July is supposed to be warm and dry in the inland northwest, and we got what we expected and then some.
For much of the Pacific Northwest, July 2013 will be forever in the record books as the driest month on record as no precipitation was recorded in some spots. So dry in fact that Quillayute, WA (just south of Forks, WA) received a scant 0.01″ for their monthly total. Not even in Twilight could their be a side story so outlandish. Nominal precipitation following a dry spring helped prime fuels for a pretty active wildfire month across the region. As July turned into August, fuel moistures approached record low territory in parts of the NW including the North Central Idaho and the Bitterroot Mountains. Fortuitously, much of eastern WA, the Blue Mountains of Oregon, Idaho panhandle and western Montana received significant summertime precipitation the first few days of August to reel in fire danger a bit.
Temperatures in July were well above normal thanks to an extremely hot start to the month. Boise ID had temperatures of 90F or above in 30/31 days during the month and topped 100 10 times. The Boise airport also had its second and third warmest days since 1940 on back-to-back days on the 1st and 2nd of the month. July featured particularly high daytime maximum temperatures (regionally about 5F above normal), whereas overnight lows were just a tad above normal regionally.
Some very strong inversions during the last couple weeks of July associated with large-scale subsidence, light winds and clear/dry conditions allowed for locations prone to cold air drainage in valley bottoms or open plains (see Moscow-Pullman airport below) to observe below normal overnight lows and above normal daytime highs. Meanwhile temperatures in the mountains above the inversions may have only cooled into the mid 60s.
NWS Spokane noted that this July was one of the warmest on record. In looking through this list you note the high number of years before 1945 making it into the top 12. While there was a string of warm years in the 1920s-1930s in the interior NW, the large number of warm July’s (6/12) prior to 1941 seemed a bit unusual to me. Graphical output here.
A glimpse at mean annual temperature for the station shows a rather odd trajectory with a step change in the early 1940s (BTW, the vertical gray line indicates an incomplete year). And from this graph one might conclude that temperature the last decade or so are similar to what was observed in the last decade of the 19th century.
If these changes were in fact real, one would expect to see a similar signature in other locations across the region. If we compare the difference between the Spokane station and an average of 20 of the most well correlated neighboring stations (pdf for details), we get a time series like the one shown below. The red line shows the raw data, and the green shows the adjusted data that I’ll briefly discuss. It is pretty clear that the Spokane station used to be significantly warmer than its neighbors prior to 1941.
So what happened?
Karin Bumbaco, Assistant State Climatologist for Washington State, helped track down the details of the station. What she found was that the Spokane station was located in the downtown area prior to the early-1940s with the location in 1938 being very close to Felts Field. Currently the station is located near the airport about a half mile south of the automated station at the airport. Described as:
“BARE, ROCKY, SPARSE, GRASSLAND COVER. TYPICAL OF SURROUNDING AREA. LEVEL AREA 5.9 MI WSW OF AND ~600 FET ABOVE CITY CTR, ROLLING HILLS S-W”
A significant change in elevation between locations is a sure bet for such differences.
The National Climatic Data Center (NCDC) tries to account for changes like these, as well as changes in instrumentation and other biases such as urban heat island effect. Processes to objectively remove these non-climatic biases from station data have been implemented for the purpose of long-term global climate monitoring. However, for local climate purposes, the raw data typically remains. NCDC’s temperature adjustment used to estimate the green line in the plot above, results involves a downward adjustment of pre-1941 temperature by 0.9C (1.6F). For a quick comparison, mean annual temperature for Felts Field from 2001-2012 was approximately 0.7C (1.3F) warmer than the official COOP station.
How about extremes?
Nearly all of the record all-time high temperatures are from the olden days.
SPOKANE INTL AP (457938) Extremes Highest Daily Maximum Temperature (degrees F) Days: 1/1 - 12/31 Length of period: 1 day Years: 1881-2013 Rank Value Ending Date 1 108 8/4/1961, 7/26/1928 3 106 7/27/1934, 7/20/1931 5 105 7/27/1939 6 104 7/16/1941, 8/8/1930, 7/25/1928, 7/24/1928, 8/8/1898 SPOKANE INTL AP (457938) Extremes Highest Daily Minimum Temperature (degrees F) Days: 1/1 - 12/31 Length of period: 1 day Years: 1881-2013 Rank Value Ending Date 1 77 7/27/1928 2 75 7/25/1937, 7/3/1924, 7/18/1918 5 73 7/23/1994 6 72 7/23/2006, 8/5/1997, 7/24/1994, 7/23/1940, 7/28/1939
Examples like this are part of the climate record and include stations that have been moved to cooler areas, warmer areas, or haven’t changed only to see the landscape around them do so.
With an observed increase in mean annual temperature of around three-quarters of a degree Celsius as seen across the region in general, present day temperatures near the Spokane airport are similar to those seen in downtown Spokane circa the late 19th century.
Explore the data yourself: http://cdiac.ornl.gov/cgi-bin/broker?_PROGRAM=prog.climsite_monthly.sas&_SERVICE=default&id=457938