Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 161916

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
216 PM CDT Wed Aug 16 2017

209 PM CDT

Through Thursday...

Low pressure centered over the mid Missouri Valley this afternoon
will lift to northern Wisconsin by midday tomorrow resulting in
active weather across northern Illinois and northwest Indiana
through tomorrow. This afternoon, a warm front is draped east from
the low center across Iowa and northern Illinois where dew points
have risen into the low 70s for many areas. Mid level lapse rates
aren`t too terribly impressive this afternoon which is helping to
keep instability in check to some degree, but still expect
moderately unstable conditions of 1000-1500 J/kg MLCAPE to
overspread the entire CWA through the afternoon. Broad ascent will
gradually overspread the area through the afternoon and evening
as upper ridge axis departs east with weak mid level height falls
begin moving overhead. An arcing band of showers have already
formed from near DVN southeast through PNT and expect intensity to
continue to increase this afternoon in response to increasing
forcing. 18Z SPC mesoanalysis indicates deep layer shear is less
than 30kt across the area which should remain the case through at
least the remainder of the afternoon, then slightly stronger winds
aloft will move overhead this evening and tonight. Overall,
severe threat is low through the rest of the afternoon given the
submarginal shear, but cannot rule out an isolated severe storm
or two this evening. Widespread severe weather is not expected.

Second area of thunderstorms is expected to develop over Missouri
and Kansas this evening in response to strengthening low level jet
ahead of a secondary low that forms near Kansas City. Storms
should track northeast into central and parts of northern Illinois
late this evening and overnight. Many of the high-res models have
trended farther south with overnight convection and have lowered
PoPs some in response. If trends continue to hold, PoPs may need
to be lowered further. Southward trend seems reasonable given the
expected orientation and position of the LLJ with the best forcing
focused south of our CWA.

Through the day Thursday, surface low will move from northern WI
into the Upper Peninsula of MI while a trailing cold front sweeps
across northern Illinois. Depending on how quickly the area clears
tomorrow and destabilizes, there is some threat for thunderstorms
to redevelop along the front across the far eastern tier of
counties in northwest IN early tomorrow afternoon. If this
happens, there is some potential storms could be severe, but
chances appear best just east of the CWA.



209 PM CDT

Thursday night through Wednesday...

We dry out Thursday night as the upper level trough axis rotates
through.  Slightly cooler and drier air moves in leading to a dry
day Friday. A secondary trough axis rotates through Friday night
through Saturday, and showers and storms may form in response to the
trough axis.  Models disagree on how widespread precip will be
Friday night. The ECMWF is the most bullish while the GFS and NAM
have precip focused south of I-80. Decided to keep a chance of
showers and storms across the forecast area.

The upper level pattern becomes more zonal over the weekend and heat
builds back into the area. Dry conditions are expected for the
beginning of next week, and we may see 90s Monday.

The next significant chance of precip arrives Tuesday with an upper
level wave and surface cold front. Cooler air and an expansive
region of high pressure move in behind the front for mid week.



For the 18Z TAFs...

Thunderstorm chances and timing make up the primary forecast
challenges today. There appear to be a couple windows of
opportunity for thunderstorms this afternoon through tonight.
First, a warm front lifting across the region this afternoon and
early evening is expected to kick off widely scattered
thunderstorms across northern Illinois and northwest Indiana. Mid
afternoon through early evening seems to be the best likelihood
for seeing thunderstorms at or near the terminals, but for now
expect coverage to remain low enough to preclude any prevailing
mention on station in the TAFs, and will maintain a VCTS in the
meantime. Confidence in specific timing remains low, and a shift
of an hour or two in either direction appears plausible.

A second round of thunderstorms is expected to develop in the
vicinity of the mid Missouri Valley this evening with
thunderstorms tracking east across central and northern Illinois
late this evening and overnight. Hi-resolution models have been
trending towards the best coverage being farther south of the
terminals overnight, so confidence in this second round of
thunderstorms is diminishing. Will maintain a prob30 for now but
would not be surprised if we remove thunder mention overnight if
model trends continue to hold.

A cold front will sweep across the region Thursday with winds
turning westerly and becoming gusty. Gusts are expected to
increase into the mid and potentially high 20kt range during the
afternoon Thursday.



209 PM CDT

High pressure over the eastern Great Lakes will reach New England
this evening. Meanwhile a low pressure system over Iowa will reach
the upper Midwest by Thursday morning and then move over the western
Great Lakes Thursday night. Winds will become southeast and increase
to 30 kt especially over the northern half of the lake tonight as
the low approaches the region.  Gusts to 30 kt are possible over the
northern half of the lake Thursday morning.  The low`s cold front
will swing across the lake Thursday evening/night, and winds will
become west to 30 kt behind it.

Winds will diminish and become northwest Friday, and high pressure
builds over the Great Lakes Saturday.  Winds will back to southwest
Saturday night into Sunday. Guidance then suggests a cold front will
slide down the lake Tuesday or Wednesday next week.






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