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182
FXUS61 KILN 291345
AFDILN

Area Forecast Discussion
National Weather Service Wilmington OH
945 AM EDT Thu Sep 29 2016

.SYNOPSIS...
A large area of low pressure will remain in place over the Ohio
Valley through the rest of the week, providing cool and occasionally
rainy conditions. Chances for rain will begin to diminish on
Saturday, as the low gradually begins to move to the northeast.
Drier and warmer conditions are expected for Sunday and Monday, with
surface high pressure building in from the west.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
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950 AM Update >
Observational platforms /radar and satellite loops/ place center
of upper low near Louisville...drifting SSE. A very broad/weak
surface low is just south of CVG and also dropping SSE. Low
clouds/fog are very slowly breaking up/lifting this morning with
vis imagery showing a far breaks in cntl OH.

Going fcst logic /read below/ largely on track. Isold/sct showers
are currently pestering the Scioto Valley in plume of deeper SSELY
flow on the ern flank of the anomalous closed low. These will
increase in number and intensity through the peak of the diurnal
cycle...and with the upper low settling SSE with time...curvature
of flow will tend to steer these into the Scioto Valley/Central
Ohio and areas of SW OH. Least chances of rain today will be west
of I-75 in wcntl OH. Localized heavier amounts seem targeted for
the lower Scioto Valley where an 1"+ could fall from several
bands/arcs of showers and storms.

Severe threat is quite marginal and discussed well below.
Northeast boundary layer flow /weak/ underneath southeast mid
level flow /moderate/ promotes enough effective shear for modest
storm organization as instby peaks mid-late afternoon. Hail the
most likely threat given cold mid level temps...but wind is not
out of the question if some localized heating can push temps
toward the upper 60s in the Scioto Valley. 29.12Z RAP/HRRR 0-1km
shear is weak the bulk of the day...briefly strengthening >20kts
over Scioto Valley in the 22Z-23Z timeframe...so a rotating storm
or two can`t be discounted early this evening before instby wanes.

513 AM Update >
Based on current observations, and expectations for the next hour
or two, the dense fog advisory has been shifted to the west. As
the wind flow has shifted slightly, areas along the eastern edge
of the original advisory have turned more to a low stratus deck,
with visibilities improving significantly. However, with KDAY/KVES
recently coming in with 1/4SM observations, a few counties also
had to be added.

355 AM Discussion >
An impressively lengthy curved band of precipitation currently
exists along a cyclonic path from near Knoxville to west of
Indianapolis. As of this writing, the entire band is wrapping
around the ILN forecast area, with little to no precipitation
actually occurring within the ILN CWA.

Of foremost importance is the issue of fog and stratus, which has
developed in earnest over the past several hours. Ground
conditions are a precarious situation, with ceiling height
differences of just a couple hundred feet making the difference
between only modest instability reductions and full-on dense fog.
The current dense fog advisory was issued where the majority of
the 1/4SM / OVC001 observations have come in -- roughly between
Wilmington and Wapakoneta, with a county or so on either side.
This has, as of now, largely spared the DAY/CMH metro areas --
which are covered solidly by stratus decks a few hundred extra
feet off the ground. While there is still a chance that dense fog
could develop outside of the current advisory, the current
placement does appear to be where conditions are most favorable
-- the calmest of winds near the center of the weak surface low,
and a dearth of mid-level moisture.

Going into the afternoon, attention will turn toward convective
potential. The center of the mid-level low will gradually move
south today, shifting from central Indiana to south-central
Kentucky by evening. The current band of precipitation is
representative of upper-level forcing and positive vorticity
advection along the inner edge of the 500mb-300mb jet max, and
this feature will gradually shift westward over the course of the
day, allowing precipitation to spread back into the ILN forecast
area from the east. This looks like one of those strange days in
which the cloud-bearing wind / storm motion vector may well be
southeasterly. In addition to the upper forcing, convergence
associated with low-level features will be on the increase as
well, though primarily southeast of the ILN CWA. Combined with an
increase in instability with a little bit of surface heating --
especially across the expected genesis region of West Virginia /
eastern Ohio / southwest Pennsylvania -- some marginal instability
is expected to develop. Steep lapse rates will be present thanks
to the cold air aloft, but although some speed shear will be
present, it is tough to say if the directional shear will be
favorable for any storm organization -- flow backs from northeast
to southeast with height, which is a fairly unusual orientation.
Nonetheless, the mesoscale setup is somewhat favorable for some
organization. Yesterday, any chance at boundary layer
destabilization was done in by the giant blob of rain that
developed very quickly and moved eastward across the area. This is
a little less likely to occur in the storm genesis region today,
but how much strength holds together as the convection moves
toward central Ohio is questionable. As such, the SPC Day 1
outlook only allows for a small extension of the marginal risk
area into the ILN CWA, which appears reasonable. Chances for
convection do exist across the entire forecast area during the
peak of the diurnal cycle, but the greatest chances for both rain
and thunder will be in the northeastern third of the CWA. With
some potential for slow motions and training, and ground
conditions that are now more saturated than before, heavy rain is
a concern that will have to be monitored.
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&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/...
Overnight, a swath of showers (associated with the upper forcing
and whatever is leftover from afternoon convection) is forecast to
move westward through the ILN CWA. If nothing else, this forecast
for rain makes the potential for fog tomorrow morning a little
more uncertain, even if conditions would otherwise be favorable.
Patchy fog wording will be included in the forecast, along with
50-70 percent chances of precipitation.

As the mid-level low starts to swing northward again on Friday,
there is reasonable model agreement in a relatively dry wedge
entering the south-central section of the ILN CWA during the
afternoon, with little in the way of forcing. PoPs on Friday
afternoon and evening will range from 20-50 percent, with the
highest values in the northern and northeastern sections of the
forecast area. With systems like this, and still plenty of room
for error in timing out the vorticity maxima rotating around, this
PoP forecast should still not be seen as an explicit forecast of
specific timing and placement.

Precipitation chances will begin a more significant decrease on
Saturday, as the low starts a quicker motion out of the Ohio
Valley and back into the Great Lakes from whence it originated.
Early on Saturday, the western half of the ILN CWA will be most
likely to receive additional precipitation, but a gradual drying
from south to north is expected by afternoon.

The relatively small diurnal temperature ranges will begin to
expand a little by Friday, and especially by Saturday, as slightly
drier overall conditions allow for a little more heating in the
afternoons. In fact, the forecasted highs on Saturday (upper 60s
to lower 70s) are only a little bit off from normals for early
October.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
This period will be characterized by a much warmer drier/period
vs the short term. On Sunday morning...the closed low will be
drifting northeast...weakening...and opening up into a more
progressive shortwave trough over New England by Monday.
Thus...there is still a small threat of showers into Sunday but
right now the timing/placement of this threat is very tough but
seems focused north of I-70 closer to mid level cold pool. Large
scale ridging ahead of the next longwave trough over the west will
allow for dry weather Monday thru Wed as high pressure at the
surface moves across the eastern U.S. Temperatures will be warming
steadily and by the middle of next week should be well into 70s if
not the lower 80s with dewpoints hanging tough in the 50s.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...
Large scale upper level low centered over south central Indiana
this morning will drift south toward south central Kentucky
through the day. The low will stall tonight, then it will slowly
move back to the north and northwest on Friday.

Daytime heating will allow ceilings and visibilities to improve,
such that western terminals will likely see predominate VFR
conditions (BKN-OVC cumuliform clouds) by 19Z. The exception will
be the eastern terminals (KCMH, KLCK) where energy rotating around
the upper level low will bring showers and perhaps a few embedded
thunderstorms. This should keep conditions at least in the MVFR
category after 19Z. How far west and southwest this precipitation
gets is still in question at this time, but have brought some
light showers to KILN/KDAY with a VCSH at KCVG/KLUK.

For tonight, operational and high resolution models indicate that
low level convergence will increase which should increase the
threat for showers as the night wears on. In addition, ceilings
will likely lower back down below a 1000 feet with visibilities
fluctuating between 1 and 4 miles in showers and mist. Threat for
any embedded thunder should diminish after 04Z.

OUTLOOK...IFR/LIFR ceilings and IFR visibilities possible Friday
morning. MVFR ceilings and visibilities possible with showers
Friday afternoon through Saturday.

&&

.ILN WATCHES/WARNINGS/ADVISORIES...
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OH...None.
KY...None.
IN...None.
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&&

$$
SYNOPSIS...Hatzos
NEAR TERM...Binau/Hatzos
SHORT TERM...Hatzos
LONG TERM...Binau
AVIATION...Binau/Hickman