Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251140

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
640 AM CDT SUN SEP 25 2016

322 AM CDT

Through tonight...
Main concerns for today will reside on precip chances and timing,
with the potential for some stronger storms to develop this
afternoon. Early this morning the downstream surface ridge continues
to have an impact on Northern IL/Northwest IN weather with a dry
easterly low-level flow and has kept clouds west of the forecast
area through much of the early morning hours. IR imagery also
indicates clouds struggling to maintain the depth with tops warming
over the recent few hours across Iowa/Missouri. The moist channel
remains west/south of the area, but as the surface ridge drifts
further east, the warm/moist boundary will advect north/northeast
across the forecast area. This will bring Td`s to the middle/upper
60s, along with an increasing convergence zone ahead of the frontal
boundary this afternoon/evening. Several of the guidance members
continue to highlight the 20z to 03z timeframe as the most probable
timing for rain/convection to develop; however, the highest
resolution guidance sets maintain less coverage.

Given the quickly warming/moist airmass ahead of the frontal
boundary, coupled with increasing low-level convergence, expect the
instability to quickly increase by early afternoon over Northcentral
IL. The wind profiles will be gusty, but should be slowly becoming
unidirectional in the late aftn/early eve hours. But expect the near
surface moisture to linger with PWAT values progged between 1.5 to
nearly 2.0 inches, which remains well above normal for late Sept.
The combination of strong winds and moisture rich atmos could offer
a few stronger storms to develop quickly. Temps will be approaching
the lower 80s to perhaps middle 80s in the far southern forecast
area. But enjoy what appears to be the last warm/humid day, as
significant changes are in store for Mon.

Tonight the frontal boundary will be arriving late, with the 500mb
trough axis pivoting east towards the Western Great Lakes. Winds
will be shifting to northwest with a tightening pressure gradient
and breezy winds. Thermal trough will bring temps overnight aloft
into the 6-8 deg c range, with lows tonight in the 50s.



322 AM CDT

Monday through Saturday...

Monday through Tuesday: 500mb trough axis will be over the Great
Lakes region Mon, with continued cold air advection over Northern
Illinois. Thermal trough aloft will remain in the single digits
through much of Mon, with the cloud shield clipping far Northeast IL
late morning through early afternoon, before lifting northeast late
mon eve. Highs Mon will struggle to warm from the mid/upr 60s, and
could see a few locations in far Northern IL remaining around 60 for
highs. Tue will only be slightly better as the trough axis begins to
drift east and mid-lvl heights rise as high pressure builds in from
the west. But the overall trend continues to be dry air returning
Mon ngt through Tue.

Wednesday through Monday: Oper EC continues to differ with GFS on
how the 500mb vort will meander in the extended periods. The
downstream ridging is progged to linger, which would have a lagging
effect on the Great Lakes cutoff shortwave, which will likely
translate into a prolonged period of a dry airmass persisting for
the Great Lakes, especially Northern IL. Then as the weekend
approaches ensembles prog the trough finally drifting east of the
Great Lakes as a trough develops across the Southwest CONUS
meanwhile ridging returns for the Great Lakes/Upper Midwest this
weekend. Temps will be steadily rebound to seasonal conditions, with
highs in the 70s and overnight lows in the upr 40s to mid 50s. Broad
surface ridging will maintain a dry extended period.



For the 12Z TAFs...

640 am...Forecast concerns this period remain thunderstorm
potential...mainly this afternoon...and timing of wind direction

Precip/convection across eastern IA has mostly dissipated early
this morning and focus is now on where/when new activity will
develop and how widespread it will become. Confidence in timing
has increased some with this afternoon the most likely time period
for thunderstorms. But there remains differences with activity
developing late this morning and spreading across the Chicago
terminals early/mid afternoon or redevelopment holding off to the
mid/late afternoon over the terminals. Some of the most recent
short term guidance suggests activity could be centered on the
early/mid afternoon. Adjusted tempo thunder mention a bit faster
but this may not be fast enough. So trends will need to be
monitored this morning and changes to this timing are possible.
Additional isolated/scattered showers could also develop later
this morning and may persist into this evening but thunder
duration appears to be just a few hours at any one location.

Southeast winds will increase into the 10-15kt range this morning
and slowly turn southerly by late morning/early afternoon and then
shift more south/southwest this afternoon. How fast this shift
occurs is a bit uncertain but winds should be southwesterly by mid
afternoon. During this transition...possible for some gusts into
the mid/upper teen kt range. Winds will shift westerly behind a
cold front this evening. Could be some higher gusts with the cold
front passage but generally 10-15kts prevailing expected
overnight. Westerly winds will quickly increase Monday morning
after sunrise with gusts into the 25-30kt range for most of
Monday. cms



210 am...High pressure over the eastern lakes will slowly move
across Quebec and New England today and tonight and then off the
East coast Monday. Low pressure over southern Manitoba will move
into western Ontario today and then slowly move to Lake Superior
Monday night. A strong cold front will move across Lake Michigan
this evening shifting south/southeast winds to the west. As much
cooler air spreads across the lake and the gradient tightens...
westerly winds will increase to 30 kt by Monday morning and likely
remain at 30 kts through Monday night...possibly into Tuesday
morning. There remains some potential for gale force winds...
especially Monday afternoon into Monday evening...but confidence
remains low.

The low pressure will weaken as it slowly drifts southeast across
the Great Lakes region Tuesday into Wednesday. As it does...winds
will turn more northerly across Lake Michigan as high pressure
builds across Ontario and the upper midwest. cms





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