FOUS30 KWBC 040814
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
414 AM EDT Sat Oct 4 2025

Day 1  Valid 12Z Sat Oct 04 2025 - 12Z Sun Oct 05 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE EASTERN GREAT BASIN ALONG WITH THE CENTRAL GULF COAST AND
COASTAL GEORGIA DOWN THROUGH THE FLORIDA EAST COAST...

...Eastern Great Basin...

An amplifying upper-level trough advancing through the Great Basin
will be driving multiple waves of low pressure northeastward along
a frontal zone draped across the Intermountain Region. The latest 
forecast guidance supports a strengthening mid to upper-level 
closed low circulation pivoting across northern UT early this 
morning before ejecting through western WY this afternoon. Strong
dynamics/DPVA will ride up across northern UT along with far
southeast ID and western WY which will be interacting with the
front and the pooling of modest instability (MLCAPE ~500 J/kg)
along it for an axis of locally heavy showers and thunderstorms.
Much of this threat will be this morning (notably in the 12Z to 18Z
time frame) as the height falls pivot across the region. The 00Z
HREF guidance suggests some organized convection that will be
capable of producing 1 to 2+ inches of rain which may include some
brief cell-training concerns for areas near and west of the 
Wasatch front including around the Great Salt Lake. Can't rule out
there being at least some isolated runoff/flash flooding concerns
from these stronger clusters of convection that evolve this
morning. Therefore, a Marginal Risk of excessive rainfall has been
introduced, but this again is mainly for early in the period today.

...Central Gulf Coast...

A low to mid-level vort center/disturbance over the northern Gulf
of America is forecast to slowly advance westward today which will
help to bring stronger low-level southeast flow and moisture into
southeast LA and coastal MS. Sufficient low-level forcing associated
with the vort energy coupled with at least modest instability with
MLCAPE values of 1000 to 1500 J/kg will support there being at
least scattered bands of convection that will develop and advance
inland across the central Gulf Coast. Some of the 00Z HREF guidance
and recent RRFS solutions suggest some linearly oriented convective
bands potentially setting up that would support some locally
enhanced rainfall totals. Given the improving PW environment with
values of 1.75 to 2+ inches, and a convergent southeast low-level 
jet of 30+ kts, some of the rainfall rates may reach or exceed 2
inches/hour. The potential banded nature of the activity may
support some localized rainfall totals going into early Sunday
morning of 2 to 4+ inches. These rains will pose at least an 
isolated threat for flash flooding, and especially if any of these 
heavier rains were to make into the New Orleans metropolitan area. 
As such, a Marginal Risk of excessive rainfall has been introduced 
for this period for the central Gulf Coast.

...Coastal Georgia and the Florida East Coast...

Low-level easterly flow will persist today and tonight across the 
FL East Coast given the placement of a well-established area of 
surface high pressure farther north over the Mid-Atlantic states. 
However, the surface ridging is forecast by the models to shift
offshore by later today which will allow for some veering and 
strengthening of the Atlantic fetch a bit farther north up into 
coastal areas of eastern GA. The latest hires model guidance 
suggests a stronger axis of low-level moisture convergence along 
with pooling of instability into coastal areas of eastern GA and 
near the SC border. Some locally focused bands of relatively
shallow/warm-topped convection may attempt impact the coastal 
areas here, with some guidance suggesting locally a couple inches 
of rain being possible. Farther south, the FL East Coast should 
continue to see scattered to broken areas of showers and 
potentially a few thunderstorms. However, the activity should 
remain disorganized given a lack of deeper layer forcing and only 
modest low-level convergence along the coast. The Slight Risk area 
that was inherited for the FL Space Coast vicinity has been removed
in favor of a Marginal Risk which will include the north/south 
length of the FL East Coast, but extending northward now to the 
SC/GA border. Any runoff concerns from heavy rainfall will tend to 
be highly isolated and primarily focused over the more urbanized areas.

Orrison


Day 2  Valid 12Z Sun Oct 05 2025 - 12Z Mon Oct 06 2025

...THERE IS MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF 
THE CENTRAL GULF COAST, THE SOUTHEAST COAST, AND THE CENTRAL PLAINS...

...Central Gulf Coast... 

A weak disturbance in the western Gulf will likely continue to focus 
enhanced convergence and convection along portions of the central 
Gulf Coast into Sunday. Heaviest rainfall totals may tend to stay 
just offshore where the better instability will reside...although 
coastal convergence and instability into the immediate coastline 
should allow for some heavier cells to make it onshore. Even our 
deterministic QPF, which is a multi model blend, brings two day 
rainfall totals as high as 3-5" over far southeast LA and right 
around 3" along coastal MS and AL. Given the convective nature of 
rainfall and PWs over 2" would expect totals to locally exceed these 
amounts, and isolated instances of flash flooding are possible. The 
00z REFS probabilities of 8" is over 90% across coastal MS/AL. This 
is seemingly an under dispersed ensemble in this case, but 
interesting nonetheless, and possibly indicative of a lower 
probability worst case scenario. Either way, will need to continue 
to monitor trends, as can not rule out the need for a Slight risk 
along portions of the immediate coast.

...Southeast Coast... 

The model QPF signal continues to trend down across the Southeast 
coast, although onshore easterly flow, coastal convergence and PWs 
around 2" will support localized instances of heavy rainfall. 
Overall the flash flood risk appears pretty low, but an isolated 
threat can not be ruled out.

...Central Plains...

Convection will likely expand in coverage Sunday evening across 
portions of northeast KS, southeast NE and southwest IA. A stalling 
frontal boundary will allow for persistent moisture convergence in 
the lower levels coupled with a divergence signature aloft. Thus it 
seems probable that rainfall will persist through much of the night, 
not moving all that much over that time. Whether we see any flash 
flood risk evolve will likely come down to the degree of instability 
present. It seems likely that we will initially have enough 
instability for deeper convection and 1"+ an hour rainfall. However 
most indications are that this instability axis will be rather 
narrow and under 1000 j/kg. Thus would expect that instability to 
get eroded fairly quickly, resulting in decreasing rainfall rates 
overnight. Nonetheless, our latest deterministic QPF has 1-2" of 
rain, with 2-3" if you include the rain that continues into Monday. 
When accounting for convection resulting in locally heavier amounts, 
and the possibility that instability ends up greater than forecast 
(which a couple models show) then a low end flash flood risk can not 
be ruled out.

Chenard


Day 3  Valid 12Z Mon Oct 06 2025 - 12Z Tue Oct 07 2025

...THERE IS MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN 
FLORIDA AND THE CENTRAL PLAINS...

...Eastern FL...

The 00z ECMWF, AIFS and GEFS mean all suggest an increase in 
convective coverage over eastern FL on Monday compared to Sunday. 
The approach of a subtle mid level wave may help increase low level 
easterly moisture transport while also providing a bit more in the 
way of upper divergence. Isolated urban flash flooding should thus 
remain a threat into Monday, although would anticipate the risk to 
stay quite localized. There has been a clear model trend away from 
bringing heavier rainfall into SC/GA and thus the northern extent of 
the Marginal risk was cut back.

...Central Plains...

Some convection will likely be ongoing Monday morning across 
portions of northeast KS, southeast NE, southwest IA and northwest 
MO. The stalled front will remain in the area through the day likely 
helping focus additional convection that should only slowly drift 
southward. Instability should increase Monday along this front, 
however this will coincide with a decrease in moisture transport and
convergence along the front. Thus not really anticipating anything 
more than a very localized flash flood risk. The threat is likely a 
lower end Marginal risk, but given the slow front and two day 
rainfall totals of 2", to perhaps as high as 3-4", across this 
corridor, localized flooding can not be ruled out. We will maintain 
the inherited Marginal risk, but shrink to to better coincide with 
the axis of highest two day rainfall totals.

Chenard
$$
--- SBBSecho 3.28-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)