Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
656 PM CDT Tuesday Sep 1 2015
225 PM CDT
An upper level low is over central Indiana and it will slide
southeast through Wednesday morning. Thunderstorms formed along the
western edge of this low earlier this morning and afternoon...and
have now subsided. A few isolated thunderstorms are possible south
and east of I-55 through this afternoon but the afternoon should be
dry for most areas. The large upper level ridge remains in place
over the Continental U.S. Tonight allowing southwest winds to weaken. Did not
make any changes to low temperatures tonight with lows in the upper 60s to
An upper level wave moves over Wisconsin Wednesday with a trailing
vorticity streamer through Iowa. Expecting convection to Blossom to
over Wisconsin and Iowa and remain out of Illinois and Indiana during the
daytime hours tomorrow. Winds will become west and there is chance
of a lake breeze tomorrow. Winds will be at 20kt or less in the
lowest 1000 feet of the atmosphere in the morning and then weaken as
the day GOES on. As such...if a lake breeze does form...it would be
late in the afternoon or early evening as the winds relax.
Therefore...raised high temperatures downtown into the low 90s...with high
temperatures across the region in the low 90s. Dewpoints will once again
be in the 70s leading to heat indices in the middle to upper 90s.
Also issued an air quality statement for lake and Porter counties in
Indiana as an air quality day was forecast for parts of Northwest Indiana
by the Indiana department of environmental management. Ozone will be
the primary pollutant Wednesday.
225 PM CDT
Wednesday night through Tuesday...
A weak middle-level wave will be sliding east across the upper Midwest
Wednesday evening...and may bring an isolated shower/thunderstorm to
southern wisc and could push into northern Illinois early Thursday. Guidance
then progressively builds the middle-level heights over the plains and
eventually arrives over the forecast area Thursday evening. This should then
limit the potential for convection/precipitation late in the week. The bulk
of convection will be displaced well north into Canada and perhaps
Warm thermal ridge remains in place through the end of the week into
the weekend...supporting highs in the upper 80s to around 90. With
generally southerly flow at the surface...low level moisture will
continue to advect north and linger over the region. This will keep
dew points elevated in the upper 60s to around 70 late in the week through
Ensembles continue to point towards a more robust middle-level shortwave
arriving across the Pacific-northwest Sat/sun and steadily sliding east.
Guidance has been leaning towards a wave developing along the Lee-
side of this shortwave early next week...with an increased chance
for showers/thunderstorms for the first half of next week. This may
bring about a pattern shift...as ensembles indicate the robust
thermal ridge will be pushing east and the arrival of cooler air
beyond the current extended periods.
//Ord and mdw concerns...updated 00z...
* potential for visibility restrctions in br late tonight.
* Low chance for lake breeze wind shift late Wednesday.
* Low chance for thunderstorms and rain late Wednesday.
Should overall be a fairly quiet taf period. Thunderstorms and rain over central Illinois
will stay well south of terminals and dissipate this evening.
Moist airmass and light winds could again support light fog/br
overnight into early Wednesday...but the setup is less conducive than
the previous two nights. Favoring dpa and rfd for MVFR br
currently. Will monitor trends for Ord/mdw/gyy.
Expecting modest west-southwest winds on Wednesday that may not
be strong enough to prevent a lake breeze from forming...but at
this time thinking it will not push through terminals with gyy
perhaps having best shot. Forecast soundings indicate unstable
conditions and minimal capping Wednesday afternoon. However...aside
from lake breeze...a trigger for convection will be lacking until
an upper disturbance over WI possibly shifts southeast late
Wednesday night. Have gone with a dry forecast in tafs...including
Ord 30-hour taf.
//Ord and mdw confidence...updated 00z...
* low in MVFR visibility restrictions late tonight into early Wednesday.
* Medium-high that lake breeze will not move through Ord/mdw late
Wednesday afternoon/early evening.
* Low in thunderstorms and rain potential on Wednesday and Wednesday evening.
//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
Thursday...mainly VFR. Low chance for rain showers/thunderstorms and rain and associated
visibility impacts. East-southeast wind shift expected in afternoon.
Friday...VFR. Southeast to east winds.
Saturday and Sunday...VFR. East-southeast wind shift possible
Monday...low chance thunderstorms and rain and associated visibility impacts. Southwest
230 PM CDT
High pressure will remain over the eastern Ohio
Valley...allowing southerly to at times southwesterly flow across
Lake Michigan. A weak frontal boundary will try to push south across
far Northern Lake Michigan and turn winds northerly...but then turn
south again late in the week. The gradient will remain light...which
will allow for the development of afternoon lake breezes. Very warm
and moist air will continue to flow across the lake...and could
allow for patchy fog to redevelop late tonight. Currently winds are
just strong enough that this does not appear to be forming. So
confidence is currently low.
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