Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211952

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
252 PM CDT Fri Apr 21 2017

243 PM CDT

Through Saturday...

High pressure continues to work its way across the region this
afternoon, with quiet and dry conditions in place. Expect this to
persist through the period, while upstream system stays just to
the south of the CWA tonight through Saturday. Outside of stratocu
in place, still feeling the effects of this upstream system
through rather dense cirrus overhead. The more dense cirrus is
sliding to the east at this time, but satellite imagery depicting
additional thicker cirrus approaching from Iowa. Did increase
cloud cover through early this evening to keep some mention of
cloud cover over northern Illinois, while the southern CWA
observers more of this cloud cover. Should see a trend for
clearing skies from north to south tonight, however, its quite
possible that the entire CWA observes additional cloud cover from
this system overnight. Cooler temps around 40 expected tonight,
with upper 30s still possible over north central Illinois. Cloud
cover and slightly elevated winds tonight may impact night time
lows, including the extent of any frost tonight. Did maintain
patchy frost mention over north central Illinois, but with this
cloud cover and steady winds, there is a strong possibility that
this front remains really limited.



248 PM CDT

Saturday Night through Friday...

High pressure will build across the region Sunday into Monday
resulting in fair and mild conditions across the region. Synoptic
flow remains out of the north-northeast albeit light Sunday which
will result in continued cooler temperatures near the lake front.
Expect an afternoon lake breeze to develop and locally enhance the
winds and help to spread those cooler temperatures farther
inland. Winds will swing around to the south-southeast Monday
behind the ridge axis and an afternoon lake breeze appears
favorable once again for areas along the Illinois lake front,
especially Lake County and northern Cook County.

More active weather is expected to develop as we approach the
middle of next week as a series of upper level disturbances and
an increasingly amplified pattern develops across the mid section
of the country. Broad corridor of deep southerly flow will promote
good moisture transport from the Gulf into the Midwest with PWats
increasing to just over and inch Tuesday as shortwave trough
lifts across portions of the Upper Midwest. Models are in decent
agreement handling the low amplitude wave though forecast
soundings do show dry low levels slow to saturate through the day
Tuesday so expect highest PoPs late afternoon into the evening.

Wednesday through the end of the week, a strong upper wave is
expected to carve out a deep trough over the Intermountain West
which will set the stage for a classic severe weather setup over
portions of the Great Plains and potentially into portions of the
Midwest. A deep upper trough axis centered over the Desert
Southwest and a Bermuda High will allow a wide open gulf and good
moisture transport into the region while aloft strong flow is
expected as a negatively tilted shortwave trough lifts across the
region. Way to early to get into specific location and timing
details, but initial indications are for the possibility of
several rounds of showers and thunderstorms locally Thursday
through the weekend along with much warmer and potentially summer-
like temperatures starting Friday. Warm front lifting across the
region Thursday will bring a shot of showers and thunderstorms
with temperatures possibly well into the 80s as we get into the
warm sector Friday and/or Saturday. As models begin to hone in on
the timing and track of the surface low and attendant front over
the next few days, any local severe thunderstorm threat should
become more clear, but in the meantime just bears close watching
until the details come into better focus.



For the 18Z TAFs...

VFR and dry conditions expected across the terminals as higher
pressure builds across the region. System tracking just south of
the area looks like it will remain to the south, with any
associated precip also staying well to the south tonight and
Saturday. Winds have switched over to the northeast at this time,
with this direction to continue through the end of the forecast
period. Some diminishing trend expected tonight, but with winds to
increase back to 10 KT or above by early Saturday morning.



248 PM CDT

Modest north to northeast flow will continue over Lake Michigan
through midday Saturday as low pressure continues to depart east
towards the New England coast and as high pressure builds into the
western Great Lakes. This ridge axis will begin to settle south
across Lake Michigan through the day Saturday with winds turning
southerly north of the ridge axis. Any southerly flow looks to be
short lived as another low moves east across Ontario into Quebec
over the weekend with a trailing cold front moving south across
Lake Michigan Sunday. Low pressure will deepen over the Plains
early next week then lift across the western Great Lakes Monday
night into Tuesday with winds swinging around to the south across
the entire lake and peaking around 30 kt during this time frame.
An active weather patten is expected from the middle of next week
into next weekend as a series of lows move across the region.



LM...Small Craft Advisory...IL nearshore waters until 4 PM Friday.

     Small Craft Advisory...IN nearshore waters until 10 PM Friday.




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