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Day 2 Outlook >
WPC Day 1 Excessive Rainfall Outlook Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 0054 UTC Thu Sep 28, 2023
Valid: 01 UTC Sep 28, 2023 - 12 UTC Sep 28, 2023
Forecast Discussion
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Wed Sep 27 2023
Day 1
Valid 01Z Thu Sep 28 2023 - 12Z Thu Sep 28 2023
...A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS
OF THE OHIO VALLEY...
...01Z Update...
Updates were made based on recent observation trends and the most
recent hi-res guidance. Overall, no major changes were from the
previous outlook.
Pereira
...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 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
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