Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 170743

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
243 AM CDT Thu May 17 2018

239 AM CDT

Through tonight...

Today should be another nice weather day overall, with more sun
than clouds, warm temperatures, and low humidity. There is a lake
enhanced cold front across central Wisconsin which will continue
through the area today and this evening. The main impact will be
a slightly stronger wind. Temperatures inland will peak similar
to Wednesday, but we will shave a few degrees off closer the
lake with a stiffer wind off still cool Lake Michigan waters.
Otherwise, expect high clouds to increase through the day as low
pressure slowly migrates into the Ohio valley. The low will remain
confined south of the region tonight, but clouds will thicken
further as ridging to the north of the low will get dislodged to
the east.



239 AM CDT

Friday through Wednesday...

Models are suggesting we will feel a bit more influence from the
upper low to our south. With blocking high pressure moving away,
south/south-east steering flow will bring more clouds on Friday,
and even some showers as the day continues. The low will slowly
shift northeastward toward the eastern Great Lakes Friday night
into Saturday. Lift will ease as the low pulls away Saturday
afternoon, thus expect a ramp down in any light shower/sprinkle
activity, with even some decreasing clouds through the day.
Precipitation never appears to be heavy and appears more like
nuisance rain. Confidence on coverage is also not that great, but
cannot ignore the increasing signal for precip and will continue
to carry lower chances of showers.

Attention will turn to the slow moving upper low across the
western US that will migrate into Plains on Saturday. Moist
southwest flow ahead of the low will increase precipitation
chances area wide gradually during the latter half of the weekend.
Our cold front from Thursday will slowly lift back northward as a
warm front. Disturbances will ride over this boundary leading to
several periods of showers, and with instability increasing even a
few thunderstorms. Guidance is not in great agreement regarding
the frontal position and progression of the low, thus confidence
on thunderstorm coverage is not that high.

Sunday night models are vastly different with the placement of the
surface low, but the upper level patterns are not all that
different. The net result from the blended model initialization
is the highest chances for precip Sunday afternoon, continued
chances Sunday night, with things trailing off Monday. With the
GEM looking more like the GFS, will not deviate from the low
precip chances Monday in spite of the EC having high pressure in
place and dry conditions.

There are some rain chances beyond Monday, but the main frontal
boundary will likely be positioned to our south and west, and
would be the more favored areas for precip in a northwest flow
aloft pattern. Temperatures remain seasonal, with continued lake



For the 06Z TAFs...

Quiet VFR weather conditions will persist through the TAF period,
with northeast winds generally prevailing.

Weak surface high pressure ridge currently overhead will result in
light and variable winds overnight. However, a cold front is
expected to move down the lake and across the terminals during
the early to mid morning hours. Once this arrives expect the winds
to shift northeasterly across the area. Wind speeds look to
initially remain around, or just under, 10 kt. However, during
the afternoon expect wind speeds to increase a bit to 10-15 kt as
flow increases off of Lake Michigan. Easterly winds up around 10
kt will also continue across the area tonight.



214 PM CDT

A high pressure ridge across southern Lake Michigan this afternoon
will gradually weaken through Thursday while drifting south. A
weak cold front will shift southward across the lake tonight into
Thursday as stronger high pressure builds across Ontario. South
winds to 20 knots across mainly the north half of the lake this
evening will transition to N to NE winds to 25 knots late tonight
north through late Thursday south. Generally E to NE winds under
20 knots will then persist late Friday through the weekend with
weak high pressure across the region. Weak low pressure will then
cross the region Monday into Tuesday, though winds should remain
under 25 knots.






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