Two Shots of Winter or Two Grinch Storms?

The upcoming week looked promising for snow lovers. But today the nasty weather grinch seemed to strike. Blah.

Storm 1

Snow on Christmas? Awesome! I do think we’ll see some and odds of a white Christmas (>1″ snow on the ground at 12z on 12/25) seem to be around 50/50 for northern Connecticut. That said, this storm appears less impressive than it did 12 or 24 hours ago.

18z NAM / 48 Hour Forecast (Valid 18z Monday)

18z NAM / 48 Hour Forecast (Valid 18z Monday)

You can see two distinct shortwaves here one near Chicago and another near Asheville, North Carolina. Some models indicated those two shortwaves would phase (i.e. the 12/22 00z GFS) but it appears that will not happen now. The two waves are within close proximity and the northern shortwave near Chicago will never have the chance to amplify much if they are unable to phase. The exact track of the shortwave will determine where the best band of snow will setup but these things generally tick north with time. I wouldn’t be surprised if we saw a light wintry mix Monday night and early Tuesday morning. I think the odds of a white Christmas are about 50/50 inland and lower at the shoreline.  A 1″-2″ kind of deal may set up north of the Pike into southern Vermont and New Hampshire but I wouldn’t get too excited. Some flakes in the air for Santa will be pretty cool, however.

Storm 2

The second storm for this upcoming week always seemed a bit more exciting than storm 1. Given that we’re still several days away we’ve seen some swings here from rainstorm to snowstorm and back to rainstorm. Typical for a 5 day or longer forecast!  Here’s the GFS from 00z to 18z. Look at the change at 500mb!

00z GFS (top) and 18z GFS (bottom) 500mb forecast valid 18z Wednesday

00z GFS (top) and 18z GFS (bottom) 500mb forecast valid 18z Wednesday

Obviously things are a bit unstable up over Canada. Big vortex over Quebec? Big vortex over Manitoba? Who the hell cares.

One of my rules of thumb is that for a day 4 or 5 forecast is that it’s hard to beat a blend of the Euro/GFS ensemble mean. When they’re in fair agreement you have a relatively high confidence forecast! Here are the GEFS mean and Euro Ensemble mean SLP for the Wednesday night/Thursday morning storm.

mthk_f108_bg_US

mthk_f120_bg_US

Hard to beat that kind of day 5 consensus. While we’ll see changes over the next few days I’m fairly confident whatever we get will likely be a messy mix (snow, sleet, rain).

The trough that digs does so far to the west. By 72 hours most models bring the storm near Nashville. The storm will be an App runner and will transfer its energy to the coast but everything is awfully far west as currently progged to deliver a big snowstorm to southern New England (outside of the distant interior). We will have to watch the trends over the next 24 hours but at this point a wintry mix seems like a  good bet – whatever we see will likely be moderate to heavy as well.

A lot to track – so that’s a start!

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