November 2011

November in review

A rather unusual circulation pattern over the North Pacific that brought record-setting cold temperatures to Alaska last month was part of a larger wavetrain that influenced conditions downstream across much of the western United States. While we failed to see the sort of temperatures that citizens of Fairbanks consider cold, much of western US had a cooler than normal November due to the more northerly orientation of the jet-stream into the West and associated advection of a cool air mass into the region.

Fig 1: Global upper-level (250hPa) wind patterns (top) and anomalies (bottom) for November 2011.  Note the poleward shift in the jet stream over the N. Pacific and more WNW flow from the Gulf of Alaska into the western US.

Most areas in the inland northwest were a couple of degrees cooler than normal in November (particularly noted for overnight low temperatures). On the precipitation front, while a series of systems slammed the westside of the Cascades, precipitation was below normal across much of Idaho and central-to-eastern Washington and Oregon being on the lee of a formidable topography. A few spots in the northern Idaho Panhandle squeezed out respectable snowfall accumulations (Priest River 20+ inches).

Fig 2: Percent of normal precipitation November 2011.

The Big Dry

The current weather situation across much of the western United States is extremely unusual. Here is a current weather map (caution: weather weenies only)

Fig 3: Sea level pressure (black lines) and 1000-500hPa Thickness ~ proxy for air temperature (magenta dashed lines) valid 4AM 12-1-11.

A couple of fun features:

  1. Sea level pressure in Seattle and parts of the NW is the highest it’s been in 15+ years (update: highest all time at SeaTac). Portland airport recorded its 2nd highest sea level pressure in 70+ years this morning.
  2. Pressure gradient (different in sea level pressure) in parts of the Great Basin and southern California is also the strongest it has been in 10+ years (source: NWS LOX). Schools have closed calling today a “wind day” in southern California.
  3. Wind gusts of 140mph in Mammoth Mnt, California; near 100mph in Utah.
What this means in the short term: A large scale blocking pattern is now set up off the west coast resulting in a highly amplified wave pattern that will make for some interesting contrasts in the coming days (like Amarillo, Texas being colder than Edmonton, Alberta). The atmospheric blocking pattern has the same effect on atmospheric flow as a big rock has in a stream.  This blocking pattern just so happens to coincide with a time of the year that we typically expect precipitation, and so is a bit out of the ordinary.  The the big block in place, we can expect unusually dry conditions the next 10 days or so (more on that in a future post), although the climate prediction center is suggesting a wetter than normal month. With high pressure dominating the inland northwest, light winds, and long nights, I would expect strong inversions with the protected low-lying areas having nighttime lows much colder than mountain locales.  There is also the potential for a cold air outbreak that I’ll be watching around the 10th of the month (still a bit in the fantasy forecast zone at this point).

Global Temperatures for 2011

As 2011 draws to a close we have a pretty good idea that global mean surface temperature for the calendar year will be around the 10th warmest on record (out of a total of 120+ years). Indeed this is cooler than last year, and 2005 and 1998. Some have claimed that the Earth has cooled in the last 15 years (or post 1998), which is a very shortsighted observation.  The winter of 1997-1998 was one of the top two strongest El Nino’s in the last century.  During an El Nino, warmer ocean surface water spreads across the tropical Pacific from west to east to set up global teleconnections that influence weather regimes well removed from the tropical Pacific. The sloshing of warmer water into the central and eastern tropical Pacific also releases a lot of energy into the atmosphere that is reflected in an increase in global surface temperatures by a few tenths of a degree. Likewise, during a La Nina the warmer waters stay in the western tropical Pacific and global mean temperatures typically drop by a few tenths of a degree. Real simple: El Nino years the Earth warms up, La Nina years the Earth cools down.  The best example of year-to-year natural climate variability. I bring this up to highlight the fact that while this year was cooler than last year (recall the winter of 2010 was a weak-moderate El Nino), 2011 will go down as the warmest “La Nina” year in the observational record.  Climate Central provided this nice figure to highlight this concept.

Fig 4: Global mean surface temperature anomalies (1950-2011).  La Nina years are highlighted in blue.


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