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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
645 PM CDT Thursday Sep 3 2015

645 PM CDT

The frontal boundary is slowly sagging southward near the WI
state line...while lake-modified front has surged ahead and is
about to pass southward through downtown Chicago as of 640 PM.
Temporary wind gusts of 20 to 25 miles per hour are expected behind this with
a 10-15 degree drop in 10-15 minutes per upstream observations.

Nearly uncapped air at 2000-3000 j/kg of MLCAPE per amdar data
and modified rap soundings has resulted in isolated storms along
the front and a few showers on the lake breeze convergence. With
the boundaries and some of their orientations in respect to the
mean wind field...there is a pool of low-level vorticity available
for any storm/shower updrafts latched to the boundary. So while
there is not much at all for wind shear through the storm
depth...cannot rule out a few storms having brief rotation and
have already seen that in Doppler velocity data with a couple
storms on the boundaries. The key is while the environment is
favorable for brief low-level is not favorable for
anything sustained. Also...with peak heating now have passed...a
general weakening in intensity is expected as low-level lapse
rates this behavior should only last another hour or two
at most.

The slow movement of storms per little light middle and upper flow
could bring a quick one inch to any areas directly under the

Minor adjustments made to forecast hourly probability of precipitation and hourly
temperature trends for the sharp cool down.



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 00z...

* isolated/scattered thunderstorms and rain/rain showers through early evening...with isolated
activity possibly lingering overnight.

* Abrupt wind shift to north-northeast-northeasterly around 00z with winds occasional higher
gusts for a short time following the passage of a lake breeze

* MVFR visible possible overnight.


//discussion...updated 00z...

A very warm...moist airmass is in place over northern IL/nwrn. An
east-west oriented boundary is slowly sagging south through southern WI
and has touched off a few ts and should continue to do so into the
evening as the boundary pushes into north central/northwestern Illinois. A lake
breeze/back cold front is pushing inland off of the lake into northestern
Illinois and as of 23z has already pushed through ugn/pwk and should
reach Ord just before 00z and then dpa/mdw right around 00z. Winds
will abruptly shift to northeasterly with occasional gusts around 15kt for a short
time following the passage of the boundary. Temperatures are also
dropping off quickly following the boundary passage. Through the
evening...winds should drop off to light/variable as high pressure still
covers the western Great Lakes and a very weak pressure gradient
will set up overnight. Diminishing winds and lowering dewpoint
depressions should help visible drop to MVFR or IFR levels overnight.

Conditions should be relatively quiet through the day tomorrow as
high pressure persists over the region. Very warm conditions and
weak winds should allow for another lake breeze to develop
tomorrow. Timing of the lake breeze is a bit uncertain at this
point...through do feel that it will push inland of the Chicago
area terminals setting up Ely winds...but expect that winds should
remain under 10kt. The lake breeze may be the focus for thunderstorms and rain
tomorrow have introduced a prob30 ts for late
tomorrow afternoon.


//Ord and mdw confidence...updated 00z...

* medium confidence in thunderstorms and rain/rain showers chances/coverage through this
evening. Low confidence of direct impact at the terminals.

* High confidence in wind speed/direction and short period of
higher gusts through early this evening.

* Medium confidence in lake breeze details tomorrow afternoon.

* Medium confidence in MVFR visible developing overnight and duration.

* Low to medium confidence in ts chances tomorrow afternoon. Low
confidence in chances of direct impact at the terminals.


//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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