No Extreme Cold Likely in the Eastern U.S. Next Week

[donsutherland1]

If one examines the NAO Indices since July 1, 2003, two noteworthy developments are underway with regard to the NAO.

<img src="http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.mrf.obs.gif">

1) The NAO is currently plunging and is heading farther negative than it has been all season so far (the timing and magnitude of the fall resemble October 2002, though there are some significant pattern differences so one should not conclude on the basis of this similarity that the overall patterns are the same).

2) Negative cycles are gradually lengthening (the last three have averaged 11 days vs. 8 days in August) and positive cycles are gradually shortening (the last three have averaged 5 days vs. 11 days in August).

At the same time, there is abundant cold air -10C and below across Canada. The area of this cold air has been expanding in recent days.

<img src="http://www5.wright-weather.com/bb/attachment.php?s=&postid=117793">

However, in terms of anomalies, the cold air in Canada is not exceptionally cold relative to normal.

Moreover, if one examines the reanalysis of air temperature anomalies for the most recent 10 days, one finds a persistent pattern in which anomalies modify fairly quickly as the cooler air presses toward the Eastern U.S.

Consequently, the upcoming week is not likely to feature extreme chill in the Eastern United States, much less for any stretch of time. Indeed, in the East temperatures will are more likely to average near normal to perhaps somewhat below normal rather than much below normal. Hence, if some of the computer guidance suggests much below normal readings for any stretch of time next week, it should probably be viewed with caution.

On the other hand, thanks in part to the continuation of slow retrogression and the lengthening of the negative NAO cycles, the potential exists for the month to conclude on a fairly chilly note (readings below normal in the East) but without extreme cold.

[Colin]

Great post and analysis, Don! I agree with the "no extreme cold in Eastern US next week." I don't expect any extreme cold for at least another 2-3, maybe even 4 weeks.

[Guest]

I pretty much agree as well................Alot of the forecast including the models have slightly backed off as it is anyways....................What else doesnt help is the simi lack of snowcover to the north................I for one have had my doubts about this anyways......................It should be noted however that yea it may not get so cold here in the US (For the most part) but up in Canada it should get cold enough to start really building on that snow pack up there which means perhaps if we see another dip in the NAO like this in say a few weeks then we may feel it alot more down here in the USA................

BTW below is a look at the snowcover!!!!See how this looks in a week or two....

[Stephanie]

That's a relief to hear! I want a normal progression into Winter and at least be able to enjoy the Fall weather.

Question - which way is the circulation of the NAO when it is negative?

[donsutherland1]

Stephanie,

In general, when high pressure (clockwise circulation) sits over the Greenland-Iceland region, the NAO is negative. When low pressure (counterclockwise circulation) sits over the Greenland-Iceland region, the NAO is positive.

Dr. Joe D'Aleo has written an excellent piece on the NAO. It can be found at: http://www.intellicast.com/DrDewpoint/Library/1137/2/

[donsutherland1]

King of Weather,

I agree that there will be opportunities to build snowcover in Canada over the next week or two. Already, there has been a slow increase over the past 7-10 days. This increase should accelerate over the next week or two as I don't envision Canada being cleared of the cold air over that time frame.

[jpp5862]

Don,

Thanks for the link to the information on the NAO, very informative and educational for someone like me who is still quite a novice.

Thanks!

[Stephanie]

That's what I thought Don. That would then to help block cold air from streaming into the US correct?

[ColdFront77]

A low pressure over the Greenland-Iceland region would cause a northwest wind
into the northeastern United States; which is obviously from a cold region.

[donsutherland1]

Tom,

The high pressure/low pressure responsible for the NAO is too far from the Eastern U.S. for it to have surface wind impacts. Its impact on the jet stream is what is important.

Dr. D'Aleo's diagrams illustrate a negative and positive NAO:

<img src="http://www.intellicast.com/DrDewpoint/Library/1137/figure26-s.gif"><br>Negative NAO

<img src="http://www.intellicast.com/DrDewpoint/Library/1137/figure25-s.gif"><br>Positive NAO

<b>Source:</i>Intellicast

[Stephanie]

So it creates an Omega block that affects the jet stream. In those graphics that you provided, it looks like it could cause cold air to come into this region, but there must be other factors at hand that will not give us that shot of cold air within the next week.

[donsutherland1]

Stephanie,

You're absolutely right with regard to "other factors." If there is a shortage of very cold air in Canada, only modestly colder air would move into the Eastern U.S.

Winter 2001-02 was classic example of this situation. When the NAO became most negative, only much warmer than normal air was available in Canada. Hence, no major arctic outbreaks occurred.

[Guest]

Stephanie,

You're absolutely right with regard to "other factors." If there is a shortage of very cold air in Canada, only modestly colder air would move into the Eastern U.S.

Winter 2001-02 was classic example of this situation. When the NAO became most negative, only much warmer than normal air was available in Canada. Hence, no major arctic outbreaks occurred.

Yep that was so true of that winter.....................Thankfully its still only Oct right now so as i said in other post here all this will do is help increase the snowcover up in Canada which in turn should help to build cold air up in Canada and allow for it to drop into the USA in the comming months as the snowpack expands................Right now with such a lack of snow cover up in Canada the coldest air has a chance to modify before it gets down into the USA................

Its ashame we dont have the record snowpack up there this year like last because im almost certain if there was with this set up it would be alot colder and snowier down this way!!!!!!!!

[PTrackerLA]

Hopefully we'll get some nice arctic air around Halloween. The fall so far has been pretty bland in the south.

[StormCrazyIowan]

From what I have been seeing, our Arctic outbreak is going to hold out until right AFTER Halloween, lol, but that is just what I saw on our local news channel, so we will see!

[Guest]

Part of the key to the Jacki is the NAO which supposed to be in the tank big time by then which means the coldest air to be had should be felt from the Midwest to the Eastcoast which is what will probably help in aiding your colder air...................Some models already hint at this but beeing that is about 2 weeks away i would wait and see how the trends go because the models do terrible that far out.................

[ColdFront77]

Tom,

The high pressure/low pressure responsible for the NAO is too far from the Eastern U.S. for it to have surface wind impacts. Its impact on the jet stream is what is important.

Understood.

It is annoying to understand something and coming across like I don't. I can't see how many of those interested in meteorology can read so many verbose articles on this. * SIGH *

[Stephanie]

Tom,

The high pressure/low pressure responsible for the NAO is too far from the Eastern U.S. for it to have surface wind impacts. Its impact on the jet stream is what is important.

Understood.

It is annoying to understand something and coming across like I don't. I can't see how many of those interested in meteorology can read so many verbose articles on this. * SIGH *

I thought the same thing at first Tom about the NAO. I thought it affected the surface winds, so I originally pictured the clockwise motion of the negative NAO would bring an on-shore flow (which is bad enough in itself, but not frigid). The graphics helped me see the affect it had on the jet stream so the notion that a negative NAO would provide warmer air (in my mind) was tossed out the window. Like KOW said, if the snow pack was larger in the Arctic and Northern Canada, we'd be hearing more reports of snow squalls and the like.

[donsutherland1]

Just a quick update...

The above to much above normal heights have continued to expand both eastward and northward from the Southwestern U.S. Over the course of the next 5-10 days, those heights will have shifted to the Eastern third of the United States.

Mirroring this northward and eastward progression of heights, above normal temperatures are also spreading in similar fashion.

What this means is that after the coming weekend, temperatures should be warming up in the Eastern U.S. While brief cooldowns are possible, the general trend should be for rising temperatures, with the month ending above normal in the East.

None of those brief cooldowns should contain extreme cold relative to the season.

As for the first half of next month?

Based on a combination of analogs, the NAO (which appears to have bottomed out and should become positive near the end of October or during the beginning of November), and the expectation that the pattern becomes progressive at around the same time, it appears that temperatures will likely run near to above normal in the Eastern third of the United States. A generally positive PNA should help preclude the possibility of extreme warmth.

In sum, a first glance at the coming month reveals that the first half of November should experience some chilly shots but temperatures should average near to above normal during that timeframe in the Eastern third of the United States.

[GAStorm]

Thanks for the update Don! I would much rather have it get warm now than in the winter months!