February's wet start:  Pattern and expected rain area Feb 2-3, 2010 (click to enlarge)
Split flow pattern expected for Week of February 1 - 5. Blue lines denote the polar jet stream; red lines denote the subtropical jet stream.
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"January Thaw" Ends; Cooler Weather, Rain Headed Back to Valley Feb. 2010

A topsy–turvy January began rather chilly, with a period of hard freezes on the 9th and 10th. The freezes were followed by a mid month coastal storm which pushed heavy rains just north of the Valley from the King Ranch through the Big Bend; a January thaw followed the coastal storm not only for Texas but for much of the nation as the cold pattern relaxed for nearly two weeks. The break would come to a screeching halt on the 29th, as a deepening storm pulled in polar air, bringing a major ice and snow storm from the Texas Panhandle to Oklahoma, Arkansas, Tennessee, and into the southern Mid Atlantic region. Chilly temperature filtered in behind the storm on the 30th and 31st, which will set the stage for a more vigorous subtropical jet and hopefully some good rains for much of Deep South Texas as we enter February.

What’s Going On Here?
While the current El Niño may be nearing its peak, the cold and wet period of December that contined with less rain but even colder weather for the first full weekend of January was quickly replaced by Valley weather that could have occurred in a La Niña or neutral year, such as 2009. Unseasonably warm, humid days and nights with frequent gusty south to southeast winds appeared on many days between January 17th and 28th. Why did this happen? Perhaps there’s more to El Niño than meets the eye. Other atmospheric teleconnections may have something to do with it. One such teleconnection is the Arctic Oscillation, or AO for short (Wallace, 2001). From December through mid January, the AO bottomed out at levels not seen since 1977, which, perhaps not coincidentally, was a time when the central Florida peninsula recorded accumulating snowfall. On January 9th, 2009 areas from Tampa Bay to Orlando had a mix of sleet and snow. Within a week of this impressive cold snap – the same one which held daytime temperatures in the 30s across most of Deep South Texas – the AO index began rising sharply. By the time the rise had peaked, much of the eastern and central U.S. had recovered back to above average temperatures, particularly locations south of 35°N latitude.

The rise in the AO (and its counterpart the North Atlantic Oscillation) would be short lived. Forecasts into mid February show the AO crashing once again, and global models indicate that colder, and wetter (rain, snow, storminess), weather is headed across the eastern half of the country, with several potential subtropical jet stream impulses returning snow and ice to the Mid South, Ohio Valley, and Mid Atlantic region while South and Central Texas hopefully sees more rainfall. A snapshot of the AO, including ensemble forecasts based on the January 30th forecast, is shown below. For more on the current and projected phase of the AO, please surf here.

Arctic Oscillation index and forecasts, January 29 2010 (click to enlarge)
Arctic Oscillation Index and ensemble projections, January 29th 2010. Area in blue shows drop in the index, which coincided with cool to cold, and generally wet weather across the eastern half of the U.S. Area in orange shows rise in the index, which coincided with the end of the cold snap and the beginning of a January thaw, especially south of 35°N latitude.

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