Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
634 am CDT Tuesday Aug 4 2015
220 am CDT
high pressure will continue to influence the weather today and
likely through Wednesday. The gradient remains rather light early
this morning...with many locations seeing a west wind around 5-
10mph. The ridge center is positioned west of the forecast area
through this afternoon...which will keep a light gradient
overhead. Mixing is poised to increase again this morning...and
may produce some occasional gusty winds to 20mph. As the day
progresses the surface ridge axis will begin to drift east towards
northern Illinois/northwest in. This will allow winds to diminish late
this afternoon/early evening...and may allow a light onshore flow
to develop for areas adjacent to Lake Michigan. Highs will warm to
near seasonal conds in the lower 80s. Closer to Lake Michigan
temperatures may be just a few degrees cooler. Tonight the surface ridge axis
will be overhead with light winds expected again. Dry weather will
continue along with minimal cloud cover...and lows in the upper 50s
to low 60s.
Wednesday will feature the surface ridge center becoming positioned over
northern lower Michigan...with a light onshore flow developing for
areas adjacent to Lake Michigan. Elsewhere with the ridge center
displaced northeast of the County warning forecast area...winds will turn southeasterly.
Moisture will begin to advect north towards the Ozarks...however
enough dry air and middle-level subsidence should maintain dry weather
through Wednesday across much of the area. Highs Wednesday will
again approach the low 80s...to the upper 70s closer to Lake
Michigan and mainly for far northeast Illinois.
238 am CDT
Wednesday night through Monday...
The middle/upper level short wave disturbance...currently noted in
water vapor imagery across Wyoming...is expected to shift east-
southward across lower Missouri value through Wednesday night.
Forecast guidance continues to suggest that this farther south
track will keep any decent moisture return with this system
largely south of my area. Therefore...I have gone dry across all
by my far southern counties Wednesday night into early Thursday
morning...though it even appears the better focus for rain may end
up south of these areas.
After what looks to be a nice dry day on Thursday...another middle
level disturbance is forecast to shift eastward across the upper
Midwest and western Great Lakes region by Friday. This could produce
or next low end chance for some showers and thunderstorms for
Friday. Then...looking ahead to the weekend...there will be some
slight chances for thunderstorms across the area as the middle level
weather pattern looks to remain disturbed across the central portion
of the country. However...confidence is low on where the better
focus and actual timing of any convective activity might end up. So
no big changes were made to chances for storms next weekend.
However...some of these slight chances may be able to be pulled out
of the forecast all together as we get a better handle on where the
focus areas will end up.
Otherwise...seasonable temperatures...mainly in the lower 80s...are
expected to continue through the period.
//Ord and mdw concerns...updated 12z...
* light west winds around 5-8kt increasing to 10kt with gusts
between 16-18kt late morning through this afternoon.
* Winds diminish around sunset...then turn northeast early Wednesday
High pressure will remain centered west of northern Illinois/northwest
in...allowing a continued feed of light winds from the west to
slightly northwest. Mixing will increase middle to late morning...and
allow some gusts to develop up to 18kt. This should be strong
enough winds to prevent a lake breeze from pushing inland. Winds
diminish shortly after sunset...but remain westerly around 4-8kt.
Dry air will continue to limit the cloud cover today...with only a
few cirrus clouds floating overhead.
As high pressure shifts east overnight...winds will turn northeast
shortly after daybreak Wednesday with speeds slowly increasing to
//Ord and mdw confidence...updated 12z...
* high confidence in all forecast elements.
//outlook for Ord/mdw for 00z Thursday-12z Tuesday...updated 12z...
Wednesday night through Thursday...VFR.
Friday...mainly VFR. Chance of showers and thunderstorms.
Saturday through Monday...mainly VFR. Slight chance of
258 am CDT
A surface ridge of high pressure will shift over the lake today into
Wednesday...resulting in mainly lake breeze driven winds over the
lake later today and on Wednesday. However...another area of low
pressure is expected to shift eastward south of the lake by
Thursday...with a second area low pressure expected to shift over
the upper Midwest later into week. These low pressure systems could
result in a period or two of some stronger easterly flow over Lake
Michigan. However...at this time the actual speeds and wind
directions are a bit uncertain due to uncertainties on the timing and
tracks of these lows.
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