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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KILN 131045
AFDILN

AREA FORECAST DISCUSSION
National Weather Service Wilmington OH
545 AM EST Thu Dec 13 2018

.SYNOPSIS...
High pressure will move off the east coast today, with
increasing moisture moving into the Ohio Valley. Chances for
precipitation will increase on Friday and Saturday, as a slow-
moving area of low pressure moves east through the region. Drier
conditions are expected for Sunday and into next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Surface analysis indicates that a weak frontal boundary has
stalled along a WSW-to-ENE orientation somewhere near the
southern border of the ILN CWA. Warm flow just off the surface
is bringing moisture just north of the boundary, resulting in
increasing low level saturation under an inversion near 875mb.
Scattered clouds have increased in coverage, and will likely
continue to do so through the rest of the morning. It should be
noted that so far, the cloud development has been a little less
robust than expected, so timing trends with regards to the sky
grids have been backed off a bit. Fog potential is also looking
a little less certain, so only some patchy fog will be kept in
the grids. Visibilities around the area are pretty much 10SM
across the board, but visibility projections from some of the
models are still indicating the potential for reductions in
west-central Ohio and surrounding areas.

The axis of a low-amplitude ridge currently extends from the
southeastern states through IN/IL, and this ridge axis will move
east through the forecast area this morning. Once it has passed,
winds off the surface will switch to the south, and surface
winds will shift to the southeast (backed from the flow aloft).
This is definitely a warm advection signal, though not at its
strongest in the boundary layer. Nonetheless, there will be
enough moisture transport and isentropic ascent to allow for
some showers to develop along and near the elevated boundary,
though chances for this will be greatest further to the west,
where deep-layer moisture is more prevalent. PoPs were kept
pretty similar to the last forecast, focusing mainly on the
western half of the ILN CWA, and only with lower-end chances for
anything measurable.

Despite the increasing clouds, there should be a reasonable jump
in temperatures today compared to yesterday. Where conditions
will be driest and most susceptible to the weak warm advection
flow in the low levels -- the southeastern portions of the ILN
CWA -- temperatures may approach 50 degrees. Middle to upper 40s
appear likely for the rest of the forecast area.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
Any warm frontal precipitation chances will likely be exiting
the northwestern CWA this evening, leading to dry conditions
through the overnight hours. If anything, the recent model runs
have been a little more pronounced with this expected break in
the action, waiting generally until after 12Z Friday morning to
bring the next precipitation chances to the area. In fact, there
may even be some partial clearing in the cloud deck overnight,
especially in southern Ohio / northeast Kentucky.

During the day, the long-advertised upper low will finally begin
its northeasterly track toward the Ohio Valley region. For
Friday, it will remain quite a ways away, only making it into
Mississippi by the afternoon. Ahead of the low, deep-layer
southerly flow will become more well defined, with an especially
notable increase in southerly 500mb flow on its forward flank.
With the ILN CWA remaining north of the low, there is a clear
signal for a weakening in the flow near where the wind flow
begins to spread out in a deformation pattern. This deformation
will likely provide the bulk of the forcing, with widespread
precipitation expected to overspread / develop in the forecast
area in the 12Z-18Z time frame. The greatest chances appear to
be in the southeastern sections of the CWA, and there should be
a clear northward limit to precipitation chances -- though the
absolute end of this will likely be north of the CWA boundaries.

Though warm advection is expected to continue on Friday, the
temperature forecast was kept somewhat conservative, given that
precipitation and clouds will be the story of the day. Mid to
upper 40s -- similar to Thursday but with a flatter gradient --
will be used in this forecast.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Operational models continue to vary on the timing and the
position of a mid level closed low to affect the area during at
least the first part of the upcoming weekend. The ECMWF and CMC
are similar with a southern track, the GFS closer to the Ohio
River, and the NAM the farthest north. Have continued to side
with the ECMWF which takes the closed circulation from the lower
Mississippi River Valley to the Tennessee Valley Friday night
into Saturday (a more northerly track does not seem to fit the
pattern given the location and strength of the upstream "kicker"
energy). The low is then forecast to move east from the
Tennessee Valley to a position off the Carolina coast by Sunday
evening. A low to mid level deformation axis is still
advertised, and this will likely make PoPs a little tricky in
that there could very well be a sharp cut off to pcpn. Higher
probabilities still favor southern sections Friday night into
the first part of Saturday with an west to east deformation
axis. By Saturday night, the axis may orient itself more
southwest to northeast before exiting the region Sunday morning.
It will be too warm for any frozen pcpn, so rain showers are
forecast. Lows Friday night will range from the upper 30s to the
lower 40s. Highs on Saturday will range from the lower 40s
northwest to near 50 southeast. On Sunday, partial clearing is
expected to occur from west to east. Highs will range from 45 to
50.

For the period Monday into Monday night, models differ on the
strength (how much digging) of upper level energy to move into
the eastern Great Lakes and New England. The GFS is the most
aggressive, the ECMWF the least, and the CMC somewhere in
between. A blended model approach has been used, which pushes a
cold front through the region on Monday with only a glancing
blow of cooler air. It should remain dry with just some CAA
stratocumulus clouds expected, particularly across the northern
and eastern zones. Highs will range from the upper 30s to the
lower 40s. Lows Monday night will drop into the mid and upper
20s.

For Tuesday into Wednesday, surface high pressure will build
into the region on Tuesday, then move east of the area on
Wednesday. Dry conditions are expected to influence the region`s
weather with temperatures near or slightly above normal.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
This set of TAFs has been trended a little bit more optimistic
with regards to ceilings today, now expecting generally only
some occasional MVFR ceilings, with IFR looking less likely to
occur. An expanse of MVFR stratocumulus clouds is spreading
north through the area this morning, and these clouds will
likely persist for a while at KCVG/KLUK/KDAY/KILN, though it is
less certain when (or for how long) they will get to KCMH.

Occasional MVFR ceilings will be possible through the afternoon,
and there is also a slight chance of some showers moving
through the Cincinnati and Dayton TAF sites during the
afternoon as well. A VCSH has been included for KCVG/KLUK but
the chances for rain look too slim to be included anywhere else.
VFR conditions are expected heading into the overnight hours, as
winds back to the southeast and eventually the east. The next
chance for rain -- which looks a little more certain to occur
than today -- will be on Friday morning.

OUTLOOK...MVFR/IFR conditions will be possible at times from
Friday morning through Sunday.

&&

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

&&

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