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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
650 PM CDT Wednesday Sep 2 2015

Short term...
258 PM CDT

Through Thursday...

A cumulus field has blossomed in the unstable environment ahead
of an upper level trough axis/vorticity maximum in western Illinois. Radar
indicates a few cells trying to go up in northeast Illinois as
suggested by the experimental hrrr...but these have been shallow.
Better forcing with the disturbance is farther north...but this
vorticity will slide through during our afternoon period with peak
heating in place. There is no surface forcing to speak of other
than subtle wind shifts in SW surface flow...noted along a line
from krpj to kugn. Have held onto dry forecast for this afternoon with
subpar lapse rates and dry amdar soundings still suggesting
nothing more than isolated activity..though expect things to
bubble for a few more hours. It is still quite muggy with
temperatures now reaching the 90s.

The upper trough axis continues slow eastward progress this
evening....and most guidance hangs onto dry conditions this evening
as it passes...with better forcing associated with the upper vorticity
maximum and lower level isentropic lift focused in Wisconsin. Later
tonight...the upper level flow subtly shifts to more northwesterly.
This will allow upper level height falls to sneak into the northeast
Illinois and out over Lake Michigan tonight. Guidance still mixed as
to the extent of convection over area. Have held to the contention
that more organized convection will remain over Wisconsin and points
northeast...but still conceivable that a broken line/area of showers
and thunderstorms passes through overnight into the early
daybreak hours closer to the Lakefront.

Pending any overnight convection...associated outflows or lingering
cloud cover...and how quickly energy slides southeast through the area will
have an impact on convective timing and coverage for Thursday...with
lower confidence on specifics during this time period. Have hit
higher probability of precipitation in the morning period as this wave continues to slide
through but still limited to the chance category for now. Cloud
cover may limit afternoon instability...which may keep the better
chance for afternoon showers/storms south and east of the area...but
with a warm and unstable air mass in place...have some lower probability of precipitation in
place. The other feature to watch is a subtle backdoor cold front
moving south Down Lake Michigan that may act as a focus farther
north late in the day. Overall forecast confidence on Friday is low-
medium. It will still be a warm and muggy day as 850/925 temperatures hold
steady...possibly a bit cooler than today due to any lingering cloud



Long term...
316 PM CDT

Friday night through Wednesday...

Continued warm and occasionally unsettled weather through the
weekend finally yields to slightly cooler highs on Monday and
Tuesday and then considerably cooler...back to seasonably
normal...highs by the middle of next week.

Chances of widespread rain and thunderstorms are not great for
any particular period over the weekend...but slight chances exist
each day just from mixing out of the low levels to dry adiabatic
and from the effects of low level boundary interactions from the
lake breeze and any convection that might happen to pop through
the very weak cap. Friday and Saturday afternoons would appear to
be more favored than Sunday due to the additional support of a
weak upper low backing into the area from the east.

Late Monday into Sunday the pattern begins to change with the
approach of a cold front toward the area. Labor Day could be
wet...but the European model (ecmwf) and GFS actually disagree on when this moves
through...with the GFS about a day or two slower. Both agree in
lifting a low pressure system out of the Southern Plains and
across the local area by Wednesday. Given the positive tilt of the
upper trough and the poor diurnal timing of the low
this range this appears to be more supportive of showers and
cooler air than severe least for the local forecast




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

* chance for thunderstorms and rain/rain showers during early morning hours.

* Chance winds may shift from southwesterly to southeasterly Thursday afternoon.


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

With the setting sun early this evening...the diurnally driven
isolated rain showers should come to an end and conditions should remain
quiet for much of the night.

The next concern will be the chances for thunderstorms and rain/rain showers during the early
morning hours. Some of the high resolution model guidance
continues to indicate the possibility for some thunderstorms and rain early Thursday
morning. However...there are some factors that are making this a
very low confidence forecast. The main forcing mechanism will be a
middle level impulse that is currently over northern Wisconsin. There is
some concern that this feature may be slower in tracking to the
southeast and could delay the timing for ts over northestern Illinois/northwestern in.
So...given the uncertainty...especially with respect to timing and
the track of the main forcing mechanism...have opted to push back
the timing a little and maintain the prob30. Still feel that there
is enough of a chance for ts to maintain the prob30 mention, but
is too low to go with a tempo at this time.

The other concern is wind direction for tomorrow afternoon. As
the previously mentioned middle level impulse tracks to the
southeast...weak warm front may develop from north central Illinois into northwestern
in...with winds shifting to southeasterly north of this boundary.
However...wind speeds is expected to be less than 10kt...closer to
5 to 7 knots. This warm front could also act as a focusing mechanism
for some thunderstorms and rain tomorrow maintain the prob30
ts mention.


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

* low confidence in ts timing/coverage Thursday morning.

* Low to medium confidence in ts chances Thursday afternoon.

* Medium to high confidence in wind trends.


//outlook for Ord/mdw for 12z Friday-12z Wednesday...updated 00z...

Friday...slight chance of thunderstorms and rain...mainly in the afternoon.

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

Sunday...VFR. Weather none.

Monday...chance thunderstorms and rain.

Tuesday...chance thunderstorms and rain.



316 PM CDT

Light and somewhat variable winds will not be supportive of
significant wave growth for the next few days but will continue to
allow for local areas of patchy dense fog over cooler portions of
the lake.



Lot watches/warnings/advisories...


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