Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 251750
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1250 PM CDT MON JUL 25 2016
259 AM CDT
A cold front bisects the county warning area along a southwest
to northeast axis. Most of the remaining precipitation is well
ahead of the front with some lighter shower activity continuing to
form along its leading edge. There are also a few showers in
southern Wisconsin associated with an upper low across west central
Ontario. Signs of a changing pattern are evident on water vapor
satellite pictures upstream of our area and at our latitude, where
the images depict a much drier story and is void of any
The leading edge of the cold front will move southeast of the area
later this morning, taking the last of the lingering showers and
thunderstorms with it. Behind the front we will welcome in a
slightly cooler and less humid air mass, along with at least a
few day period of dry weather. Comfortable dewpoints in the
60s will make more seasonably warm but certainly more tolerable
conditions for mid-summer, as highs top out in the mid to upper
80s, or just a tad above normal, though a bit cooler at the lake
with a lake breeze expected.
Surface high pressure will take firm control Monday night and
Tuesday and possibly for most of Wednesday as well. This will
result in sunny to mostly sunny skies along with lake breeze
development each day. Lows fall into the mid 60s outlying areas to
around 70 in the city, and highs hold in the mid to possibly some
upper 80s, slightly cooler lakeside, bringing of few days of
closer to normal conditions. Expect increased cloudiness on
Wednesday ahead of northwest flow and a slow moving upper low.
259 AM CDT
Wednesday night through Sunday...
An upper level disturbance generally emanating from the Pacific
Northwest will approach the Mississippi River Wednesday night. The
leading shortwave bring increased chances of precipitation
Thursday. The main shortwave moves overhead around Friday, which
would bring continues chances for rain showers, possibly a few
thunderstorms and slightly below normal temperatures in the upper
70s to low 80s. The low will pull east late in the weekend
decreasing precipitation chances.
For the 18Z TAFs...
High pressure will be in place through the period allowing Lake
Michigan to influence the winds at all sites but RFD. Large scale
flow remains north-northwest this afternoon with the lake having
helped turn winds north-northeast at ORD/MDW/GYY. Radar shows a
boundary moving inland and positioned over ORD/MDW at 1730z.
Wind obs at each site have shown an increase in variability and
even a westerly direction which has probably been caused by
convergence just ahead of this boundary. Expect the boundary to
slowly continue inland and winds to become easterly behind it in
the next few hours if not sooner. Recent MDW and lakefront obs
already show an east direction but some variability may occur with
the slow moving boundary still nearby.
Otherwise winds will diminish and may even be calm overnight.
Will continue with MVFR vsby mention at RFD/DPA/GYY but it may be
that shallow ground type fog develops vs. a larger scale uniform
vsby reduction. The high drifts east but keeps its influence in
place Tuesday. Light and variable winds will likely persist
through the morning so its tough to pin a direction down. By
afternoon, a east to southeast lake influence is favored at
ORD/MDW with an east to northeast direction favored at GYY.
205 AM CDT
The cold front associated with the low over Ontario shifts southeast
of the lake early this morning. West to northwest winds diminish as
the gradient weakens today and a lake breeze is expected. High
pressure moves over the lake early Tuesday morning. Winds become
light and variable over the southern end of the lake and southwest
10-20 kt over the northern half. Winds will be light and variable
Wednesday, but then become north to northeast Wednesday night as the
high shifts east. Winds remain northeast through the end of the
week as a weak low moves over northern IL Thursday night.
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