Area Forecast Discussion
Issued by NWS Wilmington, OH

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000
FXUS61 KILN 041408
AFDILN

Area Forecast Discussion
National Weather Service Wilmington OH
908 AM EST Sun Dec 4 2016

.SYNOPSIS...
A weak upper level disturbance will bring light rain to the area
this afternoon and evening. Drier conditions are expected on Monday,
as high pressure moves into the area. On Tuesday, an area of surface
low pressure is expected to move into the Ohio Valley, bringing rain
to the area. Behind a cold front on Wednesday night, much colder
conditions are expected through the end of the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A fairly dry air mass is currently in place over the middle Ohio
Valley. Surface dewpoints are generally running in the lower 30s in
the ILN CWA, and the 12Z KILN sounding recorded a precipitable
water value of 0.50 inches, with a significant amount of dry air
between 850mb and 500mb. This helps to set up the scenario for the
morning, as a large but not-particularly-potent area of lift moves
east into the region. A shortwave trough currently moving through
the plains states will cross the Mississippi River this afternoon,
with only some very unimpressive moisture advection / isentropic
ascent ahead of it. High-res models are stingy with their dBZs
this morning, and the dry air gives reason for disbelieving the
potential for most of the echoes over Kentucky to reach the
ground. Most of Kentucky is covered by clouds in the 8kft-10kft
range, with no precipitation occurring. Only over the southern and
western portions of the state is rain occurring, where clouds are
under 7kft. Ultimately, there is little reason to believe that
precipitation will occur in the ILN CWA early in the day, outside
of a slight chance of a quick sprinkle or flurry sneaking through
the dry layer (and a 20 PoP will be maintained to account for this
possibility).

By afternoon, the overall forcing with this system will be focused
in two locations. Right entrance dynamics will help force
precipitation over the southeastern states, which may extend into
eastern Kentucky, but will largely miss the ILN forecast area.
More focused forcing from the 500mb trough will pass through the
northern Ohio Valley and southern Great Lakes, and this will be
the primary thing to watch. All indications are for a high-PoP but
low-accumulation event, with weak ascent and a relatively quick
motion resulting in total averaged QPF of only 0.10-0.15 inches.
The highest PoPs (near 100 north of I-70, categorical for most of
the CWA except the far south) will time out from 20Z-02Z from west
to east.

The only remaining item to discuss is the impact of temperatures
on the precipitation type forecast. The diurnal curve will be
small today, with highs ranging from around 40 (NW) to 45 (SE), an
increase of only around ten degrees from this morning. There is a
surface trough that will switch winds to the west and provide cold
advection, but it lags the upper trough (and thus the
precipitation) by several hours. So, any chance for a mix with snow
will largely depend on diurnal or dynamic cooling, and this is not
a particularly favorable scenario for either this evening.
Discounting a 00Z NAM solution which appeared to dynamically cool
the boundary layer much too quickly, it only appears that
temperatures briefly get cold enough for some snow to mix into the
northern / northwestern ILN counties during the back half of the
precipitation. A rain/snow mix has been included this evening
north of a line from Richmond to Delaware. If temperatures cool a
slightly more than anticipated, some trace or slushy tenth-or-two
accumulations might be possible in Mercer/Auglaize/Hardin tier.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/...
Between 03Z-09Z tonight, a weak surface trough is expected to pass
east through the region, with a slightly cooler and drier
boundary-layer air mass coming in on the high pressure behind it.
However, the air mass aloft will be notable warm and dry, creating
an unbreakable inversion over the Ohio Valley on Monday. Thus,
trapped low-level moisture will be unlikely to break, leading to
BKN/OVC stratocumulus remaining in place through the day.

A shortwave trough and surface low currently over northern Mexico
(as of Sunday morning) will begin their trek northeast on Monday,
picking up a decent feed of moisture off the Gulf of Mexico as
they approach the Ohio Valley on Tuesday morning. The overall
forecast thinking has changed little for the ILN CWA with regards
to this system, which still looks much too warm to support a
threat for snow. In fact, max temps were raised slightly with this
forecast update, primarily in the southeastern CWA, where
temperatures may get into the lower 50s. Despite this, there still
appears to be no chance for convective activity, due to the strong
inversion mentioned above -- which will remain in place as it
moistens on Tuesday. Non-diurnal temperature curves were used
throughout the short term of the forecast.

Still three days out, it is not surprising that there are a few
model differences in exact timing and potency of this system.
However, it would take a significant change to the surface low
track to produce a change in the forecast thinking. Rainfall
amounts appear beneficial rather than hazardous -- possibly up to
an inch in some locations, with saturated soundings from the
surface to about 15kft, and precipitable water values over just
over an inch as well.

Behind this system, on Wednesday morning, a cold front is forecast
to move southeast through the region. The cold advection behind
this front is potent, and will signal the start of a period of
below normal conditions. Most of Wednesday appears likely to be
dry, but there is a rather large spread in possibilities regarding
some upper jet forcing that may or may not allow for some snow to
develop at some point in the Wednesday / Wednesday night time
frame. The going forecast will be left generally unchanged for
now.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
A broad trough will develop over the central United States by mid
week and then shift quickly east through through the end of the work
week. Short wave energy rounding the base of the trough will lead to
an increasing chance of precipitation Wednesday night as an
associated strong cold front pushes east across the area. This will
usher in a much colder airmass through the end of the week. The 12Z
GFS has trended faster with the cold push than the ECMWF and this
will ultimately affect temperature trends Wednesday afternoon into
Thursday and how fast any pcpn chances over to snow. Highs on
Wednesday will range from the upper 30s northwest to the mid 40s
southeast. Precipitation Wednesday night may start as a rain/snow mix
but as the cooler air begins to move in, it will transition over to
all snow. However, the models have trended drier with this feature
so would expect any accumulations to be fairly minimal.

Much colder air will then settle into the region through the end of
the work week as 850 mb temperatures drop down into the -14 to -16
degree celsius range. In somewhat cyclonic low level flow and with
possible fetches off of the warm water of the Great Lakes, will hang
on to some lower end pops for snow showers Thursday into Friday.
Depending on the exact timing of the strong cold push, we may likely
have non diurnal type temperatures on Thursday with highs then on
Friday only in the 20s. We will quickly transition into a more zonal
flow pattern by Saturday, allowing for bit of a moderation in
temperatures.

&&

.AVIATION /14Z SUNDAY THROUGH THURSDAY/...
Surface high pressure will move off to the east today. This will
allow an upper level disturbance to move toward the Great Lakes.
Clouds will thicken and lower through the morning hours. Decent
warm, moist advection/ascent ahead of the disturbance will bring
light rain to the terminals later this afternoon into this evening.
As it rains for a period, ceilings will lower into the MVFR
category and then eventually the IFR category. Visibilities for
the most part will remain MVFR.

The upper level disturbance will move east/northeast away from the
region tonight. Rain will end, but low clouds will persist in the
IFR/LIFR category. High pressure is then expected to build into
the region on Monday.

OUTLOOK...MVFR/IFR ceilings to linger into Monday morning.
MVFR/IFR ceilings and MVFR visibilities expected Tuesday with
rain. MVFR ceilings to linger Tuesday night into Wednesday
morning.

&&

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

&&

$$
SYNOPSIS...Hatzos
NEAR TERM...Coniglio/Hatzos
SHORT TERM...Hatzos
LONG TERM...JGL
AVIATION...Hickman


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