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FOUS30 KWBC 060819
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 AM EDT Mon Oct 6 2025

Day 1
Valid 12Z Mon Oct 06 2025 - 12Z Tue Oct 07 2025

...THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID-SOUTH AND LOWER OHIO VALLEY...

...Mid-South/Lower Ohio Valley and the Central Gulf Coast...

An elongated low to mid-level vort center near the central Gulf
Coast will lift northward into the Mid-South today and gradually
stretch northeastward into portions of the Lower OH Valley by later tonight
through early Tuesday morning. Increasingly convergent flow around
the eastern flank of the vort energy and an associated weak surface
low/trough axis coupled with enhancing moisture transport aided by
a 30 to 40 kt low-level jet will support broken bands of heavy
showers and thunderstorms. Modest instability with MLCAPE values of
500 to 1000 J/kg will be in place with the aid of diurnal heating,
but the vertical column will be quite moist with CIRA-ALPW data
showing the aforementioned vort energy embedded within a rather
deep tropical environment characterized by PWs of 2 to 2.25 inches.

Relatively warm-topped convection with high WBZ levels will support
very efficient rainfall processes for high rainfall rates as this
energy lifts north over the next 12 to 24 hours. The latest 00Z
HREF guidance supports broken convection with rainfall rates as
high as 1.5 to 2.5 inches/hour, and some concerns for some cell-
training that may foster some localized totals of 3 to 5 inches.
The more concentrated areas of convection are most likely to focus
across areas of eastern AR/western TN northeastward up into western
KY and southern IN going through Monday night and Tuesday morning
where the axis of stronger low-level convergence in conjunction
with some modest upper-level jet forcing/divergence is forecast to
become aligned. A Slight Risk of excessive rainfall will be
maintained for this period, with some southward expansion of it
depicted down across eastern AR compared to continuity given the
latest multi-model hires consensus. The setup will be conducive
for there being isolated to scattered areas of flash flooding which
will include urbanized flooding concerns.

Meanwhile, very moist and convergent low-level southerly flow off
the Gulf coupled with a moderately unstable boundary layer will
favor a threat for additional small-scale bands of convection to
impact parts of eastern LA along with coastal areas of southern
MS, southwest AL and into the western FL Panhandle. PWs running
close to or above 2 inches coupled with the instability will yield
high rainfall rates, with some localized additional rainfall totals
for this period of 2 to 4+ inches possible. This is supported by
the 00Z HREF guidance, and the setup will support isolated concerns
for runoff problems/flash flooding. As a result, a Marginal Risk
of excessive rainfall has been expanding to encompass these areas.

...Eastern FL...

Moist easterly low-level flow will continue to support scattered
areas of convection over eastern FL today through tonight. The
activity again should tend to be rather disorganized, but there
will be some potential for small-scale east/west oriented bands to
set up that may result in some isolated 2 to 4+ inch rainfall
totals. This may drive at least some isolated runoff concerns. As a
result, the Marginal Risk of excessive rainfall is maintained for
this period.

Orrison


Day 2
Valid 12Z Tue Oct 07 2025 - 12Z Wed Oct 08 2025

...THERE IS SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE OHIO VALLEY...

...Ohio Valley into the Central Appalachians...

A Slight risk was maintained from portions of northern TN into much
of KY, southern IN and OH, and western WV where isolated to
scattered flash flooding is likely. Increasing southerly flow ahead
of an approaching cold front will advect moisture northward while
also enhancing lower level convergence. Also of note is a sfc-850mb
low that is over AR today and was over the northern Gulf on Sunday.
This feature will get more elongated in nature by Tuesday, but
should still act to locally increase moisture transport and
convergence over KY and surrounding areas. PWs are forecast around
2", which is well over the 90th percentile for this time of year.
Instability is expected to only be modest, generally in the 500-1000
j/kg range, however the strong synoptic forcing provided by the
approaching front and good upper level divergence should compensate
for this and still allow for organized convection.

Both the 00z HREF and REFS show moderate to high probabilities of
hourly rainfall exceeding 1", but only low probabilities of
exceeding 2". The limited instability is likely responsible for
the lack of 2"+ per hour rainfall rates. However the deep saturated
layer and low topped convection supports highly efficient warm rain
collision-coalascence processes, leading to the likelihood of at
least 1" per hour rainfall.

The main question for flash flood potential will be the duration of
these higher rates, as it seems unlikely that 1hr FFG will be
exceeded. While the front will be progressive, it does look like we
will have enough convective coverage to support some training, and
at least a few hours of heavy rainfall rates. Thus while 1hr FFG may
be difficult to overcome, it does look like there will be an
opportunity for some exceedance of the 3hr FFG...with both the 00z
HREF and REFS showing exceedance probabilities upwards of 25-50%.
The latest WPC QPF indicates event total rainfall of 2-4" over most
of the Slight risk area, which is supported by high HREF and REFS
neighborhood probabilities of exceeding 3". Also will note that
there are even moderate probabilities of locally exceeding 5" with
this event.

There is a bit of model spread on the most favored axis for heaviest
rainfall and flash flooding. Both the HREF and REFS supports a
slightly farther southwest axis across KY and northern TN, with the
global guidance generally more over eastern KY into WV. The Slight
risk was made big enough to support a compromise of these two camps,
and will need to continue to monitor trends going forward. Generally
think the highest risk is over KY, but can no rule out that shifting
into northern TN or portions of WV.

While the rainfall does look a bit more convective now over the
Northeast, 1" per hour probabilities are still low...with most
guidance peaking rainfall rates in the 0.5"-0.75" range. Given the
ongoing drought conditions it still seems unlikely that these rates
will be high enough to overcome the dry soil conditions. Thus even
with rainfall totals approaching 1.5" we will continue with no risk
area depicted.

...Southeast FL...

Expecting another day of scattered convection within the easterly low
level flow Tuesday. Compared to previous days the threat should
mostly be focused over southeast FL by this time, and a localized
urban flash flood risk is possible.

...NM...

The convective threat looks a bit better than it did this time last
night with models trending a bit more robust with development.
Easterly low level post frontal flow will help locally enhance
convergence and upslope and scattered convection is likely to
develop. Instability is forecast to be modest, but may get towards
1000 j/kg, which would be enough for heavy rainfall rates given the
anomalous PWs in place. The high res guidance is starting to show a
threat of 1"+ per hour rainfall, which would be enough for isolated
flash flooding.

Chenard


Day 3
Valid 12Z Wed Oct 08 2025 - 12Z Thu Oct 09 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NEW MEXICO...

Looking at a continuation of the localized heavy rainfall threat over
NM into Wednesday. The moisture plume and instability axis will
expand westward which should expand the isolated flash flood risk in
that direction as well. The environmental ingredients look pretty
similar to Tuesday, as do model QPF forecasts. So would generally
expect similar convective coverage and intensity with isolated flash
flooding again a concern.

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