October 2011

Climatology is the exception rather than the rule when it comes to these monthly updates. While the last month certainly was not exceptional in terms of precipitation, temperature or any truly notable weather events, all told the inland northwest experienced a warmer and mainly wetter than normal October.  Following with my mid-month post on the meaning of mean temperatures, the month ended with daytime high temperature around 2F cooler than normal, while nighttime lows remained well above normal (4-6F).  Several locations in the inland northwest actually saw monthly averaged minimum temperatures set all time highs, including Missoula, MT and West Glacier, MT.

Many locales recorded their first freeze of the season on the 25th of the month (Spokane, Boise, Twin Falls, Pullman), several weeks to a month later than per “normal”. Likewise, prior to #snowtober in New England, much of the northeastern US including Concord, NH and Syracuse, NY had experienced unseasonable warmth and a lack of temperatures below freezing. The lack of fall onset and leaf-drop associated with the extended warmth paradoxically contributed to much of the damage in the wake of the record setting storm.

Winter outlook

By now you’ve heard she’s back for more.  Last year’s La Nina was the strongest in a half century according to atmospheric metrics, and we are on our way back to La Nina conditions for the winter of 2011-2012. Such a double-dip of sorts is consistent with strong La Nina conditions and the latest statistical and dynamical models suggest we will see a moderate La Nina this winter.

That being said, the long-term outlook for the cool season as a whole is for wetter and cooler than normal conditions due primarily to the weaker subtropical jet across the Pacific and preference for storms to ride the polar jet right into our backyard.  Also, during La Nina years, the jet stream tends to be a bit weaker that allows for a more loopy or wave-like flow over the western US.  This amplified wave pattern tends to allow for more cold air from Canada to infiltrate the area, and of course we know what happens when you mix cold with wet.  I’ll refrain from dropping any four letter words…

Typically with La Nina or El Nino events, the signatures in northern hemisphere circulation remain weak during early winter, and are more pronounced during late winter (Jan-Mar). A couple years ago Tim Brown and I developed a tool that allows one to examine the relative risk of a temperature and precipitation extremes coincident with La Nina or El Nino events.

La Nina typically brings elevated chances of anomalously wet (defined as being in the top 20% of years) and cold (top 20% of years) to much of the interior NW. Worth noting is the fact that much of the deserts of WA/OR are decoupled during La Nina events due to the trajectory of systems and orographic ratio.

Overall, La Nina winter nearly unanimously result in higher 1 April SWE across much of the northern and central mountains of Idaho as can be seen here (data from VIC 1950-2008).  The latest projections from the CPC’s coupled forecasting system suggest wetter and cooler conditions across much of the region through April.

It looks like we’ll have a good head start with a deep trough establishing itself over the western US for the next 10-15 days that should usher in much cooler air and likely snow for many.

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