Significant Pattern Change Coming To North America Next 10 Days

Written by on October 9, 2013 in United States of America with 0 Comments

As well as for Europe, we have some significant changes coming to the North America upper pattern as the NAO is heading negative along with the PNA finally shifting towards positive. The last -NAO did not deliver a typical eastern trough and in fact the warmth which was over the East, was outstanding really for the time of year and it was due to a firmly negative PNA which held the western trough and supported those big storms.

This week sees another storm and we will probably see another next week before a flip takes place.

Here comes the first system, notice this thing digs all the way into Southern California bringing a few much cooler, wetter, windier days and the first snow to the San Gabriel and San Bernardino Mountains.

Courtesy/Owned by AccuWeather Pro

Courtesy/Owned by AccuWeather Pro

As you can see the model has snow over the higher elevations of the Southwest.

Courtesy/Owned by AccuWeather Pro

Courtesy/Owned by AccuWeather Pro

That system will impact the central and northern Rockies as well as the western Dakotas only this time, it won’t have near the same impacts as the last storm. The cold air isn’t as readily available as the northern branch of the jet is cutting across Canada, stopping the cold air from coming south and wrapping into this storm.

The system winds up somewhat over the western Dakotas where it’s rain, not snow from what I can see.

Courtesy/Owned by AccuWeather Pro

Courtesy/Owned by AccuWeather Pro

Yes we’ll see snow but it’s likely to stay in the Rockies and not affect the Black Hills.

Here’s the broader snowfall.

Courtesy/Owned by AccuWeather Pro

Courtesy/Owned by AccuWeather Pro

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Here comes the next system into the west starting next week and this may bring a greater snow chance to not only the Intermountain West but further east into the Plains or Midwest as this will some colder air dragged in from Canada.

Courtesy/Owned by AccuWeather Pro

Courtesy/Owned by AccuWeather Pro


Courtesy/Owned by AccuWeather Pro

Courtesy/Owned by AccuWeather Pro

Check out the 850 temps. Although the jet stream setup still runs west-east to the north of the system, the system compared to this week’s will tap some of the colder air to the north and with it appearing to wind up more, may be ale to produce more of it’s own dynamically.

Courtesy/Owned by AccuWeather Pro

Courtesy/Owned by AccuWeather Pro

As you can see it has a fairly decent surface low. The model doesn’t really show much in the way of snow outwith the Rockies BUT I wouldn’t be surprised if modelling started to show some snow with this.

Courtesy/Owned by AccuWeather Pro

Courtesy/Owned by AccuWeather Pro

Nonetheless there is MORE snow by far with this 2nd system.

Courtesy/Owned by AccuWeather Pro

Courtesy/Owned by AccuWeather Pro

So, another TWO system in the western trough-eastern ridge setup but once next week’s system moves out into the Plains, rather than lifting NE, it appears to track due east with heights building on it’s rear. Could this signify a change in the pattern structure?

Check out the PNA and NAO



Note both signals have been negative. The -NAO can often signal an eastern trough but with a firmly negative PNA, there is no cold signal in the east. The PNA has ruled with a firm western trough and active storm setup. The heat in the East has been quite incredible for the time of year.

What will be interesting is getting that PNA positive along with the NAO going negative. This supports western ridging while a trough becomes better establishes in the east. What about cold?

The AO has went back positive and this explains why this week and next week’s system has little cold air to work with but you’ll notice the second storm is colder, perhaps because heights are building over the pole, allowing some colder air further south.


Expect to see a western ridge-eastern trough setup from the 18th onwards but how much cold comes down?

The problem is that snow cover is somewhat sparse still over Canada while the greater snowpack is over the other side of the pole in Siberia and it’s here where the coldest air is. The upper pattern doesn’t support some of this colder air crossing the pole into North America but, that doesn’t mean we cannot see some decent shots of typical autumn chill into the east. It will most certainly turn colder while the west catches a break from the recent storms.

Here’s the current NH snow cover, note it’s all on the Asian side of the pole.


Here’s North America where it’s actually a little below normal right now.


Here’s the latest ECMWF 500mb chart and 850 temps by 240 hours and you can see more western ridge-eastern trough and yes, there would appear to be some colder, arctic origin air trying to get south as heights build up the western flank of the continent.


It will be interesting to see how this pattern evolves and turns around in the next 2 weeks. Certainly looks as though we have a very different end to October on the way compared to how it started off in the east that’s for sure.

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