Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Sun Aug 18 2019
 
Day 1
Valid 01Z Mon Aug 19 2019 - 12Z Mon Aug 19 2019 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF 
GEORGIA AND FLORIDA...

...Northern Florida and Southern Georgia...
The main focus for flash flooding over the overnight hours is 
expected to be near and along the inflow associated with the 
surface wave over southern GA. 

The activity over northern and western FL has diminished 
temporarily as earlier activity has overturned the airmass in 
place. As the surface wave over the western FL Panhandle tracks 
into southern GA late this evening, in the low level inflow ahead 
of it continues to pump deep moisture across the Big Bend area 
into southwest and south central GA. Convection is expected to 
form in the instability axes over western FL into southern GA in 
areas that have not been affected by earlier storms. 

The most recent RAP/HRRR showed MUCAPE values recovering to 
between 1000/2000 J/KG between 19/01z and 19/05z in this area, 
which should be sufficient to support increasing convection. Low 
level convergence in conjunction with the surface wave allow the 
storms to form into southwest/northeast lines from east of KTLH 
into southern GA (an idea supported by the most recent HRRR). Weak 
mid level flow around the mid level low is expected to result in 
short term training and cell mergers. 

At this point, it is not clear just how extensive the convection 
will be. The deep moisture associated with the strengthening 
inflow could allow for very efficient rainfall makes (with warm 
cloud depths in excess of 4000 meters), and hourly rainfall rates 
between 2.00/3.00 inches are possible. The HRRR has had a multi 
run signal for local 6.00+ inch rainfall amounts somewhere over 
west of KTLH into southernmost GA, though the placement of the 
maximum rainfall amounts has changed from run to run. Given the 
potential for a flare up of the deep convection in the low level 
inflow (mainly between 19/04z and 19/10z), the Slight Risk area 
was left mainly intact from the previous forecast. 

...Southern Plains, Mid MS Valley into the Ohio Valley and the 
Northeast...
Several impulses in the fast mid level flow interacting with 
instability and deep moisture will feed convection that continues 
to support a low lend flash flood threat through the late evening 
hours (for the most part).

The first area is across PA/NY into NJ, where short term training 
along bands of convection will continue to pose a low end flash 
flood threat until the instability in place is exhausted. That 
process has already begun. and is expected to persist though about 
18/04z. The next impulse is moving through IN/lower MI/OH, also 
located in an axis fo 1500/2000 
J/KG of SBCAPE (per 00z soundings). The impulse is in the southern 
edge of the westerly mid level flow, and it is allowing the 
convection to remain progressive as it moves east. As the 
instability in place becomes elevated (or exhausted), the 
convection is expected to weaken as it moves across OH headed for 
western PA. Three hour flash flood guidance values are as low as 
1.50 inches over portions of Ohio, so the Marginal Risk was left 
in place through the end of the activity (which is expected to be 
before 19/06z).

The final area is across the Southern Plains into the Lower and 
Mid MS Valley. convection over northeast AR/far western is 
expected to follow the instability axis present across AR into far 
northern LA and western OK. there is a fair amount of spread 
concerning how this occurs, with much of the high resolution 
guidance sending the convection south with the propagation 
vectors. The most recent HRRR, however, is not quite as bullish 
bringing the activity into portion of OK and western AR, keeping 
it closer to the MS River before it runs out of instability and 
weakens. Since there is some spread, the Marginal Risk was 
modified to account for current activity, while left in place over 
OK to account for the potential shown by the 12z WRF ARW/NMMB. 
 
Hayes
 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt