Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231613

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1113 AM CDT Sat Sep 23 2017

1113 AM CDT

Morning Update...

Minor adjustmentswest of about a Pontiac to Harvard line to going
forecast, mainly account of weak onshore winds/lake breeze
already developing along the lake.

Weaker surface pressure gradient today with surface high pressure
center having shifted farther west across the southern Great Lakes
region since yesterday. In addition to overall lighter wind field,
slight backing to the south-southeast is expected, allowing more
substantial lake breeze to develop. O`Hare TDWR already depicting
weak boundary pushing inland a couple miles to the north of
downtown Chicago, with shoreline obs indicating a light southeast
or east wind component. Northerly Island obs is holding at 79-80
degrees currently with a light east wind. Expect onshore flow to
strengthen into this afternoon as lake breeze circulation becomes
better established, thus have lowered max temps a bit into the
lower 80s right along the lake shore this afternoon. Inland areas
already warming quickly with both ORD and MDW having touched 90 in
the past 30 minutes or so, and no changes planned there, though
some cooling from max temps can be expected by late afternoon as
lake breeze pushes further inland.

Farther west, 925 mb temps sampled at 25C at DVN in their morning
sounding, and progged 925 mb temps of +25/26C across the far
western cwa this afternoon suggests highs in the 92-93 range.
Bumped temps about a degree especially far west near Sterling

Otherwise, weak moisture return from central into northwest and
north central IL was depicted in morning 925-850 mb analysis. Some
of the high-res guidance has been hinting at the potential for a
few very isolated showers/thunderstorms across the western third
or so of the cwa later this afternoon. Max temps do approach
convective temps during the peak heating of the afternoon, though
with weaker wind fields would expect any development to remain
quite isolated. While an isolated afternoon shower can`t be
completely ruled out across the west/southwest parts of the FA,
expect coverage to be low enough to not include mentionable pops.



328 AM CDT

Through tonight...

Main forecast concerns/challenges are with continued warmth and
near to record breaking heat, and low potential for an isolated
shower this afternoon across north central IL.

Pattern still supportive of mainly dry/quiet conditions across
the region today, as large upper level ridge remains in place.
With this in place, focus for precip development will stay tied to
the stronger and more energetic westerlies across the central and
northern CONUS. However, similar to yesterday, there remains a
possibility for isolated convection later this afternoon. Very low
chance, but still a possibility as there will be some weak upper
level support in place. This coinciding with some possible low
level convergence and steepening low level lapse rates across
northern Illinois, can`t rule out an isolated shower or an isolate
storm. With low confidence, have excluded at this time though. In
this location, scattered CU will support partly cloudy skies in
the afternoon. However, don`t think either will limit expected
warmth today. Latest trends and guidance now supporting another
hot day today, with temps likely getting well into the 90s across
much of northern Illinois and northwest Indiana. Did raise high
temps today to reflect latest trends, and think record high temps
at both Rockford and Chicago may be reached today. Locations near
the lake will stay slightly cooler though, as lake breeze
development is likely by around midday. These temps and
anticipated dew points will support heat index values in the mid
to upper 90s.



328 AM CDT

Sunday through Friday...

Large upper level ridge and surface high will remain in place
late in the weekend, with similar conditions expected to persist.
As some slight moderation to the air mass is expected, temps will
not be as warm Sunday. Still think many locations will have a
chance at reaching the 90 degree mark though. By early next week,
upper level ridge shifts to the east while a trough moves through
the central CONUS. As this trough and surface low push east
through the region, will see precip chances returning Tuesday into
Wednesday. This will also be an end to the warmth, as a much
cooler air mass will move in behind this system.



For the 12Z TAFs...

Expect generally VFR conditions, outside of some patchy MVFR fog
early this morning, and once again tonight. Light southerly winds
continue this morning into midday but then expect lake breeze
development to impact the terminals this afternoon. Some possible
slight time differences with the wind shift today, but do think a
slight delay with this wind shift is appearing more possible. Dry
conditions should continue but am still monitoring for possible
isolated showers with a very low chance of a thunderstorm across
northern Illinois this afternoon. Very low confidence and not
worthy of including in the TAFs at this time, but will monitor
this potential later today.



438 AM CDT

Southerly winds in place across the lake as high pressure is
still situated to the east, and while low pressure is over
Ontario. This low will continue to the northeast, with this high
not really making much movement to the east through the weekend
into early next week. As this high remains in place, additional
trough of low pressure will be in place to the west, helping to
maintain a southerly direction for much of the period.






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