Area Forecast Discussion
Issued by NWS Wilmington, OH

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FXUS61 KILN 042044
AFDILN

Area Forecast Discussion
National Weather Service Wilmington OH
344 PM EST Sun Dec 4 2016

.SYNOPSIS...
An upper level disturbance will move across the region today
bringing mostly rain with snow possibly mixing in across the
north. 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 AM THIS MORNING/...
Starting to get reports of light rain across our northwestern
zones (and sleet in Wayne County, IN). This is thanks widespread
isentropic lift across the area along with a shortwave approaching
from the west. Looking at the latest high res models confidence
has built that rain will likely transition to a rain/snow mix and
then transition to all snow possibly across our far northern
counties (Mercer, Hardin, Auglaize). This transition will likely
be driven by stronger upward motions and adiabatic expansion from
lower pressures (dynamic cooling). The latest HRRR run shows
these northern counties actually transitioning to all snow around
23z with some light accumulations on grassy surfaces possible.
There may be some accumulations on roadways (esp. elevated
surfaces) which could cause some slick spots but this should be
transient. Have covered this in the HWO. All the three metro areas
and south should stay all rain. The upper level shortwave will
start to take on a negative tilt and eject northeast late this
evening with precipitation coming to an end across the area around
midnight.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH TUESDAY NIGHT/...
Monday morning a weak surface high pressure will move overhead
behind the shortwave trough that moved through the area Sunday.
Global models are showing some clearing Monday but there remains a
sharp inversion in both GFS and NAM forecast soundings around 920
mb which could help trap some low level moisture. Looking at
ECMWF, GFS, and NAM rh fields a potent surge of dry air is
expected to move into the area and have allowed skies to clear a
bit. High temperatures Monday will likely have trouble getting
much above the mid 40s as 1000/ 850 mb thicknesses (~1305 m)
support highs only in the mid 40s. Given the above mixing is only
forecasted to be around 925mb so have backed off on high
temperatures a bit for Monday.

The weather pattern will be quick to change Monday night as an
upper level low currently over northern Mexico (as of Sunday
afternoon) will begin to head northeast Monday afternoon with a
coastal low forming. This surface low will be near the Tennessee
valley Tuesday morning and move just southeast of the area Tuesday
afternoon. Current thermal profiles support rain across most of
the area with the NAM and GFS trying to show a brief change over
to snow across the far northern zones. Overall the GFS, ECMWF, and
NAM are in pretty good agreement with the timing of the surface
low which would still support non- diurnal temperatures. Have
adjusted the timing of the precipitation onset a bit for Tuesday
while also adding a brief period of rain/snow for our far northern
counties.

The surface low will also help bring deep gulf moisture northwards
with PWATs on GFS/ NAM forecast soundings rising to above 1"
Tuesday. These PWATs are also shown on the NAEFS which are 1 to 2
SDs above normal for this time. Given the above have kept PoPs
categorical with storm total amounts of 0.50" - 1.00" of rain
possible Tuesday.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
This period will be characterized moreso by temperature impacts
rather than precipitation impacts - though the latter will still
need to be watched on late Wednesday night/Thursday morning...and
again later next weekend.

Confidence assessment - there`s really pretty decent agreement in
larger scale evolution of the flow into next weekend between higher
resolution deterministic runs and parent ensemble means. There`s
much higher confidence on temperature evolution Wed-Sun vs.
precipitation...with the season`s first /well advertised/ cold spell
centering on Friday with temp departures some 10-20 degrees below
normal. Wednesday/Sunday can be viewed as transition days into /Wed/
and out of /Sun/ the thermal trough but this entire period will
average out well below normal. Precipitation type/timing is much
more problematic and of much lower confidence - with notably two
decent changes at synoptic scale forcing/vertical motion. The first
is with shortwave energy rippling along the advancing cold front
Wednesday night/Thursday morning...the second is progressive
shortwave energy across the Great Lakes next weekend. Very low
confidence with the forecast next Sat/Sunday - as it is noted that
ensemble plumes /GEFS and ECMWF/ get extremely noisy in Days 7-10
suggesting the pattern is of very low confidence with how the energy
next weekend evolves. Both data show the deterministic runs trending
away from their respective ensemble means by Day 8...suggesting
caution in next weekend`s forecast.

All that being said - however - Wednesday there is rather high
confidence in a dry day downstream from approaching height falls
moving through the Plains. Most recent /04.12z/ ECMWF has caught up
in speed to GFS/GEFS which was a disagreement source early on.
Advancing strong cold front and strengthening shortwave trough have
had hints of developing a weak frontal wave as it approaches the
area Wed night/Thursday morning...with deterministic ECMWF /04.00Z
and 04.12Z/ consistent with a stripe of modest QPF enhancing on the
cold side of the strong cold front which may be worthy of some light
snow accumulations north of the Ohio River...but agreement in this
scenario has less /GFS/ to much less /CMC/ agreement outside the
ECMWF/ECMWF ens spectrum for anything more than mid-range snow
chances at this time.

Once this front/frontal wave passes...cold air blasts in but is
progressive. -16C at 850mb is significant for mid-December on Thurs
night and thus have no problem accepting mid/upper teens for lows
and mid/upper 20s for highs Fri through this brief cold spell.
Moderation begins on Saturday with temps back toward 30 on Saturday.

Some cold advection flurries/snow showers are possible both
Thursday/Friday...particularly across central Ohio in favorable
trajectory off Lake Michigan...but coverage expected to be spotty.

Per confidence assessment above - not a great deal of confidence how
shortwave energy and associated rain/snow chances look for Sunday
and this lingers beyond Day 7 depending how energy ejects within the
upper trough.  For right now have middle-range snow chances
developing Sunday with transition from south to north given warm
advection...but we`ll need to see more data before any potential
impacts are.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
Rain associated with a trough of low pressure will overspread TAF
sites this afternoon and this evening. Ceilings and visibilities
will lower as the boundary layer saturates, with IFR ceilings
likely and LIFR possible tonight as rain departs east. Mixed snow
in at CMH and LCK with no accumulation expected.

Slow improvement to MVFR will occur Monday morning under lingering
low level moisture. CVG may see a return to VFR around 18z as per
latest models.

Winds starting out of the southeast will shift to west overnight
with speeds near 10 knots. Light west to southwest winds are
expected Monday.

OUTLOOK...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...Haines
NEAR TERM...Haines
SHORT TERM...Haines
LONG TERM...Binau
AVIATION...Coniglio



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