Weather Update for Chicago Area Tuesday, November 7, 2017


 National Weather Service


High pressure remains over the region through Tuesday, and cloudy conditions will persist. The clouds will impact temperatures by keeping them a bit higher overnight and cooler during the day. Lows tonight will be in the low to mid 30s away from the city, and highs Tuesday will be in the low to upper 40s. Tonight and Tuesday will also be dry with northeast winds.


Today: Mostly cloudy, with a high near 44. Northeast wind around 10 mph, with gusts as high as 15 mph.

Tonight: Mostly clear, with a low around 31. Northwest wind around 5 mph.

Wednesday: Sunny, with a high near 44. Northwest wind 5 to 10 mph becoming west southwest in the afternoon.

Wednesday Night: Mostly clear, with a low around 34. West southwest wind 5 to 10 mph.

Thursday: Mostly sunny, with a high near 40. West southwest wind 10 to 15 mph becoming north northwest in the afternoon. Winds could gust as high as 25 mph.

Thursday Night: Partly cloudy, with a low around 23.

Friday: A 30 percent chance of snow showers before noon. Partly sunny, with a high near 35.

Friday Night: Partly cloudy, with a low around 29.

Veterans Day: A chance of snow before 9am, then a chance of rain and snow between 9am and 11am, then rain likely after 11am. Mostly cloudy, with a high near 41.

Saturday Night: Rain likely before 4am, then a chance of rain and snow. Mostly cloudy, with a low around 35.

Sunday: A slight chance of rain and snow. Partly sunny, with a high near 43.

Sunday Night: Partly cloudy, with a low around 35.

Monday: Mostly sunny, with a high near 45.

Detailed Forecast Discussion

Today and Tonight…

Fast westerly flow aloft, with a series of minor amplitude short
wave ripples, will continue to bring mid-level cloud cover to the
area through much of the day. A few weak radar returns have been
noted with the mid-deck overnight, though 00Z soundings from
upstream locations and current AMDAR aircraft soundings with vapor
sensors out of KRFD depict very dry air below 750-700 mb. Given
little organized forcing, the weak nature of the echoes and 8-10 kft
of dry air beneath them, am not expecting any precip to reach the
ground across the LOT cwa. High-res guidance indicates an overall
decrease in coverage over the next few hours as well. Light rain
currently observed over downstate IL/IN is associated with the
stronger low level baroclinic zone located over the lower MO and OH
river valleys, and will not be a factor this far north this morning.
Thus mainly partly sunny/mostly cloudy skies are expected. Cooler
than average temperatures will persist, with highs generally from
the lower 40`s over far northern IL to the mid-upper 40`s across
the southern half of the cwa.

Farther north, GOES-16 water vapor depicts a stronger short wave
propagating east across the upper Midwest/northern Great Lakes early
this morning. A positive-tilt trough trailing from this feature is
progged to push southeastward across the region by this evening.
Subsidence/mid-level height rises behind it will erode cloud cover
from the northwest from late afternoon into this evening, and will
shift surface high pressure ridge from MN/WI into the area
overnight. Clearing skies, light winds in the vicinity of the
surface ridge axis and dry low level air (surface dew points in the
20`s) will support a chilly night across the forecast area. Coldest
temps are expected to be across north central IL, where the best
radiational cooling conditions are expected. Slightly colder NAM MET
guidance favored for overnight mins ranging from the lower 20`s north
to the upper 20`s/near 30 farther south/southeast. May see some lake
induced clouds hang on near Lake Michigan across northwest IN


330 AM CST

Wednesday through Monday…

The main concern is the quick blast of unseasonably cold Arctic
air Thursday afternoon through Friday, which could yield some lake
effect snow showers late Thursday night into Friday morning. The
temperatures prior to this will merely be below normal chill for
early November, but with plenty of sun during the day on Wednesday
and highs generally in the lower to mid 40s. A west- southwest
breeze will develop in response to elongated high pressure
centering just south of the area. Warm advection aloft Wednesday
night and light southwest winds will result in overnight lows not
as chilly as tonight.

Turning to the arrival of the Arctic air mass on Thursday,
guidance is coming into better agreement on the timing of arctic
front, which will sweep across the area by mid day with a wind
shift to breezy northwest winds gusting up to 25 mph. Temperatures
should rise to the mid 30s to around 40 north of I-80 and lower
to mid 40s south by the early afternoon prior to the sharp lower
level cold advection response. Once this occurs during the
afternoon, temperatures will quickly fall the remainder of the
afternoon to the upper 20s to mid 30s by sunset. 850 mb
temperatures as cold as -15 Celsius will pour into the area
during the evening as strong 1037-38 mb Arctic high pressure moves
into the western Great Lakes.

Winds are expected to diminish enough for a combination of
radiational and advection driven cooling Thursday night, which
will result in bitterly cold temperatures for this time of year to
start Friday. Am anticipating lows mainly in the mid teens to
lower 20s, if not colder (per various MOS products), except mid
20s in the downtown Chicago heat island and near the lake in
northwest Indiana where the lake effect clouds will move in. Wind
chill values early Friday could bottom out in the single digits
outside of Chicago in northern Illinois.

Regarding the lake effect response, 850 mb to lake delta Ts will
be extreme in the 20-25 Celsius range, which will generate about
400-500 j/kg of lake induced CAPE. On the negating factor for more
significant lake effect is the very dry air with the high pressure
and then fairly marginal equilibrium levels under 7kft. Low
level flow will turn northerly and then northeasterly with a
convergence signal in the guidance and then quickly southeasterly
Friday morning before the lake effect shuts off. Have increased
PoPs some into chance range for NW IN late Thurs night and then
points west along the Illinois shore Friday morning. Any
accumulations will likely be limited at most.

After a very cold day for November 10th with highs in the mid 30s
on Friday (normal highs lower 50s), the Arctic air mass will
quickly lose its grip. Models continue to indicate a system
potentially affecting the region over the weekend, but with
continued significant differences in timing and evolution.
Thus, confidence is on the lower side, including with
precipitation type, which could still start as snow on Saturday.


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