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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
108 am CDT Friday Sep 4 2015

850 PM CDT

Early evening isolated storms have dissipated as the mixed layer
has largely decoupled and the frontal speed and its associated
convergence have diminished. Still a good circulation associated
with the vorticity maximum in Wisconsin heading south over our area
overnight. The modest forcing for ascent with this may still be
enough to offset the loss of instability for an isolated shower
or two overnight...but for most if not all the rest of the night
should be dry.

Earlier lake-cooled air had generated some clouds and even fog
near the Lake Shore but that has since eased as dew points have
dropped. The area where the front is stalling /Rockford through
Northwest Indiana/ may be most susceptible to some fog overnight
as the dew points remain the highest. Have patchy fog mentioned
in the forecast from that corridor northeastward to the lake



Short term...
254 PM CDT

Through Saturday...

Main forecast concerns over the next couple days will be continued
unseasonably warm and humid well as precipitation

This afternoon...satellite imagery and surface observation show a weak
trough draped from near lse through msn and mke and across Lake
Michigan. This trough will continue to sag south the the remainder
of the afternoon and into the evening with winds turning from west-
southwest ahead to northeast behind. This boundary is expected to
enhance/become coupled with a lake breeze this afternoon which
should help push temperatures back into the 70s along the lake
front. The boundary could also serve as the focus for
isolated/widely scattered thunderstorm development. Latest Storm Prediction Center mesoscale-
analysis Page shows axis of moderately strong instability across
northern Illinois...weakly capped or uncapped...however lack of any
forcing and weak midlevel height rises appear to be keeping
convection in check for the moment. As the aforementioned trough
moves into the area this should be sufficient to kick
of a few thunderstorms though chances will then begin to diminish
late this evening as the boundary layer begins to stabilize
nocturnally. With the boundary in place...isolated development may
linger overnight...but not expecting much coverage.

For Friday...

Trough axis settles over the area on Friday with fairly weak flow at
the surface. Low pressure over the plains will advance eastward with
modest warm air advection spreading into portions of northern Illinois. Also...a
lake breeze forms in the afternoon. Another warm and humid day is
on tap...thus an unstable environment will develop again. Guidance
is mixed with convective development...but have leaned away from
the more agressive guidance at this point...and have kept the
highest probability of precipitation...still in the chance category...for the later
afternoon along the lake breeze.



Long term...

Saturday through Thursday...

308 PM...multiple forecast concerns including high temperatures Sunday
and Monday...precipitation chances/timing Sunday night through Wednesday
then cooler/less humid end of next week.

The upper ridge will be building across the area on Saturday and
models are in general agreement with decreasing probability of precipitation through the day
and likely dry conditions into Saturday evening. Winds will likely
be light enough that a lake breeze turning winds southeasterly
along the Illinois shore should develop. Not sure if this will be strong
enough to aid any isolated convection to develop. Possible that
there could still be some isolated showers or thunderstorms
Saturday afternoon so maintained slight chance probability of precipitation with dry
conditions expected Saturday night and Sunday.

High temperatures in the upper 80s/around 90 look reasonable Saturday...
cooler along the Illinois shore. Sunday looks to be warmer with highs
generally in the lower 90s most locations. Perhaps slightly
cooler along the Illinois shore with south/southeast winds. Winds may be
a bit stronger on Sunday...perhaps in the 10-20 miles per hour range
especially across northwest Illinois but with dewpoints in the upper
60s/lower 70s both days...afternoon heat indices in the middle 90s
possible both days.

A frontal boundary will move southeast across the Midwest Sunday
into Sunday night and 12z GFS/European model (ecmwf) are in good agreement with
precipitation holding off until after midnight...perhaps not until closer
to dawn Monday morning so Sunday evening looks dry. As this front
sags into the County Warning Area Monday...forcing begins to weaken so confidence
regarding precipitation coverage and timing is fairly low. Could be
decaying showers/convection in the morning and then new
additional development with daytime heating later Monday
afternoon/evening. At this point...maintained chance probability of precipitation both
late Sunday night and on Monday. High temperatures will be tricky for
Monday. Cooler across northern areas seems on track but southeast
areas could easily get back into the upper 80s/lower 90s.

This front then lays out across the area as it weakens Monday night
into Tuesday while a stronger cold front pushes across the northern
plains Tuesday and reaches the area Tuesday night into Wednesday.
While this is still several days out...given the high dewpoint air
likely pooling along this boundary...its possible there could be
periods of heavy rain at least across some parts of the region.
Once the second stronger cold front moves across the area on
Wednesday...precipitation should come to an end with cooler and less
humid air moving back into the region. Cms



//Ord and mdw concerns...updated 06z...

* MVFR ceilings and visibilities in the area overnight.

* Light and variable winds with favored easterly direction.

* Isolated thunderstorms and rain Friday afternoon.


//discussion...updated 06z...

A pretty tough but hopefully low impact forecast overnight. MVFR
ceilings are being reported at several upstream stations and
satellite pics seem to show this lower area of clouds spreading
toward the mdw/Ord terminals. Despite mostly clear metar at taf
time...went ahead and included a 3kt broken ceiling. This certainly
will look like an odd decision if trends do not progress as
expected...but may also be too high based on a few pockets of IFR
ceilings out there as well. As for visibilities...opted to delay the lower
values till around sunrise since trends are not as clear and far
fewer observation are heading that direction at this hour.

Friday afternoon is the next period of concern...with support for
isolated thunderstorms and rain activity...perhaps with a bit more coverage than
Thursday afternoon but with low confidence on timing and location.


//Ord and mdw confidence...updated 06z...

* low confidence on ceilings and visibilities overnight.

* High confidence regarding light wind speeds early in taf period.

* Low confidence on timing and coverage of thunderstorms and rain Friday afternoon.


//outlook for Ord/mdw for 12z Saturday-12z Thursday...updated 00z...

Saturday...slight chance of thunderstorms and rain in the afternoon.

Sunday...slight chance of thunderstorms and rain late.

Monday...chance of thunderstorms and rain.

Tuesday...chance of thunderstorms and rain.

Wednesday...chance of thunderstorms and rain.




308 PM...a weak frontal boundary will move south of the lake this
evening with a period of north/northeast winds expected into
Friday which will slowly diminish first across the southern part
of the lake. Winds will slowly turn southerly Friday night into
Saturday. These southerly winds will slowly increase into the
10-20 knots range by Sunday as the gradient tightens some between a
large trough of low pressure over the plains and large high
pressure over Atlantic coast. Its possible winds may increase into
the 15-25 knots range... especially on the northern part of the Lake.
A weakening cold front will move across the lake Sunday night into
Monday morning with winds shifting northerly by Monday evening. It
appears winds will shift back to southerly Tuesday until a
stronger cold front arrives late Tuesday night into Wednesday...
which will shift winds to the northwest. Cms


Lot watches/warnings/advisories...


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