Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250528

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1228 AM CDT Tue Apr 25 2017

215 PM CDT

Through Tuesday...

Generally quiet weather expected through tomorrow.

For the remainder of the afternoon, main concern will focus on
dewpoints, and in this case, how low dewpoints will be. As of
early this afternoon, sfc dewpoints were in the upper 20s to low
30s as sfc temps rise into the low to middle 70s. This will allow
afternoon min RH values to drop to the upper teens to lower 20s
percent range. While this will be pleasant for most people, for
those planning on burning field brush will need to exercise
caution as the low relative humidity values, combined with winds
gusting to 15 to 20 kt could cause some fire spreading issues. For
tonight, there should only be some increasing mid and high level
cloud cover. Combined with winds of 5 to 10 mph, radiative cooling
should be tempered by the increasing cloud cover and low level
moisture increasing overnight as winds turn to more southerly. By
daybreak tomorrow morning, sfc dewpoints are expected to rise to
the middle to upper 40s. So, overnight lows should be in the lower
50s. For Tuesday, weather conditions will remain quiet, with
strengthening south winds further allowing for a continued
moisture increase through the day with little in the way of
afternoon drying or mixing down of drier air. Highs tomorrow
should be in the middle to upper 70s, with dewpoints in the upper
40s to lower 50s. Temperatures along the lakefront, from Chicago
northward will likely remain a bit lower as a slightly east of
south wind directions should allow for lake cooled air to spread a
short distance inland, keeping highs there in the lower to middle


328 PM CDT

Tuesday night through Monday...

Continued dry conditions likely at the start of the period
Tuesday night with upper level trough and energetic flow still
situated to the west. However, do anticipate forcing associated
with lifting shortwave energy and WAA to spread over northern
Illinois throughout the night, closer to the early morning hours
on Wednesday. Have continue chance mention for showers and
thunderstorms, giving increasing instability, but it still appears
that the higher focus will be mainly over the western CWA across
north central Illinois. Most locations in northeast Illinois and
northwest Indiana will likely remain dry during this time.
Although instability will increase Tuesday night, at this time,
thunderstorm intensity should remain low. Still worth monitoring
for an isolated stronger storm though.

Any initial showers and thunderstorms will likely lift to the
north Wednesday morning with dry conditions appearing possible for
much of the day. This is due to surface cold front remaining
situated just to the west of the CWA. Latest guidance coming in
this morning is showing increasing trends with instability, with
moderate instability appearing possible ahead of this front
especially by the afternoon. Guidance showing fairly good signal
of showers/thunderstorms developing along this front by the
afternoon and continuing into Wednesday evening. This seems like a
likely scenario given the upper level trough will be pushing
overhead. Some hazards to monitor for Wednesday afternoon and
Wednesday night will be possible strong to severe storms as well
as heavy rainfall. This instability in the presence of a highly
sheared environment supports this possibility. However, with cloud
cover likely overhead from the early morning convection,
confidence is lower with how unstable the environment gets in the
afternoon. Nonetheless, worth continuing to monitor as well as the
potential for heavy rainfall, as PWAT axis above one inch
situates overhead. Guidance varying with the end time of these
showers/thunderstorms, but it is possible that this convection
continues to stream overhead for much of Wednesday night before
departing Thursday morning. After a lull period by the end of the
work week, pattern becomes highly active once again going into the
weekend. Exact solution/details to fall out later, however, worth
noting that the weekend could see several periods of showers and
thunderstorms some of which could produce additional heavy
rainfall and possible stronger development.



For the 06Z TAFs...

1228 am...Southeast winds will remain around 10kts for much of
the early morning hours and then increase into the 12-16kt range
later this morning with higher gusts developing. Winds will turn a
bit more to the south/southeast later this morning as well. Gusts
will diminish this evening with speeds remaining around 10kts
tonight into Wednesday morning.

Scattered high clouds to start will likely lower to a bkn mid deck
later today with few/sct cu possible. Cigs are expected to lower/
develop into the 4-6kft range tonight. While vfr is expected
through the period...some patchy/shallow ground fog will be
possible in the usual locations through sunrise this morning and
again early Wednesday morning.

A few showers may be approaching northwest IL/rfd toward the end
of the current forecast and then possibly spread across northwest
IL early Wednesday morning...after the end of the current taf. cms


355 PM CDT

High pressure situated across the Great Lakes region, a weaker
pattern just to the south of the lake, and a trough of low
pressure over the Central Plains is supporting highly variable
winds across the lake. Lighter winds are expected initially late
this afternoon into the early evening across the south half, with
speeds increasing up the lake and especially over the far northern
part of the lake. East winds up to 30 KT sill being observed in
this location. Do expect speeds to increase all across the lake
tonight, with winds of 15 to 25 KT likely while any easterly
component turns more southeast especially into Tuesday. As low
pressure approaches the lake Tuesday night into Wednesday expect
these similar winds to be in place across the lake. Do anticipate
some increase in speeds, with speeds up to 30 KT possible. At this
time, hazards conditions for small craft are not expected along
the nearshore, but its quite possible that speeds may be higher
than forecast.






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