214
FOUS30 KWBC 042013
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
413 PM EDT Sat Oct 4 2025

Day 1
Valid 16Z 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...

16Z Update: Upon examination of current radar trends in northern
Utah, it appears the mesoscale convective vortex associated with
the mid-level upper low is still producing widespread moderate to
heavy rainfall from the greater Salt Lake City area to near the
Idaho border. Movement of the vortex is slowly towards the NNE, and
recent CAM guidance suggests this will persist through about 18-20Z
before lifting out of the region and weakening. Therefore, the
existing Marginal Risk area will remain valid for now.

Previous discussion...

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
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.

The existing Marginal Risk of excessive rainfall has been
maintained for this period for the central Gulf Coast, but trimmed
back to southeastern Louisiana south of Lake Pontchartrain where
the highest rainfall coverage is likely to reside.

...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. Any runoff concerns
from heavy rainfall will tend to be highly isolated and primarily
focused over the more urbanized areas. No changes were necessary to
the ongoing Marginal Risk area for this region.

Hamrick/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...

20Z Update: There has been an additional lighter trend in the
guidance for the coastal portions of Georgia, and the majority of
heavier showers in onshore flow is expected to be across eastern
Florida, so the new Marginal Risk area is trimmed back to
Jacksonville. For the Midwest, the Marginal Risk is extended a
little more to the northeast to include northeastern Iowa where a
relatively narrow band of higher QPF ahead of a slow moving cold
front is likely to be, with the potential for some convective
training that may result in a few instances of flooding where cells
persist the longest. All seems on track with the Marginal Risk area
for the central Gulf Coast. /Hamrick

Previous discussion...

...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 Florida...

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.

...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 is possible. The existing Marginal
Risk has been maintained with this update.

...Ohio Valley...

A Marginal Risk area has been introduced from eastern Arkansas to
southern Indiana. Moist return flow around a surface high off the
East Coast, in tandem with a frontal boundary slowly approaching
from the northwest, will result in a corridor of warm air advection
with scattered to numerous showers and thunderstorms developing.
Dewpoints rising into the upper 60s to lower 70s, in combination
with increasing mixed layer CAPE, will provide enough instability
for higher rainfall rates that may result in some instances of
flooding, mainly in the 18Z Monday-6Z Tuesday time period.

Hamrick/Chenard


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt