July 2011

The streak continues.

You have probably heard of the record setting heat across most of the country during July, including all time monthly records in places like Washington DC, Oklahoma City.  Some have suggested that these records may be contaminated due to instrumentation biases and the urban heat island; but the scientist in me is also very skeptical that some of the all time temperature records for US states, might have occurred with faulty instrumentation that was not properly ventilated. Per NCDC preliminary figures, nearly 9000 warm records (record high maximum or minimum temperatures) were set in July, compared to about 1200 cool records(record low maximum or minimum temperatures).

Records set in July 2011 for the US. Source: NCDC

No matter how one dissects July 2011 for most of the US, hot sums things up succinctly. The Pacific and Inland Northwest was a different story.

July 2011: Most of the country not including the northwestern US was under the evil heat "dome". Thanks to Linda Sharps for the humor.

For six consecutive months (Feb 2011-July 2011) we’ve had below normal temperatures. Temperatures were a good 3-6 degrees F below normal across much of the Idaho panhandle and eastern Washington for July 2011, making the inland northwest the winner of the coolest anomalies for July 2011 climate sweepstakes (CONUS only).  The guy in the picture above looks thrilled with this award. Snow to persists in the Cascades into August due to the combination of a record setting wet spring and our current cool streak which lowered the freezing level.

Monthly mean freezing elevation for the last 12 months over the central WA Cascades. Source: http://www.wrcc.dri.edu/cwd/products/, NCEP-NCAR reanalysis

The cool temperatures in July, included a few record minimums on the Palouse for places like Moscow, ID and Rosalia, WA.  A persistent trough and very low total column water vapor over the inland northwest (10-15% below normal, BTW, much above normal across the eastern 2/3 of the US) coupled with relatively light nocturnal winds were suspected to have enabled for the cool overnight lows in some of the valley locations prone to decoupling.

July 2011: TMAX departure from new 1981-2010 "normals". From inland northwest climate tracker, http://www.wrcc.dri.edu/research/jtwrcc/idaho-mon/

The current six-month streak of subpar maximum temperatures across the area is historically quite interesting.  I qualified the “streak” by taking a four long-term stations across eastern WA and northwestern ID (Spokane, WA, Lewiston, ID, Moscow, ID and Priest River, ID) and qualified “normal” using the full period of record, rather than 1971-2000 or 1981-2010.

Histograms of (top left)cool streaks and (top right)warm streaks 1895-2011. Bottom panel shows timing and duration of all cold and warm streaks at least 6-months in length.

You can see how the distribution of warm/cool streaks plays out from the above two histogram plots, following a log-normal distribution.  The bottom panel shows the dates and duration of all streaks at least 6-months in length, with the blue bar at the far right our present cool streak. It doesn’t take a genius to immediately see the overall shift in the frequency of warm streaks vs. cool streaks over the past century and the overwhelming number of warm streaks since about 1985. In fact, we have to go back to the 6-month cool streak of Sep 1984-Mar 1985 to find the last time we’ve been in such a slump.  Overall, we’ve seen about 30 cool-streaks of 6-months or more since 1895, but this is the first in 26+ years.

The persistent trough that has been with us since Feb 15th can be seen below in the 500hPa geopotential height anomaly. The bulls-eye of low height anomalies over Washington state (see unhappy guy in cartoon above) indicates enhanced troughing and cyclonic flow across the western United States, responsible for the cool-wet regime we’ve been in. As a quick and dirty proxy, a 60m anomaly translates to about a 5F cooler air mass in the lower-troposphere using a few simplifying assumptions.  You can also see that wave train of height anomalies extending in a great circle route from the subtropics near the dateline and arching over the United States.

500hPa geopotential height anomaly Feb-Jul 2011. From NOAA ERSL, NCEP-NCAR reanalysis data

The stubborn pattern has really cut back on the number of warm days (90F and greater) so far this summer.

Location    thru 7/31    average # July/Year    Record low (year)   Record high (year)

Moscow, ID           2         7/16                          3 (1993)               40 (2003)

Lewiston, ID         10       16/41                          20 (1993)             63 (1958)

Spokane, WA         3        9/20                          3 (1948)               39 (1958)

By contrast, Dallas TX hasn’t been below 90F since May 26 and now has had 30 days above 100F threatening their all-time streak of 42 days set in 1980.

Enjoy the cool summer