Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 PM EDT Tue Sep 27 2022
 
Day 2
Valid 12Z Wed Sep 28 2022 - 12Z Thu Sep 29 2022 

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

...2030Z Update...

...Florida...

In collaboration with all of the Florida NWS offices, a high risk 
area has been issued for portions of central Florida with this 
update. The high risk includes the Tampa, Fort Myers, and Orlando 
areas. This area is in association with the landfall and slow 
track of major hurricane Ian. As the hurricane approaches the Gulf 
Coast of Florida, expect the forward speed to slow down 
significantly. As a result, expect a prolonged period of very 
heavy rain, likely exceeding 3 inches per hour at times, across 
much of Florida, especially in the high risk area. Expect at least 
a foot of rain over this period from Tampa south through at least 
Venice. The high risk extends southward along the coast to around 
Naples. These southern communities can expect somewhat less rain 
as the Ian's eye moves overhead, but the combination with storm 
surge and several inches of rain will locally cause flooding 
concerns. The latest HiRes model output strongly favors the area 
ahead of and north/west of the eye to be the area most likely to 
see excessive rainfall totals. As such, the high risk area's 
southern boundary is close to the latest NHC track.

Some uncertainty still persists regarding the exact track and 
speed of the hurricane. These factors will have a big influence on 
how much rain any one area sees over this multi-day event. 
Fortunately, the trend has been for a nominally faster forward 
speed in latest forecast updates, but since an overall very slow 
forward motion remains in the forecast, expect localized totals 
over 2 feet to be likely somewhere in the high risk area by the 
time all is said and done. Any training thunderstorms could 
compact the time frame it takes to get to this rainfall total into 
a much shorter time, which would enhance flooding impacts.

Outside of the HIGH risk area, uncertainty increases over 
northeastern Florida as to how far north and west the northern 
extent of the precipitation will reach. This uncertainty is 
largely the result of continued disagreement on the exact track of 
Ian as it weakens over the Florida Peninsula, along with its 
interaction with a front currently parked over the northern part 
of the state. This is likely to result in a very sharp gradient in 
precipitation. The MRGL, SLGT, and northwestern edge of the MDT 
risk areas over FL and GA are likely to be adjusted with future 
updates to the track of Ian.
South of the HIGH risk area, expect multiple feeder bands to 
criss-cross the state from southwest to northeast over southern 
FL. The exact location of these feeder bands is highly uncertain, 
but should they set up over the urbanized I-95 corridor, the 
combination of the wet antecedent conditions from today and likely 
training convection will enhance the flash flood risk. Thus, in 
coordination with the MLB/Melbourne and MFL/Miami offices, 
expanded the MDT south along the coast, and expanded the SLGT 
through the southern tip of the FL Peninsula.

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.

...AZ & UT...

A very small expansion of the preexisting MRGL risk was done to 
the north into Salt Lake City and the southwestern corner of WY. 
There was some indication in the 12Z guidance that there would be 
a bit more precipitation in the mountains that could pose an 
isolated flash flood risk. No other changes were made to the area, 
as meteorologically all remains nearly the same.

Wegman

...Previous Discussion...

...Ian...
Maintained the Moderate Risk of excessive rainfall over the 
Florida peninsula as Ian approaches the west coast and begins to 
slow its forwards speed during the latter portion of the period.  
With the potential for greater rainfall amounts resulting from the 
slower forward speed...expanded the southern side of the Moderate 
and Slight Risk areas and began to shrink the area of Slight and 
Marginal risk areas in the southern peninsula.  Saw little reason 
to make more than tweaks on the east coast where persistent 
frictional convergence should lead to enhanced rainfall totals 
during the period. 

...Southwest U.S...

Maintained a Marginal Risk area extending from the international 
border region in Arizona northward into the southern Wasatch range 
in Utah given model signal for moisture return and some 
interaction between the moisture and a shortwave trough ejecting 
out ahead of a broader synoptic scale trough. Spaghetti plots of 
the SREF 1-inch rainfall amounts in a 24-hour period develop over 
the higher terrain...focused primarily in northern Arizona and 
southern Utah.  The axis of highest precipitable water values 
should be along this corridor by late afternoon...with amounts 
slightly greater than 1.5 inches confined 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 
for this time of year.  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 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt