Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211916

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
216 PM 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




Saturday night through Friday...

216 pm...Two main forecast concerns this period...first high temps
Sunday with a cold front late afternoon/early evening...second is
precip/thunder chances with a storm system middle of next week.

High pressure will be departing to the southeast Saturday night as
low pressure moves from the Dakotas Saturday night to lower MI by
Sunday evening. A trailing cold front will move across the area
Sunday afternoon into Sunday evening. Timing of this front will
have a large impact on temperatures...with possible quickly
falling temps behind the front but also how warm temps rise Sunday
afternoon. Ahead of this front...temps will warm well into the 60s
and some lower 70s are also possible especially across the
southern cwa. Possible front may accelerate down the lake into
far northeast IL with a wind shift more off the lake. An earlier
arrival would limit how much high temps rise across the north.

A fast moving high pressure will build across the area Monday into
Monday night with high temps Monday/Tuesday near seasonal levels...
upper 50s/lower 60s. Another low will develop over the central
plains Monday and Tuesday and move across the area Wednesday
afternoon into Wednesday night. This timing and track has been
fairly consistent for several runs now. There is a chance of
precip along a warm front Tuesday afternoon/Tuesday night which
sets up mainly over southern WI...possibly affecting far northern
IL. But the bulk of the precip will hold off until Wednesday and
Wednesday evening as the low moves across the area. Chance of
thunderstorms also likely Wednesday along with gusty southeast
winds as the low approaches. Flow quickly turns back southerly
Thursday night into Friday with high temps possibly remaining in
the 60s for the end of the week. cms


For the 18Z TAFs...

- Wind direction varying between NW and N with speeds at 10 kt or
higher this afternoon

High pressure to the west and low pressure off the Atlantic coast
is resulting in occasionally gusty and erratic winds for the
terminals. While the direction is generally NW to NNW they will
occasionally vary to more of a 300-320 direction and not out of
the realm of possibility to occasionally drift to 010. Have held
to a 340 idea at this has been the general consensus. Winds should
decouple fairly quickly this evening as the high noses in.

Expect a broken VFR deck this afternoon in the 4000-5000 foot
range away from any lake effect clouds in NW Indiana. Expect these
to also erode after dark. Winds shift to southwest on Saturday as
the high drifts south and east of the area Saturday afternoon. We
should see some mid to high clouds tonight into Saturday that
could lower some.




216 pm...Northerly winds will slowly diminish tonight into
Saturday as a ridge of high pressure approaches the lake tonight
and moves over the lake Saturday. Winds will then turn west/
southwest ahead of low pressure which will move across the lake
Sunday afternoon. This low will be absorbed by stronger low
pressure over Quebec Sunday night but strong high pressure will
build across the plains Sunday night and then across the Great
Lakes region Monday. The gradient will tighten Sunday night with a
brief period of northerly winds to 30 kts possible...mainly on the
south end of Lake Michigan. These northerly winds should steadily
diminish Monday as the high moves across the lake. Another low
will move from the central Plains Tuesday to southern lower MI
Thursday. Southeast winds will increase as this low approaches and
may reach 30 kts on Wednesday. Winds will then shift to the north/
northeast Wednesday night into Thursday as the low shifts east.


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

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




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