It’s been a warm October from the Plains east while cold from the Rockies west but we’re set for a big mid month flip and that’s largely down to the PNA flipping positive while the NAO and AO takes a second dip.

Check out these departures from normal.

Temperature Departures From Normal Oct. 1 to 13, 2013

Location	Departure (Degrees F)
Minneapolis	+6.0
Chicago	+7.1
Detroit	+6.8
St. Louis	+6.3
Omaha, Neb.	+4.6
Cincinnati	+5.8
Boston	+4.6
New York	+6.6
Washington, D.C.	+6.4
Raleigh, N.C.	+2.7
Atlanta	+3.9

The current storm on the map is now heading into the Plains and snowfall is once again breaking out over the western Dakotas. It’s as the system taps Gulf moisture and draws colder air out of Canada, so we see conditions significantly go downhill over the Dakotas with wind and snow likely to produce another round of blizzard or near blizzard conditions. It’s this system that once up into the Great Lakes, will draw south the coldest air of the young autumn season.

Here’s the surface chart of the GFS by Tuesday morning.

The 48 hour GFS snow chart shows a large area of the Dakotas seeing snow with a few inches over North Dakota and likely the Black Hills. Wouldn’t be surprised to see a foot around Lead and Deadwood.

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By later Wednesday the system is tracking through the Great Lakes with a line of showers and thunderstorms pushing east through the Ohio and Tennessee Valley but it’s the cold that gets forced south on the backside of the storm system that will be noteworthy.

Check out the 850mb temps by Saturday.

That’s the kind of air mass which can produce the first freeze in Minneapolis and Chicago with highs stuck in the low 40s.

The GFS lifts the first of two troughs out, never really penetrating the Northeast and Mid-Atlantic as you can see from the 850 chart below.

However, check out the 168 hour snow chart for Canada. Notice how it has the current system covering all of Ontario, Quebec and Labrador with snow. Impressive given the fact it’s only mid-October.

Perhaps a little overdone? Probably to a degree.

Let’s take a closer look at the Northeast US.

Note it has some snow for downwind areas of the eastern Great Lakes. Could the model be sniffing out some lake effect snow? Quite possible given the cold air mass, strong NW winds and warm lake waters.

Once this thing has been and gone. Look at what the model has coming down after!

That’s for the 25th and that means business.

Plausible? Absolutely given the indexes.

Here’s the CPC 6-10 day temperatures.

Some modelling suggests snow in Chicago and freezing temps into NYC even DC before the end of the month. What a turnaround coming…

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