FOUS30 KWBC 150820
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
420 AM EDT Sat Mar 15 2025

Day 1
Valid 12Z Sat Mar 15 2025 - 12Z Sun Mar 16 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-SOUTH INTO THE TENNESSEE VALLEY...

...Ohio and Tennessee Valleys...

The upper-level pattern remains amplified today across the central
U.S. as one upper-low lifts northward into Canada while a second 
upstream shortwave/upper jet dives southeast into the southern 
Plains this morning and spreads eastward over the Lower Mississippi
Valley/Southeast into the afternoon/evening. A surface wave 
developing along the leading system's trailing cold front will help
focus deep Gulf moisture tapped by a very strong 850 mb jet while 
supported aloft by increasing upper-level divergence ahead of the 
approach of the upper jet. This will help lead to the continued 
development and maintenance of both open warm sector convective 
development along the leading edge of the moist warm air 
advection/850mb jet as well as along the slowing eastward moving 
cold front. The updated 00z CAM guidance supports the most likely 
corridor of overlap between the warm sector development as well as 
potential repeated/training rounds of storms along and just ahead 
of the cold front across portions of the Tennessee Valley from 
northeastern Mississippi east-northeastward into northern Alabama, 
much of Tennessee into southern Kentucky, and northwestern Georgia 
into far southwestern North Carolina. This region coincides with
HREF PMM QPF values of 3-6", with locally higher amounts (as well 
as HREF 40-km neighborhood probabilities of 5" exceedance of 
20-60%). Strong convection with an anticipated storm mode 
supporting discrete supercells will allow for locally extreme 
rainfall rates of 2"+/hr at times, leading to flash flooding over 
more sensitive terrain without a consideration of training. But as 
much of this discrete convection will occur out ahead of the main 
line of storms, some training is also anticipated (which could lead
to the localized totals of 6"+). 

The inherited Moderate risk area was expanded with this update,
mostly to the north and west to include more of Tennessee, and a
bit more of northern MS and edging into south-central Kentucky.
This generally encompasses the area where the HREF exceedance
probabilities suggest a heightened risk of 3-5"+ totals (40-km
neighborhood 3" exceedance probabilities of 50-100%, and 5" values
of 20-60%). The axis of highest probabilities exists from far
northwestern Mississippi into Middle Tennessee and far south-
central Kentucky (40-60% corridor for 5" exceedance), where this
could be considered a higher-end Moderate risk (as 6-hr FFGs are
2.5-3.0", and the vast majority of this rainfall will occur over
that short a period). As is typically the case with Moderate risk
events, there is an elevated risk of localized significant flash 
flooding, particularly if extreme training/totals develop near
sensitive terrain and/or urban areas. 

...Pacific Northwest...

A Marginal Risk remains in place for portions of the Pacific
Northwest (mostly coastal/western Oregon and a bit of far
northwestern CA) where guidance continues to indicate the 
potential for 2-5" localized rainfall totals along the coast (with 
the highest amounts indicated in the OR/CA coastal border area). 
This is due to an the arrival of an AR this evening with forecast 
IVT of 400-600 kg/ms indicated perpendicular (or nearly 
perpendicular) to the coast (which will continue well into Day 2). 

Churchill/Putnam


Day 2
Valid 12Z Sun Mar 16 2025 - 12Z Mon Mar 17 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE EAST COAST, FROM THE SOUTHEAST TO NEW ENGLAND, AND FOR PORTIONS
OF THE PACIFIC NORTHWEST...

...East Coast...

As the aforementioned low lifts northward into Canada, what's left
is a weakening cold front as the upper-level pattern becomes a bit
less amplified (shortwave over the Southeast lifting a bit and
cutting off from northern/polar stream). The result still looks to
be scattered localized totals of 1-2" over a large region from the
Southeast (central FL, southeast GA, and Carolinas) into the Mid- 
Atlantic and portions of the Northeast/New England (though some
2-3" totals are possible in the coastal Carolinas region). The
Marginal risk was maintained as these relatively limited totals
should only present a localized flash flood risk as the front 
begins a faster progression towards the Atlantic Coast.

...Pacific Northwest...

The aforementioned AR looks to bring in stronger moisture
transport into Day 2 period with the potential for IVT in the 
600-800 kg/ms range near the OR/CA border region, peaking for a 
period early in the day on Sunday. Model consensus suggests an 
additional 2-4" of QPF through Day 2, and a targeted upgrade to 
Slight risk remains possible (depending on the evolution of the
forecast QPF in the Day 1 period).

Churchill/Putnam


Day 3
Valid 12Z Mon Mar 17 2025 - 12Z Tue Mar 18 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent. 

Churchill
$$
--- SBBSecho 3.20-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)