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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KILN 171047
AFDILN

AREA FORECAST DISCUSSION
National Weather Service Wilmington OH
547 AM EST Thu Jan 17 2019

.SYNOPSIS...
As low pressure moves across the Ohio Valley today, a mix of rain
and snow will occur. High pressure is expected to move into the area
for Friday, bringing dry conditions. A significant winter weather
event is expected over the weekend, as a strong area of low pressure
moves northeast through the area. This will bring a mix of
precipitation types to the area. Very cold air will move into the
region for Sunday and next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Current surface analysis indicates that low pressure is located
over northeastern Oklahoma, with a weak trough extending
northeastward into southern Illinois. The surface low is
expected to move off to the northeast into the Ohio Valley
today. A mid-level wave is also crossing Missouri as of this
writing, with a subtle swirl visible on GOES-E water vapor
imagery, and an obvious area of favorable ascent out ahead of
it. The combination of these features will bring precipitation
to the ILN CWA today, with accumulating snow as the main
concern.

Though light radar echoes have been showing up for a few hours
across the southern and southwestern sections of the forecast
area, observations of precipitation have been sparse thus far.
Ceilings are still around 2000 feet in this area, with thicker
low-level moisture still upstream. As the boundary layer
saturates, a brief period of drizzle does appear possible.
Depending on surface temperatures, this could be freezing
drizzle, as is being observed at KIND as of 0754Z.

As moisture becomes deeper (through about 10kft) the chances for
measurable precipitation will rapidly increase in the 10Z-14Z
time frame. This activity appears initially to be forced well
aloft, with vorticity advection ahead of the mid-level wave, and
the first push of theta-e advection noted at 850mb and 700mb at
this time. As the surface low gets a little closer, eventually
the boundary layer flow will also switch to more of a southerly
orientation, with temperatures warming through the saturated
layer. This will also increase the strength of the forcing, with
precipitation absolutely certain to occur (at least
intermittently) during the 14Z-20Z time frame across the ILN
forecast area. PoPs were increased to 100 percent CWA-wide.

The precipitation type question is a little bit trickier,
though certainly not to the extent of the weekend system that
will be discussed in the Long Term AFD section below. There
continues to be very little evidence of any sort of traditional
mixed precipitation (outside of the freezing drizzle possibility
near onset) thanks to low level conditions that are isothermal
or cooling slowly with height. Without any signs of a strong
inversion, sleet or freezing rain will be very unlikely.
It is true that there will be an inversion in place initially,
but this inversion will weaken considerably as moisture
advection occurs. Precipitation should generally begin as snow,
gradually mixing with rain from south to north as the event
progresses. If there is any chance of sleet as the column
saturates near precipitation onset, it might be across the far
western sections of the CWA.

On a similar subject, the strength of the warm advection near
the surface is not expected to be overly impressive, and the
antecedent conditions will still be relatively cool with the
presence of a snow pack. Because of this, there are concerns
about just how warm it will get. This also leads to a
precipitation type question, because the nearly-isothermal
temperature profiles will be very sensitive with respect to
which type of precipitation (rain or snow) is favored. It could
be a day where wet snowflakes and cold rain mix back and forth
from time to time. Nonetheless, there should be enough time with
pure snow to allow for most of the CWA to at least receive minor
accumulations (perhaps excepting the far south / southwest). In
general, forecast amounts are around an inch in the northern
half of the CWA, tapering off to near zero in the far south. Two
notes about 00Z models with regards to snowfall. First, GEFS
plumes have generally come down in amounts over the past couple
runs, and the going forecast numbers (as described above) are
actually well on the high side of the spread. With that said,
operational models (including the 00Z ECMWF) still support
totals of around an inch, especially along the convenient
Interstate 70 corridor. Second, a few higher-resolution runs
(00Z 3km NAM / 06Z HRRR) are indicating the potential for up to
an inch in south central Ohio with the initial push of
precipitation. Given that it is a small window for temperatures
to remain cold enough for snow in this part of the CWA for any
sort of lengthy period of time, the 00Z 3km NAM projection in
particular seems a bit unlikely. Nonetheless, numbers were
increased slightly in this area to account for the possibility.

Temperatures should have a fairly small range today, with highs
in the lower to upper 30s. Given the location of the low,
temperatures may not begin to fall for a while on Thursday
evening -- especially in the southeastern sections of the
forecast area.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
Precipitation should become notably lighter as forcing weakens
by mid to late afternoon today, though some very light
rain/snow/drizzle mix may continue into the evening. However,
once the axis of the shortwave has passed the area, the surface
low will also be moving over the region -- ending any isentropic
ascent from the previous warm advection. Thus, there should be
a lack of forcing heading into the overnight hours, cutting off
chances of precipitation. The remaining low level moisture will,
however, have nowhere to go and no means to mix out. There is
very high confidence (SREF probabilities running quite high) in
thick stratus remaining in place through the overnight hours,
with fog and mist also developing. Dense fog is not out of the
question. The line between drizzle and mist is probably a fine
one, but it would be preferred to have some sort of lift in
place to put any actual precipitation into the wx grids. Fog,
however, will be included -- along with 100% sky cover. Min
temps are expected to range from the upper 20s to mid 30s,
representing a drop of only around 3-5 degrees from max temps
during the day.

Outside of the thick clouds and possibly some persisting fog,
there is very little to discuss with regards to the weather on
Friday. As high pressure moves into the forecast area, advection
will remain weak, and temperatures will gain only a few degrees
from overnight lows. Max temps in the lower to upper 30s are
actually fairly close to normal -- and /normal/ is not a word
that will be used too frequently to describe the weather from
Saturday and beyond.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Low pressure over the southern Plains Friday night will track
northeast to the middle Mississippi River Valley by Saturday
morning. This system will bring a multiple set of hazards for
the region through the weekend.

The biggest challenge as we head into Saturday and Saturday
night will be the exact track and strength of the low pressure
system. Operational model wise, the NAM is the fast outlier,
followed by the GFS, then the slower CMC and ECMWF. The first
interesting thing to note, as pointed out by WPC, is that the
mid level trough associated with the low is not undergoing a
strong negative tilt as it passes across the Ohio Valley region.
That being said, this may give support to the operational ECMWF
and its ensemble members which take the low a little farther
south then the other operational models. There is a noted pause
for concern as low pressure systems passing through the Ohio
Valley tend to trend farther north than expected as one closes
in on an event. But, this low is not winding up as it passes to
our south, so the ECMWF may be on to something. Regardless, we
should hopefully have a better idea Thursday night into Friday
as the energy will be fully inland from the West Coast. The
track of the low and its thermal fields will be key to ptype as
it affects our area with considerable moisture. A blended model
approach that puts some weight to the ECMWF solution now favors
a more narrow ribbon of mixed pcpn for locations between I-70
and the Ohio River. This should result in a transition zone of
snow, sleet, and freezing rain that pivots slowly northward
during the first part of the day. Given the potential for
heavier rainfall rates (more runoff) and temperatures near
freezing, it appears that ice accumulations may be kept down in
this zone, and also due to other ptypes being mixed in. That
being said, current forecasts of 0.10" to perhaps 0.20" of ice
accumulation (planar flat ice) will be possible on Saturday. To
the north, ensemble probabilities look likely for a significant
snowfall, six inches or more, with the potential for amounts
peaking in the 8"-10" range near and just north of the I-70
corridor. For points along and south of the Ohio River, a brief
wintry mix or just rain can be expected for much of the day.
Rain may be moderate to locally heavy at times, which may result
in some localized flooding issues. The low will be pushed
east/northeast Saturday night by an upstream mid level trough of
Arctic origin dropping into the Great Lakes. As this occurs,
colder air will filter southeast, allowing ongoing pcpn to
change from rain to snow. Another thing to note at this juncture
is the the GFS and its faster and more northerly solution
places the southern CWFA in a dry slot where as the southern
track and slower motion continues deformation pcpn going longer
into the night. Have sided with WPC at this time which allows
accumulating snow to affect the remainder of the region. Pcpn
will gradual taper off overnight as the low pulls away. As this
occurs, the pressure gradient will have tighten between low
pressure to the east and Arctic high pressure to the northwest.
This will increase winds, which will become gusty at times,
perhaps up to 35 mph. This will likely result in some blowing
and drifting of snow where much of it fell in generous amounts.
In addition, with falling temperatures, wind chill temperatures
will start to become a concern, especially across our northwest
zones. All in all, the first stab at total accumulations (some
interrupted by mixed pcpn) looks like 6 inches to perhaps up to
10 inches along and north of the I-70 corridor, 4 to 6 inches
south of I-70 and north of a line oriented southwest to
northeast between the Tri- State region to Hocking County, with
2 to 4 inches elsewhere. Certainly, winter headlines will have
to be issued at some point where confidence increases, which
will probably happen on the day shift. Will mention all the
hazard possibilities in the HWO. Lows Friday night will range
from the mid 20 north to the lower 30s south. Highs on Saturday
will range from the upper 20s northwest to the lower/mid 40s
southeast. Lows Saturday night will range from near 10 northwest
to the lower 20s east/southeast.

For Sunday into Sunday night, Arctic high pressure will build
into our region. There will be a chance of light snow east early
on, otherwise, skies will be partly cloudy to cloudy, with
perhaps a chance of CAA flurries. Stiff and gusty northerly
winds will allow temperatures to slowly fall through the day. By
Sunday night, it should be dry with clearing skies. This will
set the stage for cold temperatures, especially where an
appreciable snow cover results. Lows will range 5 below to 10
below northwest to 0 to 10 above elsewhere. Winds will stay up,
resulting in cold wind chill temperatures from 10 below to 20
below north, with 10 below to near zero south.

Arctic high pressure will move east on Monday with a dry day
expected. After cold lows, highs will struggle, especially
north. Highs will range from near 10 north to the lower 20s far
south.

There is quite a bit of timing uncertainty with the approach of
the next area of low pressure Tuesday into Wednesday. This
could also also have an impact on temperatures and thus
precipitation type. Leaned towards an ensemble mean for now.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
Deteriorating conditions are expected through the TAF period.
Currently, ceilings are MVFR/VFR across the region, but this
will change over the next couple hours. Precipitation is
expected to spread into the region from southwest to northeast.
There may be a brief period of drizzle or freezing drizzle near
onset, but snow is expected to be the primary precipitation type
through the morning at KDAY/KILN/KCMH/KLCK, with a mix with rain
starting at KCVG/KLUK. Rain will eventually mix in northward to
the other TAF sites, though not until later in the day. Where
snow falls, IFR ceilings and visibilities are both possible,
with accumulations of an inch or so at KDAY/KCMH/KLCK. There
may be a slight improvement to conditions once the snow switches
to rain, but no better than the pessimistic end of the MVFR
category.

Overnight, precipitation will come to an end, but ceilings will
continue to lower into the 200-600 foot range. Some drizzle or
fog is also expected, reducing visibilities significantly as
well.

OUTLOOK...IFR conditions are expected into Friday morning, with
MVFR/IFR conditions will continuing through the day. IFR
conditions are likely with precipitation on Saturday and
Saturday night, with MVFR/IFR conditions and gusty winds on
Sunday.

&&

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

&&

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