Positive NAO

If you want to see what a “positive” NAO (North Atlantic Oscillation) looks like, see below.  The chart shows pressure departures aloft  (500mb/~15,000 ft up) from late Weds night (10/30) over the northern Atlantic.  Blues are areas where pressure is lower than normal (stronger lows/storm centers), while oranges are areas where the pressure is higher than normal (stronger highs).  The contours basically show you the alignment of the jet stream.  The tighter these lines are together, the faster the winds… and the wind flows parallel to these lines.  The low spinning counter clockwise and the high spinning clockwise act to pull the jet stream “tight”, and the net result is 60- to 70-degree weather in late autumn.  The bar chart at the bottom shows the phase and intensity.

Anyhow, these phases tend to last 4 to 6 weeks, and then it resets…  sometimes to the same phase, other times it flips.

500mb departures from normal for 00z, October 31 (8 pm EDT on October 30, 2013).  The bottom of the chart shows the daily evolution of the NAO since July.  This is a classic "positive" NAO... the "North Atlantic Shop Vac", which acts to suck the cold air out of North America.  This setup typically yields, warm, wet weather over the central and eastern U.S.

500mb departures from normal for 00z, October 31 (8 pm EDT on October 30, 2013). The bottom of the chart shows the daily evolution of the NAO since July. This is a classic “positive” NAO… the “North Atlantic Shop Vac”, which acts to suck the cold air out of North America. This setup typically yields, warm, wet weather over the central and eastern U.S.


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