Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Wed Sep 27 2023
 
Day 1
Valid 01Z Thu Sep 28 2023 - 12Z Thu Sep 28 2023 

...A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS 
OF THE OHIO VALLEY...

...01Z Update...
Updates were made based on recent observation trends and the most 
recent hi-res guidance.  Overall, no major changes were from the 
previous outlook.  

Pereira

...16Z Update...

The main change this forecast is the upgrade to a Slight Risk 
across central KY the borders of IN and TN for heavy rain concerns 
later this evening and overnight. In coordination with the WFO at 
Louisville, in agreement for a heightened signal for flash flood 
concerns across much of central KY. More information on the 
upgrade within the "Ohio Valley" synopsis below...

Marginal risk was expanded to include all of South FL, including 
the FL Keys after coordination with WFO Key West. The premise was 
to account for heavy rainfall potential being exacerbated with the 
King tide cycle expected overnight when rainfall will be the 
heaviest. Local rainfall amounts >5" will be possible across 
portions of the middle and lower Keys with 12z HREF neighborhood 
probabilities of 3"/hr rain rates pushing up into the 20-30% range 
with much higher percentages within the 2"/hr probability. Couple 
with expected 3 standard deviation above normal PWAT indices 
across extreme southern FL, this was enough to warrant the 
extension further south.

...Ohio Valley...

An upper-low is currently positioned over northern IL as indicated 
on UA analysis and WV satellite. The trajectory of the low is 
forecast to slide eastward with the ULL center situated over 
northern IN by the end of the period. The slow motion will be a 
factor in the forecast as limited progression will maintain large 
scale forcing over generally the same areas within the next 24+ 
hrs. Forecast soundings within the confines of the ULL show steep 
lapse rate reflection classic of a mature upper disturbance which 
will be present downstream as the day evolves. A stationary 
boundary in-of the the western portion of the Ohio Valley will 
become a focal point within the convective scheme that is expected 
this afternoon and evening. Hi-res deterministic is consistent on 
two origin points of convection today; the first being over 
eastern KY up through Ohio as area difluence downstream of the 
mean trough within a core of modest instability will trigger 
scattered convection across the aforementioned areas. There's some 
lower grade enhanced rainfall signatures within the precip 
footprint to allude to localized flood concerns, mainly shown 
within the 1-2"/hr rain rate, 12z HREF neighborhood probabilities 
where percentile ranges confirm at least a local threat and within 
the 5% MRGL risk threshold. 

The main area of interest is across western and central KY this 
evening as a stronger vorticity maxima swings through the region 
this evening along the base of the mean trough. The enhanced 
ascent will be focused over a stationary front centered from 
southern IL through KY as noted by latest WPC forecast and 
forecast thermal gradient within all deterministic guidance. 
1000-1500 J/kg of MLCAPE centered near and south of the stationary 
boundary is plenty sufficient for instability within a corridor of 
elevated PWATs between 1-1.5 standard deviations above normal. A 
persistent mean steering flow will present an opportunity for 
training storms within the axis of best instability creating an 
environment favorable for flash flooding concerns. 12z HREF mean 
QPF is now over 2" for portions of central KY with 12z 
neighborhood probabilities a whopping 30-40% for at least 2" 
encompassing all of central KY. This rainfall is likely to occur 
within a window of 6-10 hrs overnight and not over the course of 
any entire day, thus putting the area within a higher risk than 
normal. As a result, have included the area with a SLGT risk and 
was in agreement with the local WFO. 
 
...Southeast U.S....

A near carbon-copy outlook of recent days given a pool of 2-2.5" 
PWs to go along with a quasi-stationary front draped over Florida 
and an upper trough over the Gulf of Mexico. Wednesday features a 
little more instability across the Gulf side of the Florida rather 
than the Atlantic for now, maintained the focus for Excessive 
Rainfall from southwest Florida on north to southern Georgia and 
southern Alabama. As much as 500-1,000 J/kg of MLCAPE will be 
present across central and northern Florida, and with such a 
tropical air-mass in place, rainfall rates could approach 3"/hr in 
the strongest storms. A Marginal Risk for Excessive Rainfall 
remains in place as urbanized communities and poor drainage areas 
are most vulnerable to possible flash flooding.

Kleebauer/Bann

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