Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Fri Nov 27 2020
 
Day 3
Valid 12Z Sun Nov 29 2020 - 12Z Mon Nov 30 2020 

The probability of rainfall exceeding flash flood guidance is less 
than 5 percent.

...Central to Eastern Gulf Coast...
At the start of the forecast period (Sun-29.12z), the mid to 
upper-level low will continue to advance eastward supporting 
continued surface low development along the coastal front.  Deep 
moisture and instability will maintain ongoing convection 
along/just ahead of the surface low across Southeast LA into 
Coastal AL/Western FL panhandle.  As the shortwave/jet structure 
pivots, low level warm conveyor belt will become more vertically 
unidirectional to support individual elements with the ongoing 
convection to momentarily train with slow eastward progression of 
the pre-frontal trof.  Modest diffluence aloft and veering 
low-level flow may also support further back-building along the 
line, focused with stronger frictional surface convergence at the 
coast.   Global guidance trends continue with steady increase in 
rainfall efficiency and totals across the area with 2-4"totals 
forecast, particularly focused across the western FL Panhandle. 
Recently dry conditions of 5 to 50% of normal (per AHPS 2-week 
anomalies), suggest high FFG values in the area are reasonable; so 
these totals would be nearing this range, but not quite there, 
especially given storm-scale nature to the potential flash 
flooding.  As such will keep the area below categorization at this 
time, but with increasing confidence/consistency in the guidance, 
a Marginal Risk may be required. 


Lower Tennessee/Central Ohio Valley
By 29.18z, the southern stream shortwave/closed low will begin to 
shear  but swing negative-tilt toward the northeast under 
influence of digging northern-stream trough across the Central 
Plains.  The southern stream jet will press eastward, exposing the 
area of concern to increased divergence aloft.  Surface and 
low-level cyclogenesis will see a strengthening of the warm 
conveyor belt/TROWAL bend back against the flow and sharpen the 
low to mid-level deformation zone across western TN, providing 
strong moisture flux convergence and isentropic ascent for 
increasing rainfall efficiency.  Some embedded weak instability 
(100-250 J/kg) may allow for some isolated vertical growth 
increasing the rainfall rates too, but quite unlikely to exceed 
short-term rates needed for flash flooding.  However, the duration 
of this moderate rain suggest some pockets of 1-2" totals possible 
per the guidance suite over a 6-12 hour period.  This area has 
been very dry recently with AHPS precipitation anomalies well 
below normal, however, ground conditions are dormant or 
per-dormant to suggest some increased run-off could be a problem 
in traditionally prone streams/areas.  All considered, this still 
appears to be just below threshold for increase to a Marginal Risk 
category at this time, with agreement from local forecast offices. 
 

Gallina
 
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt