Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241942

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
242 PM CDT Sat Jun 24 2017

240 PM CDT

Through Sunday...

Mixed clouds and seasonally cool conditions continue this
afternoon, with readings holding the 70s. Clouds are thinning
across north central Illinois behind a weak moisture gradient and
with falling dewpoints.

An organized closed upper low over MN will lead to an expansion of
showers and even a few thunderstorms to our northwest this
afternoon. Guidance is in good agreement in sliding the upper low
through south central WI this evening, then quickly east to the
eastern Great Lakes Sunday. With the less than optimal arrival
time of the upper low and with fairly dry low level conditions in
place, expect the showers and storms to weaken as they move west
to east across the area this evening. The forcing is still decent
enough to include the scattered shower mention for northern IL,
with a lowest chances south and east. By midnight most activity
should be done.

The surface pattern changes very little into Sunday, thus expect
continued breezy west winds. The cool northwest flow pattern will
also continue, thus expect quick development of a broken cloud
field again. Shortwave disturbances Sunday will be more subtle,
but there appears a slightly better chance that one comes through
during the peak heating time of day. Therefore, isolated showers
are possible in the afternoon. Thunder chances still appear low,
but slightly higher than today. With the cool air aloft and broken
clouds, expect temperatures to again be held down in the vicinity
of 70 possibly a shade higher.



225 PM CDT

Sunday night through Saturday...

For the longer term forecast period, general trends remain
consistent with previous model runs and the short term forecast
period trends.

By Sunday night, the longwave pattern will continue to trend to a
high amplitude, short wavelength, slowly progressive longwave
pattern, with broad upper troughing east of the Rockies and strong
upper ridging over the west coast.  The general trend for the
remainder of the weekend and into early next week will be for well
below normal temperatures and low precipitation chances.  A series
of weak shortwaves will drop through the nwly flow aloft helping to
deepen the upper trough Sunday and Monday and max temperatures will
be the lowest of the period with highs only in the upper 60s to
lower 70s. This would give max temperates of 10 to 15 degrees below
seasonal normals, which are in the lower 80s for late June.  With
broad cyclonic flow aloft persisting across the region through early
next week, there will be periodic chances for scattered showers or a
few isolated thunderstorms, particularly in the afternoon hours.  By
Tuesday and through the remainder of next week, the upper ridge will
begin to progress east, with heights building across the region.  At
the surface, the high pressure that will cover the region through
the weekend and early next week, will shift east, setting up a
return flow of warm/moist air.  A warming trend will begin on
Tuesday with highs increasing to the middle to upper 70s. The
warming trend will continue through the week, with temps rebounding
back to the lower to middle 80s.  The next significant chance for
precipitation will come by Wednesday night into Thursday as low
pressure develops over the northern plains and tracks to the western
Great Lakes. Increasing warm/moist advection in advance of this
system will bring increasing chances for showers and thunderstorms
for the middle to later portions of next week.


For the 18Z TAFs...

Satellite depicts a broken lower VFR stratocumulus field across
the flying area this afternoon along with gusty west winds. Expect
the cloud field to stick around for a decent portion of the
afternoon, lifting a bit but also clearing out a little from west
to east. The more organized disturbance upstream in MN will slide
across southern WI this evening. Forcing does appear strong enough
even across the Chicago terminals for a broken line of showers,
but moisture is somewhat scant. Confidence high enough for TEMPO
at RFD, and gave some credence to the forcing to carry VCSH at the
Chicago TAFs, but guidance does continue to show a weakening
trend with eastward extent.

Forecast soundings show a near repeat performance tomorrow with
similar steep low level lapse rates suggesting another quick
arrival of a lower VFR stratocumulus field again that lifts some
with daytime heating. Another wave in northwest flow will lead to
a more typical afternoon isolated shower chance. Thunderstorm
potential is still low, but a tad higher than chances tonight.
West winds continue Sunday, likely a tad weaker than today.



235 PM CDT

A relatively quiet period of marine weather is expected across
Lake Michigan through the remainder of the weekend and into early
next week, as weak low pressure slowly moves across the eastern
Great Lakes and high pressure builds across the northern and
central Plains. This will set up modest west to northwest winds
across the lake, though there will be some occasional gustiness
especially along the west shore with 20-25 kt winds during the
afternoon hours when daytime heating and stronger winds aloft can
mix down to the surface over land and a short distance offshore.
Eventually, the Plains high will move southeast into the lower
Missouri and Ohio Valleys, with weaker ridging moving east into
the western Lakes region by Tuesday. Low pressure is expected to
develop over the northern plains Tuesday night and then lift
northeast to the northern Great Lakes late Wednesday night into
Thursday. The low is expected to deepen as it tracks northeast,
will result in increasing south winds across the lake Wednesday
into Thursday, with 30 kt winds likely and a few higher gusts





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