Significant Snow Possible Sunday/Monday

Normally, I'd wait another 24-48 hours before posting about a storm that's a good 4 days away, but (1) I know a lot of folks will be traveling Sunday as they wrap up Thanksgiving weekend, and (2) there has been some model consistency on this storm for the last couple of days, which increases confidence.

Here's the quick take, and then we'll get into some detail:

• There will be a storm in our part of the country late this weekend;
• Current thinking on timing is late Sunday/Monday;
• It will bring rain to some, snow to others, and a rain-to-snow changeover for others;
• Because of the path of the storm, there should be abundant moisture, which could result in heavy (6"-plus) snowfall for some;
• Timing, storm track and accumulations will come into better focus sometime on Friday.

The two main mid-range weather models, GFS and Euro, have been literally alternating their interpretations of this storm.  The GFS model runs every 6 hours; the Euro every 12 hours.  During one run, GFS will show heavy snow in Wisconsin and Euro will show rain...then in the next run, GFS goes light snow and Euro goes heavy.  Since Monday, they've vacillated between 0" and 20" (GFS FV3) of snow in our general area.  This is pretty normal at this range.

Here's the set-up:

Low pressure comes on shore in the northwest early Friday.  You see it on the surface map west of Oregon:

In the middle of the country, a separate system will bring us rain Friday night and Saturday morning.

The second system moves southeast toward Utah and Colorado 24 hours later:

 

After dipping into the southern plains (and grabbing significant moisture from the Gulf), the low starts to head northeast, and is centered in Missouri Sunday morning:

 These tracks will wobble and change as we get closer to the actual event.  There are three key variables I'm watching that will greatly impact the type of precipitation we get, and how heavy it is:

1. How far south will the low get before it tracks northeast?  If it gets into northern Texas, it can pick up significant Gulf moisture. 
2. How much cold gets wrapped to the southeast with the passage of the first low on Saturday? This will help determine our precipitation type, and the timing of a rain-to-snow changeover on Sunday.
3. Where will the low track once it starts heading northeast?  If you scroll back up to the Sunday morning surface map, you can see high pressure in North Carolina.  This ridge, if it sets up, could serve as a "block" for our low, sending it slightly west and bringing us mostly rain.  Remember that a low cannot "move" a high; the high always wins when it comes to steering.  But if that high pressure is weak, or sets up father south or east, that allows the low to track through central Illinois and into lower Michigan, which would be the "ideal" storm track if you're hoping for heavy snow.  

Storm track is vital because the heaviest snow will fall the northwest and west of the low, as its counter-clockwise circulation pulls cold air in behind it.  The low position shown below (0z GFS-FV3 model; showing 7pm Sunday) is ideal for heavy snow in southern Wisconsin.  So it's no wonder that this is the model that's currently throwing down 20" in Janesville.

As we get closer to the weekend, shorter-range mesoscale models will come into play, so we'll be able to see how NAM, WRF and the Canadian model are interpreting track and intensity.  It's at that point that the National Weather Service will determine if we have enough confidence and model consistency to issue a Winter Storm Watch.

I'll be updating this storm throughout the weekend, and will probably have predictions for you on Friday afternoon.

In the meantime, let me know if you have any questions!

~Scott