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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 192348

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
548 PM CST Mon Nov 19 2018

239 PM CST

Through Tonight...

Main forecast concerns/challenges are with the lake effect snow 
likely late tonight into early Tuesday morning primarily for Lake 
IL, Cook, Lake IN, and Porter counties with some snowfall amounts 
possibly impacting the morning commute.

Mainly dry and quiet, with well below normal temps still in 
place. Temps struggled to rise much into the 30s for most 
locations given the solid clouds in place much for much of today. 
With some holes developing in both the low stratus and mid/high 
clouds this afternoon, some locations will observe a window of 
sunny/clear skies this afternoon into early evening. However, 
approaching shortwave energy dropping southeast along with digging
upper level trough will help push a surface low/trough and front 
over IA and WI across the area this afternoon into the evening. 
Cloud cover will fill back in during this time frame and with 
increasing ascent this evening, do think there will be the chance 
for either flurries or light snow over much of the area. However, 
at this time, not anticipating any snow accumulation. Despite 
cloud cover in place, increasing CAA will provide another cold 
night with low temps expected to fall to the low to mid 20s.

Turn attention to likely lake effect snow showers to develop 
across locations in far northeast IL and northwest IN late tonight
into early Tuesday morning. Behind the main surface low/trough 
and front, continued CAA will support highly favorable 
thermodynamic setup for lake effect snow, with increasing 850mb 
delta Ts and with inversion heights approaching 8kft to 10kft 
later tonight. With boundary layer flow off the lake by around 
late in the evening or closer to the midnight time frame, will 
likely see light lake effect snow development for both IL and IN. 
Also behind this surface trough and front, will observe a 
secondary surface trough swing south down the lake supporting a 
more defined band of snow late tonight. Given the thermodynamic 
setup and strong low level convergence/forcing guidance is 
showing, this band of snow could be of more moderate to possibly 
heavy intensity. Many pieces of guidance hinting at this 
possibility and with this in mind, did trend the forecast towards 
this solution. However, confidence is still lower with this 
possible moderate to heavy snow potential, mainly with exact 
placement and duration. At this time, think there is enough 
potential for this snow to move across the eastern sides of Lake 
IL and Cook counties late tonight and then focus briefly into 
northwest IN early Tuesday morning. During this time though, it 
does appear this snow should be more progressive in nature, with 
residence time likely low.

So, at this time, expect light snow showers area wide this 
evening with lake effect snow developing late this evening. 
Persistent lake effect snow showers, with a possible band of 
moderate to heavy snow, is expected for portions of Lake IL, Cook,
Lake IN, and Porter counties late tonight into early Tuesday 
morning. Window of possible heaviest snow is from 2 AM CST to 9 AM
CST. At this time, thinking that a half inch up to an inch will 
be possible for Lake IL and Cook counties, with 1-2 inches 
possible for Lake IN and Porter. However, will need to monitor 
this possible heavier band of snow and where it sets up and then 
how it evolves, as high snowfall rates could support isolated 
amounts of 1-2 inches or even 1-3 inches in anyone location in 
northeast IL and northwest IN. Morning commute will likely be 
impacted, outside of this possible heavier band of snow with some 
slippery conditions. If the heavier band materializes, then snow 
covered roads and severely reduced visibility will provide 
hazardous driving conditions. Snow development will likely 
continue through mid morning across northwest IN, however, will 
focus east of there by late morning.



239 PM CST

Tuesday through Sunday...

Not much has changed from previous thinking during the Holiday

The weather should remain on the quiet but cool side to start the
holiday weekend Wednesday and Thanksgiving day. Another cold 
front is expected to shift southward across the area on Wednesday 
and this act to reinforce the chilly weather over the area 
through Thanksgiving day. The good news is that there should not 
be any major storm systems around the area to impact travel. 
Temperatures will warm closer to normal into the mid to upper 40s 
on Friday the the lower level flow turns southerly. Rain chances 
have been boosted for Friday night and early Saturday as at least 
that period is starting to show a stronger precipitation signal in
the guidance envelope.

A short wave disturbance will ride the northwest upper flow into 
the Great Lakes region by early Wednesday, with its forcing 
remaining east of the area. However, a backdoor surface cold front
is depicted by the NAM and GFS to move into the area Wednesday 
morning, turning winds northeast and the pattern favors clouds. 
Some of the forecast soundings from the NAM even give a hint of 
patchy drizzle, but it may be overdone on the magnitude of its 
low-level saturation and inversion. Have highs on Wednesday in the
mid to upper 30s from north to south.

Also a trend in especially the GFS is some clouds lingering into 
Thanksgiving day for northeast Illinois and northwest Indiana 
before south-southeast winds being to usher them northward. Tough
to say at this distance, but forecast highs for Thanksgiving are 
down a handful of degrees from a couple days ago, with upper 30s 
to lower 40s north to south. The 00Z longer range guidance 
generally keeps Friday morning and likely at least part of the 
afternoon dry, with a south wind helping temperatures into the mid
to upper 40s. Even with more clouds by afternoon, advection 
should drive the temperatures up.

By Friday, the influence for the regional weather becomes more 
driven by the subtropical jet than the northern stream/polar one,
as a strong jet progresses across the southern Plains into the 
mid-lower Mississippi Valley. A fairly strong wave is forecast to
gradually become negatively titled within this pattern as it 
traverses over the area Friday night or early Saturday. The EC 
continues to be more amplified and 6-12 hours slower, and the 00Z 
GEFS members do support this more so than the GFS. This would mean
better chances for rain into Saturday. PWATs are predicted to get
into the 0.75 inch area which is not too high, and guidance 
predicted QPF shows good member clustering for four-five days 
out, with one quarter to one half inch.

For the rest of the holiday weekend into the start of the final 
week of November, the pattern is active and progressive, and given
the spread also messy on the forecast front. A stronger wave is
forecast to ride the subtropical jet in that period, with
influence/phasing from the northern stream hinted at. So there 
could be potential for a bigger precipitation system in that 
period. The pattern is forecast to quickly slow by the EC and 
especially the GFS, thus northwest flow on the backside of that
system with a pattern favoring below normal temperatures next 
week. This is in line with the current CPC 6-10 day outlook.



For the 00Z TAFs...

548 pm...Main forecast concerns are lake effect snow showers 
early Tuesday morning and wind directions tonight.

A cold front will move south of the terminals this evening
shifting winds to the north/northwest with speeds increasing to
10kts. Some higher gusts are possible. Winds may turn northerly
overnight but confidence on specific wind directions is low. Winds
will turn back to the northwest Tuesday morning and then to the 
west/southwest Tuesday afternoon with speeds increasing through 
the afternoon with gusts 20-25kts possible by Tuesday evening.

A few snow showers or flurries are possible this evening but these
are not expected to be significant or reduce visibility. Lake
effect snow showers are expected overnight into Tuesday morning
and will move along the Illinois shoreline. Confidence for how far
inland these move is low and this activity may remain east of ord.
There will be a better chance of snow showers at mdw with the best
chance of snow showers in northwest Indiana and at gyy. Duration
will likely only be 1-2 hours but may persist longer in northwest
Indiana. cms


LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...10 PM Monday to 
     noon Tuesday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...midnight Tuesday 
     to 4 PM Tuesday.