Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 181732

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1132 AM CST Wed Jan 18 2017

244 AM CST

Through tonight...

Expect mild but otherwise quiet weather today and tonight across
northern Illinois and northwest Indiana as high pressure builds
across portions of the mid Mississippi Valley and Ohio Valley. Early
this morning, stratus and fog persist across the mid and upper
Mississippi Valley, but as low/mid level anticyclonic flow
overspreads the area through the morning and afternoon, do
expect clearing  to eventually occur over the forecast area from
the west sooner or later. For now, leaning a little slower than
guidance given how models typically struggle with clearing
stratus. Afternoon highs are expected to be in the low to mid 40s
and overnight lows will be around 30.



244 AM CST

Thursday through Tuesday...

A series of upper lows will make for an active weather pattern
throughout much of the extended forecast period. Quiet and mild
weather will continue Wednesday, but attention will turn to the
southwest Thursday as an upper low which is currently spinning
across the desert southwest lifts across the Central Plains
towards the Upper Midwest. A weak surface reflection will
develop over the Missouri Bootheel Thursday and lift to northern
Illinois by late Thursday night with warm advection precipitation
overspreading the forecast area from the south through the day
Thursday. A very warm column will eliminate any precip type
concerns with this initial low but guidance does show some modest
instability and moisture convergence associated with a low level
jet that should be sufficient for scattered thunderstorms. Another
weak wave will lift across the central and northern Plains Friday
into Saturday which will bring another chance of rain Friday
night. Temperatures look to peak with this wave as 850mb temps
warm to 6-8C across the local area Saturday which climo suggests
would support highs around or into the 50s. A more substantial
upper low is progged to undercut us and move east across the lower
Mississippi Valley Sunday and lift across the Appalachians early
next week. Temperatures should moderate some as cooler air wraps
back in over the local area behind the low, but confidence in the
forecast details (precip amounts and type in particular) during
the early part of next week is pretty low due to considerable
spread among GEFS ensemble members and operational models.



For the 18Z TAFs...

Low CIGs and VSBYs will continue to be the main aviation concern
through tonight as low-level moisture remains trapped under a
stable layer aloft. As a result of this, we have continued with
IFR CIGs through mid morning on Thursday. While there could still
be a period late this afternoon through early this evening with
some improving CIGs, it appears any improvement would be short
lived, with CIGs and VSBY likely to come done again through the
evening. For this reason, we have chosen to go IFR through this
period. LIFR CIGs and VSBY will be possible again tonight, but
will hold off on going this low in the TAF.

Conditions should improve some Thursday ahead of our next weather
system. This next weather disturbance will result in our next
period of rain, likely beginning Thursday evening. Winds are
expected to become easterly Thursday afternoon in association
with this weather disturbance.



323 AM CST

As low pressure exits to the eastern Great Lakes early this
morning, high pressure is building over the mid Mississippi Valley
and will continue to slide to the east today into tonight. Winds
have generally stayed on the lighter side over much of the lake
but as this high continues east today, do expect the gradient over
the lake to increase. This will allow southwest winds to increase
throughout today, across the entire lake. Although the highest
winds will occur over the north half, it does appear now that
increasing winds will be observed over the south half as well.
Have winds increasing to 30 KT over the north half today, while
winds stay in the 15 to 25 KT range across the south half. Speeds
over the nearshore should stay in the 10 to 20 KT range, where the
gradient is a little more relaxed. With this high departing to the
east later tonight into Thursday, the gradient will relax with
winds diminishing once again.






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