12-1 and 12-2

11-30  Morning update.

Still looking at a snow, sleet, freezing rain, rain, snow event.  NAM has 2" on the front end now, here, which I think is going to be more.  But it loses the dump after it turns back to snow and adds only another inch.  This is alarming, but we'll see. 

11-29

Been watching this for a while.  Massive storm hits west coast, regenerates days later in Colorado, occludes quickly, starts moving across the country.  As the surface low weakens due to occlusion, the upper low continues across.  A warm front will be waayyy out in front of this which will kick things off.  Using GFS for depiction. Red line warm front, blue line cold front, black circle upper low,red L is surface low.

Precip starts as snow, we are 600 miles from the surface low at this time.  Typically this set up just results in a frontal passage as these lows tend to go up through the lakes. But this time there's a pretty strong low at 50N50W (called a 50/50 low, highlighted) in the way, which holds the high pressure in place and shunts the upper low SE.

This is an unusual situation - a Colorado low moving up to WI, then down to our area.  Now the warm front triggers another low, called a triple point, off NJ. 

The location and strength of this low is what will determine if we stay snow, mix or go to rain.  You can see the blue/pink/green all over the area. You can also see the Colorado low essentially dying as the triple point takes over.  But we're not done.   The upper low still is strong and heading our way.  If you end up north of this low, you get snow.  

The triple point low is slow to move out (written in L), and as the upper low moves in, its energy triggers another low to pop.  This is our snow on Monday. 

The new low becomes stacked with the upper low which almost always produces snow this time of year.  The storm moves slowly as the upper low is pretty cut off and a long duration snow event unfolds. 

This results in the GFS snow map as such.

Euro, Canadian and Ukmet are all showing similar events, but timing and low placement differences lead to slightly different results.  What is not represented in the pivotal maps is whether its snow or sleet.  Freezing rain is factored out and accounted for in the bottom map.

So for right now I'm leaning for CT and NNJ north of I80  to south of the Catskills, snow to start, 2-4 inches likely and could come down hard.  Start time late morning NJ, noon for CT.  Typically this arrives earlier than modeled unless the atmosphere is super dry and it dries before making it to the ground.  By Sunday evening its a toss up.  No idea if it goes to sleet, freezing rain or rain, but temps between 5,000 and 10,000 feet (850mb and 700mb) seem to warm for almost all the area.  Monday morning that final low forms, cold air rushes in throughout the column and everyone in NJ, PA, LI, CT goes to snow.  The trick with this second part is that the snow should fall in bands, and it should vary light and heavy.  Parts north of I80 could again see anywhere from 3 to 10 inches during this time frame.   So my map looks like this for now.  I'll get better at making them soon. 

So text isn't showing up... Gray is 1-3.  Aqua is 2-4. Blue is 4-8.  Indigo is 6-12 and Purple is 12+