Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
638 am CDT Monday Sep 1 2014
340 am CDT
Entering September and meteorological autumn we still find the
forecast challenges to be more Summer-like...at 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.
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 timing...it 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 jet...do 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 occur...limited 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
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 column...at 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 it...as 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 12z...
* period of rain showers/thunderstorms and rain possible middle morning into early afternoon.
* Another small chance of of few showers/storms this evening.
* Short 1 to 2 hour period of light east-southeast winds early
this morning...before becoming gusty out of the southwest later
in the morning.
Convective chances and timing continue to be the primary weather
concern and focus for the area today.
The overnight thunderstorm activity to our west has dissipated as
it moved across northern Illinois. However...its effects on the
surface winds and ceilings remain. As such...it appears that a short
period of low MVFR ceilings and an east-southeasterly wind component
are expected through around 14 UTC this morning before the winds
revert back out of the southwest and become a bit gusty by middle to
Uncertainty remains somewhat high in whether or not any convective
activity will impact the terminals today. Overall it appears the
best chances in the near term will be roughly between 16 and 19
UTC as a weather disturbance across northeastern Missouri shifts
eastward over the area. This could lead to some development
near...or just south of the terminals during this period.
Thereafter...it also appears there may be another chance for some
scattered storms this evening after 23 UTC as another Stout upper level
disturbance moves over northern Illinois as a cold front
approaches. Timing at this juncture remains very difficult to pin
down...and this 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 12z...
* fairly low confidence in thunderstorms and rain timing/chances through the day.
* Medium confidence in wind trends.
* High confidence in other forecast elements.
//outlook for Ord/mdw for 00z Wednesday-12z Monday...updated 12z...
Tuesday night through Thursday...predominantly VFR.
Friday...chance of shra/tsra.
Saturday and Sunday...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 occurs...an 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
Lm...Small Craft Advisory...lmz740-lmz741-lmz742-lmz743-lmz744-
lmz745...10 am Monday to 10 PM Monday.
Visit US at http://weather.Gov/Chicago (all lowercase)
Follow US on facebook...twitter...and youtube at: