Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 090302

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
902 PM CST Fri Dec 8 2017

902 PM CST

No big changes planned to going forecast. Did low temps upward
just a bit tonight.

Potent jet is diving south into the Upper MS Valley region this
evening with RAP analysis suggesting 150kt+ 300mb winds at the
core with 125kt sampled on MSP`s evening sounding. Strong ascent
in the left exit region of this jet is resulting in strengthening
ascent and blossoming area of virga over northern IL. Radar
presentation would suggest that snow is very close to reaching the
ground if it isnt already over the northwest suburbs. Recent ACARS
soundings suggest the dry layer is only on the order of about
3-4kft deep and virga will likely be able to penetrate through and
saturate the low levels this evening.

Once low levels are saturated, expect light snow to develop withe
some bursts of occasionally moderate snow showers. Most intense
snow showers should occur near and just ahead of the approaching
cold front later tonight. See no reason to make any adjustments to
forecast snow amounts tonight with most areas likely to see a
dusting to an inch; though areas of NE IL and NW IN could see an
inch to perhaps 2" on the high end.

One change to forecast for Saturday morning was to add post
frontal snow showers and flurries in the CAA regime, particularly
eastern CWA in closer proximity to the cold upper low. Moisture
will be limited, but what little moisture there is should be
squeezed out fairly easily given the steep low level lapse rates
and strong CAA. Not expecting much more than isolated amounts of a
few tenths to maybe a half inch, but that should be spotty given
the expected isold-wdly sctd nature of the activity.

The same steep low level lapse rates and high RH in the low level
conducive for CAA driven snow showers over land will make for a
stellar lake effect snow (LES) set-up tomorrow. LES parameters
are quite impressive and it looks like an intense single band
could develop. Tropopause drops to near 500mb which will also
serve as the equilibrium level for lake induced convection (very
high for LES) and combined with 850-lake delta Ts approaching 20C,
suggests a very potent LES band should develop. Given the progged
parameters, if single LES band develops as expected, would expect
snowfall rates of at least 2" per hour (probably higher) in the
heart of the snow band. The unseasonably warm lake and deep
instability also raise the specter of potential isolated
thunderstorms with the LES band tomorrow.

Big question for snowfall amounts will be residence time of the
snow band at any given location. Hires models strongly suggest
that lake induced mesoscale enhancement of the surface low will
take place Saturday. The primary LES band is expected to develop
to the west of this low, with the low likely to result in some
meandering/wavering of the position of the band. Gave very strong
consideration to upgrading Porter to a winter storm warning, as
such an intense band could drop 6" of snow in just a couple hours.
If rates of that magnitude occur, impacts could be very
significant, particularly in light of the expected 30-45 mph winds
near the lake which could result in white-out/blizzard conditions
at times in the more intense snow squall(s). The primary factor
keeping me from upgrading to a warning at this time is the
uncertainty in location of the LES band. The operational HRRR
keeps it primarily east of Porter County, but many other models
have it farther west and keep Porter County well within the line
of fire.

- Izzi


112 PM CST

Through Saturday...

Expect a burst of dry snow tonight area wide, mainly after
midnight. Gusty northwest winds will arrive Saturday morning
creating low lying drifting snow. Northwest Indiana, particularly
Porter County, will likely see some lake effect snow, along with
blowing snow on Saturday.

We did not see any big changes in the 12z guidance to change the
message of a quick hitting band of accumulating snow overnight.
This system is still across southern Canada, but will make a bee-
line southeast into our area tonight, then quickly exit east on
Saturday. This will be a drier snow with fairly decent -12 to -18C
omega shifting through the area just ahead of a cold front
overnight. Areas east stand chance of a bit more accumulation.
Expect a several hour period of snow, and with cold pavement,
accumulation will happen quickly once the initial top down
saturation occurs this evening. The main snow will occur after
midnight. This looks like a solid 1-2" event across the center of
the CWA, with lower totals west (north central Illinois and
across Central Illinois) and higher numbers possible east across
Indiana. A few showers will linger Saturday morning, though
accumulations would be limited to a dusting or a few tenths.

Concerns on Saturday will shift to gusty northwest winds,
drifting/blowing snow. In northwest Indiana, lake effect snow is
expected. Earlier in the shift, we added a winter weather
advisory for Porter County beginning tonight and ending late
Saturday. The synoptic snow does not appear to be significantly
different than other areas, although northwest Indiana is in a bit
more favored track for initial synoptic snows. After these few
inches of snow fall, strong winds (gusts to 30 mph or so) will
occur behind the snow as a compact surface low will pass right
over southern Lake Michigan. Forecast soundings across northwest
Indiana depict several hundred J/Kg of lake induced CAPE, with
this CAPE square through the dendritic growth region, and
reasonably high equilibrium levels. The heaviest lake effect snow
(which is typically highly variable dependent on location) will
fall into LaPorte and counties east where stronger convergence is
depicted, but there is a decent signal into Porter county for at
least a time. These combined concerns of strong winds, especially
for northern portions of the county seem to warrant an advisory.



147 PM CST

Saturday night through Friday...

A fairly active weather pattern is still expected for the next
several days, with periodic chances for pcpn and generally below
normal temperatures.

By Saturday evening, any remaining snowfall will likely be some
lingering lake effect snow for northwestern Porter County as low
level winds back from nwly early in the evening to swly by morning.
Wind speeds should also be on a diminishing trend as a weak ridge of
high pressure builds across the midwest.  The upper level pattern
will continue to trend toward high amplitude and slow progression
with upper ridging building over the wrn 1/3 of the CONUS and broad
troughing over the ern 2/3 CONUS.  The slowly progressive, high
amplitude long-wave pattern lends relatively higher confidence in
the overall trends of the long term forecast period, with persistent
nwly flow aloft keeping the region under persistent, deep layer
cold advection.  This would suggest that the trend of below normal
temperatures will continue through the period.  Toward the end of
the forecast period, from Wednesday through Friday, the long term
models are trending a bit warmer than the large scale pattern would
suggest and feel that model guidance temperatures are a bit too
high, so have trended the temperature forecast a few degrees lower
than much of the model guidance would suggest.  The other, more
complicated, problem for the extended forecast period will be pcpn
chances.  Under nwly flow aloft, pcpn chances will largely be driven
by shortwaves dropping out of Canada and tracking across the
midwest.  After a dry period Saturday night and sunday, general
forecast confidence is relatively high in periods of pcpn, likely
snow, from Monday through the remainder of next week.  There is
relatively good agreement among the models in the idea of a quick
moving clipper bringing another round of snow to the region.  The
fast nature of the system would suggest that any new snow
accumulations should be light.  After Monday, confidence in the
exact timing, track and intensity of individual shortwaves
diminishes as model solutions of the finer scale details diverge.
So, will continue the idea of periodic chances for pcpn, which
should mostly be snow.  Each shortwave dropping out of Canada will
bring a reinforcing shot of cold air, helping to maintain the
general trend of below normal temperatures through next week.


For the 00Z TAFs...

The main concern is the period of snow tonight. Light accumulations
of up to an inch or so of dry/fluffy snow are expected for the
eastern terminals, with less west. There is some uncertainty on
the snow start time later this evening, which will depend on the
low levels fully saturate. However, confidence is medium-high in
timing of the highest rates, which will be up to about 3 hours
after 06z. Looking at upstream observations, visibility of 1 mile
or less is probable during the period of highest rates, along with
brief low MVFR to IFR CIGs. The steadier snow will quickly taper
off prior to 12z, with on and off snow shower activity likely for
the eastern terminals through mid day/early afternoon. There looks
to be a window where 2-4 hours of more robust snow showers are
possible, starting in the mid to late AM. In addition, lake effect
snow could set up near GYY, but most likely just east.

Southwest winds will continue in advance of the clipper low
pressure system this evening and much of the overnight. Winds will
then shift to strong northwest Saturday morning as the low tracks
to western lower Michigan, with gusts in the 25-30 kt range,
possibly higher at times especially at GYY. Any lingering MVFR
CIGs will clear by Saturday evening, along with diminishing west-
northwest winds.



200 PM CST

Headlines...A Gale Warning is now in effect for the southern 2/3
of Lake Michigan, from Saturday morning into Saturday evening.

North to northwest winds are expected to increase rapidly late
tonight as deepening low pressure lifts through the eastern Great
Lakes and high pressure builds across the Upper Missouri Valley.
The far northern portions of the lake will likely remain below
Gale Force as the gradient should be a little weaker there. The
overall period of gale potential should be relatively short as the
ridge builds across the western Great Lakes by Saturday evening,
allowing winds to diminish and back to southwesterly.

The series of low pressure systems will continues through next
week. Another weak low will form near Lake Superior Sunday
morning followed by a stronger low that moves over Western Great
Lakes Monday afternoon. Guidance differs if the low will pass over
central Lake Michigan or south of the lake entirely, which would
have some impact on the forecast wind trends. However, at this
time, it looks like Gales are again possible behind the low
Monday night into Tuesday. Another high pressure ridge will
quickly pass over the western Great Lakes Wednesday bringing a
short period of diminishing winds, before ramping up again as the
next low approaches the Great Lakes late next week.


IN...Winter Weather Advisory...INZ002...midnight Saturday to 6 PM

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...6 AM Saturday to 9
     PM Saturday.

     Gale Warning...LMZ743-LMZ744-LMZ745...9 AM Saturday to 9 PM

     Small Craft Advisory...IN nearshore waters until 9 AM Saturday.

     Gale Warning...LMZ777-LMZ779...6 AM Saturday to 9 PM Saturday.




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