Thursday, November 10, 2011

When will Winter Weather finally come around?

We are currently thinking the first 2 weeks of December will be the time when winter weather begins to surface over the US. While we have already had a snowstorm in the Plains and Upper Midwest, winter is still not 'here', so to say. So why is it still acting like fall?
The PNA is playing a part.
The PNA (Pacific North American Pattern) is a variable which oscillates between positive and negative. Figure 1 shows the positive phase while Figure 2 shows the negative phase.
Excerpt from Wikipedia:
The positive phase of the PNA pattern is associated with above-average temperatures over western Canada and the extreme western United States, and below-average temperatures across the south-central and southeastern U.S.


Figure 2
The PNA is in a strong negative phase at this moment, and likely won't come out of it for the rest of the month. However, there are signs that it will begin weakening in the final days of November, shown below by weather models.
Should this PNA become positive, there is a chance that colder weather may begin to have an effect on the US.

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