Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
413 PM EDT Wed Sep 27 2023

Day 1
Valid 16Z Wed Sep 27 2023 - 12Z Thu Sep 28 2023

...A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS
OF THE OHIO VALLEY...

...16Z Update...

The main change this forecast is the upgrade to a Slight Risk
across central KY the borders of IN and TN for heavy rain concerns
later this evening and overnight. In coordination with the WFO at
Louisville, in agreement for a heightened signal for flash flood
concerns across much of central KY. More information on the
upgrade within the "Ohio Valley" synopsis below...

Marginal risk was expanded to include all of South FL, including
the FL Keys after coordination with WFO Key West. The premise was
to account for heavy rainfall potential being exacerbated with the
King tide cycle expected overnight when rainfall will be the
heaviest. Local rainfall amounts >5" will be possible across
portions of the middle and lower Keys with 12z HREF neighborhood
probabilities of 3"/hr rain rates pushing up into the 20-30% range
with much higher percentages within the 2"/hr probability. Couple
with expected 3 standard deviation above normal PWAT indices
across extreme southern FL, this was enough to warrant the
extension further south.

...Ohio Valley...

An upper-low is currently positioned over northern IL as indicated
on UA analysis and WV satellite. The trajectory of the low is
forecast to slide eastward with the ULL center situated over
northern IN by the end of the period. The slow motion will be a
factor in the forecast as limited progression will maintain large
scale forcing over generally the same areas within the next 24+
hrs. Forecast soundings within the confines of the ULL show steep
lapse rate reflection classic of a mature upper disturbance which
will be present downstream as the day evolves. A stationary
boundary in-of the the western portion of the Ohio Valley will
become a focal point within the convective scheme that is expected
this afternoon and evening. Hi-res deterministic is consistent on
two origin points of convection today; the first being over
eastern KY up through Ohio as area difluence downstream of the
mean trough within a core of modest instability will trigger
scattered convection across the aforementioned areas. There's some
lower grade enhanced rainfall signatures within the precip
footprint to allude to localized flood concerns, mainly shown
within the 1-2"/hr rain rate, 12z HREF neighborhood probabilities
where percentile ranges confirm at least a local threat and within
the 5% MRGL risk threshold.

The main area of interest is across western and central KY this
evening as a stronger vorticity maxima swings through the region
this evening along the base of the mean trough. The enhanced
ascent will be focused over a stationary front centered from
southern IL through KY as noted by latest WPC forecast and
forecast thermal gradient within all deterministic guidance.
1000-1500 J/kg of MLCAPE centered near and south of the stationary
boundary is plenty sufficient for instability within a corridor of
elevated PWATs between 1-1.5 standard deviations above normal. A
persistent mean steering flow will present an opportunity for
training storms within the axis of best instability creating an
environment favorable for flash flooding concerns. 12z HREF mean
QPF is now over 2" for portions of central KY with 12z
neighborhood probabilities a whopping 30-40% for at least 2"
encompassing all of central KY. This rainfall is likely to occur
within a window of 6-10 hrs overnight and not over the course of
any entire day, thus putting the area within a higher risk than
normal. As a result, have included the area with a SLGT risk and
was in agreement with the local WFO.

...Southeast U.S....

A near carbon-copy outlook of recent days given a pool of 2-2.5"
PWs to go along with a quasi-stationary front draped over Florida
and an upper trough over the Gulf of Mexico. Wednesday features a
little more instability across the Gulf side of the Florida rather
than the Atlantic for now, maintained the focus for Excessive
Rainfall from southwest Florida on north to southern Georgia and
southern Alabama. As much as 500-1,000 J/kg of MLCAPE will be
present across central and northern Florida, and with such a
tropical air-mass in place, rainfall rates could approach 3"/hr in
the strongest storms. A Marginal Risk for Excessive Rainfall
remains in place as urbanized communities and poor drainage areas
are most vulnerable to possible flash flooding.

Kleebauer/Bann



Day 2
Valid 12Z Thu Sep 28 2023 - 12Z Fri Sep 29 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
FLORIDA AND OHIO VALLEY...

...2030Z Update...

Very little deviation from the previous forecast as the overall
synoptic setup is on track and broad scale precip coverage showed
little change from the prior forecast issuance. Two main areas of
focus for D2 will be across the southern Ohio Valley and the
coastal areas of FL. Our upper trough will continue its slow
progression through the Great Lakes, northern Ohio Valley through
Thursday. Ascent will be maximized across OH/KY/TN with a primary
focus over southern KY and TN where remnant boundary from the
previous round of convection the overnight period will linger and
a backbuilding convective scheme will persist on the front end of
the MRGL outlook. An elongated vorticity maxima will continue
pivoting over the area of interest with training cells from
western KY into northwest TN by Thursday AM. Ensemble
bias-corrected QPF and 12z HREF blended mean indicates a cluster
of 1-1.25" of precip within a short time frame Thursday (12Z-18Z)
which correlates well with the HREF signal for neighborhood
probabilities exceeding 30% for at least 2"/3 hrs and a more
robust 50-60% for at least 1"/3 hrs. Considering the nature of
training precip from the previous D1 period, a MRGL was maintained
to match consensus and act as a good pivot point from the slight
on D1. The rest of the MRGL will be for scattered convection
across eastern KY up through OH where isolated flash flood
concerns will persist within a core of elevated PWATs and
continued ascent from the upper trough traversing overhead.

No change within the Florida MRGL risk as isolated 2-3"/hr rain
rates will be possible within any areal convective development as
noted by the continued 40-50% neighborhood 3" or greater
probabilities Thursday afternoon for much of the state as it
resides within an anomalous moisture environment south of a
stationary front to the north. Best chances for flooding will
occur within urban environments along the coast where runoff is
most likely and sea breeze convergence occurs during diurnal storm
patterns.

Kleebauer

...Ohio Valley...

Maintained the Marginal Risk area that was introduced on Tuesday
afternoon with only a few adjustments.  The latest runs of
numerical guidance continue to show increasing ascent ahead of an
approaching longwave trough over the northern Midwest/Ohio Valley
with some overlap with areas expected to get locally heavy
rainfall on Day 1.  Global deterministic guidance became much more
aggressive compared to previous runs in terms of QPF...with local
1-2+" totals focused within the region...and that largely
persisted into the most recent model runs. The HREF probabilities
that are available...through the first 12-hours of the Day 2
period during this outlook cycle...tends to focus higher amounts
in the southern Ohio Valley closer to the better instability and
better precipitable water values.  Made a southward
nudge/expansion as a result.

...Florida...

Locally heavy rainfall across the coastal areas of central and
south FL will pose a non-zero risk for flash flooding within a
tropical environment entrenched over the Sunshine state. Best
convergence is still being depicted over the adjacent waters, but
elevated instability across both coasts will pose a threat for
enhanced rainfall rates where thunderstorm activity is prevalent. 
The model consensus is that most activity will be off-shore...but
the model consensus was for more inland convection than shown by
earlier runs. 

Bann


Day 3
Valid 12Z Fri Sep 29 2023 - 12Z Sat Sep 30 2023

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST....THE FLORIDA PENINSULA AND PARTS OF THE
MID-ATLANTIC REGION INTO SOUTHERN NEW ENGLAND...

...2030Z Update...

Only minor modifications were made to the overall outlook(s) in
place as guidance continues to be persistent in primary QPF
placement for the evolving pattern. The MRGL in place over
portions of the Northeast U.S is the most likely to see future
deviation pending the evolution of the inverted trough/surface low
off the east coast heading through Friday. There is some
discrepancy in the handling of the surface reflection and a lot
stems from the handling of the 500mb progression as to where
guidance closes off the shortwave trough. Guidance with the
furthest west QPF field and most robust precip maxima inland are
early to close off at 500mb leading to a prolonged dynamic
structure across interior portions of PA/NY/NJ. Models that are
further east are later to close off, creating more robust precip
footprints along the coastal plain up through areas like Long
Island to southern New England. 12Z ensemble bias-corrected QPF
takes into consideration the two camps and generates a pretty
solid blend that seems reasonable within the realm of continuity
from the previous forecast issuance. However, the extended periods
of hi-res deterministic that reach out to D3 are little more
focused further inland, thus have a greater areal coverage of
higher precip values closer to the I-95 corridor up into northeast
PA and northern NJ. This was factored into a decision to expand
the western fringe of the MRGL a little more to encompass the
major metros from Philadelphia up to just south of Boston. A
general maximum is forecast over the central NJ coast which aligns
well with the CIPS analog avg QPF outline and NBM 75th/90th
percentile 24 hr precip forecast. It will be an area of focus the
next few days until we have a better consensus across the board.

The Upper Midwest will be active Friday afternoon and evening
within a core of incredibly high PWAT anomalies up near 2.5-3
standard deviations above normal centered over northeast ND and
northern MN. Lower FFG's currently in place over the northern
Midwest within the corridor of higher PWATs will present a risk
for localized flash flooding concerns in any convection,
especially along the approaching front where moisture convergence
will be highest.

No major changes were necessary for the FL Peninsula as the
overall pattern and forecast remains steadfast as the state sits
within a core of well above normal PWATs and in the area of a slow
moving frontal boundary.

Kleebauer 

...Upper Midwest...
Locally heavy rainfall will break out across parts of the Upper
Midwest on Friday as surface low pressure develops along the front
and heads northeastward.  South to southwest winds around 30 kts
develops at 850 mb...resulting in increasing moisture during the
day with precipitable water values increasing to between 1.25
inches and 1.50 inches over parts of Minnesota by late afternoon. 
That pattern will also support strong thetae advection helping to
support locally heavy rainfall rates. Given how progressive the
storms should be...will maintain a Marginal at this point,.

...Mid-Atlantic Region into Southern New England...
Low pressure developing off the Mid-Atlantic coast will be close
enough for some enhanced rainfall rates within the deformation
zone that scrapes the coastline during the day.  By Friday
afternoon or evening...the rain shield should be approaching the
southern New England coast with heavier rates not expected until
later at night.  Given that part of the area has had wet
antecedent conditions...will keep the previously issued Marginal.

...The Florida Peninsula...
A cold front attached to the area of low pressure moving northward
off the Mid-Atlantic region mentioned above will provide a focus
for additional convection capable of producing locally heavy
rainfall that results in isolated flooding...with at least a small
chance that the heavy rain occurs in areas that received a dousing
in the previous two or three days. Precipitable water values will
be ranging from 2 to 2.25 inches with persistent flow of moisture
from the south and east being drawn towards the front and a stream
of mid-level vorticity tracking from southwest to northeast
throughout the day and into the evening to help support the storms
and their rainfall rates.

Bann










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