AgWeatherNet September 2015 Weather Review for Washington

Finally: Cooler Than Normal Temperatures in September

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

Overview
Although September often seems like a time of endings in terms of summer heat and the growing season, last month felt much more like a time of beginnings for healing and renewal. Following a long, parched summer, many locations received their first rainfall and first stretch of cool weather since the spring, as Washington inched closer to the start of the cold season. Mercifully, September 2015 was Prosser's coolest since 2005, and also the state's first cooler than normal month since last November.

While last month represented the most significant positive step relative to drought and heat relief in a long while, there is still much work to be done.

Below normal daytime temperatures were observed for much of early to mid to September, with a brief hot spell around the 12th separating two cool and unsettled periods of occasional rain and wind. The latter part of the month was generally calmer and warmer, especially relative to normal, although a few cold nights occurred near the end of September.

In early September, Mt. Vernon recorded rainfall on 8 consecutive days (August 28th to September 4th) for the first time since early February. September 5th was a notable day for Pullman, as the largest daily rain total (0.66 inches) since February 9th was recorded. The rain-chilled high temperature of only 54 degrees was their coolest since May 13th. However, heat returned a week later, as Omak soared to 94 degrees on the 12th. Still, many areas saw a rapid temperature decline thereafter. From the 12th to the 14th, Moxee's high temperature decreased by 31 degrees (from 92 to 61 degrees) in only two days, before a frost occurred on the morning of the 15th. In other words, Moxee shifted from 90s to frosty conditions in about 60 hours. Similarly, Vancouver's highs fell from the 90s to the 50s in just three days from the 11th to the 14th.

Warmer and drier days generally resumed during the latter part of September. However, a few clear and calm nights were very cold at month's end. On September 28th, Pullman fell to 26 degrees. The last time that station was colder than 26 degrees was March 4th. Overall, Prosser's September mean high temperature was a modest 1.3 degrees below normal. Even so, this was their largest cool departure from normal for a monthly high since February 2014, which is yet another testament to the unusual lack of prolonged cool periods during the last year and a half.

September 2015 Average Temperatures (°F)
Location (Period of Record) Maximum Anomaly Minimum Anomaly Mean Anomaly Soil Temperature
8 inches
Prosser (WSU IAREC; 1989-2015)
77.0
-1.3
46.0
-1.9
60.9
-1.6
65.3
Moxee (1989-2015)
76.7
-2.4
39.3
-2.7
57.6
-2.2
65.0
Mt Vernon (WSU NWREC; 1994-2015)
66.5
-1.7
48.3
-0.6
57.2
-0.9
61.2
Wenatchee (WSU TFREC; 1994-2015)
76.3
-2.4
48.2
-0.9
61.9
-1.3
68.8
Tri-Cities (1995-2015)
79.7
-1.3
50.2
-1.4
64.6
-1.3
73.7
Walla Walla (1993-2015)
76.1
-1.4
50.2
-1.2
62.4
-1.6
70.4
Moses Lake (1989-2015)
76.1
-2.2
46.4
-0.6
61.1
-1.4
70.0
Omak (Pogue Flat; 2005-2015)
75.2
NA
48.5
NA
61.5
NA
67.9
Royal City East (2008-2015)
75.1
NA
49.6
NA
61.4
NA
68.6
Pullman (2008-2015)
70.6
NA
41.6
NA
56.1
NA
61.2
Long Beach (WSU Long Beach; 2005-2015)
66.9
NA
47.2
NA
56.8
NA
61.4
Sequim (2008-2015)
67.1
NA
45.2
NA
56.2
NA
59.4
Seattle (2011-2015)
68.6
NA
51.1
NA
59.2
NA
65.4
Vancouver (WSU RE; 2008-2015)
72.3
NA
48.4
NA
60.1
NA
63.8
2015 Rainfall and Evapotranspiration (ET) (inches)
Location (Period of Record) Rainfall ET Net
September 2015 September (Rain - ET)
Prosser (WSU IAREC; 1989-2015)
0.17
3.78
5.66
-5.49
Moxee (1989-2015)
0.10
2.72
4.54
-4.44
Mt. Vernon (WSU NWREC; 1994-2015)
2.12
16.55
2.76
-0.64
Wenatchee (WSU TFREC; 1994-2015)
0.37
4.81
4.81
-4.44
Tri-Cities (1995-2015)
0.11
3.83
5.77
-5.66
Walla Walla (1993-2015)
1.01
8.98
4.43
-3.42
Moses Lake (1989-2015)
0.25
3.15
5.63
-5.38
Omak (Pogue Flat; 2005-2015)
0.00
3.46
4.90
-4.90
Royal City East (2008-2015)
0.03
2.33
5.19
-5.16
Pullman (2008-2015)
0.95
9.52
5.53
-4.58
Long Beach (WSU Long Beach; 2005-2015)
1.31
25.25
2.56
-1.25
Sequim (2008-2015)
0.91
9.26
2.74
-1.83
Seattle (2011-2015)
1.56
18.98
2.72
-1.16
Vancouver (WSU RE; 2008-2015)
0.89
17.38
3.67
-2.78
June 2015 Daily Rainfall Time Series Graph for Prosser June 2015 Daily Rainfall Time Series Graph for Long Beach

September Weather
Early September-Cool and Unsettled
The beginning of the month was cool and unsettled, as Walla Walla recorded 0.84 inches of rain on the 5th, which was their largest daily total since May 13th. Moxee dropped to below 35 degrees on the morning of September 3rd, which was their coldest reading since May 9th.

Mid September-From Hot to Cool
Summer's last gasp was observed on September 12th, as the Tri-Cities climbed to 94 degrees. However, rapid cooling followed for the mid-month time period, as daytime highs at numerous locations fell from the 90s to the 60s in a couple of days. Later in the period, Pullman recorded a 32 mph wind gust on the 20th.

Late September-Delightful by Day, Cool by Night
Highs were in the 70s for many areas for much of late September, as generally sunnier and calmer weather returned. However, the state experienced several cold nights near the end of the month. The low temperature at Moxee dipped to 29 degrees on the 28th.

September 2015 Daily Temperatures
2015 temperature Time Series Graph for Prosser 2015 temperature Time Series Graph for Moxee
2015 temperature Time Series Graph for Wenatchee 2015 temperature Time Series Graph for Tri-Cities
2015 temperature Time Series Graph for:Walla Walla June 2015 Temperature Time Series Graph for WSU
June 2015 Temperature Time Series Graph for WSUMoses Lake June 2015 Temperature Time Series Graph for WSU Omak
June 2015 Temperature Time Series Graph for WSU Long Beach June 2015 Temperature Time Series Graph for WSU Mt. Vernon

September-By The Numbers
The average September temperature at Prosser (WSU IAREC) was 60.9°F, which is 1.6 degrees below average. The average high temperature was 77.0°F, which is 1.3 degrees below average. The average low temperature was 46.0°F, which is 1.9 degrees below average. The maximum temperature recorded last month was 93.6°F on September 12th, while the coolest daily high was 64.9°F on September 15th. Low temperatures ranged from 55.4°F on September 13th to 34.7°F on September 27th. Daily average temperatures ranged from 72.0°F on the 12th to 51.9°F on the 27th. The monthly precipitation total was 0.17 inches. The average wind speed was 3.9 mph, and the average 8 inch soil temperature was 65.3°F. Wenatchee (WSU TFREC) recorded an average temperature of 61.9°F, which is 1.3 degrees below average, while Walla Walla's average monthly temperature of 62.4°F was 1.6 degrees below average. Pullman was one of the cooler spots in eastern Washington, with an average September temperature of 56.1°F. In western Washington, Mt. Vernon's average temperature was 57.2°F, which is 0.9 degrees below average.


Notable September Events
  • September 3th: The temperature at Moxee fell to 35 degrees.
  • September 5th: 0.66 inches of rain fell at Pullman.
  • September 5th: The high temperature at Pullman was just 54 degrees.
  • September 12th: The temperature at the Tri-Cities climbed to 94 degrees.
  • September 28th: The temperature at Pullman plunged to 26 degrees.

September Temperature Extremes (°F)
  Observed in 2015 Record Monthly Extremes
Site (Period of Record) Warmest Date Coldest Date Warmest Date Coldest Date
Prosser (WSU IAREC; 1989-2015)
93.6
9/12/2015
34.7
9/27/2015
97.0
9/2/1998
30.7
9/23/2000
Moxee (1989-2015)
92.3
9/12/2015
29.0
9/28/2015
98.3
9/5/2003
23.5
9/23/2000
Mt. Vernon (WSU NWREC; 1994-2015)
75.3
9/12/2015
39.6
9/30/2015
83.0
9/3/2003
35.8
9/23/2000
Wenatchee (WSU TFREC; 1994-2015)
93.4
9/12/2015
35.3
9/28/2015
98.9
9/1/1998
29.6
9/23/2000
Tri-Cities (1995-2015)
93.8
9/12/2015
37.4
9/28/2015
98.3
9/1/1998
32.8
9/23/1996
Walla Walla (1992-2015)
90.9
9/12/2015
38.8
9/28/2015
97.6
9/15/2013
30.6
9/23/2000
Moses Lake (1989-2015)
90.9
9/12/2015
36.5
9/28/2015
98.3
9/3/2006
27.6
9/28/1999
Omak (Pogue Flat; 2005-2015)
93.5
9/12/2015
39.4
9/23/2015
97.3
9/9/2011
30.6
9/29/2007
Royal City East (2008-2015)
88.7
9/12/2015
41.5
9/15/2015
93.3
9/12/2013
36.9
9/30/2009
Pullman (2008-2015)
89.4
9/12/2015
26.3
9/28/2015
94.7
9/24/2011
25.3
9/12/2012
Long Beach (WSU Long Beach; 2005-2015)
77.4
9/11/2015
37.3
9/28/2015
96.0
9/22/2009
30.2
9/17/2010
Sequim (2008-2015)
76.6
9/11/2015
35.1
9/22/2015
80.9
9/8/2011
33.0
9/29/2011
Seattle (2011-2015)
80.9
9/12/2015
42.4
9/27/2015
NA
NA
NA
NA
Vancouver (WSU RE; 2008-2015)
92.0
9/11/2015
37.6
9/27/2015
95.9
9/11/2013
35.8
9/23/2008

Agriculture Report
As of early September, some areas had to re-seed winter wheat due to crusting conditions, and ripe pumpkins were ready in some fields. In mid-September, corn and potato harvest continued, and dry corn harvest began two weeks early in some areas. As of late September, apple harvest was underway in Snohomish County. The winter wheat crop was suffering due to low soil moisture at seeding, although soil moisture improved in parts of western Washington. Otherwise, hop harvest was in full swing, along with peaches and onions (National Agricultural Statistics Service).

April 1 to September 30 Accumulated Growing Degree Days
  Base Temperature (°F)
32 43 50
Location 2015 5 Year Avg Difference 2015 5 Year Avg Difference 2015 5 Year Avg Difference
Prosser (WSU IAREC)
6188
5777
411
4177
3772
405
2932
2561
371
Tri-Cities
6888
6365
523
4878
4355
523
3613
3113
500
Walla Walla
6452
5977
475
4443
3969
474
3191
2748
443
Wenatchee (WSU TFREC)
6593
6112
481
4581
4105
476
3343
2887
456
Moxee
5722
5316
406
3719
3327
392
2536
2169
367
Omak (Pogue Flat)
6250
5775
475
4238
3777
461
3025
2595
430
Moses Lake
6263
5856
407
4251
3849
402
3008
2633
375
Pullman
5038
4608
430
3055
2657
398
1923
1583
340
Royal City East
6313
5839
474
4300
3832
468
3057
2623
434
Mt. Vernon (WSU NWREC)
5064
4726
338
3051
2718
333
1814
1516
298
Long Beach (WSU Long Beach)
4433
4216
217
2420
2212
208
1233
1059
174
Sequim
4768
4325
443
2755
2326
429
1575
1190
385

GDD are calculated by subtracting a base temperature from the average daily temperature (°F). If the resulting value is less than 0, then it is set to 0. Thus GDD are always 0 or greater.

GDD = (Tmax+Tmin)/2 - Tbase


This Month in History
On September 18th, 2010, the high temperature at Moxee was only 58 degrees.

Climate Outlook
According to the Climate Prediction Center, above normal temperatures and near normal precipitation are likely for October, and then above normal temperatures and below normal precipitation are favored for later in the autumn and well into 2016. El NiƱo continues to strengthen, and is expected to peak at near record values during the early winter.

Washington State University