Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191820
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1220 PM CST Sat Jan 19 2019

.UPDATE...
1045 AM CST

The winter storm is gradually ending across the area, with much of
the winter storm warning looking to end on time. Still, bands of
heavier snow occasionally are seen in the deformation area of the
departing system. One such band is just south of I-55 over the
Chicago metro into Grundy County as of 1045 am. Another couple
such bands may be seen south and east of there before ending by
early-mid afternoon, given frontogenesis and decent instability
above 600 mb (as observed on 12Z DVN and ILX raobs). Disorganized,
light lake effect snow showers are expected from time to time in
northeast Illinois this afternoon with little accumulation from
just that.

In terms of blowing snow after the falling snow, reports from law
enforcement and webcam images thus far have not shown much for
reduced visibility or major road impacts. Will continue to check
with locations as the snow ends. If the message is still just
patchy blowing snow mainly in open locations, will just continue
that in the forecast with no additional headline needed.

Continue to assess the lake effect snow showers, and the
likelihood of a moderate to strong single band late evening into
overnight in northeast Illinois, and the behavior/pivoting of this
likely into northwest Indiana Sunday morning. A Winter Weather
Advisory may be needed for a period of heavy snowfall with this
and quick accumulations of three plus inches, especially Lake and
Cook Counties in northeast Illinois, and Lake County in Indiana
mainly for Sunday morning.

MTF

&&

.SHORT TERM...
347 AM CST

Through today...

Main forecast concerns/challenges are with light to moderate,
occasionally heavy, snow continuing across much of the area this
morning. Although snow was slow to develop in some locations
across east central IL and northwest Indiana last night,
increasing large scale ascent with approaching mid level
trough/short wave and persistent FGEN/WAA across the region has
supported snow development over the entire CWA early this morning.
Short term guidance continuing to indicate this
synoptic/mesoscale forcing will persist over portions of north
central and northeast IL through early/mid morning, and east
central IL and northwest IN through mid to late morning. During
this time, expect light to moderate snow with some periods of
heavier snow, with locations in/around the RFD area possibly
staying in the light to moderate range. Latest RAP analysis and
surface obs indicating there is some slight lake component to the
snow across far northeast IL. Although not overly
thermodynamically favorable across the lake at this time, onshore
flow and some weak convergence is supporting some lake enhancement
in this location. This can be noted on the TORD and TMKE radars,
with narrow bands of snow spilling inland across areas near the
lake and likely providing brief bursts of more moderate snow. This
will likely continue to be a persistent feature this morning, and
likely for much of the day. This system snow will likely exit
northern IL later this morning and locations across east central
IL and northwest IN later this afternoon, with no change to the
current warning end times at this time.

Highest snowfall totals at this time are likely along and north
of I- 88, especially towards the IL/WI border. Have highest
observed totals around 3-6 this morning and think additional
totals through the remainder of the morning will still reach
current forecast amounts. However, did make some slight adjustment
based on latest trends. Across north central IL, anticipate storm
totals to be in the 4-7 range. Across far northeast IL, mainly
Lake IL, DuPage, and Cook, think 5 to 9 are still likely.
Locations along and south of I- 80, expect snow totals of 3-6
inches. Based on reports received overnight and observing latest
webcams, hazardous conditions are occurring over much of the CWA.
Snow covered roads and blowing/drifting snow will continue to
provide hazardous driving conditions this morning over all of
northern IL and northwest IN. Blowing and drifting will likely
only worsen through the morning, as winds/gusts further increase.
System snow once again likely ends area wide later this morning
and afternoon, but do think blowing snow will be an issue and will
continue to monitor trends later today. Locations near the lake
will likely observe periodic snow showers off the lake this
afternoon. However, don`t anticipate any heavier snow or amounts
during this time.

Rodriguez

&&

.LONG TERM...
405 AM CST

Tonight through Friday...

Lake effect snow showers will continue to be a concern tonight
and Sunday across both far northeast IL and northwest IN. Once
again, after the synoptic snow ends later today, expect lake
effect development mainly on the IL side. Thermodynamic conditions
over the lake still not appearing overly great at the start of
the period, with marginal delta Ts and low inversion heights still
likely. So, do anticipate some development to still occur but
don`t anticipate this to be too impactful at that time. However,
conditions do change through late this evening and early Sunday
morning with both delta Ts and inversion heights increasing along
with the potential for strengthening convergence on the western
side of the lake. This should support higher intense snow, though
confidence is still lower with exact placement and duration for
locations in northeast IL tonight. With time, should see this
shift more into northwest Indiana by Sunday morning. Confidence
lowers further on Sunday with how the lake effect snow will trend,
as guidance continuing to vary with placement of persistent
development. As surface ridge shifts overhead and gradient
relaxes, it`s possible for this snow to shift back west. This will
be something to continue to monitor, as it could once again
impact locations in far northeast IL. Have continued previous
forecast of 2 to 5 inches for locations near the lake in northeast
IL and northwest IN tonight into Sunday. Have not issued any
headlines at this time, but will continue to monitor the need for
one later today.

Cold nights expected through early next week with wind chills
below zero likely. An active pattern appearing likely next week
with the next system to impact the area arriving Tuesday. The
arrival of this system will bring some warmer air, with both a
wintry mix and rain appearing possible.

Rodriguez

&&

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

Forecast concerns for the Chicago area airports are:

- Gusty northeast winds becoming closer to due north this evening

- Light lake effect snow showers this afternoon along with blowing
  snow

- Heavier accumulating lake effect snow showers tonight,
  especially later evening into overnight

The main snow shield has moved beyond the Chicago area airports,
however light, disorganized lake effect snow showers are likely to
continue into northeast Illinois through the afternoon. These will
gradually increase in intensity this evening as conditions become
more favorable for lake effect snow. Occasional visibility in the
IFR range is possible by late afternoon into the evening. As a
more well-defined lake effect snow band moves into northeast
Illinois later this evening (after 03Z), the potential for LIFR
visibility will increase. The challenge is what timing...start
time and duration. Continued a similar message to previous TAFs,
though right now it does not look like ORD or MDW would have a
full four hour window of such low conditions, but left that window
due to uncertainty. This snow band will shift more toward GYY on
Sunday morning.

Northeast winds gusting around 30 kt will continue through early
evening with high confidence. The direction may become more 020
degrees by late afternoon, but again will remain east of due
north. The direction tonight will be closer to due north and
depending on when the lake effect snow band passes, the direction
will shift from north-northeast to north-northwest. Speeds will
generally ease tonight, but near the heaviest lake effect snow
showers, sporadic gusts around 30 kt are possible.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Warning...ILZ023-ILZ033-ILZ039 until 3 PM Saturday.

     Lakeshore Flood Advisory...ILZ006-ILZ014 until 9 AM Sunday.

IN...Lakeshore Flood Advisory...INZ001 until 9 AM Sunday.

     Winter Storm Warning...INZ001-INZ002-INZ010-INZ011-INZ019 until
     3 PM Saturday.

LM...Small Craft Advisory...nearshore waters
     until 3 PM Sunday.

&&

$$

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