Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 242029

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
329 PM CDT Mon Apr 24 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 18Z TAFs...

Main concern for the period will be wind speed/direction.

Weak high pressure is moving across the region and the light sely
winds have increased through the morning. With a weak gradient in
place, expect winds to shift to more ely as a lake breeze
develops. As of issuance time, there is no sign of the lake breeze
on radar, but anticipate that it should develop over the next
couple hours. With a modest push of cooler air, some ocnl gusts
are possible, so have added a tempo for gusts to 20kt for a few
hours following the passage of the lake breeze. Winds should drop
off with sunset, but there should be strong enough of a gradient
to keep 5 to 10 kt winds overnight, with direction veering to sely
overnight and more sly tomorrow as the high pressure slides to the
east and low pressure approaches the middle/upper Mississippi
Valley. As low level moisture begins to increase overnight into
Tuesday morning there may be some patchy/shallow ground fog
possible in sheltered, low-lying areas.



215 am...A weak ridge of high pressure over the southern lakes
region will slowly move southeast today and tonight...while a
stronger area of high pressure remains over northern Ontario. The
gradient is expected to remain tight over far northern Lake
Michigan today and tonight with easterly winds 15-25kts expected.
Over the rest of the lake...east/southeast winds will increase to
15-25kt later tonight as low pressure over the western plains
moves northeast to the upper midwest tonight and dissipates. A
second area of low pressure will develop over the central plains
tonight and move northeast to the western lakes region Wednesday.
This low will then slow as it moves into Ontario Thursday. West/
southwest winds may increase to 30 kt Thursday as this low
departs. cms





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