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Posted at 11:00 AM ET, 12/17/2009

A look back at last week's historic weather

By Steve Tracton

* Weekend snow? Accumulation Potential | Full Forecast *

Can anyone put the size of the storm moving in tonight in historical perspective? Locally it may not amount to more than an inch or two of rain, but a glance at the national radar shows this system is HUGE. You could leave DC heading west and drive through unending precip all the way to olorado (or from Nebraska to Florida.) Strikes me as not something I've seen in recent memory.

Such was the comment left by CWG reader manatt last week on the evening of Dec. 8 ahead of a storm that, for the D.C. area, began briefly as a wintry mix before soaking the region with a general 1 to 2 inches of rain.

Nationally, the storm resulted in one of the most active weeks weather-wise in many years. The sheer size, strength and menagerie of wild weather caused by the powerhouse storm was remarkable. Keep reading for a look back at how the storm evolved, what its impacts were, and from where and how it originated. (Hint: El Niño is not to blame.)

Weather maps show progression of massive storm last week. Credit: NOAA.

At the beginning of the week a low-pressure system strengthened and the breadth of its circulation increased as it migrated from central California over the inter-mountain region to Colorado. Thereafter, the storm intensified rapidly as it tracked toward the Upper Midwest by midweek and then to northeastern Canada by week's end.

The storm significantly affected two-thirds of the nation. At its peak intensity, the minimum central pressure (974 millibars; 28.76" mercury) and strong winds were comparable to a Category 2 hurricane. Elsewhere across the country, the wild weather ride included:

*Snowfall totals of 30 to 40 inches over the mountains of California and near Flagstaff, Ariz..;
*Blizzard conditions with winds up to 90-100 mph and thundersnow whiteouts over the Upper Midwest;
*Up to 3-4 feet of Lake-effect snow over parts of western New York state;
*Intense rains and flooding in the country's southern region; and
*Mixed precipitation and freezing rain between the rain-snow boundaries from California to New England.

The storm was blamed for at least 17 deaths, most of them (not surprisingly) the result of traffic accidents. See here for an excellent wrap-up of the storm with videos.

Now for the origin of the events just described. During the preceding week a tremendous change occurred in the mid-to-upper levels of the atmosphere over the eastern Pacific Ocean with development of a classic, textbook Omega Block, in which a broad area of high pressure blocked the general west-to-east flow in a way that resembles the Greek letter Omega. The magnitude of this event and its dominance of the large-scale circulation over the Pacific basin were unusually impressive, but not unprecedented.

Without getting too much more technical, it appears the blocking pattern was very likely the consequence of a series of intensifying storms emerging from Asia into the Pacific combining with the remnants of Super Typhoon Nida. The net result, as shown generically in the graphic below, was a split in the jet stream with the northern and southern components converging over western North America complete with a tropical source of moisture.

Typical Omega Block blocking pattern. Credit: NOAA and CWG.

More often than not such convergence is a storm-making machine as very cold air from the northern branch of the circulation bumps into the much warmer and moisture-laden air to the south. In this particular case, the convergence occurred over central to southern California and led to last week's blockbuster storm.

Blocking episodes such as this occur relatively infrequently. More generally it's important to note that many of the significant storms affecting the United States, including the D.C. area, have their origins in weather systems over the Asian continent that subsequently interact with storms over the Pacific and sources of tropical moisture. Not incidentally, that was the case with the so called "surprise snowstorm" of Jan. 24-25, 2000. I plan to address this on or about the storm's 10th anniversary next month.

Almost forgot!... While last week's southern jet stream was comparable in appearance to that characteristic of El Niño -- the periodic warming of waters along the equatorial Pacific Ocean that impacts weather patterns worldwide -- it had totally different origins. And the overall circulation, temperature and distribution of precipitation was not consistent with El Niño. More importantly, according to Gerry Bell of the National Oceanic and Atmospheric Administration's Climate Prediction Center, the conditions that "lock in" El Niño via the distribution of atmospheric heating associated with tropical convection has not yet developed, but is expected to do in the near future.

By Steve Tracton  | December 17, 2009; 11:00 AM ET
Categories:  Tracton, U.S. Weather  
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Next: Reading material: Pattern favorable for snow


It's looking more and more likely than a snowstorm will hit us this weekend. NAM this morning looks like a blizzard for DC but the latest GFS shows a significant storm for us but not a blizzard.

Either way, I like the chances of accumulating snow.

Posted by: NoVAredsox | December 17, 2009 11:27 AM | Report abuse

It's really cool to have this sort of in-depth analysis. If the storm this weekend turns out to be historic in itself, I hope you'll do another one of these. :-)

Posted by: KBurchfiel | December 17, 2009 8:56 PM | Report abuse

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