Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191841

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
141 PM CDT Thu Oct 19 2017

140 PM CDT

Through Tonight...

A quiet night ahead with the high pressure ridge shifting from
overhead this afternoon to east of the area by daybreak. The air
mass is quite dry with 40-45 degree temperature/dew point spreads
providing relative humidity of 15-25 percent, especially
along/north of I-80. The light winds, dry air, and clear sky on an
October night are a recipe for strong evening radiational
cooling, and have sharpened the slope of the early evening
temperature drop in the hourly forecast. The 9-10 pm readings
should already be down to within 5 degrees or so from the
nighttime lows. A slight south wind late will ease the rate of
cooling with lows forecast in the mid-upper 40s in outlying
locations, to the lower to mid 50s in the metro.



219 AM CDT

Saturday through Wednesday...

Saturday still appears to be a fairly nice day across the region in
spite of steadily increasing cloudiness. Mild southwest flow will
continue with some upper level energy spreading in ahead of a deep
upper trough across the west. Some of this energy splits between the
northern and southern jet streams, but there is a pacific origin
surface front that will support a steady blossoming of rain and,
given modestly impressive mid level lapse rates ahead of the front,
even a few embedded thunderstorms Saturday night into Sunday, with
Sunday still holding the better chance for rain.

There are some pretty significant model differences in how each
model handles the eastward movement of the upper trough. The general
idea is a trend toward a cooler and showery pattern given the
influence of a deep upper low, but the way the guidance develops the
precipitation is not all that consistent as a farther western
solution will place the region under more significant rain chances
per the EC/GEM whereas the GFS which is a bit quicker with the
deeper souther track surface low. At this point chances of showers
and slightly below normal highs generally tell the story, but expect
some tweaks to the forecast in the farther extended once a better
sense of the low progression becomes apparent.



For the 18Z TAFs...

Few to no concerns for aviation-related weather through Friday,
with the one possibility being gusty winds on Friday afternoon
near due south in direction.

A surface high pressure ridge over the area this afternoon will
want to keep the winds light and variable. A lake breeze pushing
inland in northeast Illinois should reach ORD around 19Z and
provide a more consistent northeast wind, though speeds will
likely remain at or below 7 kt. The light winds will turn
southeast this evening and then south early Friday morning. Deep
mixing in the dry air mass within a tighter pressure gradient will
support gusts Friday afternoon. Some of these should near 20 kt.
The confidence in the wind direction being near due south is



235 AM CDT

The southerly gales are finally easing down into the 25 to 30 kt
range early this morning, so the gale warning will be allow to
expire at 3 am. Expect the winds to become westerly for a period
today following the passage of a weak surface boundary. However,
stout south-southwesterly winds are likely to return to the lake
by Friday. This as another storm system takes shape over the
Northern High Plains and then shifts into Saskatchewan and
Manitoba by Saturday. Expect southerly winds to increase up to
around 30 kt again Friday into Saturday, especially over northern
Lake Michigan. A period of low end gale force gusts will also be
possible on Saturday over northern Lake Michigan. An associated
cold front then looks to sweep across the lake some time on
Sunday, with a period of showers and storms possible.

The lakes region looks to remain in an active weather pattern next
week. It appears that a much colder airmass could spill over the
region by the middle of next week. With this would come the
increased potential for enhanced northerly winds and lake effect
showers/storms and waterspouts.






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