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Area forecast discussion 
National Weather Service Wilmington Ohio
423 PM EST sun Dec 21 2014

Synopsis...
high pressure will continue moving east of the region tonight...as
a trough of low pressure begins to move into the Mississippi
Valley. With southerly winds beginning to increase...warmer
conditions are expected through Wednesday. As the trough and a
cold front move through the region...chances of precipitation will
increase for Tuesday and Wednesday...with cooler conditions for
the end of the week.

&&

Near term /until 6 am Monday morning/...
it took some time...but the low clouds have all eroded from the
iln forecast area...under the influence of warming temperatures
(especially at around 900mb). The weather pattern is gradually
evolving into one that is favorable for deep southerly flow...but
it will still be another forecast period or two before this
alignment is maximized. For now...the ridging over the
middle-Atlantic is rather flat...and the surface high is still close
enough to provide an easterly component to the low-level flow.

As the 925mb/850mb gradient begins to tighten overnight...moisture
will begin to be transported north...eventually reaching the Ohio
Valley before morning. Until then...it appears that skies will be
covered only in cirrus...leading to some fairly favorable
radiational cooling conditions (at least during the first half of
the overnight period). Temperatures may begin to warm somewhat
before 12z...especially once the low clouds move in. Raw model
temperatures all indicated a sharp drop in temperatures between
21z and 03z...followed by little change after that.

&&

Short term /6 am Monday morning through Tuesday night/...
the changes in the weather pattern over the Ohio Valley will
become more pronounced on Monday. A surface pressure gradient
will begin to increase...between a developing surface low in
western Iowa and a departing surface high over the Canadian
maritime provinces. A large middle-level trough over the central
states will develop into a closed low...and with ridging off the
Atlantic coast...a regime of deep southwesterly flow will set up
over the Ohio Valley. Moisture transport from the Gulf will reach
the iln forecast area early Monday morning...with precipitable
water values gradually increasing through Tuesday...leading
through the first of two main waves of precipitation. The second
wave on Wednesday will be covered in the long term section of the
forecast discussion.

A weak lead shortwave is forecast to rotate northeast through the
Ohio Valley on Monday morning...combining with the front edge of
the northward-moving moisture. There appears to be just enough
isentropic lift to support some weak showers...mainly in the
southern and southeastern portions of the forecast area...where a
20-pop has been used in the forecast. However...this shortwave
will be weakening with time...and widespread precipitation is not
likely to occur. As this light precipitation moves in...the region
will be coming out of another chilly morning...with temperatures
only expected to rise above freezing just before the precipitation
arrives. A worst-case scenario could support some light freezing rain near
the Ohio River at around 12z...but it appears that the
precipitation will be light/sparse enough (and temperatures
warming just soon enough) to make this a low-probability scenario.

Temperatures on Monday will warm significantly beyond values on
Sunday...thanks to warm advection...and even in the absence of
much insolation. Temperatures across the iln forecast area should
range from the lower-to- middle 40s in the north to the lower 50s in
the south. These warming temperatures will be aided by a lack of
precipitation...as the forward progression of the developing low
pressure is not particularly fast.

Chances for precipitation will increase ahead of the progression
of the middle-level low...which will be moving northeast into the
Great Lakes on Tuesday morning. As moisture convergence increases
over the lower Ohio Valley...a swath of showery precipitation is
expected to translate northeastward as well. This activity will
eventually outrun the slowing cold front...but will be aided by
favorable diffluent flow ahead of the approaching upper jet
streak. While the greatest chances for precipitation (and greatest
model output qpf) appear to be in the northwestern County Warning Area...the focus
for any potential for thunder will be in the northern Kentucky and
south central Ohio. Surface-based instability will be very
unlikely...with some level of inversion still remaining in
place...as also evidenced by the slightly decoupled wind
directions between the surface and 850mb. However...850mb parcels
will have a roughly moist-adiabatic profile above them...with 15z
Storm Prediction Center sref parameters indicating at least a chance of equilibrium
levels cold enough to support some weak convection. Thus...a brief
period with a slight chance of thunder was included Thursday
morning and afternoon in the southern/southeastern portions of the
forecast area.

Probability of precipitation will decrease on Tuesday evening as the middle-level wave moves
off...and upper support is lost. Chances will increase again early
Wednesday morning...as low-level forcing and convergence begins
to increase ahead of a developing surface low. Throughout this
period...temperatures will be dominated by advection...with
non-diurnal curves required for the forecast grids. Highs on
Tuesday should range from the lower 50s to upper 50s from north to
south...with little change in temperature expected Tuesday night.

&&

Long term /Wednesday through Sunday/...
low pressure will deepen along a cold front at the beginning of the
long term period. GFS/CMC are similar in the intensifying process as
the low moves northeast from the middle Ohio Valley to southeast
Ontario into Wednesday night. The European model (ecmwf) is a little stronger and the
NAM is a weak outlier. Have taken a blend of the GFS/CMC. As the
deepening process occurs...widespread showers are expected Wednesday
morning. By Wednesday afternoon...the cold front will push northeast
through the region. The coverage of precipitation will decrease behind
frontal passage as drier slot of air aloft moves in briefly. By
Wednesday night...some wrap around moisture will occur...but
coverage looks to be chance at best. It will be warm ahead of the
front with temperatures dropping Wednesday afternoon into the night.
Highs on Wednesday will range from the 50s to near 60 with lows
bottoming out into the lower and middle 30s by Christmas morning. It
should become cold enough for a chance of rain/snow showers and then
a chance of snow showers Christmas evening night. However...little is
expected in the way of accumulations given recent warm temperatures
and no real focus/forcing for appreciable snowfall rates. It will
become brisk to locally windy during the period...but winds are
expected to stay below advisory criteria at this time.

On Christmas day...deepening low pressure will continue to move
northeast away from the region. Any threat for snow showers will
quickly come to an end during Christmas morning. By
afternoon...skies should become partly cloudy as surface ridging
quickly builds into the area behind departing system. Highs on
Christmas will range from the upper 30s to the lower 40s...near
seasonal normals.

For Thursday night...surface ridge will move east with return
southwest flow ensuing along with increasing clouds ahead of the
next system.

For Friday into Saturday...low pressure and a cold front will move
across the Great Lakes and Ohio Valley. System appears to be
moisture starved so will continue with low chance probability of precipitation. We should
see a warm up on Friday followed by a cool down Saturday in the wake
of the system.

By Sunday...high pressure and much colder temperatures are expected.
Highs by then will range from the upper 20s to the lower 30s.

&&

Aviation /21z Sunday through Friday/...
last taf location with MVFR ceilings is Kiln and skies will see a
rapid breakup this afternoon to come in line with remaining taf
sites in County Warning Area.

Southeast winds will help erode any other low clouds today and
lighten up a bit overnight before picking back up tomorrow during
the morning. Another slug of low level moisture will come up from
the south tomorrow during daylight hours and could bring yet
another MVFR deck to the region in the morning. It should mix out
a bit during the late morning and afternoon...but be topped by
some middle to upper level clouds streaming over the region yet
again.

Outlook...MVFR to IFR ceilings and visibilities are possible Monday night
into Thursday. Winds will pick up significantly starting
Wednesday.

&&

Iln watches/warnings/advisories...
Ohio...none.
Kentucky...none.
In...none.

&&

$$
Synopsis...hatzos
near term...hatzos
short term...hatzos
long term...Hickman
aviation...franks

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