Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 291509

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1009 AM CDT Wed Mar 29 2017

332 AM CDT

Through today...

Main forecast concerns/challenges are with dense fog this
morning, and then shower and thunderstorm chances increasing
tonight into Thursday with a threat of strong to severe
thunderstorms along/south of the Kankakee river in east central
Illinois and northwest Indiana Thursday afternoon/evening.

Continue to monitor fog trends here this morning, with the
overall trend overnight has been for the dense fog to slowly move
west. Can see low stratus and fog on satellite, however, surface
obs and webcams show the more dense fog situated from northern
Lake/Porter counties northwest through Cook, and into
Boone/McHenry counties. The most dense fog is still likely right
along areas near Lake Michigan, and then closer to the IL/WI
border. Have expanded the advisory overnight with the most recent
updates to include Boone, McHenry, and now Winnebago counties.
Although there is likely fog across DeKalb, Kane, and DuPage
counties at this time, the more dense fog is likely situated just
to the north. Will continue to monitor these areas including Will
for possible dense fog and expansion of current advisory. Short
term guidance which has a fairly decent handle on the trends, has
further expansion of dense fog into these areas in the 9-11Z time
frame. Morning commuters should be prepared for this dense fog to
impact travel this morning, with vis at or below one quarter mile
creating hazardous travel. Improvement still anticipated by mid
morning, but areas closer to the lake may see this dense fog
linger longer into late morning or midday.

Expect most locations to remain dry for much of the day, but
guidance is still varying to the extent of any upstream precip to
move overhead later today into the afternoon. Drier air in place
and strongest focus for much of the day remaining to the west are
likely complicating factors with these precip trends. So kept
areas dry through midday but I do introduce slight chance pops by
early afternoon for areas along/west of I-39, and then slowly
increase to low chance pops by late afternoon. Areas elsewhere
should stay dry, although latest runs of the HRRR are showing some
light showery development by late afternoon. Can`t completely
rule out this light precip, but do expect to remain thunder free
with any development today.



332 AM CDT

Tonight through Tuesday...

Upper level low will continue to slowly move through the central
CONUS through tonight, with attendant surface low likely reaching
northern MO. Increasing ascent from approaching shortwave energy,
lifting boundary, and strong LLJ will really help precip spread
across the area tonight. However, LLJ and isentropic ascent do
appear to initially be focused over northern Illinois for much of
the evening. So have highest pops situated over north central
Illinois for the evening but then as LLJ begins to veer, should
see precip spread into remaining areas in northeast Illinois and
northwest Indiana overnight. Despite weaker instability tonight,
strong/focused ascent should allow isolated thunder to spread
across the CWA. So have expanded thunder mention tonight. Don`t
anticipate severe tonight, however, can`t rule out an isolated
stronger storm across the southern CWA.

Showers and thunderstorms still likely to be ongoing over the
entire area Thursday morning, with similar trends to intensity
possible over the southern CWA during that time. Surface low and
boundary expected to lift northeast throughout the day Thursday,
with guidance now indicating a strong possibility for this
boundary to lift north into the CWA by the afternoon. Although
some variability with placement, do think at least the far
southern area along/south of the Kankakee river could get into the
warm sector. This presents an increased threat for strong to
severe weather for this area Thursday afternoon into the early
evening. Guidance indicating surface based instability south of
the boundary as well as the potential for steeper lapse rates.
However, confidence to the extent of any instability in this area
remains lower given the high likelihood for cloud cover to be
present for most of the day. As upper level trough/low lift
northeast in the afternoon and forcing and mid/upper level flow
increases, the possibility for more robust development will
arrive. This area appears to be right on the northern extent of
the stronger shear though. Nonetheless, enough ingredients in
place to monitor for a damaging wind/hail threat Thursday
afternoon for areas along/south of the Kankakee river in east
central Illinois and northwest Indiana. Although thunderstorms
will likely linger Thursday evening, the threat of any stronger
development should end during this time.



15z Update...

Fog and stratus have largely eroded this morning with some
lingering fragments of low stratus in spots. Otherwise, mid level
cloud cover is thickening and bases will be lowering through the
afternoon but remain VFR until tonight. Have added a VCSH mention
for late afternoon with ascent ahead of the approaching low
pressure system increasing notably for a several hour window.
Still have a wedge of drier low level air in place to overcome but
expect that radar returns will increase with some intermittent
light rain being able to reach the ground in spots after about


Previous discussion for For the 12Z TAFs...

Fog and low ceilings, increasingly gusty winds, along with showers
and possibly thunderstorms all forecast concerns today and
tonight. Early this morning, low stratus and fog moved inland from
Lake Michigan and have overspread much of northern Illinois and
northwest Indiana. Expect fairly steady state conditions through
the morning with ceilings of 002-004 and vsby between 1/2 and 1
1/2 miles, though cannot rule out brief 1/4SM. Should start to see
some improvement mid to late this morning, between roughly 15-17Z
with ceilings lifting and eventually scattering back to VFR. ENE
winds will prevail throughout most of the TAF period and are
expected to become gusty early to mid afternoon, with gusts into
the low to mid 20 kt range. Winds may diminish some with sunset,
but are expected to pick up again overnight as low pressure
approaches the region and the gradient tightens. Gusts may
increase into the mid to high 20 kt range overnight. Showers and
thunderstorms are expected to develop over Kansas and Missouri
this afternoon and spread northeast across northern Illinois late
this evening and overnight. Instability is fairly modest this far
north, so mainly expect showers, but there will likely be a few
isolated thunderstorms in the area as well. Showers will continue
off and on through the morning and into midday Thursday.



312 AM CDT

Northeast to east flow is expected to increase again this
afternoon and evening as low pressure lifts from the southern to
central Plains. The low is expected to track east from Missouri
Wednesday night to northern Indiana Thursday night. East winds are
expected to peak late Wednesday night into Thursday around 30 kt
with occasional gale force gusts to 35 kt expected, mainly across
the south half of the lake. The low will continue east to the
Atlantic Coast Friday night with winds over Lake Michigan backing
to the north, but remaining moderately strong around 30 kt. High
pressure will begin to build over the region on Saturday allowing
winds to relax some over the weekend before another low takes a
similar track south of Lake Michigan early next week.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...4 PM Wednesday to
     4 AM Saturday.




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