Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211746

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1247 PM CDT Tue Mar 21 2017

251 AM CDT

Through tonight...

No major wx concerns in the short term. Lake enhanced secondary
cold front will move across the area today bringing a re-enforcing
shot of cooler air later this afternoon. Behind the front temps
will level off and begin to fall, especially near the lake where
temps will probably drop into the 30s by evening. Weak shortwave
will move across the region this afternoon, but guidance has
continue to trend dry with this feature. Have removed pops for
this afternoon, but plan to maintain a chance of sprinkles just in
case the virga from the mid level deck manages to penetrate the
low level dry air over far southern portions of the CWA.

Deep trough and healthy late season push of cold air expected to
dig into the NW CONUS Wednesday. Associated surface high will move
into the western Great Lakes and provide for a glancing blow of
the cold air locally, which should keep temps in the 30s near and
downwind of Lake Michigan to the low 40s well inland.



251 AM CDT

Wednesday through Monday...

Upper trough will quickly exit stage right as upper ridge gets
shoved east into the region by strong upper trough digging into
the four corners region Thursday. Temps will begin to moderate
Thursday as a warm front lifts north across the area during the
afternoon. Could see some showers associated with the front, but
as the front lifts north the better precip chances appear to be to
our north Thursday night. Maintained a chance of showers and
thunderstorms northern CWA Thurs night in case the northward trend
in the models reverses, but as it looks now, we could be dry Thurs

Various medium range models remain in good agreement on having our
entire CWA in warm sector of approaching cyclone Friday. Blended
model guidance temps appear a bit too cool compared with local
925mb climo temps, which would tend to support at least lower 70s.
Have nudged high temps up a bit, though with more sunshine and
less clouds we could be even warmer Friday.

The latest runs of the GEM, ECMWF, and GFS all continue to close
of a significant mid-upper level low associated with the
approaching storm system late in the week into the weekend.
Suspect that models could trend a bit slower with time in precip
arrival, with Friday likely remaining dry and perhaps even into
Friday evening before the big upper low wobbles east into the
region. There remains some spread in the models regarding the
track of this large slow moving closed low, and while the track
differences aren`t terribly large early on, they do result in
pretty drastically different wx locally. The slightly more
southerly solution would mean a stout, chilly wind off the lake
Saturday and Sunday, while a more northerly track could keep temps
more moderate Saturday. Given low confidence in picking any one
particular solution, made no changes from the blended model
guidance initialization. The slow moving upper low could also
result in at least periodic chances of showers and perhaps
t-storms Friday night through the weekend.



For the 18Z TAFs...

N/NW winds of 10-15kt will gradually become N/NE this afternoon as
high pressure builds into the Upper Midwest.  Will continue to
indicate wind direction change to N/NE at KORD by 19z, although some
high-res models such as the HRRR are now suggesting it may be
delayed until 20-21z.  Mid/high clouds will prevail through the
afternoon before MVFR clouds develop/advect off Lake Michigan this
evening into the overnight hours.  Forecast soundings show ceilings
lowering to around 1500ft at KORD after 03z.  The lower ceilings
will persist through the night before clouds scatter by mid to late
Wednesday morning.



207 AM CDT

Another cold front will shift southward over the lake today. In
its wake, a surge of much colder air is expected to shift
southward over the lake. This airmass will help set the stage for
a period of enhanced northerly flow up to 30 KT down the lake
later today and tonight. These stout northerly winds will also be
aided by a potent surface high, which will gradually build
southeastward from south central Canada into the Upper Great Lakes
today through Wednesday. With this period of stronger northerly
winds over the lake, expect high waves up to around 10 FT over the
southern shores of the lake. These hazardous conditions to small
craft will continue over the southern Lake Michigan near shore
waters through Wednesday afternoon.

Once this surface high shifts over the eastern lakes by early
Thursday morning, the winds over the lake will shift southward
for a period again during the day Thursday. However, another cold
front is likely to shift down the lake Friday night into early
Saturday. Following this frontal passage, another potent surface
high is likely to build over the Upper Great Lakes and southern
Canada as a storm system tracks eastward from the Plains to the
southern lakes region by Saturday night. These two pressure
patterns could result in a period of strong northeasterly winds
again on the lake for a period this weekend.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...4 PM Tuesday to 4 PM Wednesday.




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