Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
402 am CDT Sat Aug 29 2015
328 am CDT
Through Sunday night...
Main forecast challenges/concerns are with precipitation trends across
the entire County Warning Area this morning into the afternoon...with precipitation
chances lingering tonight into Sunday across a portion of the
Latest radar imagery depicting an area of light to at times
moderate showers moving through north central Illinois early this
morning. This is in response to middle/upper level trough and surface
reflection slowly pushing east across the region...with fairly
energetic waves lifting out ahead of it. With the departure of a
stronger wave/better forcing...the intensity of these showers has
diminished and while instability axis remains to the west of the
County Warning Area...lightning has been limited just to the west of the County Warning Area.
Nonetheless...a solid area of showers continues to slowly move
east this morning...slowly overcoming drier air in place. With
upstream energy to reinforce lift this morning through
midday...expect this precipitation to continue moving into remaining
areas in northeast Illinois and then eventually Northwest Indiana.
Have Cat probability of precipitation for areas in northern Illinois through middle to late
morning with chance to likely probability of precipitation elsewhere...but then lower to
high chance probability of precipitation across the entire area by early afternoon. While
the middle/upper level trough further dampens today large scale
forcing will still be present...but wonder if it will be enough to
sustain such a wide area of precipitation into the afternoon. Think more
scattered development will likely be the trend later today...thus
the lowering of probability of precipitation...with best chances for precipitation shifting more
eastward. Guidance still indicating an increase in instability
through midday and the afternoon...and have maintained mention of
slight chance for thunder. However...with weak lapse rates in
place...not expecting any strong/severe development today. Despite
a warming airmass...clouds/precip in place today will limit
warming and have lowered high temperatures for today. Have lower 70s
north and then middle to upper 70s across the southern County Warning Area...but may
have not lowered enough as its possible that some locations may
not make it out of the 60s today.
Further weakening of this system will occur tonight into Sunday
but with this continued slow exit...precipitation chances may linger for
the eastern County Warning Area tonight. This could continue on Sunday with
guidance still advertising for redevelopment of showers and
thunderstorms on Sunday...mainly south of Interstate 80. Sunday
will be much warmer with highs in the low to middle 80s. Middle to upper
80s could be realized in some locations but with potential for
clouds/precip...did not go higher at this time.
328 am CDT
Monday through Friday...
Stronger westerlies will be situated well north of the region
beginning of the period...with ridging/weaker flow across the
area. Continued warming trend will continue early next week with
middle to upper 80s likely on Monday...with temperatures likely approaching
90 for most locations on Tuesday and Wednesday. With increasing
moisture/instability during the period...afternoon thunderstorms
do appear possible throughout the week. Have tried to indicate
best chances during this next week...but with these probability of precipitation likely
being refined with later forecasts. However...would think most
locations will remain on the dry side with any development more
than likely isolated to widely scattered.
//Ord and mdw concerns...updated 09z...
* period of rain overnight into Saturday morning
* MVFR or lower ceilings and visibility possible Sat night
Weakening storm system will bring a period of rain with possible
periodic MVFR visibility late tonight into Sat morning before rain
tapers off and ends by midday. Small chance exists for some isolated
rain showers or perhaps a thunderstorms and rain during the late afternoon as what is left
of the storm system festers...but chances look too low to include
precipitation in tafs at this time. Light east to southeast winds could
turn more southerly Sat morning before probably turning fairly
light and variable during the afternoon. Should see a tendency for
winds to become light northeast Sat night and with ample low level
moisture and recent rains could see some light fog and/or stratus
//Ord and mdw confidence...updated 09z...
* medium-high confidence in rain late tonight/Sat morning
* high confidence in winds below 10kt...but low confidence in
* Low confidence in ceilings/visibility Sat night
//outlook for Ord/mdw for 12z Sunday-12z Friday...updated 00z...
Sunday...VFR. East wind.
Monday...VFR. Southeast wind.
Tuesday...VFR. South wind.
Wednesday...slt chance thunderstorms and rain. Primarily VFR. South wind.
Thursday...VFR. South wind.
207 am CDT
Narrow ridge of high pressure stretching from near Detroit to
Green Bay combined with weakening low pressure over Iowa will
result in a narrow ribbon of stronger easterly winds early this
morning over portions of Central Lake Michigan before backing more
southeast later this morning then ultimately easing this
afternoon. Weak low over Iowa will dissipate tonight and remainder
of the upcoming week will see weather over the lake dominated by
high pressure anchored over the middle Atlantic and northeast.
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