Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 151209

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
609 AM CST Sun Jan 15 2017

237 AM CST

Through Monday night...

Going forecast remains largely on track with the biggest change
being slowing the onset of the wintry precip. Strong Canadian high
over the area will move off to the east today as a storm system
organizes over the southern plains. As is typical with closed
off upper system, model guidance has been gradually trending a bit
slower each run, and it now appears that precip will hold off
until after midnight tonight.

Dry low levels will need to be overcome via top-down saturation
late tonight into Monday morning and its possible a lot of the
initial band of precip will be lost to virga, especially eastern
CWA. Typically, top-down saturation events like this start out as
sleet once precip does penetrate the dry layer, though as column
saturates and gradually warms would look for precip to transition
to light freezing rain showers. Temperatures should be slowly
climbing Monday with the rain vs freezing rain line likely
steadily moving north through the morning, likely making it to the
WI border by early afternoon. Moderately confident that a winter
wx advisory will eventually be needed, particularly northern and
western CWA, but given we`re looking at mainly a 3rd period event
will allow day shift to get another look at things and hash out
details like which counties to include and timing.

Precip looks to continue Monday night as system closes in on the
area. Guidance generally a bit slower weakening the system
compared with previous runs, so given the fairly strong forcing
and model depiction of a couple hundred j/kg of MUCAPE, thanks
largely to colder mid levels, have opted to introduce a slight
chance of thunder Monday night. Bigger story Monday night in
addition to the showers, could end up being the areas of fog,
potentially dense, that develop as higher dewpoints surge north
into the area ahead of the sfc low.



237 AM CST

Tuesday through Saturday...

Showers look to have ended by Tuesday morning, but a weaker
upstream short wave could be enough to squeeze out a couple light
showers Tuesday and certainly should keep the cloud cover around.
Temps Tuesday into Wed look to remain modestly above average for
what is climatologically the coldest time of the year.

By late in the week and through the weekend, medium range models
develop a strong/high amplitude northern stream upper level ridge
over eastern NOAM. ECMWF and GFS have flip flopped in their
handling of southern stream shortwave potentially affecting the
area in the Friday time frame bringing rain and keeping temps
"only" solidly above normal rather than WAY above normal. Until
medium range guidance is more consistent with each other and run
to run, no changes made to blended model solution for late week
into the weekend. Any day (Fri-Sun) that is dry without fog or
low stratus being a big factor could see highs soar well into the
50s, if rain, stratus, or fog are prominent then highs may "only"
reach into the upper 40s/low 50s. Normal highs are near 30 and
lows in the teens, so temps during this period should generally be
20F+ above average during the day and >25F above average at night.



For the 12Z TAFs...

Quiet weather and primarily VFR conditions (outside of some patchy
early morning fog) will prevail through the day today with the
main forecast concerns centered around an initial wave of
precipitation lifting across the region Monday morning.

High pressure over the area today is resulting in light winds,
variable at times, but expected to trend to a more southerly
direction this afternoon. Exact wind direction (SE vs SW) remains
a bit uncertain, but there is high confidence in light winds
speeds of 5 kt or less. Patchy fog early this morning has reduced
vsby to 4-5 miles at some locations, and with a mid and high
clouds over the area, do not anticipate much more reduction.

Low pressure is expected to lift across the Southern and Central
Plains tonight into Monday with lowering clouds expected locally
overnight and eventually saturating enough for precipitation to
reach the surface. There remains a large spread between models in
timing the onset of precipitation, however, forecast soundings do
indicate the potential for a window of freezing rain and sleet
late tonight into early Monday morning before the column warms
enough for a transition to rain by mid to late Monday morning.
Expect ceilings to eventually fall to IFR with LIFR a possibility,
especially beyond the current TAF timeframe as the surface low
nears the region.



304 AM CST

High pressure is centered over the Upper Midwest and western Great
Lakes early this morning resulting in light flow across lake
Michigan. The high will shift to the eastern Great Lakes today and
then to New England on Monday with southwest flow increasing
behind the high. Winds are expected to peak in the 20 to 25 kt
range across the north half of Lake Michigan late this afternoon
through tonight. Winds should begin backing to the southeast and
increasing from the south late tonight into Monday as low pressure
lifts across the plains. The low is expected to reach Lake
Michigan early Tuesday with winds turning westerly behind the low
through the day Tuesday and continuing Wednesday, again peaking
around 25 kt.






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