AgWeatherNet December 2011 Weather Review for Washington

Unusually Dry and Calm Weather Prevailed Until the Final Week of December

Nic Loyd, Meteorologist, 509-786-9357
Gerrit Hoogenboom, Director, 509-786-9371

Overview
December of 2011 was one of the driest Decembers on record thanks to a persistent and anomalous ridge of high pressure that blocked strong storms from reaching the Northwest for most of the month. In fact, heavy rain in parts of western Washington during the last week of the year prevented western Washington from experiencing the driest December on record. Poor air quality and low clouds were the greatest weather concerns for much of the month through Christmas. Although a weak sun and strong inversions caused cold lowland temperatures, temperatures in the mountains were well above normal and the overall weather pattern resembled August more closely than December. The generally calm and uneventful conditions were themselves noteworthy and unusual since December is normally an active winter weather month that can feature any combination of rain, wind, snow, and arctic cold. Monthly rainfall was below normal in western Washington, while snowfall was well below normal during what is normally one of the snowiest months of year in the Cascades. The dry conditions were a particular surprise since both the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) are in their cold phase this winter, which tilts the odds toward more active winter conditions in Washington. January 1st snow depths in the Cascades were somewhat below normal overall, and ranged from 55 to 103% of normal. The White Pass and Paradise snow depths of 20 and 50 inches, respectively, are slightly below the previous lowest La Niña winter readings for that date. Many mountains sites reported record low New Year's snow depths based upon a sample of all La Niña winters.

The first of a series of storms finally affected Washington beginning on Christmas Day. More typically unsettled conditions caused heavy rain at the coast, and strong winds and warm temperatures in eastern Washington during the last week of December. The warm Chinook winds and unseasonable warmth in central Washington on December 28th and 29th countered the cold conditions earlier in the month, and caused temperatures to be near to slightly below normal. Although there was significant spatial variability again in temperature departures from normal, there were several clear trends. Overnight low temperatures were below normal to a greater extent than daily high temperatures. Also, locations such as Moxee and Prosser were cooler than normal since much of the month was dominated by stable conditions and cold air trapped in the low levels of the Columbia Basin. By contrast, Wenatchee was warmer than normal thanks to its close proximity to the mountains and inability to remain mired in the strong inversion and cold pool that was present in the deep basin regions.

December 2011 Daily Average Temperatures (°F)
Location (Period of Record) Maximum Anomaly Minimum Anomaly Mean Anomaly
Prosser (WSU IAREC; 1990-2011)
37.9
-0.3
24.6
-2.4
30.6
-1.8
Moxee (1990-2011)
38.4
1.0
19.7
-3.6
27.9
-2.1
Mt. Vernon (WSU NWREC; 1994-2011)
45.0
-0.8
33.9
-0.9
39.6
-0.6
Wenatchee (WSU TRFEC; 1994-2011)
37.9
2.5
23.5
-0.5
29.9
0.4
Gramling (Tri-Cities; 1989-2011)
37.1
0.4
25.3
-1.7
30.8
-0.8
Walla Walla (1993-2011)
39.8
-0.9
27.4
-2.2
33.3
-1.6
Royal City East (2008-2011)
36.8
NA
25.7
NA
30.5
NA
Pullman (2008-2011)
37.7
NA
24.6
NA
31.0
NA

December Weather
Early December
December began with dry and mostly clear conditions, as the Northwest lay under the influence of a strong ridge of high pressure. Light northerly flow and stable conditions led to cold low temperatures, with many areas in eastern Washington falling into the teens at night. At the same time, the mountains were warmer than normal, and enjoyed sunshine and relatively mild temperatures. While there was abundant sunshine east of the Cascades, much of western Washington had persistent low clouds which lingered throughout the day for much of the first week of the month.

Mid December
Low clouds and fog expanded into many valleys in eastern Washington on December 8th, keeping many locations in the 20s even during the day. Much of the lowlands experienced poor air quality for several weeks due to the stagnant conditions. After a weak storm system on December 14th and 15th brought a dusting of snow to eastern Washington and some light rain to western Washington, high pressure returned to dominate the weather. Around one-quarter of an inch of rain fell on December 18th in northwest Washington. Additional very light rain fell on Tuesday, December 20th as another weak system passed through the area and finally mixed out the stagnant conditions in eastern Washington. High pressure regained control of the weather again until Christmas Day.

Late December
Conditions finally became more typically unsettled during the last week of the year, as a series of mild storm systems brought periodic rain to western Washington beginning on Christmas Day. The heaviest rain fell at the coast on December 28th and 29th, as Long Beach recorded about two and a half inches of rain during that time period. While eastern Washington experienced a bit of rain during the final week of 2011, most of the week was dry but windy and warm. Temperatures climbed into the 60s at certain AgWeatherNet sites near Walla Walla and the Tri-Cities area on December 28th and 29th, while winds gusted to over 50 mph at St. John, Pullman, and Wallula. After a strong cold front delivered much needed snow to the mountains and cooler temperatures to Washington on Friday, December 30th, the year ended with relatively uneventful weather.

December Daily Temperatures



December-By The Numbers
The average December temperature at Prosser (WSU IAREC) was 30.6°F, which is 1.8 degrees below average. The average high temperature was around average at 37.9°F, and the average low temperature was somewhat below average at 24.6°F, which is 2.4 degrees below average. Only 0.15 inches of precipitation fell during the month, while the average wind speed was 3.7 mph. The warmest temperature of the month at Prosser (WSU IAREC) was 57.1°F on December 28th, while the coldest temperature of December was 15.7°F on December 9th. December 28th was a very warm day overall, since the daily low temperature of 51.6°F was nearly four degrees warmer than the previous warmest December low temperature. The coldest high temperature was 23.1°F on December 10th. The average daily temperature of 54.8°F on December 28th was also a December record for warmth. The coldest day of the month was December 9th, which had an average temperature of 20.8°F. The average December temperature at Wenatchee (WSU TFREC) was close to average at 29.9°F, while the average temperature at Pullman was near to slightly above average at 31.0°F.


Notable December Weather Events
  • December 9th: Temperatures dropped to 15.7 degrees at Prosser (WSU IAREC).
  • December 18th: 0.21 inches of rain fell at Mt. Vernon.
  • December 27th: 1.63 inches of rain fell at Long Beach.
  • December 28th: Temperatures rose to near 60 degrees in the Walla Walla area.
  • December 28th: Winds gust to over 50 mph at St. John, while sustained winds were around 40 mph.

December Temperature Extremes
  Observed in 2011 Normal Range Record Monthly Extremes
Site (Period of Record) Warmest Date Coldest Date Warmest Coldest Warmest Date Coldest Date
Prosser (WSU IAREC; 1990-2011)
57.1
12/28/2011
15.7
12/9/2011
56.0
13.2
62.6
12/20/1994
-7.8
12/29/1990
Moxee (1990-2011)
55.6
12/28/2011
8.5
12/22/2011
54.4
9.4
61.3
12/16/1999
-13.5
12/29/1990
Mt. Vernon (WSU NWREC; 1994-2011)
54.9
12/25/2011
21.4
12/12/2011
56.7
22.2
65.0
12/24/2005
9.5
12/20/2008
Wenatchee (WSU TFREC; 1994-2011)
51.5
12/30/2011
14.6
12/22/2011
49.5
11.2
59.5
12/16/1999
-6.6
12/20/2008
Gramling (Tri-Cities; 1989-2011
55.2
12/28/2011
12.1
12/12/2011
54.7
13.4
63.4
12/14/2002
-11.0
12/29/1990
Walla Walla (1993-2011)
59.4
12/28/2011
18.2
12/23/2011
60.7
14.6
69.6
12/31/1996
-3.3
12/16/2008
Royal City East (2008-2011)
57.3
12/28/2011
14.3
12/9/2011
NA
NA
NA
NA
NA
NA
Pullman (2008-2011)
52.0
12/28/2011
14.0
12/8/2011
NA
NA
NA
NA
NA
NA

Agriculture Report
Washington's 2011 apple crop is expected to be the third largest in history, but smaller than the 2010 crop. 104.3 million boxes of apples should be packed and shipped. The estimate has increased from previous expectations due to more apples making it safely to storage than expected (The Spokane Spokesman-Review). Eastern Washington winter wheat is in good to excellent conditions, but the dry December may limit the crop. The dry and calm conditions were beneficial for the western Washington vegetable crop and for tree fruit pruning in central Washington, and also made for an easy Christmas Tree harvest (Washington Agricultural Statistics Service).

This Month in History
On December 30th, 1996, Yakima observed a record snow depth of 27 inches. On December 29th, 1990, the temperature at Prosser (WSU IAREC) dropped to a monthly record minimum of -8 degrees.

Climate Outlook
Although quiet conditions prevailed in Washington during the end of 2011 and the beginning of 2012, it is unlikely that the entire winter will be so calm. Since we are in a La Niña winter, which tilts the odds toward more severe Northwest winters, it is likely that Old Man Winter will make at least a brief appearance in Washington before spring arrives in 2012. The Climate Prediction Center (CPC) outlook for Washington calls for enhanced odds of cooler and wetter than normal conditions through the spring of 2012. However, current forecast models suggest that near to above normal temperatures will continue for at least the next ten days, with little chance of lowland snow.

Washington State University