Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
901 PM CDT Tuesday Aug 4 2015
901 PM CDT
For evening update...
Only tweaks made to hourly grids...mainly to add detail to winds and
bump up sky cover slightly. Otherwise forecast generally on track.
Western Great Lakes region within weak surface pressure gradient
this evening...with the main low level frontal/baroclinic zone well
to our west and south from the middle-Missouri to the Ohio River
valleys. Locally across northeast Illinois/northwest in...a fascinating
combination of lake breeze/synoptic trough and weak cold frontal
boundaries were noted on klot 88d and tord/tmdw tdwrs late this
afternoon and early this evening...though main sensible weather is
only shifting of winds near the lake and a temperature drop of a few
degrees with winds off of the water.
Overnight...light winds...partly cloudy skies and low humidity
will be the rule. Have bumped up sky cover to mention just partly
cloud most areas with intermittent jet streak cirrus and convective
blow-off from distant storms closer to the aforementioned baroclinic
zone. Going temperatures look on track similar to last night with some middle-
upper 50s in areas away from Lake Michigan and the Chicago heat
221 PM CDT
A weak surface trough remains in place across Southern Lake Michigan
this afternoon...with high pressure to our west...and a stationary
front stretched from the Southern Plains and eastward across the
Ohio/Tennessee valleys. Aloft...northwest flow persists on the back side of
the upper low just south of Hudson Bay...ridging across the Central
Plains into the upper Midwest...with a disturbance under cutting the
ridge closer to the surface front.
Thunderstorms are ongoing though weakening closer to the
wave/surface front...but with high pressure just to our west...we
are just seeing some high clouds from this activity...along with
some daytime heating induced cumulus associated with the upper
low. The upper low will pull east tonight into
Wednesday...allowing the upper ridge to slide overhead tomorrow.
Precipitation associated with the disturbance undercutting the
ridge will remain to our south as surface high pressure moves
along and east of Lake Michigan and anchoring the surface front
across central and southern Illinois. This will mean dry
conditions for the forecast area tomorrow during the daytime
hours. With the high east of the lake...onshore flow will keep
lake adjacent areas cooler in the 70s...while areas inland stay
close to where they are peaking today in the low 80s.
308 PM CDT
Wednesday night through Tuesday...
Morning guidance finally coming into better agreement with
evolution of upstream energy...and associated surface reflection
and precipitation. Have removed or lowered probability of precipitation from Wednesday night into
Thursday night as guidance consistent with keeping precipitation axis to
the south of the County Warning Area...into central and southern Illinois/Indiana.
Some guidance trying to inch this precipitation further north on
Thursday...but I would think a continued southerly trend with this
system and precipitation would likely occur. Have left a slight chance of
showers across a portion of east central Illinois and Northwest
Indiana for that time...but could see this being removed with
later forecasts. Despite precipitation staying to the south...northern
Illinois and Northwest Indiana will see middle/high level cloud cover
continue to spread north. Extent of cloud cover is little
uncertain at this time Wednesday night but should see the bulk of
these clouds slide southeast on Thursday with skies becoming more
partly cloudy...and with temperatures expected to be near normal around
Model guidance in fair agreement with the general weather pattern
towards the end of the work week and into the start of the
weekend...although some slight uncertainty is in place with exact
timing and coverage of expected weather. Energetic westerly flow
will try to usher in better forcing and chances for showers and
thunderstorms by Friday and Friday night...but once again
confidence on timing and coverage/placement of associated precipitation
is low. At this time...best chances for precipitation will be Friday
afternoon as surface trough and boundary push through with areas
across northern Illinois the location with highest probability of precipitation. With the
best forcing remaining to the north of the area its possible that
this precipitation will stay further north. Still maintained chance probability of precipitation
though...as forcing across northern will be sufficient for at
least some type of scattered development. Instability will
increase on Friday and do think thunder will be possible but with
the instability axis still west of the area during this time...am
not overly concerned at this time for severe weather. However...if
instability were to increase quicker...than some isolated stronger
development could be possible.
//Ord and mdw concerns...updated 00z...
* east/northeast winds 10-15kt early this evening.
* Lake breeze/east winds around 10kt Wednesday.
Three separate boundaries on radar this evening...a lake breeze
moving inland across southeast Cook County/Northwest Indiana.
Another boundary moving south from dkb to Ord...and a third
frontal boundary moving inland from ugn to just east of downtown.
Winds have shifted north at Ord in the past few minutes and will
turn east in the next hour or so as the frontal boundary moves
inland. Winds will remain easterly at mdw as the lake breeze moves
inland. These east/northeast winds will slowly diminish during the
middle/late evening and likely become variable as high pressure moves
across the area overnight. This high will result in light
northeast winds Wednesday morning with a lake breeze expected
Wednesday afternoon...shifting winds to the east at Ord/mdw.
Speeds will likely be near 10kts...at least for a few hours before
diminishing some by late evening. Cms
//Ord and mdw confidence...updated 00z...
* high for east/northeast winds through the period...medium for
speeds/timing this evening. Low for timing/speeds Wednesday. Cms
//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
* Thursday...dry/VFR. East winds.
* Friday...chance of thunderstorms and rain. Southeast winds.
* Saturday...dry/VFR. Southwest winds.
* Sunday...chance of thunderstorms and rain. Southwest winds.
* Monday...chance of thunderstorms and rain. Northeast winds.
448 PM CDT
Weak northerly flow is in place across the lake this afternoon
while weak high pressure is situated just south of the lake.
Stronger high pressure to the west and northwest of the lake will
drop southeast across the lakes through middle week...and then
continue to the east towards the end of the work week. Speeds
during this time will remain relatively light...before turning
more southerly and increasing by the end of the work week.
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