Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 261858
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
158 PM CDT MON SEP 26 2016

.SHORT TERM...
158 PM CDT

Through Tuesday...

Following the passage of a strong cold front last night, much
cooler, drier air has filtered across the region.  Also, sfc high
pressure is building across the middle Mississippi Valley, helping
keep mostly clear skies across the region.  Under persistent deep
layer cold advection, modest pressure rises and a strong pressure
gradient between a deep low over the upper Great Lakes and the high
pressure building across the middle Mississippi Valley, strong,
gusty west winds will persist through the day.  Conditions should
remain rather breezy into the night tonight.  There should be a
brief lull in the winds as a weak sfc trough rotates around the swrn
periphery of the low, but expect brisk, gusty winds to return for
tomorrow.  Latest satellite imagery shows a widespread area of
backwash cloudiness over the the upper midwest, but latest low-mid
level RH progs suggest that the moisture should remain north of the
WI border and not not expect any sgfnt cloud cover to work south of
the border in the short term forecast period, so expect mostly clear
skies to persist through the day tomorrow.  Temperatures across the
CWA will top out at 15 to 20 degrees lower than yesterday, with
highs in the upper 60s to lower 70s.  Under clear skies tonight,
conditions should be rather chilly, with lows in the mid to upper
40s away from downtown Chicago where lows will only be in the low
50s.  Max temps should be a couple degrees lower tomorrow due to a
combination of persistent cold advection and lowering heights aloft
as the deep upper low begins to move south to near lake Superior by
tomorrow evening.

&&

.LONG TERM...
319 AM CDT

Tuesday through Sunday...

Tuesday through Wednesday: Main focus for Tue/Wed will be on the
placement of the 500mb vort and surface low. Current guidance is
indicating the shortwave will be stalled over Lake Superior early
Tue, then begins to drift south Tue aftn/eve. This will likely
spread precip chances further south towards Northern IL late in the
day Tue through Wed as the shortwave drifts south towards Southern
Lake Michigan Wed morning. Guidance continues to trend cooler for
Wed as cloud cover will be considerable and limiting much direct
heating. Have nudged temps cooler towards the lower 60s, but could
see this not being cool enough with the possibility of several
locations remaining in the upper 50s.

Thursday through Monday: In the extended periods there remains
considerable uncertainty with the shortwave that will start the week
across the Northern Great Lakes and likely trend towards the Ohio
Valley/Middle Altantic region Thursday. Downstream 500mb ridging
over the Northern Atlantic will act to block or stall the shortwave,
which may result in the shortwave retrograding back towards Lower
Michigan late in the week. Ensembles continue to struggle with how
to resolve the downstream ridge, which is resulting in several
solutions diverging with differing forecasts. Have opted to not
stray too far from a blend of EC/GFS in the extended periods, with
temps slowly expected to moderate towards the low 70s. Towards the
second half of the weekend ensembles are returning to greater
confidence in the 500mb wave pattern, with weak troughing over the
Southwest CONUS and a downstream ridge for the Great Lakes/Upper
Midwest. This should help to lift the lingering shortwave towards
the Northeast CONUS.

Beachler

&&

.AVIATION...
For the 18Z TAFs...

Only concern this period remains winds. West winds have been
gusting to 25-30 kt with a few higher gusts. Expect that
prevailing gusts to 30 kt through arnd sunset should hold though
an isold higher gust is not out of the question, with direction
remaining prevailing wly through the late evening hours. A brief
lull in winds should occur overnight as a weak trough rotates
around the swrn periphery of a deep sfc low north of Lake
Superior. Expect wind speeds/gusts will increase again after
sunrise tomorrow morning, though gusts should only be to arnd 25kt.
Otherwise, dry/vfr through the period.

&&

.MARINE...

220 am...Low pressure over western Ontario will slowly move
southeast to Lake Superior Tuesday afternoon and then move south
across western lower Michigan Tuesday night...as it slowly
weakens. Westerly winds will persist across the lake through
Tuesday with speeds to 30 kts. There remains the potential for
some gale gusts this afternoon into tonight but confidence on how
widespread these will become remains low and will maintain gale
gust mention. Winds will turn northerly Tuesday night as the low
moves south with speeds diminishing. As the low moves to the mid
Atlantic Wednesday...high pressure will build across Ontario and
the upper midwest...tightening the gradient again across the
western Great Lakes. North to northeast winds may increase back to
30 kt Wednesday night before diminishing on Thursday.

With this low slowly moving across the lakes region and associated
cold air aloft...there will be a chance of waterspouts Tuesday
into Wednesday. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...Small Craft Advisory...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 7 PM
     Tuesday.

     Small Craft Advisory...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 10 PM Wednesday.

&&

$$

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