Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
251 PM CDT Tuesday Oct 13 2015
201 PM CDT
Lower level clouds are eroding from the west this afternoon...but
much of the region remains in northwest/cyclonic flow on the back
side of an upper level low over the eastern Great Lakes/southern
Ontario. There is a fast moving shortwave on the back side of the
upper low which will spread some middle level clouds through the area
this afternoon and maybe a sprinkle. But this wave will quickly
depart this evening...with more significant drying in the
subsidence region behind this wave.
Models depict low level cold advection tonight as flow remains
markedly northwesterly. Surface high pressure will move in from
the west tonight which should allow for significant breaks in the
clouds. The exception would be in Northwest Indiana where some
lake effect clouds persist. With clearing skies and lighter
winds...readings will drop into the upper 30s to low 40s...coldest
Fox Valley westward and away from the urban influence.
After continued cold advection overnight...a weak low level warm
advection regime ensues on Wednesday as flow turns more westerly.
We remain under the influence of progressive northwest flow aloft
and cyclonic flow which will keep things on the cooler side. But
with the weak warm advection and considerable sunshine...expect
some recoveries with highs getting back into the 60s...possibly
upper 60s far southwest...more in line with seasonal norms.
213 PM CDT
Wednesday night through Tuesday...
Amplified West Coast ridge will be in place to start the period
with broad troughing across the middle and eastern portions of the
country. Northwest flow will be in place across northern Illinois
and Northwest Indiana with cold advection resulting in a cooling
trend for the latter half of the week. A weak surface low is
prognosticated to move across the northern Great Lakes region Thursday
with a cold front sagging south across northern Illinois. NAM/GFS
have trended a little faster once again with the front while the
European model (ecmwf) has maintained an upper wave that digs a little deeper into
the Midwest. There continues to be a mixed signal on whether or
not there will be precipitation locally so confidence remains low...and
maintaining slt chance to low chance probability of precipitation. Either way though...there is
a fairly vigorous reinforcing shot of cold air with 850 mb temperatures of
7-8c Thursday falling below 0c Friday and bottoming out around
-5c Saturday. Surface high will gradually build from the northern
plains Friday directly overhead Saturday night. This should set
the stage for favorable radiational cooling. Anticipate much of
the forecast area outside of the urban heat island will drop near
or just below freezing Saturday morning...with some areas seeing a
hard freeze Sunday morning under the more favorable conditions for
nocturnal cooling. Continue to carry areas of frost for each
morning. Both the GFS and European model (ecmwf) continue to advertise a warm up as
we head into next week...with some chances for precipitation as warm air advection
overspreads the region.
//Ord and mdw concerns...updated 20z...
West-northwesterly winds (around 300 degrees) will continue to be a
bit gusty for time to time this afternoon. However...speeds will
gradually be diminishing through the afternoon as a surface low over
southeastern Canada continues to shift away from the western Great
Lakes region. Lighter west-northwesterly winds are expected by late
this afternoon through Wednesday.
Cloud cover continues around 2800 feet above ground level. However...satellite trends
suggest conditions are improving across north central
Illinois...with a scattering out of these MVFR ceilings. These trends
are expected to gradually shift eastward over the main terminals
this afternoon...with VFR conditions likely within the next couple
//Ord and mdw confidence...updated 20z...
* high for all elements.
//outlook for Ord/mdw for 00z Thursday-12z Tuesday...updated 12z...
* Wednesday night...dry/VFR. Light west winds.
* Thursday...chance of afternoon/evening rain showers. Westerly winds.
* Friday...dry/VFR. Gusty northwest winds.
* Saturday...dry/VFR. Northwest winds.
* Sunday...dry/VFR. Southwest winds.
* Monday...dry/VFR. Strong/gusty south/southwest winds.
213 PM CDT
Expect moderate flow primarily out of the west to northwest to
continue through the remainder of the week into Saturday as high
pressure remains anchored over the plains and a series of lows
rotate across the eastern Great Lakes into the Canadian maritime
provinces. The high will eventually translate east across Lake
Michigan late Saturday into Sunday with winds turning southerly
across the lake later on Sunday into Monday. Expect moderate south
flow then to be in place into the middle of next week.
Lm...Small Craft Advisory...Indiana nearshore waters until 10 am
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