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981 
FXUS63 KLOT 252158 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
358 PM CST Fri Jan 25 2019

.SHORT TERM...
1209 PM CST

Through Saturday...

Differential water vapor RGB imagery this morning shows a well
defined shortwave trough digging southeast into the Midwest. Late
morning sfc analysis has a 1018mb low near KFSD with short range
guidance in good agreement in tracking this low east-southeastward
across central IL this evening. The 12z soundings out of ILX and
DVN both were extremely dry with PWATs of barely 0.05" with very
little in the way of upstream moisture to advect north. However,
continued and strengthening ascent should result in deepening
of the saturation in the column and eventually the development of
a swath of light snow that will track across northern IL and
northwest IN later this afternoon into this evening. 

Biggest change to going forecast was to nudge the axis of higher
(albeit still very light QPF) farther north. Going forecast used
snow:liquid ratios close to 20:1 and stuck with that range though
forecast soundings and time-height cross sections suggest the
ascent in the better DGZ is fairly weak and the isothermal layer
is barely within the prime DGZ range. This raises the possibility
that SLRs could end up lower, perhaps 15:1 or less if the dominant
flake type is more needle and crystals. With the higher QPF moved
north a bit, the 1-2" amounts now extend up to or just a bit north
of I-80. Depending on flake size and subsequent SLR, could end
up with max amounts closer to or just below an inch. 

Regardless of snowfall is a half inch or 2 inches, the potential
impacts won't vary much. The bitter cold air mass will allow snow
to immediately stick on untreated roadways and the very cold temps
could also make chemical treatments on roads less effective. So
main forecast message for this afternoon is light snow will
develop and result in potentially hazardous travel conditions
during rush hour, especially along and south of I-88/I-290.

Light snow will quickly end from north to south this evening with
the primary forecast challenge tonight being cloud cover. MOS 
guidance, which typically handles cold season post-frontal stratus
well hangs onto cloud cover through most of the night. However, 
satellite trends would seem to favor a faster clearing trend 
tonight and will hedge the grids in that direction. Given the sfc 
high moving nearly overhead tonight, if skies clear out there is 
potential for temps to really get out of hand, particularly in the
favored cold spots. Given the expected clearing, plan to nudge 
lows toward the lower end of the guidance spectrum tonight. 

- Izzi

&&

.LONG TERM...
240 PM CDT

Saturday through Monday night...

The primary forecast challenge during the period continues to 
focus on the potential for significant snow accumulations Sunday 
night and Monday.

The period will begin Saturday with continued cold conditions.
Temperatures are expected to top out near 10 north, to the upper
teens far south. Similar temperatures are expected for Sunday. Another
mid-level disturbance is likely to shift southeastward across the
area during the day Saturday. This should result in another decent
chance of some light snow and flurries, with the best chances 
over the south.

The main period of interest for our next winter storm continues to
focus on the late Sunday evening through Monday timeframe. During
this period, a clipper type system is expected to dig
southeastward across the Dakotas and into the lower Great Lakes.
Some of the American 12z operational model runs have tried to 
shift the track of the clipper a bit farther north, namely across
far southern WI on Monday. This certainly adds some uncertainty 
to the forecast of snow amounts to the area, as such a track could
keep the heaviest snow into Southern WI. However, the ensembles 
do not necessarily support such a slow and northward track of 
this low. For example the 12Z GEFS mean favors a track farther 
south and a slightly faster than the 12z operational run of the 
GFS. This farther south solution also continues to be favored by 
the 12z ECMWF and its ensemble. 

Overall, this is a very complex pattern in which this clipper 
will be evolving in, and as such the individual model runs are 
likely to continue to waver a bit in the track of the system into
the weekend. While the farther north track seen in the GFS and 
NAM do add some uncertainty, we have opted to not change forecast
snow amounts much at this time. As such, at this time, we 
continue to favor the heaviest snow along and north of I-80. 
However, this will have to be fined tuned in the coming days. 

KJB


Tuesday through Friday...

Record breaking and potentially historic outbreak of cold still 
appears to be in the offing for the middle of next week. Greater
than normal degree of forecast uncertainty in temperatures, due in
large part to models showing all time records being broken and in
some cases shattered.

Generally made only very small tweaks to the temperatures from the
model blend, due to the uncertainty. It is worth noting, that this
model blend includes bias-corrected grids which still reflect a
bias correction warmer from the unseasonable warm spell back in
late Dec and early Jan and also contains a MOS component which
tends to have some weighting toward climo, which obviously midweek
temps will not look anything like climo. Typically, in record
breaking temperature situations, a blend of the raw 2m model
temperatures verifies best and those values are colder than our
official forecast and approach or break all time records for ORD
and MDW. 

The GFS, GFS-FV3, GEM, and ECMWF 12z operational runs all continue
to drop 850mb temps to below -30C & in the case of the GFS to- 
40C. To put this into perspective, using the SPC sounding climo 
page, the coldest 850mb temp even observed in IL (ILX, DVN, RAN, 
and PIA and dates as far back as 1949) is -32.9C. Over the past 
couple days there have been some minor run to run variations in 
how low 850mb temps bottom out, but there has been a good deal of 
consistency of 850mb at least reach -30C. Even the 51 member ECMWF
ensemble has 850mb temps reaching -30C over northern IL. 

Assuming we have a fresh snow pack from Monday's system, the stage
would be set for temperatures to really get out of hand and end up
colder than currently forecast. Initially, moderate to strong
northwest winds look to send wind chills bottoming out in the -35
to -50F range late Tues night into Wednesday. Some minor
improvement in wind chills is expected Wed night into Thursday
morning, due primarily to diminishing winds. Also worth noting 
that the 35 to 50 below wind chills are based on the somewhat 
conservative official forecast temps, but should the colder raw 2m
model temps verify we could see wind chills approaching 60 below 
in the coldest areas and with most of the CWA reaching close to 50
below. 

There is still time for model guidance to back off on the
intensity of Arctic air mass. Keeping that in mind, I felt quite
comfortable with the more conservative temperature forecast. It is
much easier and preferred to slowly trend a forecast closer to 
all time records this far out, rather than jumping out forecasting
all time records only to later have to back off on those extremes
if model guidance trends less severe with the cold. 

To put this cold into perspective some, during the past 20 years,
Chicago has only had a sub zero high temp twice and only 22 times
in the past 100 years. Our current forecast has air temps below 
zero between 40 and 60 hours across most of the CWA, which a much 
longer period of sub-zero wind chills. 

While still very cold, temps do look like they will begin to
moderate Friday.

- Izzi

&&

.CLIMATE...

Here are the record lows and record cold highs for next week's
cold...

                     Tue Jan 29   Wed Jan 30   Thu Jan 31   
   Chicago 
Record Low           -16 (1966)   -15 (1966)   -12 (1985)
Record Low Max        -6 (1966)     3 (1966)     1 (1971)

   Rockford 
Record Low			 -20 (1966)   -19 (1966)   -18 (1966)
Record Low Max		  -9 (1966)    -3 (1951)    -3 (1996)

   Chicago
All Time Record Low:      -27 (Jan 20, 1985)
All Time Record Low Max:  -11 (Jan 18, 1994 & Dec 24, 1983)

   Rockford
All Time Record Low:      -27 (Jan 10, 1982)
All Time Record Low Max:  -14 (Jan 18, 1994 & Jan 6, 1912)

- Izzi

&&

.AVIATION...
For the 12Z TAFs...

The forecast item of note for the Chicago area airports today is
the likelihood of light snow mid-late afternoon into mid-evening. 

Westerly winds early this morning will become more southwest by
late morning and settle in at 8-10 kt for much of today. An upper
level disturbance will move across the region late today into the
evening. Its forcing will result in light snow spreading eastward 
over the region during the mid-late afternoon. Given the Alberta
Clipper archetype of this system and the path it is taking over
the area, all five local area TAF sites are most likely to 
experience visibility in the 1-2 SM range during the snow with 
medium-high confidence. Brief lower visibility is possible. 
Blowing snow is not expected to be a noteworthy issue on the 
airfields given wind speeds quite a bit less than the past couple 
episodes of blowing snow. As for timing of the snow, confidence 
is medium in what is forecast. The snow will be a lighter 
composition, with less than an inch of accumulation expected at 
the TAF sites.

During the snow, ceilings are favored to bottom out into the lower
MVFR category with any IFR ceilings being brief. Behind the snow 
late tonight into early Saturday morning there may be some 
lingering MVFR clouds but most are expected to have departed by or
shortly after daybreak.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Heavy Freezing Spray Warning...LMZ740-LMZ741-LMZ742-LMZ743-
     Gary to Michigan City IN until 9 PM Friday.

     Small Craft Advisory...IN nearshore waters until 3 PM Friday.

&&

$$

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