Anatomy of a transient cold intrusion.

There is a surge of Arctic air moving out of northwestern Canada, and chances are this cA regime will affect most of the USA next week into the first few days of November. I stress that word “transient”, because the cold will not last more than three or four days, and likely will not involve the Southwest at all. Also I must suggest that the way this cold spell plays out will not resemble what the upcoming winter season looks to be like.

I spent all of the past weekend finishing the WEATHERAmerica Winter Season (DJFM) forecast, finally sending out the discussion and graphics last night. Using the analogs based on recent observed trends in actual weather (not indices….), a 500MB pattern emerged that bears no similarity to the currently evolving upper atmospheric flow. Namely, the ridge amplification in the -EPO position, which will break down as energy undercuts the positive height anomaly and heads into Mexico. If you review the low sun period projections, an increasingly strong -AO/-NAO signature takes shape.

Also notice that there is a defined moisture stream into the south central states that was started by the arrival, and break-up, of what was once Hurricane Norma. I believe that Tropical Storm Otis will become a hurricane, and get drawn into this moisture fetch across central Mexico and then Texas. When the energy and higher dewpoints run into the Arctic front, severe weather is quite likely from the Rio Grande Valley into the Ozark Plateau.

Then a refreshing, but chilled, domain will make its appearance east of the Rocky Mountains. California will stay warm and dry, which is another sign that this is not your typical El Nino. More later….

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