December 16-18 Winter Storm Threat

[Thundersnow12]

Looking at the GFS, it looks like the srn stream wave is a touch faster this run on the which leads to a little sharper trof overall, more ridging out ahead, stronger at 850mb and slightly better advection into the cold air leading to more cold sector QPF on the backside 

[CoachLB]

I'm not sure what the GEM saw on the 00z. But it gave me 3 inches on the front end and 3 inches on the backend. On the 12z it pounded me with sleet and freezing rain.

[Chicago Storm]

GGEM looks pretty nice again.

Indeed.

Continues to the be wettest among OP guidance.

[Hoosier]

1 minute ago, Thundersnow12 said:

Looking at the GFS, it looks like the srn stream wave is a touch faster this run on the which leads to a little sharper trof overall, more ridging out ahead, stronger at 850mb and slightly better advection into the cold air leading to more cold sector QPF on the backside 

Man this looks like another tough one for LOT, right through a highly populated corridor.  Snow amounts, how far north does mixing get, etc.  

[Hoosier]

Here are the GFS and GGEM freezing rain maps.  The GGEM has a band of sleet while the GFS is almost non-existent with that, so the northern edge of the mixing on the GFS is close to what you see here. On the GGEM, add about 20-30 miles north to get the northern edge of the mixing on that model.

 

[Jackstraw]

Runs tomorrow will be interesting once this start's to get fully sampled.

[Hoosier]

00z ECMWF looks like an improvement overall.

[Chicago Storm]

00z ECMWF looks like an improvement overall.

Wetter across the main snow axis.

0z GEFS look like a step up too.

[Harry Perry]

18 minutes ago, Hoosier said:

Here are the GFS and GGEM freezing rain maps.  The GGEM has a band of sleet while the GFS is almost non-existent with that, so the northern edge of the mixing on the GFS is close to what you see here. On the GGEM, add about 20-30 miles north to get the northern edge of the mixing on that model.

 

Ahh... not what I was hoping to see. GFS trending towards some ice. Ice accretion on top of 6" of snow plus temps sub zero 12 hours prior.

[Sidewinder]

Anyone on the roads here Friday night when it changes over is going to get a nasty black ice surprise with these ground temps

[kevlon62]

Although precip type issues still might creep into the picture for parts of Chicago, it has looked more likely as we get closer to the event that surface temps will stay below freezing except for maybe far south suburban areas. Toeing a fine line there but I like how things are panning out on that side of the coin.

Also, we could use some Midwest Chaser up in here again.

[ConvectiveIA]

Based on current radar trends I suspect that it's possible the current NWS forecast is underestimating the potential southward extent and coverage of snow showers throughout IA for Today. 

[purduewx80]

despite whatever qpf or mesoscale setups guidance is throwing out - one synoptic thing that is fairly well agreed upon is the storm track from just north of STL, thru LAF to ~DTW. that's classic for heavy snow in chicago. 

water vapor loops are pretty sick over the northeast pacific right now. that's a lot of unique energy for the models to decipher. should note this is being fed by tropical convection. slam some of that into this arctic outbreak we're having - and boom.

[Nelson]

Someone asked the question about consistency of model runs. Here are the GEM and GFS over the past few runs. GFS has been more consistent with SLP placement. It is also a decent amount east of where the GEM settles in at.

Last 4 runs of the GEM, centered on 18z Saturday.

[Nelson]

Last 6 runs of GFS centered on 18z Saturday

[Chicago Storm]

12z NAM shifted the main snow axis south overall.

[Chicago Storm]

12z NAM shifted the main snow axis south overall.

[UMB WX]

Prob should just give in and finally by a snowblower. I'm stubborn..been without one for 43 yrs.. why give mother nature any more reason to flip me the bird.

I thought this one was over when it was congrats MN.  Now every run since is 6+

[SchaumburgStormer]

5 minutes ago, UMB WX said:

Prob should just give in and finally by a snowblower. I'm stubborn..been without one for 43 yrs.. why give mother nature any more reason to flip me the bird.

I thought this one was over when it was congrats MN.  Now every run since is 6+

Once you pull the trigger, you will never shovel again. Best purchase I have ever made 

[DAFF]

Owning a new snowblower is a form of snow repellant. If you want waggons south then go for it !! I too was waiting to see if the NAM follows suit with the rest of guidance. As per the last system I found the NAM did very well and caught on early with the northern placement of snow in Michigan. Was it a fluke or the model to watch and trust, time will tell.

[blue60007]

Crazy. 12z NAM show Indy barely getting above freezing while getting almost 1/2 qpf. GFS OTOH, has temperatures going up to 55 on Saturday.

[DaveNay]

The most obvious trend I see in the GFS is that it seems to be morphing into a more concentrated and localized event in the I94 north of Milwaukee area. Seems like the gradient gets tighter and tighter onto that area.

 

[RyanDe680]

4 hours ago, purduewx80 said:

despite whatever qpf or mesoscale setups guidance is throwing out - one synoptic thing that is fairly well agreed upon is the storm track from just north of STL, thru LAF to ~DTW. that's classic for heavy snow in chicago. 

water vapor loops are pretty sick over the northeast pacific right now. that's a lot of unique energy for the models to decipher. should note this is being fed by tropical convection. slam some of that into this arctic outbreak we're having - and boom.

By that, are you saying that the model are undercutting the qpf?

[Hoosier]

FWIW, the Canadian tag team is more bullish on the southern extent of snow tomorrow.

[Hoosier]

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

1148 AM CST Thu Dec 15 2016

.UPDATE... 1138 AM CST

Other than the bitterly cold weather, no other weather concerns today. Allowed the Wind Chill Advisory to expire at 10am, though very low wind chills will continue through the afternoon as highs only reach the single digits to low teens. Assessing the 12z guidance thus far, main concern is that our snow amounts in going forecast are a bit too low for Friday afternoon through early-mid evening, right in line with the commute. There is a consistent signal on the guidance for lift driven by strong isentropic ascent and lower level frontogenesis, along with synoptic lift from a fairly robust mid-level shortwave moving over the area. This will be in concert with good moisture on the isentropic surfaces, with mixing ratios of 3-4 g/kg. After initial dry air, expecting that cold air mass should allow top down saturation and steady snow at least down to I-80. Thinking is that there will be a burst of steady light to moderate high-ratio snow that will have no problem accumulating on road surfaces, with a few to several inches by early evening, highest north. Given the poor timing with the Friday afternoon commute, the commute could be significantly impacted. Have additional concerns with mixed precipitation issues, especially south of I-80 later Friday night into Saturday, as well as continued snow potential along the IL/WI border and with the arctic front passage on Saturday but still assessing these concerns. The main message is that confidence is increasing for significant impacts to the Friday afternoon commute, including the Chicago metro area. Will not be issuing any headlines until the full afternoon forecast package, but will be issuing winter headlines for at least a portion of the area at that time. Messaging wise this is a three pronged event (precipitaiton portion) 1) Snow coming on quickly mid afternoon and impacting the evening commute tomorrow, especially I-80 north. 2) Mixed precipitation and ice accumulation potential overnight into early Saturday, especially Chicago southward. 3) Additional snow Saturday afternoon/evening with significant blowing snow.

Castro/KMD

[TugHillMatt]

23 minutes ago, Hoosier said:

FWIW, the Canadian tag team is more bullish on the southern extent of snow tomorrow.

What models do local news stations tend to use? The news stations around here always show models that have ridiculous snowfall maps that are the most abstract and stray from the common and general snowfall maps we discuss on here. Is it an RPM model? The one I just saw showed the heaviest snow waaay north of West Michigan, while most od the ones we use on here have West Central Michigan in a heavy snow area.

[TugHillMatt]

It must be one of the versions of the NAM?

[Hoosier]

4 minutes ago, blackrock said:

What models do local news stations tend to use? The news stations around here always show models that have ridiculous snowfall maps that are the most abstract and stray from the common and general snowfall maps we discuss on here. Is it an RPM model? The one I just saw showed the heaviest snow waaay north of West Michigan, while most od the ones we use on here have West Central Michigan in a heavy snow area.

Yeah, when they say it's their in-house model or something like that, it's RPM. Depending on who you're watching, some people also show the GFS or Euro.

[SchaumburgStormer]

Will this be sampled for the afternoon runs? 

[Hoosier]

34 minutes ago, SchaumburgStormer said:

Will this be sampled for the afternoon runs? 

No.  And actually, it won't be until the 00z Saturday run that we get good RAOB sampling of the trailing wave on Saturday/Sunday.