Area Forecast Discussion
Issued by NWS Wilmington, OH-- Highlight Changed Discussion --
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FXUS61 KILN 271751
Area Forecast Discussion
National Weather Service Wilmington OH
151 PM EDT Tue Sep 27 2016
A large area of low pressure will gradually move south from the
Great Lakes over the next day or so, eventually remaining in place
over the Ohio Valley through the end of the week. After a dry and
breezy day today, cooler and rainier conditions are expected from
Wednesday through Friday.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
The Ohio Valley remains situated between a cold front which
passed east of the region Monday morning and a large stacked area
of low pressure across the northern Great Lakes. In between these
two features, extremely dry air has filtered into the area, as
evidenced by the 0.31 inches of PWAT sampled on this morning`s 12z
ILN sounding. As such, expect mostly clear skies today except for
perhaps a few high based cumulus during peak diurnal heating.
Still do expect that breezy conditions will develop early this
afternoon with a tightening pressure gradient, especially for
northwestern parts of the ILN FA. Expect that fairly efficient
mixing should allow for gusts between 25-30 kts in far NW part of
CWA during the middle of the afternoon. Gradient will be weakest
in far SE, so expect that winds may only gust to 15 kts across
far southern OH this afternoon.
Boundary layer RH values are probably not quite low enough today
to run the risk of fire weather concerns, but with the gusty winds
in place, it is something that should at least be kept in mind.
Thankfully, much of the CWA saw at least some light rain during
the past 24 hours, with the heaviest rain occurring in the
northwest -- where winds today are going to be the strongest.
With dry air in place and abundant sunshine, surface temperatures
are rebounding nicely from this morning`s start in the 40s. Highs
are still on track for lower to mid 70s across FA.
.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY/...
As model agreement strengthens in the overall scenario for
Wednesday through Friday, the forecast will continue to be
adjusted as has been done tonight -- slightly cooler temperatures,
slightly greater cloud cover, and slightly higher chances of
precipitation. For the most part, it still looks well too early to
try to time out the vorticity maxima rotating around the huge
upper low, especially as model agreement in the exact position of
the system is still only marginal at best.
With that said, attention will focus to the first such area of
upper support, which will begin to develop directly over the
forecast area early on Wednesday. This will occur as the overall
envelope of the low is just getting into the area from the north.
NAM/GFS projections show a jet streak rotating around the southern
periphery of the low, with its left exit region tracking directly
through the ILN CWA between 12Z-18Z. Combined with a weak surface
boundary (and a boundary layer wind shift to the west) there is
enough evidence to allow for an increase to likely PoPs in the
eastern half of the forecast area for Wednesday (late morning
through mid-afternoon). Owing more to the cooling temperatures
aloft than anything else, some marginal surface-based instability
is expected to develop, and a few low-topped thunderstorms will
likely result from this -- especially if any breaks in the clouds
end up allowing for some slight surface heating.
Beyond Wednesday, it is much less certain where precipitation
chances will be favored -- or even which quadrant of the upper
low the ILN CWA will fall under. PoPs will be kept in the 20-40
percent range all the way into Saturday, favoring the diurnal peak
periods slightly. Though a little more questionable than
Wednesday, a slight chance of thunder was included for Thursday as
Max temps will be held in the middle 60s through the period, owing
to considerable uncertainty in potential insolation, and the
expectation of occasional rain. This forecast is about 4-8 degrees
below normal for max temps. The air mass would support values
near 70 if the upswing of the diurnal cycle could coincide with
dry and partly sunny conditions, but confidence in a window like
that occurring is fairly low.
.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
The GFS has continued to trend more in line with the ECMWF,
lingering the upper level low across the region through at least
the end of the week. As a result, have continued to nudge up pops
each day through Saturday and tweaked down temperatures a bit.
Beyond that, both the GFS and ECMWF are trying to lift the low
northeastward and out of the area, although the ECMWF is still
slower in that process. At this point, will keep Sunday and Monday
dry but again, if things continue to trend toward the slower
ECMWF, we may need to introduce some low end pops into early next
week and lower temperatures a bit more toward the end of the long
.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
-- Changed Discussion --Mostly clear skies will continue through the afternoon across the
area, with just a few VFR Cu for northern sites.
A large and pronounced stacked low pressure system will continue
to drift south from the Great Lakes area into the Ohio Valley
through the TAF period. For this afternoon, a tightening of the
pressure gradient will combine with efficient mixing to allow
winds to gust to 25-30kts for northern sites of KDAY, KCMH, and
KLCK, with gusts to 20-25kts possible elsewhere. These
west/southwesterly gusts will be diurnally-driven, with a fast
drop off expected towards 23z as temperatures begin to cool with
the approach of sunset.
Between 00z-12z Wednesday, moisture will gradually be on the
increase as the low moves south from the Great Lakes. Past 12z,
dynamic lift will increase in association with upper- level energy
rotating around the low pressure system. As such, expect showers
will develop and move west to east from mid- morning through early
afternoon. Expect a quasi- linear cluster of showers to move east
through terminal sites toward 18z, with best chance of showers for
eastern sites of KCMH and KLCK. Additional (re)development is
possible during the afternoon and early evening hours behind
(farther west of) initial activity.
While VFR conditions are anticipated through the majority of the TAF
period, some brief MVFR CIGs/VSBYs are possible in heaviest shower
activity Wednesday. Slight model differences on location and
timing of activity precludes anything other than a VCSH at this
While instby will be on the increase after 18z, expect potential
isolated TS development to be fairly limited.
OUTLOOK...Local MVFR ceilings and visibilities possible with
showers on Wednesday. An isolated thunderstorm is also possible. Widespread
MVFR ceilings with local MVFR visibilities possible with showers
Thursday through Saturday.
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