Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 132031

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
231 PM CST Fri Jan 13 2017

230 PM CST

Through Saturday...

The forecast message remains similar for northern Illinois and
northwest Indiana tonight. There is a chance of light freezing
precipitation in the southern third, or basically along and south
of U.S. Highway 24 (Pontiac to Rensselaer). If this materializes
it would seem to be spotty between mainly midnight and 9 a.m., and
impacts look limited enough to just handle with a Special Weather
Statement for the far southern three counties noting potential
light freezing precipitation.

Modified arctic high pressure of 1045 mb centered across Wisconsin
will slowly weaken through Saturday but hold its own, with
northeast winds supplying some dry lower level air (most
pronounced between 875-950mb). This will limit some virga/flurries
along the WI/IL border late this afternoon, and also the northern
extent of light precipitation tonight as isentropic ascent and
expands across central Illinois. Lift and saturation are limited
in the mid-levels/ice presence zone, and should rule out snow for
a weather type tonight. Also, a fair amount of precipitation in
the upstream moisture feed could cut off a better moist fetch too.
So if precipitation can reach into the southern forecast area
tonight and Saturday morning, it likely would be light spotty
freezing rain, with possibly ice pellets at first due to wet bulb
cooling in the lower levels.

Clouds will continue further north with a dry forecast north of
I-80. Temperatures may drop a few degrees initially this evening
but with thicker cloud cover they should hold steady the rest of
the night. Saturday should see highs around 30 for most of the



321 AM CST

Monday night through Thursday...

In the wake of this system, guidance strongly suggests a quick
transition to a more zonal flow with above normal heights and
temps over the eastern 2/3rd of the nation. Later next week,
strong Pacific jet is forecast to carve out another deep western
trough leading to downstream amplification of a very impressive
ridge over the eastern half of N America. In fact, medium range
models with strong support from their ensembles are developing a
pattern that could send temperatures into near record warm
territory by late next week. Have nudged temps upward from the
blended model forecast initialization cocktail, but these temps
appear too cool (possibly significantly so). The forecast 925mb
temps on the operational GFS/ECMWF both support potential for
highs to reach 60F by Friday and possibly into the weekend, but
forecasting record highs this far out is risky with so many
potential things to go wrong (like cloud cover), so conservatively
forecasting 50s for now for Friday.



For the 18Z TAFs...

Little for concern in this TAF period, with precipitation expected
to remain south of the TAF sites through Saturday.

Strong high pressure centered across Wisconsin will maintain a
north-northeast wind this afternoon that will become more
northeast by late in the day. Sporadic gusts to 17-20 kt are
possible this afternoon, namely at MDW and GYY based on
observational trends. The northeast winds had steered lake effect
MVFR clouds into GYY and MDW and near ORD, but these will be
fading this afternoon. Otherwise just mid-high clouds into

Wintry precipitation will lift northward across central Illinois
later tonight, but should remain well south of the TAF sites
thanks to low-level dry air. Areas of 4000-6000 ft cigs are
expected to develop over the airports late tonight and persist
into Saturday morning, with light and variable winds at the



230 PM CST

As high pressure moves from Wisconsin over the lake later tonight,
the northeast winds will lighten and become east. Waves should
slowly continue to ease along the Illinois and Indiana nearshores.
A quick-moving low across Canada will tighten the pressure
gradient across the northern part of the lake Saturday, with a
brief increase in gusts to around 25 kt near the northern
straits. A prolonged period of southerly flow is forecast Sunday
afternoon through most of next week (at times southwest and others
southeast). While gales are not presently forecast, there does
look to be possible 30 kt winds during midweek as low pressure
passes the western/central Lakes.







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