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

Day 3
Valid 12Z Sat Sep 14 2024 - 12Z Sun Sep 15 2024

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

...Tennessee Valley into the Southeast...
A similar setup to day 2 continues into the day 3 saturday time
period. The environmental setup remains favorable for a narrow
axis of significant rainfall. Broadly convergent flow will exist
across the region...with easterly flow off the Atlantic and south
to southwesterly flow east of the remnants of Francine. Within the
broader flow there will probably be a narrower axis of stronger
low level convergence. And with the upper flow broadly divergent
and plenty of moisture and sufficient instability forecast...the
stage remains set for localized heavy rainfall. The trickiest part
of this forecast will likely be pinning down the axis of heaviest
rain, as it will probably be quite narrow in nature. At the moment
portions of central TN, eastern AL and central/northern GA appear
most favored. Like day 1 and 2, there is high impact potential on
Saturday. Locally considerable flash and urban flooding is
certainly possible in this setup. Hopefully the axis will shift
some each day and not repeat...but there is some chance similar
areas could see training convection on successive days...which
would only exacerbate the flood potential. Given the inherent
uncertainty in a day 3 forecast and the expected narrow nature of
heaviest rainfall, can not really go any higher than a higher end
Slight risk at this time. However will need to closely monitor
model trends over time.

...Southwest...
We will maintain a small Marginal risk over southern AZ. Moisture
looks to stream northward out of the Eastern Pacific ahead of a
tropical disturbance. This increasing moisture and some weak
forcing may result in convective development and a localized flash
flood risk. The 00z model consensus is for a slightly different
tropical evolution over the eastern pacific, which would result in
less of a convective threat over AZ on day 3. However the
predictability of this pattern appears low, and run to run model
consistency has been poor. Thus while the 00z guidance verbatim
would support removing the Marginal risk, the preference was to
maintain it for now and continue to monitor trends.


Chenard


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt