Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 072020

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
220 PM CST Wed Dec 7 2016

218 PM CST

Through Thursday...

Temps warmed more than expected today as cloud cover briefly gave
way to sunshine.  Cirrus has spread over the region again bringing
temps back down by a few degrees. Cloud cover will remain overnight,
but colder air moves in with the upper level trough.  Low
temperatures will be in the mid to upper teens. Combine that with
wind gusts up to 25 MPH and min wind chills Thursday morning will be
+1 to +5 degrees near the lake, and +4 to -1 away from the lake.

Gusty winds continue Thursday with gusts to 30 MPH, possibly higher
at times. High temps are forecast to be in the low to mid 20s, but
it will feel like +5 to +10 degrees.  Flurries are possible
throughout the day as the snow growth zone will be just above the
surface. Soundings lack significant forcing, but with the growth
zone so close to the surface and adequate saturation, flurries are
possible. Accumulating snow is NOT forecast through Thursday.



328 AM CST

Saturday through Wednesday...

The long term story is on accumulating snowfall for Saturday PM
into and Sunday. Nothing earth shatteringly different with the 0z
suite of guidance with the GFS, EC, and Canadian models generally
staying in their respective camps this go around, continuing
considerable uncertainty on storm evolution. What do have to note
that the energy driving this system is still spinning off the
coast of Alaska, so it has some time go before this portion of the
storm will be sampled by radiosondes. So let`s at least discuss
what they have in common for messaging purposes. The baroclinic
zone separating the warmer air to our south and our colder air
mass will lay out across Iowa and northern Illinois Saturday
afternoon. Strong southwest lower level flow will feed into this
frontal boundary as the upper flow becomes more cyclonic. Upper
level jet energy will also aid in lift and expect a quick ramp up
in snow during the afternoon and even more so in the evening. The
models do also largely agree that precip type for most if not all
areas will be snow. The Canadian model is about 6 hours faster
then the EC and 9 hours ahead of the GFS in starting
precipitation, and even gets things going Saturday morning, though
it looks like there may be some initial dry air to overcome. The
EC while lighter on precip amounts initially lingers the snow
longer. This snow appears to be a bit fluffier than our last one.

Where things differ is in the strength of the upper flow pattern and
whether a surface low will completely develop over our area as
the GFS/GEM say, vs remaining as an inverted trough axis like the
EC before the surface low develops to our east. This leads to
significant differences in the strength and timing of forcing and
therefore associated QPF, with the GFS double the amounts of the
EC (more in some places), and spread in GEFS ensembles of 0.2 to 1
inch of QPF. The GFS ensemble mean is around 0.55 and the Euro
ensemble mean is around 0.3. Therefore while forecasting amounts
are futile at this juncture, no matter how you paint it,
accumulating snowfall of at least several inches is expected
during this time.

The back side of this system will not be like our last snow where we
stayed mild. This will be the more typical one-two punch of snow
and then brisk and cold. Even colder air will invade the region
mid to late week (see the CPC 8 to 14 day outlook, which has our
entire area with well below normal conditions). To give you a
hint, even the model blend has highs in the mid single digits to
low teens, which is very likely overdone in an anomalously cold
situation with a snowpack in place.



For the 18Z TAFs...

Broken high level cirrus is over the region with west winds
occasionally gusting to around 20 kt. Cloud cover increases this
evening as the upper level system shifts overhead. Ceilings lower
to low end VFR/high end MVFR by tomorrow morning. Ceilings could
be lower than forecast, around 1500 ft, but do not have enough
confidence to go that low. West winds increase with gusts arnd 25
KT possibly up to 30 kt Thursday. Scattered flurries are also
possible throughout the day, but no accumulation nor impact are
expected from the flurries.



218 PM CST

Headlines: No changes were made to the ongoing gale warning.
Extended the small craft advisory due to lingering hazardous waves.

The low over James Bay continues east through the end of the week
while high pressure builds over the plains. West winds will remain
in place into Saturday as the high moves over the lake.  The next
low forms over the plains this weekend. Guidance still show
significant differences in the low`s path and strength; however,
the low will impact the lake over the weekend.  Winds become south
to southeast ahead of the low Saturday evening.  The low itself
passes over or near the lake early next week, and winds become east
behind it.  Have low confidence in the wind and wave forecast
Saturday evening onward due to low confidence in the weather pattern.



LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 6 AM Friday.




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