A pattern shift in the making

My apologies for having taken some time off from the blog after the recent Arctic outbreak. I guess I was thawing my fingers. The finger-thawing weather will continue for the foreseeable future, though with some temps in the 60s the past few days, I don't think there is any thawing left to be done!

While temps averaged over 15 degrees below normal for the first 10 days of January, highs have rebounded into the mid 50s to lower 60s the last 4 days and the forecast points to more of the same for at least the next week. Also of note, yesterday and today's rainfall of between 1.0-1.5" was the first rain of 2010, a full 16 days into January! Of course, we can't forget the few hundredths of an inch of liquid precip we picked up with the snow on January 7, but this was the first rain since December 31. This marks the latest "first rain day" to start a year in at least 15 years, which is how far my available records go back.

Looking ahead, following the Arctic intrusion that blanketed the eastern half of the nation to start the year, we are now in the midst of a "pattern shift" which will signal a significant change in U.S. weather patterns over the coming couple of weeks perhaps. During the first two weeks of January, the upper level pattern was marked by a large scale trough (upper level low pressure) over the eastern U.S. and a ridge over the west (upper level high pressure) (see graphic of the upper-level pattern on January 5, courtesy Storm Prediction Center, above).

Gradually that pattern has shifted to something more akin to El Nino years. We now will be seeing a flatter upper level pattern (lower amplitude troughs and ridges) with a strong jet stream pointed straight into the southwestern U.S. and northwestern Mexico (see graphic above valid mid-week; click for larger version). This pattern will mean frequent and strong storms for California, which will then move across the southern Rockies, southern Plains, and into the southeast U.S. More benign weather, outside of clipper systems from Canada, are expected across the northern tier of the U.S. So, if you are watching The Weather Channel or even national news, you'll definitely be seeing stories on the emergence of an El Nino weather pattern and probably flooding and mudslides in southern CA and feet upon feet of snow for the Sierra range. For more on the pattern shift and effects on our west coast neighbors, see Paul Yeager's excellent post "Storms to Blast California."

For the Mid-South, this will mean milder weather with storm systems approaching from the west or southwest that will bring rain every few days and perhaps setting the stage for one or more of these systems to bring the threat of some thunderstorms. The milder weather is due to the fact that these storms will be of Pacific origin and not Canadian origin. Between systems, with some sun, temperatures will have no problem reaching the 50s and 60s and overnight lows should remain "mild for January." More details will be found in the forecasts posted on MemphisWeather.net.

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