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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
415 am CDT Monday Sep 1 2014

340 am CDT

Entering September and meteorological autumn we still find the
forecast challenges to be more least early on in
the forecast. That is storm and heavy rain potential today and
tonight...then storm chances Wednesday night and Friday which
Sandwich a possible 90 plus degree day on Thursday. Less but
still some time was spent on the final challenge of how much of a
cool down occurs the first full weekend of this new meteorological

a long wave trough extending southward through Canada into the north
central U.S. Has a very progressive short wave feature moving
through its southern base. This is speeding east-northeast across
Iowa early this morning. A surface cold front moving east lags this
short wave...extending from northern Minnesota through the Texas Panhandle
early this morning...with multiple surface waves in the 1003mb
neighborhood along it. Northwest surface flow behind this front is
not particularly strong so this front will continue to be a slow-
mover today...not expected to enter the forecast area until this
evening. Being within a warm sector under quickening upper/mid-level
flow presents the storm chances these 24 hours.

This morning...
a loosely defined mesoscale convective system moving eastward into northwest Illinois early this
morning waned considerably with warming cloud tops and outflow
extending well ahead of it. Seeing an uptick across west central Illinois
likely associated with lift of the aforementioned progressive short
wave. High resolution models greatly vary on what will happen even
just a few hours out...but would expect synoptically to see some
convection evolve into the forecast area this morning...possibly
only the western/southwestern area but still cant rule out scattered
activity moving eastward through the entire area including
Chicago during middle/late morning. Despite a fairly strong low-level
jet into any activity...the continued broad area of storms across
eastern Kansas into west/southwest MO through early to middle morning
should lessen moisture transport and thus ability to realize steep
lapse rates aloft for more robust activity...and maybe even
thunder in general north of Interstate 80.

This afternoon and tonight...
despite this being in the first 12 hours of the forecast...due to
influence of morning storms and large model differences...confidence
on specifics in these periods remain low. There should be some
form of outflow boundary or boundaries from morning activity/cloud
cover that linger into the forecast area into the afternoon...most
favored south of Interstate 80. As warming and destabilization
occurs with dew points around 70f...could see scattered activity
develop near any such boundaries during the middle to late afternoon.
The second upper wave across western Wyoming this morning will begin to
approach the region as well by late in the day...with forcing from
this possibly aiding in scattered storm development across much of
the region. Even early this morning there are a handful of storms
across the upper Midwest not linked to any specific surface
feature. Increasing deep layer shear of 35 to 50 knots will support
isolated to scattered severe discrete storms in the region.

While it remains difficult to pinpoint would seem the
greatest coverage in redeveloping storms this evening would be in
the southern forecast area where aforementioned boundary may occur
and modest southwest 850-925mb flow and moisture transport
remains. As the area becomes under a right entrance region to the
upper expect increasing ascent tonight in the region for
more robust convection and potential severe and heavy rain
threats. Right now a guidance overlap would indicate the far
southeast forecast area is most favored for this. Precipitable
waters around two inches and storm motions nearly parallel to the
upper and middle level flow will present heavy rain threats in
the general central to southern Illinois into in regions...but again at
this time confidence varies on placement. Because of that will
continue with the hydrologic outlook /esf/ for the areas hit hard
on Saturday night with heavy rainfall.

Tuesday and Wednesday...
the front should clear the forecast area sometime Tuesday morning.
While dry air advection will convergence remains
in the southeast forecast area for possibly a storm or two Tuesday
but it looks to be a low chance. Temperatures only cool slightly
as high pressure quickly moves atop the area Tuesday night and
then eastward.

Wednesday night through Friday...
warm air advection and a rapid precipitable water increase is
forecast Wednesday night on all guidance. Feel that models may be
under-doing the storm chances given forecast profiles of steepening
lapse rates within a rapidly moistening least in the
western forecast area. So have increased probability of precipitation in that area. As for
Thursday...have gone dry in many areas collaborating and
undercutting a model blend of probability of precipitation. The synoptic pattern favors a
hot day and barring any stratus issues with Wednesday nights warm
air advection and possible precipitation...should see the 90 degree mark
reached or exceeded across a good part of the forecast area based
on strong southwest flow and forecast 850mb and 925mb temperatures
compared to climatology. For now have kept the north a little
below that with the possibility of morning clouds. The system
cold front is presently forecast by the ec and the GFS to come
through the area sometime on Friday which would bring storm
chances with well as flow turning off the lake behind its

no changes made to this weekend where highs look to be around 70
with dry conditions behind the aforementioned front. High pressure
around 1023mb is forecast by a blend of guidance. Given the 925mb
temperatures forecast if that high is centered over the forecast
area sometime Sunday or Monday nights /7th or 8th/...lows in
outlying areas would likely be in the 40s.




//Ord and mdw concerns...updated 09z...

* period of rain showers/thunderstorms and rain possible this morning.
* Additional showers/storms possible either later this afternoon
or this evening.
* Gusty southwest winds during the day.


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

Convective chances and timing will be the primary weather concern
and focus for the area today.

An area of ongoing convective activity over eastern Iowa and into
Missouri is gradually shifting to the eastward. Although a weakening
trend is expected overnight with this appears that the
remnants may hold together to produce at least some showery
activity...with some embedded thunder this morning. Current timing
brings this activity across krfd after 09 UTC...through around 11
UTC...with the activity shifting over the eastern terminals after
13-14 UTC this morning. The winds could temporally be messy during
the morning as this active pushes over the terminals...but overall a
southwesterly wind component should be the rule during the day. It
also appears to be a breezy day...with southwesterly wind gusts up
to 25 knots expected.

Uncertainty is high in whether or not any afternoon and evening
convection will fire over the terminals. The morning activity looks
to subdue afternoon destabilization some...and it is quite possible
that the main axis of afternoon convective activity occurs just south
of the terminals. spite of these uncertainties...I felt
justified in adding a probability 30 group for thunder during the evening
today. Another Stout upper level disturbance is expected to move over
northern Illinois as the cold front approaches this evening and I
think there is a decent possibility that if the afternoon activity
fires south of US that there could be some additional activity
farther north associated with this approaching disturbance. Timing
at this juncture is very difficult to pin down...and will need to be
ironed out as we get a better Handel on trends through the day.
Either way...there will likely be areas of convective activity this
afternoon and tonight south of the terminals.


//Ord and mdw confidence...updated 09z...

* fairly low confidence in thunderstorms and rain timing/chances through the day.
* Medium confidence in winds Monday
* high confidence in other forecast elements


//outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...

Tuesday through Thursday...predominantly VFR.

Friday...chance of shra/tsra.

Saturday...predominantly VFR.



232 am CDT

It looks to be an active weather day across the lake...with Stout
southerly winds lake-wide...and the threat for some strong to severe
thunderstorms across Southern Lake Michigan. These strong southerly
winds will result in winds around 25 knots and the ongoing Small Craft
Advisory still appears good. The surface low...responsible for these
strong southerly winds today...will shift across Ontario...and to
near James Bay by this evening while deepening to around 29.2
inches. As this associated cold front will be drawn
eastward across the lake by early Tuesday morning. Winds will become
westerly and abate significantly with the passage of this front.

Another period of stronger southerly winds looks to set up again
Wednesday night and Thursday as another potent area of low pressure
tracks eastward across Ontario. Winds speeds could again be around
25 knots out of the south during this period. Another stronger cold
front looks to shift down the lake in the wake of this low late
Friday and this could set the stage for a decent northerly wind
event across the lake as a Stout area of high pressure quickly
builds eastward across the upper Midwest. Strong northerly winds of
25 to 30 knots appear quite possible in the wake of this cold front
Friday night into Saturday...which would also result in large waves
in excess of 5 feet across Southern Lake Michigan into next



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...10 am Monday to 10 PM Monday.



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