Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201915

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
215 PM CDT Fri Oct 20 2017

214 PM CDT

Through Saturday...

One more day of mild and dry left on Saturday before the pattern
starts to shift. Little in the way of note weather-wise through
the day Saturday besides gusty winds.

In the wake of a departing high and ahead of a northern Plains
deep low, the pressure gradient will strengthen over the area.
This will keep some wind tonight and provide lows several degrees
warmer than last night.

Strengthening winds on Saturday will provide gusts of 25-30 mph,
likely modulated some by whether there is sun within cirrus
breaks. Temperatures too may see slight dependence on sun, but
advection should mainly drive highs into the mid 70s given
850/925 mb temperatures near/just a tad cooler than today.




Saturday through Thursday...

230 am...Forecast concerns include rain chances Saturday night
through Sunday evening...then two cold fronts and associated cold
air burst behind each front.

The gradient will tighten further on Saturday with southerly winds
into the 15-25 mph with higher gusts. There will likely be more
cloud cover Saturday...perhaps in the afternoon and this lowers
the confidence for high temps which for now are near guidance
highs but its likely that highs on Saturday could be similar to
Friday...especially across eastern areas.

A cold front will move across the area on Sunday with showers
developing ahead of the front late Saturday night and continuing
for much of Sunday. Timing appears to have slowed just a bit with
the best chances for rain from daybreak Sunday through mid/late
afternoon. Thunder chances look fairly low and will continue with
slight chance mention. QPF amounts look to be mainly under an
inch...storm total but there could be band/axis of heavier
rainfall though too difficult to pin down where this may occur
just yet. Models differ on end time across the eastern cwa Sunday
night into Monday morning and trends will need to be monitored as
to the exact end time of precip.

The first of two stronger cold fronts will arrive Monday as an
upper low closes off over the Lakes region Tuesday into Wednesday.
This will spread much cooler air across the region through
midweek along with showers and potentially lake effect showers in
northwest Indiana though too early for exact locations which will
be dependent on wind direction. Also currently looks to stay warm
enough that this precip should remain all rain but if precip can
become heavy enough and cool the column enough...then some mixed
snow can`t be completely ruled out.

This midweek upper low shifts east by Thursday with a short-lived
ridge building across the area Thursday then another strong cold
front moves across the area from late Thursday afternoon through
Thursday night...still some timing differences. This cold blast
will likely be colder than the first and while its just beyond the
end of this also looks to be short lived as temps
begin to moderate going into next weekend. cms


For the 18Z TAFs...

Concerns with the Chicago area airport forecasts are:

- Wind direction backing slightly early this eve possibly to SSE

- Chance for low-level wind shear overnight

- Southerly wind gusts to near 25 kt Saturday afternoon

A pressure gradient will gradually tighten across the area through
Saturday leading to a strengthening wind field. Sporadic gusts in
the mid teens should be seen this afternoon before diminishing
around sunset. Winds will naturally want to back this evening and
could go as far as south-southeast in direction. Speeds are
favored to be 7-9 kt. Off the deck, a strengthening low-level jet
will provide southwest wind speeds of 40 kt around 1000 ft, and up
to 45 kt over RFD. So there is low-level wind shear potential.
Confidence is not high enough in true LLWS to include in the
Chicago TAFs at this time.

Southerly surface winds will strengthen on Saturday with gusts in
the 20-25 kt range being common Saturday afternoon. While clouds
will thicken, these will all be mid/high VFR.



244 AM CDT

High pressure is centered east of Lake Michigan today while low
pressure is slowly deepening over Alberta. Moderate south to
southwest winds will be in place across Lake Michigan through most
of the weekend. These winds will gradually strengthen to around 30
kt from late tonight through midday Sunday. Shallow mixing and
stable conditions just off the surface will help limit the
magnitude of the wind gusts, but cannot rule out a sporadic gale
force gust Saturday into Sunday. A cold front will move across
Lake Michigan on Sunday resulting in winds shifting out of the
west to northwest later Sunday into Sunday night. Winds briefly
back to the southwest once again early Monday ahead of a second
and stronger cold front which will move across the lake Monday
evening and night with winds swinging back to the northwest to
north at around 30 kt and gales once again a possibility.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...9 AM
     Saturday to 4 PM Sunday.




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