Preview of NWS' New Version of Forecast
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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 180531

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1131 PM CST Thu Jan 17 2019

852 PM CST

No further changes made to going forecast through tonight. First
glance at 00Z NAM arriving continues to favor areas north of I-88
for heaviest snowfall amounts with storm developing Friday
afternoon and night. No changes to headlines at this time.

In the near term, spotty very light drizzle/freezing drizzle and
flurries continue to work their way across northern IL at this
hour. No impacts associated with this very light precip, which
already appears to be ending from the west across northwest and
north central IL per radar trends and lifting cloud bases out
west. Most of this should be out of the area by or shortly after
midnight, as sheared mid-level vorticity axis moves off to the
east of the forecast area. Extrapolation of trailing edge of 
stratus currently over northeast IA/western WI would bring the
potential for some partial clearing into Rockford and vicinity by
sunrise, while cloudy skies persist farther east/south. Surface
high pressure ridge approaches Friday morning, with light
northwest winds diminishing with time overnight. May see some
patchy light fog persist across the far south/southwest counties
of the cwa where the gradient will remain weakest, though
widespread dense fog is not expected.

As indicated above, just getting a quick look at the new 00Z NAM
guidance. Fairly similar to previous runs, and continues to 
highlight areas along/north of the I-88 corridor across far 
northern IL for the greatest snowfall potential with similar
timing going into Friday night. Perhaps a slight (half county)
shift southward with the main frontogentically-forced QPF axis 
Friday evening, though basically little change to current 
forecast. In collaboration with surrounding offices have made no 
changes to going headlines at this time. Midnight shift will have 
full suite of 00Z guidance available for possible upgrades.

Updated digital/text forecasts and watch statements available.



217 PM CST

Through Tonight...

The short term forecast period should be the calm between the 
"storms" as a sfc low largely responsible for the light pcpn earlier 
today lifts newd through central Indiana this afternoon.  

Latest radar imagery shows the last of the lingering pcpn exiting 
off to the east. A weak sfc trough/secondary cold front is moving 
through ern Iowa this afternoon and should push across the local 
area later this afternoon and early evening.  winds will gradually 
become nwly and increase to 5-10 mph.  High pressure The leading 
edge of a large area of high pressure is expected to push through 
the Upper Mississippi Valley overnight tonight, bringing some 
partial clearing to the nwrn portions of the CWA.  The secondary 
cold front will likely wash out over central IL by early Friday 
morning, so the cloudy skies will likely hang in over the remainder 
of the area.  With the partial clearing and stronger cold advection, 
the nwrn portions of the CWA, including the Rockford area, should 
see temps drop into the middle to upper teens overnight, while the 
remainder of the area sees lows in the lower to middle 20s.  


245 PM CST

Friday through Sunday night...

Significant winter storm likely to impact the region late Friday
through early Saturday afternoon, followed by a period of
accumulating lake effect snow near the lake Sat night into Sunday. 

No significant changes to previous thinking and the forecast
today. Highlights from the storm include:

* Accumulating snow starting Friday PM northern CWA, spreading
  south to I-80 corridor Fri evening and south of the Kankakee
  River late Fri evening or after midnight.
* Prolonged period of light to moderate snow with periodic bursts
  of heavy snow likely
* Increasing winds Saturday will result in blowing and drifting
  snow, especially in open and outlying areas. 
* Lake effect snow showers likely into NE IL Saturday afternoon
  and night, and Sat night into Sunday NW IN. Most significant
  lake effect snow accums expected late sat evening through 
  Sunday morning

Still appears as though frontogentically driven band of snow will
quickly develop east into northern IL Friday afternoon, mainly 
north of I-88. Model forecast cross sections Friday afternoon and 
evening do show multiple bands of f-gen with fairly deep, 
moderate ascent through the column. The stronger lift is forecast 
to intersect the prime dendritic growth zone, however, forecast 
ascent is not particularly strong, so assuming these are accurate 
representations of the ascent it isn't looking like extreme 
snow:liquid ratios. 

Widespread light to moderate snow should overspread the remainder
of the CWA Friday evening, possibly not until after midnight
southeastern most counties. It does appear as though there will be
transient bands of f-gen likely to result in at least periods of
heavier snow CWA-wide Friday night into Saturday morning. In
addition, while thermodynamics aren't great, they are probably
sufficient Friday night into Saturday morning for at least some 
lake enhancement to the snowfall over NE IL during this time 

As it appears now, the northern CWA looks to be most likely to see
the highest snowfall totals given the expected longer duration of
the f-gen band setting up Friday afternoon and night. Elsewhere
across the CWA, as it looks now, f-gen banding should be more 
transient Friday night into Saturday morning. Widespread 3-7"
totals look likely central and southern CWA and 4-8" totals
roughly along and north of I-88. Still appears as through 
somewhere near or south of I-80 could see a minimum of snowfall 
with highest totally, locally over 8" (especially near the lake) 
north. Additionally, the northward shift in the guidance does 
raise the chances of some local totals >7" far southern CWA as 

After collaboration with neighboring offices, opted to include the
entire CWA in a Winter Storm Watch. There will probably be a
sizable area within the watch that sees totals in the 2-5" range,
but the potential exists area wide for totals of 6"+. In addition,
strengthening winds should result in some blowing and drifting
before the accumulating snow ends. Wind will likely be strong
enough for continued blowing and drifting in open areas after the
accumulating snow ends Saturday, but assuming the watch is
upgraded to a warning tonight or Friday morning, thinking was that
any post accumulating snow blowing and drifting could be handled
by an advisory after the fact. Attempted to frame the watch timing
around the most impactful winter weather and not necessarily
capture every winter impact (ie some areas of blowing and drifting
late Sat afternoon and evening).

After the synoptic snow ends Saturday, lake effect snow showers
will likely continue into NE IL during the afternoon and into the
evening. Initially, thermodynamic profiles do not favor moderate 
to heavy snow showers during that time frame, however, as cold
advection continues thermodynamics will grow increasingly
favorable for moderate to perhaps locally heavy snow showers later
Saturday night into Sunday morning. Inversion heights are forecast
to only climb to perhaps around 6000ft with convective cloud
depths of around 4000ft, so somewhat marginal for a big lake
effect snow event. However, the magnitude of the low level
instability beneath the inversion combined with strengthening 
land breeze convergence zones to help focus the ascent could be 
enough for a single band of moderate to perhaps heavy snow snow, 
especially late Sat night through Sunday morning. With a sizable 
break expected between the end of the synoptic snows midday 
Saturday and the more meaningful accumulating lake effect snow, 
felt for messaging purposes it was worth not lumping the lake 
effect potential into this watch. Also, it isn't clear that this 
will be a warning worthy lake effect event given the marginal 
inversion heights and increasingly nearby dry air mass. 

In addition, bufkit forecast soundings show boundary layer mean 
winds nearing 30kt Saturday into Saturday evening, so gusts could 
reach 35 mph. Given the onshore the flow, also a potential for 
some lake shore flooding given the elevated lake levels. Later 
shifts may need to consider a lake shore flood advisory if 
guidance remains this strong with winds. 

Temperatures will really drop Saturday night into Sunday and could
see some areas of north central IL struggle to get much above 10F
Sunday. Guidance has trended quicker with the progression of
things, so now appears as through high cloudiness in advance of
the next system as well as developing southeast winds could keep
temps from getting too out of hand Sunday night. It is going to be
close, so confidence in temps Sunday into Sunday night is low. 

- Izzi

Monday through Thursday...

Monday and Monday night: A mid-level ridge and associated surface 
high will begin to shift east of the CWA on Monday. Meanwhile, lee-
cyclogenesis will be ongoing across the central High Plains. Weak WAA 
and low-level isentropic lift in response to the return southerly 
flow will support a band of light precip lifting NE across the CWA 
during the day Monday into early Monday evening. Thermo profiles 
support all snow as the precip type. Timing of the precip band 
varies throughout the day among guidance. However, most locations 
will likely see only a 3-6 hr period of precip. A light dusting of 
snow of less than 0.5" will be possible.

Tuesday through Wednesday: Significant model disagreement develops 
during this time, with the GFS, CMC, and ECMWF all showing vastly 
different solutions regarding a potential strong winter storm in the 
region. In fact, the difference in the GFS and ECMWF solutions 
increased from the 00Z to 12Z runs. As such, there is a considerable 
lack of uncertainty with the forecast during this time. 

The GFS has trended warmer as the strong lee cyclone tracks NE 
across the NW third of the CWA Tuesday afternoon. This solution 
supports rain for nearly the entire CWA on Tuesday, with a NW to SE 
changeover to snow late afternoon through the evening before precip 
diminishes. The ECMWF solution has trended to a stronger low from 
near STL to northern IN Tuesday evening into Wednesday morning. With 
this solution, significant snowfall would occur across a majority of 
the CWA, with some rain potential across far southern portions. In 
stark contrast to the GFS/ECMWF, the CMC keep the surface low and 
much of the precip south of the CWA. Additionally, the GEFS members 
cover the entire range of solutions from the deterministic guidance.

With the considerable uncertainty, those that could experience 
impacts from any of these potential solutions should monitor 
forecast updates over the coming days.

Thursday: Long-wave troughing will settle over eastern Canada by 
this point. However, the core of the coldest air looks to miss the 
CWA well to the northeast. Light LES showers could continue across 
portions of NW Indiana during this time. Additionally, indications 
are that a weak clipper system with the potential for light snow 
will cross the area sometime Thursday into Friday. 



For the 06Z TAFs...

Multiple forecast concerns this forecast period continue including...

Cigs through Friday afternoon.
Snow start time/location Friday afternoon.
Associated vis/cigs with snow.
Winds/blowing snow Saturday morning.

No significant forecast concerns through midday Friday. The threat
of drizzle/freezing drizzle has ended and not expecting any precip
chances overnight. Northwest winds generally under 10kts will turn
more northerly by morning then northeasterly by late Friday
morning. Mvfr cigs are likely to fill back in over the next few 
hours and while some clearing is still possible into far northern 
IL by morning...confidence is decreasing and maintained mvfr cigs 
Friday for all but rfd.

A band of snow is still expected to develop across far northern IL
by late Friday afternoon...first at rfd and then into dpa/ord.
Adjusted tempo timing slightly but still not confident enough to 
go prevailing snow just yet. If these trends continue...prevailing
snow with vis under 1sm may be needed in this corridor. This band
also may shift a little further south...possibly affecting mdw by
late afternoon. Snow will then spread across the entire area 
Friday evening and continue into Saturday morning. Prevailing vis 
is expected to be below 1sm for much of this time period and 
likely in the 3/4sm to 1/2sm range with some vis restrictions from

Temperatures will be slowly falling Friday night into Saturday
morning and the snow will begin to get drier/slightly more fluffy.
Northeast winds will increase to 10-15kts Friday afternoon with
gusts into the 20kt range Friday evening and then speeds/gusts
will continue to increase Saturday morning. Thus blowing snow will
likely develop at some point early Saturday morning. cms


IL...Winter Storm Watch...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014...3 PM Friday to noon Saturday.

     Winter Storm Watch...ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-
     ILZ032...7 PM Friday to noon Saturday.

     Winter Storm Watch...ILZ033-ILZ039...midnight Saturday to 3 PM 

IN...Winter Storm Watch...INZ001-INZ002...7 PM Friday to noon 

     Winter Storm Watch...INZ010-INZ011-INZ019...midnight Saturday to 
     3 PM Saturday.