Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 210804

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
304 AM CDT Fri Oct 21 2016

304 AM CDT

Through tonight...

Primary concern in the near term is with lake effect showers
today. Coastal observations show mid-lake convergence band has
developed with regional radar imagery depicting band of lake
effect showers developing at this hour. Forecast soundings depict
a stout inversion around 5000ft AGL now, but with time, weaken
this inversion this morning, so would anticipate gradually
intensifying shower activity. Cannot rule out a thunderstorm, but
equilibrium levels look to remain largely below the -20C, so
didn`t think thunderstorm chances were high enough to keep in the
grids. Conditions do look favorable for waterspouts and cannot
rule out a waterspout near the shore. Plume of lake effect showers
will likely wiggle around a bit today with a tendency for band to
shift more into Porter Co later this afternoon and evening, before
shifting east of our CWA tonight.

Other than lake effect, other concern is with frost potential.
Any frost this morning will likely be patchy, given winds still up
in the 5-10kt range and some cloudiness near the WI boarder. While
there may still be patchy frost in sheltered areas, risk is low
and time for action has passed, so plan to drop the current frost
adv with morning forecast issuance. Conditions look far more
favorable for frost tonight, however it is getting late in the
season and lows are only forecast to be a few degrees below
average, so after coordination with ILX/DVN, decided to defer
decision on need for further frost advisories (including tonight)
to the day shift who can collaborate with our agricultural



304 AM CDT

Saturday through Thursday...

Quiet weather over the weekend into early next week. Fast moving
northern stream wave will pass to our north over the weekend with
WAA ramping up and leading to some moderation in temps Saturday
and a respectable warm up Sunday. By Sunday, highs will probably
making a run at 70F most areas. Cold front will pass with little
fanfare Sunday evening, though northwest winds will pick up behind
the front and usher slightly cooler temps (closer to normal).

Another shortwave trough is progged to move through the region
midweek, with WAA potentially leading to showers break out as
early as late Tuesday, but more likely Tuesday night into
Wednesday. Both GFS/ECMWF forecast a fairly sharp trough and a
narrow ribbon of respectable moisture return (Tds in the 50s/near
60), which would support at least a potential for some thunder
Wednesday. Initially, cooler Pacific origin air mass is forecast
to filter in behind the system. Some indications exist that a more
pronounced long wave trough could lead to better cold air
intrusion late next week and into next weekend.



For the 06Z TAFs...

Main forecast concerns/challenges are this morning with MVFR
ceilings and potential for showers. Ceilings continue to move
across the area at this time, but with generally VFR ceilings in
place at this time. Do think there will be a slight downward trend
in the ceilings with MVFR becoming more probable in the next
couple hours, primarily for MDW and GYY. It`s during this time and
place where increased shower development will occur, and likely
persist through mid morning. These lower ceilings and scattered
showers will shift to the east with time later today. Northwest
winds will prevail through the period, diminishing later today and
then becoming more west northwest late in the period.



304 AM CDT

Northerly winds persisting this morning as low pressure is
situated over the mid Atlantic, and while large high pressure is
over the Plains. Prevailing winds/gusts of up to 30 KT will likely
continue through midday and although did remove mention of gale
force gusts, do think a sporadic gale force gust may still be
possible over the next couple hours. Did not make any changes to
the small craft advisory, with hazardous winds and waves likely
for much of today. Diminishing trend occurs later this afternoon
and especially tonight as surface ridge of high pressure moves
through the region. Winds will then return to a west southwest
direction by Saturday through much of the rest of the weekend.
However, next low pressure system to move through the region will
arrive Sunday and help bring a shift back to the north while
speeds increase.



IL...Frost Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
     ILZ012 UNTIL 8 AM Friday.

LM...Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 4 PM Friday.

     Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 4 AM Saturday.




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