Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1003 PM CDT Tuesday Sep 16 2014
908 PM CDT
Only minor changes made to the rest of tonights forecast. With
clear skies and light winds allowing for ideal radiational cooling
in the continued cool and dry air mass...temperatures dropped as
much as 15 degrees in two hours early this evening. Considering
minimum temperatures last night and dew points running lower at
most locations...have nudged minimum temperatures down a couple
degrees. Would expect to see some isolated sub 40 degree readings
in the usual outlying locations.
Slightly higher boundary layer moisture exists in parts of east
central Illinois and northwest in...as evidenced by dew points and
earlier daytime cumulus. This probably will allow for patchy
shallow fog in this area. Have added into the forecast but should
not be a big impact.
254 PM CDT
high pressure has become centered over the region today...providing
abundant sunshine and light winds. This evening is setting up to be
a beautiful one with mostly clear skies and nearly calm winds for
much of the overnight hours. Temperatures will radiate once again into the
low 40s for much of the forecast area...with the exception being
downtown Chicago where temperatures will likely hold in the upper 40s/around
Wednesday through Friday...
middle-level trough will remain overhead through Wednesday with a continued light
northwest flow. At the surface broad surface ridging will remain in
place...with abundant sunshine once again Wednesday afternoon. A backdoor front
will slip south across the lake/northern Great Lakes Wednesday early
morning...eventually arriving across northern Illinois/northwest in late
Wednesday night/early Thursday morning. This will likely produce an area of
stratus as it arrives...however at this time dry conds are expected
to persist through Thursday. Then another surface ridge is poised to slide
south from central Canada...and linger over the region through early
As the surface ridge Friday begins to drift east...the middle-level flow will
begin to flatten. This will allow a steady turning in the low level flow
to the south/southwest and begin advecting warmer/moist air into the
region Friday night. Guidance has been steadily pushing probability of precipitation back until
Friday night...and could see the surface ridge holding into Friday night with probability of precipitation
holding off until early Sat.
Temperatures Wednesday/Thursday will generally be in the middle/upper 60s...with a few
points possibly touching 70 mainly south of I-80. With the backdoor
front slipping south it is possible the clouds could linger long
enough Thursday that temperatures hold in the low 60s to possibly upper 50s
across the northern Illinois sections. Then for Friday temperatures will likely
begin to climb into the low to possibly middle 70s. Overnight lows will
generally be in the 40s to low 50s.
Saturday through Monday...
ensembles have come into slightly better agreement in the Sat/sun
timeframe with the 500mb flow remaining somewhat flat...which should
help to bring the expected channel of moisture/convection into the
region but remain progressive. Then for the second half of the
weekend ensembles return the middle-level ridge to the western
Continental U.S....with downstream troughing developing over the Great Lakes
region early next week. It appears the best day for convection will
end up being Sat night/Sun morning. Temperatures will likely approach 80 degree
Sat...and perhaps even the low 80s across the southern County warning forecast area. However
this warm-up appears to only be brief with temperatures sun back into the
low 70s and 60s for Monday.
//Ord and mdw concerns...updated 03z...
* lake breeze late Wednesday afternoon. Cms
The lake breeze has surged inland and is currently moving west
through Ord/mdw. Speeds may reach 8-10kts with its passage but
then diminish back to 4-6kts for a few hours. High pressure over
the region tonight will allow winds to become light and variable
or calm by late evening. Winds will then become light southwest
or west Wednesday morning with the weak gradient remaining as the
center of the high shifts east. Another lake breeze should
develop Wednesday afternoon...possibly reaching Ord/mdw by late
//Ord and mdw confidence...updated 03z...
* medium for a lake breeze reaching Ord/mdw Wednesday afternoon.
//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
* Thursday and Friday...dry/VFR.
* Saturday...chance thunderstorms and rain. Gusty southwest winds.
* Sunday...slight chance of thunderstorms and rain.
228 PM CDT
High pressure has built across Southern Lake Michigan this
afternoon. However the south to southwest gradient has increased
across the northern half of the lake ahead of the next cold front
over Lake Superior. The front will sag south across the
lake for the latter half of Wednesday into Wednesday night. High
pressure is right on the heels of this front causing a sharp
increase in winds behind the front into the 20 to 25 knots range out of
the north-northeast Wednesday evening. Expect winds will be slightly
less farther south as the pressure gradient will not quite as tight
for the southern half of the lake. Winds will veer back around to
the southeast Thursday and Thursday night while diminishing...then
become south to southwest Friday increasing into the 25 to 30 knots
range as broad low pressure develops over the plains and Canadian
prairies. Winds may strengthen to near gale force Sunday as the low
strengthens east toward Lake Superior then shifts east.
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