Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 180848

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
248 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 06Z TAFs...

Low ceilings remain across the terminals early this morning, and
will likely stay at least through late morning. This stratus deck
is quite expansive, and will likely take some time to scatter/exit
later today. So will continue to monitor the end time of this low
stratus through today. Have maintained IFR ceilings this morning
but am monitoring for the potential for ceilings to drop further,
possibly to the 300-500 FT range. Especially as this is already
occurring in isolated spots over northern Illinois. Vis in fog
this morning should remain MVFR, however, some periods of lower
vis will also remain possible. Precip not anticipated this
forecast period, with west to southwest winds expected today into



232 PM CST

Fog across the lake should be slow to improve tonight and more
swiftly show improvement Wednesday morning. Northwest winds
tonight will become southwest Wednesday morning and remain as such
through the day Thursday before turning east or northeast. All in
all, speeds remain in check through midweek. Higher dew point air
creeps northward Friday and Saturday, and may support some fog,
especially later Saturday. Confidence in wind direction drops
during the weekend into early next week, as the weather pattern
has multiple cutoff lows across the central U.S. Presently,
increasing northeast winds are forecast Sunday and Monday that
would support Small Craft Advisory criteria in the Illinois and
Indiana nearshores.






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