Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
207 PM CST Monday Nov 30 2015
207 PM CST
Rain mixed with a little sleet is continuing to shift north and east
through northern Illinois. Did not expect sleet to persist this long...but
the dry layer at the surface is still rather impressive leading to
evaporative cooling and sleet. Thinking sleet will dissipate this
afternoon as the column moistens. The dry layer is also slowing the
rain/S progress through the warning area with most areas in Northwest
Indiana still dry.
The next low is over eastern Nebraska and southwest Iowa and will
shift over southern Minnesota early Tuesday morning. The next wave
of precipitation is over Iowa closer to the low. Additional vorticity
streamers will move across the region tonight. The added forcing
along with additional moisture will lead to widespread rain. Total
rainfall amounts will vary from up to 0.25 inches south of I-80 to
0.25-0.5 inches north of I-80. Could see areas of fog develop after
midnight. Dense fog is not expected...but visibilities will be
reduced by rain/mist/fog. Low temperatures will be above freezing and
forecast soundings only have saturation below the dgz...so ice
crystals will be hard to come by...giving ME high confidence in rain.
Rain ends from southwest to northeast Tuesday morning as the vorticity
streamer pushes through the region. Raised high temperatures for tomorrow
as gusty SW winds will help usher in warmer air. However...at the
same time cooler upper level air will be pushing in from west to
east. Therefore high temperatures will vary from the upper 30s by Rockford
to the middle 40s east of I-55. Lowered wind gusts as the winds above
the surface do not look as strong. Have gusts maximum gusts of 25-30
miles per hour. A secondary vorticity wave associated with the upper level low
approaches from the west Tuesday afternoon/evening...but guidance is
keeping precipitation north of the Illinois/WI state line. Therefore...continued
the dry forecast through Tuesday afternoon.
325 am CST
Thursday through Sunday...
The extended period looks relatively quiet. Following the departure
of the cool upper trough...ridging aloft settles in for Thursday...
resulting in near normal conditions with increased chances for
sunshine...though their could still be some lingering low clouds
Thursday morning in addition to some lake effect clouds in Northwest
Indiana. Upper level heights generally rise Friday into the
weekend...and as this occurs a very strong surface high will
builds over the eastern half of the country. Increased southerly
flow will result temperatures increasing through the 40s with
continued sunshine. Maybe a few more clouds find their way into
the region toward Sunday...but still quiet and relatively mild for
//Ord and mdw concerns...updated 20z...
* easterly winds today/this evening.
* Rain with possible sleet in the near term...with rain becoming
more prevailing this evening/overnight.
* MVFR ceilings quickly falling to IFR and LIFR this evening.
Latest radar imagery depicting rain impacting rfd/dpa...and slowly
reaching the other terminals at this time. Drier air still in
place is likely limiting eastward development but expect that to
not be a limiting factor early this afternoon...with this
showerly activity likely reaching the remaining terminals. An
initial rain/sleet mix will still be likely at the onset...but
with sleet becoming less likely there after. More widespread rain
is still expected later this evening into the overnight
hours...exiting by early Tuesday morning. MVFR ceilings will
finally fall to MVFR this evening and then likely quickly fall to
IFR and then LIFR. Winds will vary throughout the forecast period
with the current easterly winds becoming southeast tonight...and
then turning to the southwest Tuesday morning while also observing
an increasing trend into midday.
//Ord and mdw confidence...updated 20z...
* medium high with wind speed/direction.
* Medium high with rain/sleet mix.
* High with rain this evening/overnight.
* High with ceiling trends.
//outlook for Ord/mdw for 00z Wednesday-12z Monday...updated 12z...
Tuesday night...MVFR with IFR possible. Chance of light rain
changing to snow. Southwest winds.
Wednesday...MVFR with early morning IFR possible. Chance of light
snow. West winds.
Thursday-Sunday...primarily VFR. Southwest winds.
215 am CST
It will be an active first half of the week across Lake Michigan as
ridging departs eastward today and low pressure develops to the
southwest. Easterly winds in advance of this low will encompass
the lake today and strengthen this afternoon. Small craft criteria
winds and waves will be seen by middle-afternoon across the Illinois
nearshore...especially from downtown Chicago northward. Winds will
turn southeast later tonight with waves building over northern
parts of the lake and a temporary easing over the far south.
With the low occluding over southern Minnesota...the system will
spiral a cold front northeastward over the southern and central
part of the lake Tuesday morning. The setup favors southwest winds
to quickly increase early Tuesday morning with gusts likely to
temporarily exceed 30 knots. Cips analog guidance for similar events
as to that forecast indicate 30 to 40 percent of them produce
gale force gusts near the lake...so will need to monitor for a
brief gale period over the south Tuesday morning. Winds will turn
westerly behind a secondary cold front on Wednesday morning. Wind
speeds will remain elevated...in the 15 to 25 knots range through
Thursday...before dropping on Friday as ridging moves across.
Lm...Small Craft Advisory...Illinois nearshore waters until 4 am Tuesday.
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