario sees the heaviest 
rainfall stay offshore...however we could see some heavier 
convective cells along coastal areas, warranting a Marginal risk.

Chenard
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