Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Thu Jun 22 2023
 
Day 2
Valid 12Z Fri Jun 23 2023 - 12Z Sat Jun 24 2023 

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF 
THE HIGH PLAINS AND THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA...

...Plains...

Impressive synoptic forcing will move into the northern Plains 
Friday as a strengthening shortwave intensifies into a closed low 
by Friday night. The flow aloft should be highly divergent with 
dual left exit and right entrance upper jet support between 2 
separate jet streaks. In the lower levels a developing low 
pressure and associated frontal structure will provide plenty of 
lower level convergence to go along with the mid and upper level 
ascent. PWs are not off the charts anomalous, but still plenty 
above average for late June. Thus many ingredients are coming 
together to support a heavy rainfall event over portions of 
Montana, Wyoming and the Dakotas. The magnitude of the flash flood 
threat is likely going to come down to the placement of the low 
level front and higher instability, and how progressive things end 
up being. This does look like a pretty quick moving system, which 
should put an upper bound on total rainfall potential. 

Besides this area of the country having been abnormally wet over 
the past 2 weeks, an LLJ bringing Gulf moisture northward into the 
Dakotas, and perhaps a bit towards the west into MT, and some 
instability being advected into the region, it will be interesting 
how all of this interacts with the favorable upper level 
environment. The guidance is hinting at portions of south central 
MT, near Billings, where leeside troughing may locally amplify the 
forcing and allow for more persistent heavy rains in the broad 
southwesterly flow. The Dakotas will have the benefit of more 
moisture, but being somewhat displaced from the best upper level 
divergence, may see lesser amounts of rain, though any storms that 
form over that area still have the potential to produce 2 
inch/hour rainfall rates.

For the southern Plains, the typical afternoon showers and 
thunderstorms will redevelop again Friday afternoon, but since 
they will be a bit more isolated and less organized as the upper 
level flow diminishes, the flash flooding risk is expected to be 
more isolated as compared with previous days. 

...Eastern U.S...

A low-end Slight risk area was introduced for all of the Florida 
Panhandle and into southern Georgia with this morning's forecast 
update. The stalled out front that has acted as a corridor of 
forcing for storms the last several days will remain stuck in 
place, as a drier air mass over central MS/AL/northern GA tracks 
east on the north side of the front. Thus, the front will continue 
to have a significant enough difference between the air masses on 
either side of the front to continue to provide a source of 
lifting. With those differences maximized, the winds on the 
moisture-laden/stormy side of the front across the FL Panhandle 
should allow the storms that form to be moving a bit more quickly 
off to the ENE. This should result in less overall rainfall over 
this area, despite the tendency for storms to backbuild. 
Nevertheless, given the incredibly wet conditions already present 
over this area, and the potential for another 1-3 inches of rain, 
with those higher values falling in a short time remaining very 
capable of resulting in additional flash flooding, think the 
potential is high enough for a Slight Risk area. 

Further north across the Mid-Atlantic, the upper level low will 
begin to open up into a trough, but will become negatively tilted. 
This will increase the overall southerly wind flow over the area, 
allowing for faster storm movement, but the added lift from the 
divergence associated with the trough will support storms capable 
of producing heavy rainfall to 2 inches per hour. Given the still 
overall dry conditions over much of the Mid-Atlantic from a few 
months of abnormally dry conditions, think any instances of flash 
flooding will be localized to urbanized and poor drainage areas.  

Wegman
 
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt