Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
436 AM EDT Wed Sep 28 2022
 
Day 1
Valid 12Z Wed Sep 28 2022 - 12Z Thu Sep 29 2022 

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS THE CENTRAL 
FLORIDA PENINSULA...

...Florida...
Maintained the high risk area over much of the central Florida 
peninsula as widespread life-threatening and catastrophic flooding 
spread inland during the day in association with Hurricane Ian.  
The corridor of heaviest rain still looks to be across the central 
portion of the peninsula given a combination of a broader shield 
of rainfall developing ahead of the hurricane itself and a period 
where the forward speed begins to slow down.  There continued to 
be some adjustments to Ian forecast track which required some 
adjustments to the High and Moderate Risk areas across the 
peninsula...although the shifts have been increasingly small.  As 
a result, a prolonged period of very heavy rain with rainfall 
rates likely exceeding 3 inches per hour at times is 
expected...especially in the high risk area...today that results 
in at least a foot of rain. Based on the southward shift in the 
NHC track at 28/03Z, the southern boundary of the High Risk area 
was adusted southward. 

Outside of the HIGH risk area, The Moderate and Slight Risk areas 
continued to cover some of the uncertainty as to how far north and 
west the rainfall spread either as a result of the track or how 
much interaction there is with a southwesterly upper level jet at 
250 mb and 200 mb extending from Georgia northward into the 
Carolinas.  

IMPACTS: It is important to note that these high to extreme 
rainfall totals have the potential to result in catastrophic 
flooding. Preparations in this area for the arrival of Ian should 
be rushed to completion. Those in this area are urged to heed all 
local evacuation orders. Also note that freshwater/inland flooding 
can also be life-threatening, even well away from any storm surge 
impacts near the coast.

...Southwest U.S...
There is a Marginal Risk of excessive rainfall over portions of 
the southwest U.S. given a model signal for moisture return and 
interaction between the moisture and a shortwave trough ejecting 
out ahead of a broader synoptic scale trough. The axis of highest 
precipitable water values should be along this corridor by late 
afternoon...with precipitable water values of slightly less than 
1.5 inches near the international border tapering off to 0.80 
inches or so in southwest Utah (where the anomalies shown by the 
GEFS were in excessive of 3 standard deviations and within the 
95th to 99th percentiles of climatology). Forecast soundings 
showed enough dry air in the sub-cloud layer to hold the risk 
category as a Marginal...for the time being.

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