fxus63 klot 171940 

Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois 
140 PM CST Sat Nov 17 2018 

Short term... 
344 am CST 

Through today... 

Main concern for today will be snow trends associated with a weak 
clipper tracking across the region today. 

Latest radar imagery indicates that the heaviest and most widespread 
snow associated with the clipper is over ern Iowa/nwrn IL, as of 
330am, with an area of more broken light snow extending across nrn 
Illinois and into nwrn in. As the main vort Max and mid-level forcing 
tracks ewd, expect that snowfall rates should increase, with the 
main period of accumulating snow expected to be from arnd the 
current time through arnd noon for the Rockford area and arnd 5am- 
1pm for the Chicago Metro area. Have not made any significant 
changes to the going forecast, with the greatest accumulations still 
expected to be along and north of the I-80 corridor. There is still 
some concern for locally higher amounts due to localized banding, 
even at this point, confidence in where any enhanced banding may set 
up is still low and radar trends will need to be closely monitored 
as this type of banding is difficult to pin down. Still expecting 1 
to 2 inches of widespread snowfall north of the I-80 corridor. 
Should any mesoscale banding develop, localized areas of 2 to 4 
inches may be possible. There is a chance that pcpn could mix with 
or change over to rain for far nern Illinois/nwrn in by early afternoon 
ans the pcpn begins to wind down as temperatures climb into the 
middle 30s. Temperatures across ncntrl/nwrn Illinois will likely remain 
low enough to support all snow through the duration of the pcpn. 
Also, the models continue to indicate that an elevated frontal zone 
will develop this afternoon and then become stationary across the 
southern portions of the County Warning Area later this afternoon and into tonight. 
This could bring some light snow accumulation to areas south of the 
I-80 corridor. The elevated forcing is expected to be relatively 
weak and sfc forcing should be minimal, so any accumulation 
over the southern portions of the County Warning Area should be light. 


Long term... 
140 PM CST 

Sunday night through Saturday... 

Sunday night through wednesday: deep troughing over Hudson Bay will 
only gradually shift eastward to northern Quebec during this time, 
with multiple embedded shortwave troughs rotating across the Great 
Lakes. The first shortwave and associated surface trough will cross 
the County Warning Area late Monday afternoon into Monday evening. Isolated to 
scattered snow showers will be possible for a 2-3 hr window as the 
trough passes. 

North-northwest winds behind the front will support les for Northwest Indiana Monday 
night into Tuesday morning, with a window for les backing into 
eastern Cook County late Monday night into Tuesday morning. With 
800 mb/SST Delta temps of ~14c, only isolated to scattered showers are 

A second shortwave rounding the broader trough will pass NE of the 
area Tuesday night. Enough dry air should be present to prevent 
precip across the NE cwa, but cannot rule out a few showers. 

Thanksgiving day through saturday: guidance has come into better 
agreement that the eastern Canadian trough will move eastward as a 
broad mid-level trough shifts from the Great Plains to the Great 
Lakes region. Though light warm air advection precip should remain north of the 
area on Thanksgiving, the County Warning Area may end up in a persistent shield of 
stratus through much of the day. Warmer conditions arrive on Friday, 
with temps at or possibly slightly above normal for late November. 
Another trough axis will track eastward across the area and bring a 
period of rain Friday night into Saturday morning. 



for the 18z tafs... 

Snow is gradually winding down across the terminals early this 
afternoon, with improvement from IFR to MVFR conditions expected. 
With snow slowly moving southeast, gyy will be the last to 
improve. Potential for light lake effect precip exists overnight, 
with gyy and to a lesser extent mdw possibly seeing a period of 
precip and lower vis/cig conditions. 

Mid-level disturbance was moving east and weakening across the 
forecast area this mid-day. Local radar at 1740z depicts some 
moderate banding still across the Chicago area terminals, though 
this should be translating east shortly, with improvement expected 
at dpa/Ord within the hour. Transient bands of IFR light snow will 
continue to shift slowly southeast, clearing mdw a little slower 
and affecting gyy into late afternoon. Light snow/rain mix will 
likely persist south of the terminals into tonight, as another 
less-amplified disturbance tracks across central IL/in. Winds are 
shifting north behind the current disturbance, with speeds 
increasing to 10-15 kt this afternoon through early evening. 

As indicated above, models indicate the development of lake effect 
clouds/precip tonight, which is expected to impact gyy and 
possibly shift as far west as mdw overnight. Forecast soundings 
indicate fairly shallow inversion heights, and dry air aloft which 
along with marginal surface temps may result in light rain or a 
rain/snow mix. Terminals northwest of mdw should not be affected. 

VFR conditions return area-wide by mid-morning Sunday. North winds 
will back more westerly at less than 10 kt during the day. 



Lot watches/warnings/advisories... 


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