Monday, February 8, 2010

Welcome Weather Underground Visitors
Some Further Thoughts on Super Storm 2010

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Welcome Wundergrounders, and thank you Jeff Masters for your generous compliment. I've been a fan of Weather Underground since the earliest days of the WWW. Along with UIUC Weather World, it was one of my first bookmarks on Netscape 0.5.

I'd like to take this opportunity to endorse Dr. Masters' commentary on the relative significance of moisture and temperature in monster winter events and try to expand the discussion a bit. Every individual weather event is unique in its own way, and extreme events, by definition, are even more unique; otherwise, they wouldn't be rare. However, there are also a lot of similarities. In these days of increasingly accurate computer models (and this latest event was very well captured as much as 6 days out), it may seem old-fashioned, but looking at analogs, or similar situations from the past, can be very instructive. Considering the historic nature of this storm, I went back and looked at the weather maps for the other record February snowstorms of the past century in Washington. Overall (and somewhat simplified), the basic elements and their evolution were very similar to what we've all come to expect from these situations:
  • a low pressure area generally originating in or near the Gulf of Mexico, moving northeastward and then deepening rapidly off the Mid Atlantic coast
  • a blocking cold high pressure area to the north
  • another deep low pressure area to the northeast helping to anchor the high in place
These things are all very well documented and in more more detail in Kocin and Uccellini's now-classic books. One thing that really struck me in comparing these, however, was the big difference in the intensity of the cold air to the north. The other cases all had extremely cold surface temperatures in northern New England and southeastern Canada, frequently in the -20°F to -40°F range. That was not true in this case. As Dr. Masters explained, once all the other elements are in place, it doesn't need to be ultra cold in order to snow, just cold enough.

Take a look at the 12Z (7 am EST) surface map for Saturday, Feb. 6. All the right elements are there, including a honking deep 986 mb low parked near the mouth of the Chesapeake Bay, and heavy snow is pummeling the Nation's Capital. Temperatures to the north are cold, of course, but the 0° line is way out of the U.S. Single digits extend from Maine across northern New England and into Quebec. Below-zero readings are confined to central and western Ontario, too far west to be feeding into this storm.

Now consider the 12Z map closest to the heaviest snow for the 2003 event, Feb. 17. The zero line (blue dash-dot) goes from northern Nova Scotia across the northern third of Maine (-22° at Houlton), along the Canadian border and encompasses nearly all of Quebec and eastern Ontario.

In 1983, the low development is less advanced at 12Z on Feb. 11, but the freezer is working overtime to the north. The zero line (black dash-dot) takes in essentially all of New Brunswick, Maine, New Hampshire, Vermont, about half of New York, Quebec, and eastern Ontario. Peak cold, as plotted, is -26° to -27°.

For the President's Day storm of 1979, the Feb. 19 map shows all the usual suspects, although the orientation of the high is somewhat different. Temperature-wise, however, the zero line cuts across a large chunk of New England (-18° at Houlton), northeastern New York, all of Quebec, and most of Michigan. Readings in the -20° range are widespread.

The Feb. 7, 1967 picture shows a little less extent for the 0° intrusion into New England, but Earlton, Ontario checks in with -33°, and Armstrong sports a mercury-freezing -39° and an unmeasurable dewpoint.

Feb. 16, 1958? Armstrong is back with its frozen instrument, and Pagwa plays "Can you top this?" with -40°.

On Feb. 7, 1936, the U.S. has embarrassingly not yet embraced the polar front theory of analysis, but the icebox is stoked with -20° to -40° temperatures.

The conclusion I would draw from all of this is that the 2010 storm was distinct from other similar events in the past by having moisture be the dominant element over temperature in producing the extreme snow amounts.

Images (click to enlarge): Surface weather maps at 12Z (7 am EST) from National Weather Service for major Washington, DC February snowstorms (top to bottom):
  • Feb. 6, 2010
  • Feb. 17, 2003
  • Feb. 11, 1983
  • Feb. 19, 1979
  • Feb. 7, 1967
  • Feb. 16, 1958
  • Feb. 7, 1936

1 comment:

vanzare apartamente cluj said...

Thank you very much for this information... you have clarify many questions about the storms and weather changes. Thank you

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