Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 152017
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
217 PM CST Fri Dec 15 2017

.UPDATE...
212 PM CST

Current wave of snow will have a burst of big flakes and has a
history of reduced visibility to about 2 miles, though lasting
less than 1/2 hour with the bigger flakes and gusty west winds. Do
not expect any accumulation.

KMD

&&

.SHORT TERM...
1202 PM CST

Through Saturday...

There are several short lived periods for snow chances
today, first with a weak disturbance moving through early this
afternoon. Flurries have been reported fora few hours or less at
any one location upstream in northeast Iowa. This evening a bit
stronger NW flow disturbance will interact with a a northward
moving warm frontal feature. Isentropic lift over this front will
lead to some snow or more likely flurries, mainly northeast of a
RFD-MDW-VPZ line during the 0z-5z time frame, or mid to late
evening. This would also be brief, only a few hours or so. After
this, the final push with this clipper will come overnight, but by
this point the forcing will have shifted north of the Wisconsin
border. Some guidance tries to break out snow farther south, but
at this point the main frontal boundary will have shifted into
Wisconsin and the air temperatures in the cloud layer will be less
than optimal for snow production.

The story for Saturday will be the milder air mass that will advect
into the area from the southwest as we at least temporarily break
the cycle of northwest flow aloft. There is a closed positively
tilted trough well south into the Baja peninsula.  Surface high
pressure to our south will also shift into the southeast on
Saturday, which will promote deep layer southwesterly flow. With the
front to the north, we will be in the dry and largely cloud free
warm sector. The surface front may not actually get north of the
Wisconsin border, so cooler southeast winds will keep readings
closer to 40 in these areas, where across central Illinois highs
will approach 50. The gulf moisture is still far enough away to
make for a pleasant and not too breezy day than typically occurs
when we get into southerly winds in the winter time.

KMD

&&

.LONG TERM...
215 PM CST

Saturday night through Friday...

Main concern for the long term forecast period remains precipitation
type Sunday morning.

The upper level pattern is trending toward a higher amplitude,
slowly progressive longwave pattern.  By Saturday night, broad upper
ridging will build over the ern 2/3 CONUS with deep troughing over
the Rockies, with upper ridging building onto the West Coast.  This
will shift the pattern from cooler nwly flow aloft to more moderate
swly flow. Southern stream shortwave energy will be the primary
weather driving systems for the long term period.  In general,
temperatures for the long term forecast period will trend toward
above normal for much of next week as the area remains under upper
ridging, with highs in the upper 30s to upper 40s with more
seasonable lows generally in the mid 20s to mid 30s until late next
week.

The longer range models continue to come into a little better
agreement on the timing of the closed upper closed low located over
the southern end of the Sea of Cortez Saturday morning lifting
northeast through the lower Missouri Valley Sunday morning.  The
longer term model consensus is a little better in both timing and
strength of the system as it opens up into an open wave, with timing
that is trending slightly slower than yesterday`s guidance.  By
Sunday morning, Expect that there will still be sub-freezing surface
temperatures over nrn IL/IN as far south as the southern tier
counties from Livingston to Benton Counties.  The mid-level pattern
will be transitioning to strong warm advection, with the warmer air
overrunning the colder surface air.  for example, the 925mb
temperatures indicated by the ECMWF will be above 0 C by Saturday
evening and climbing through the night as sfc temperatures drop below
32 F, with persistent easterly flow locked in over nrn IL/IN/srn
lake Michigan by strong high pressure parked sern Canada and the ern
Great Lakes.  The warm front, induced by the srn stream shortwave is
expected to still be just south of the CWA Sunday morning as
associated pcpn begins to spread in from the southwest. GFS is
advertising a relatively similar solution, but is trending a little
faster in lifting the warm front to the north, likely due to a
slightly faster progression of the associated shortwave.  At this
point, would tend to favor the slower solution which is supported by
the NAM, GEM and SREF guidance.  So, while the faster timing
indicated by the GFS may only be on the order of 3 to 6 hours, this
would have a significant impact on precipitation type.  With the sfc
colder air more likely to be scoured out by the warm advection
aloft.  Thermal profiles should be a shallow sub-freezing layer at
the surface with the warmer air aloft.  The shallow cold air would
be more supportive of freezing rain, and have carried that scenario
for early Sunday morning.  While there is still a chance that the
warm air aloft could mix down to the surface, setting up a rain
scenario, there is also a chance that the initial pcpn could also
generate some evaporative cooling near the surface, delaying sfc
temperatures from rising above freezing.  So, to make a short story
long, will carry a short period of freezing rain. possibly reaching
as far north as the I-88 corridor.  However, the window of
opportunity for freezing rain will be relatively brief, quickly
changing over to rain as the warmer air spread north.  Given that
pcpn amounts are likely to be light and the period of freezing rain
brief, any glazing of surfaces should be brief.

Otherwise, have made little change to the going forecast for the
remainder of the period.  The relatively mild should continue
through much of next week.  the longer term models are still
advertising another pattern shift by late next week with an upper
trough carving out over the Upper Mississippi Valley and Great Lakes
regions as a series of northern stream shortwaves break down the
upper ridging and bringing a return to more seasonable
temperatures.

&&

.AVIATION...
For the 18Z TAFs...

After a brief clearing period in wake of this morning`s flurries,
there are several additional short lived periods for snow chances today
and tonight. With each wave expect at least some MVFR clouds.
The first is with a weak disturbance moving through early this
afternoon. This evening a bit stronger NW flow disturbance will
interact with a a northward moving warm frontal feature.
Isentropic lift over this front will lead to some light snow or
more likely flurries, mainly northeast of a RFD-ORD line during
the 0z-5z time frame, or mid to late evening. This would also be
brief, no more than a few hours. After this, the final push with
this clipper will come overnight, but by this point the forcing
will have shifted north of the Wisconsin border.

Gusty west/northwest winds will shift to southwest tonight. Later
Saturday expect a wind shift to southeast, but confidence on
timing is not all that high just yet.

KMD

&&

.MARINE...
215 PM CST

A relatively quiet period of weather is setting up for Lake
Michigan for the next several days as a series of weak clipper-
type lows track across the upper Great lakes region. The next
chance for impactful weather and Gale Force winds will not be
until early next week when the passage of a cold front. There is
still some uncertainty as to the timing of the cold front passage,
but the latest model guidance continues to suggest Monday night
or early Tuesday for the frontal passage, with west to northwest
gales for Tuesday into Wednesday. The active pattern looks to
persist late next week with another system bringing another round
of strong winds to the lake.

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters
     until 9 PM Friday.

&&

$$

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