The quality controlled Iowa COOP data for December 2017 has been uploaded to the IEM. These observations are kindly provided by Harry Hillaker, our state climatologist, who retired earlier this week! This will be the last monthly update of quality controlled data from him. The IEM will continue to curate this data, so no need to panic! He passes along these notes on the month:

General Summary. Temperatures averaged 24.1° or 1.2° above normal while precipitation averaged 0.42 inches or 0.92 inches less than normal.   This ranks as the 61st warmest and 12th driest December among 145 years of records.   A drier December was last recorded in 2002.   Windy conditions were frequent early in the month with gusts over 50 mph recorded on the 4th, 5th, 11th and 13th.   Estherville Airport reported the highest winds with a gust to 63 mph on the 13th.

Temperatures. The first three weeks of December were mostly much warmer than usual, averaging 7.7° above normal.   However, a turn toward much colder weather began on the 22nd and intensified on Christmas Day with temperatures over the final ten days of the month averaging 14.4° lower than normal.   Each of the first four days of the month averaged more than ten degrees above normal, as did the 13th, 15th, 16th, 18th and 19th.   The warmest day was the 4th when temperatures averaged 20.1° above normal with Indianola and Lamoni reaching 70°.   This was only the twelfth date on record for temperatures to reach the 70 degree threshold in Iowa in December (last occurrence was Dec. 3, 2012).   The season’s first zero degree temperatures were recorded on the morning of the 7th at Primghar and Sibley.    Meanwhile temperatures averaged twenty degrees or more below normal on the 26th, 27th, 28th, 30th and 31st with New Years’ Eve averaging 30.3° below normal.   Sioux City Airport recorded the lowest temperature of the month with a -24° reading at midnight New Year’s Eve.   Meanwhile subzero daily maximum temperatures were recorded somewhere in Iowa locations on six of the final seven days of the month.   Northwood reported the lowest maximum temperature of the month with a minus 14° ‘high’ on the 31st.   The lowest wind chill reading was -46° recorded at 9 p.m. on the 31st at Storm Lake.

Heating Degree Day Totals. Home heating requirements, as estimated by heating degree day totals,  averaged 3% less than last year and normal.   Heating requirements thus far this season (since July 1) are running 13% more than last year at this time and 7% less normal.

Precipitation. The very dry weather pattern that began in mid-October continued through December 20th.   A statewide average of only 1.25 inches occurred over the 67-day period ending December 20 while normal for the period is 4.30 inches.   Precipitation for the remainder of the month averaged near normal.  A statewide average of only 0.2 inches of snow fell through December 20.   A slower start to the snow season has not occurred since 1960.   However, light snow was frequent for the remainder of the month.   The largest precipitation event of the month came on the 29th when snow fell across the northeast three-fourths of Iowa with greatest storm totals of 4 to 6 inches occurring from the Spirit Lake area southeastward through Fort Dodge and Marshalltown to Muscatine.   The statewide average snowfall was 5.5 inches or 2.8 inches below normal.   This ranks as the 63rd least December snowfall among 130 years of record.   Monthly snow totals varied from 2.8 inches at Newton to 9.3 inches at Muscatine.   Monthly precipitation totals (rain plus the liquid equivalent of snow) varied from 0.17 inches at Forest City to 0.91 inches at Iowa City.   Precipitation totals were below normal at every reporting point in the state for the second consecutive month.

2017 Summary

General Summary.   2017 temperatures averaged 49.9° or 1.8° above normal while precipitation totaled 33.10 inches or 2.17 inches less than normal.   This ranks as the 9th warmest and 61st wettest year among 145 years of records.   A warmer year was recorded just last year while a drier one in 2012.   Snowfall averaged 18.9 inches or 13.3 inches less than normal.   This was the least snowy calendar year since 2006 and ranks 11th least among 130 years of records.

Temperatures.   Above normal temperatures were persistent through 2017 with only two calendar months (May and August) falling below the historical median.   The most unusual temperatures arrived in February when readings reached seventy degrees or higher somewhere in the state on eight days during the month, double the previous maximum frequency of such temperatures for February.   Temperatures averaged an amazing 18.3 degrees above normal for the two weeks ending February 23rd.   The most noteworthy streak of unseasonably cool weather came from July 28 through September 8 when only six days averaged warmer than normal (and just barely at that) over this 43 day period.   This cool spell helped crops cope with a rather dry summer with surprisingly high yields in most areas.   Despite this extended late summer cool spell the year averaged 19 days with daily temperatures above 90 degrees, three more days than normal and the highest total since 2013.   Subzero cold occurred an average of just 10 days, eight less than normal and the smallest annual total since 2012.   Annual temperature extremes varied from highs of 101°at Ottumwa on July 20 and at Des Moines the next day to a low of -24° recorded at midnight on New Year’s Eve at Sioux City.   Overall, warmer than normal days outnumbered cooler ones by a count of 220 to 145.

Precipitation.   Precipitation totals were generally near to slightly above normal for the first five months of the year.   Hefty April rainfall resulted in considerable planting delays in some areas, particularly in south central Iowa.   Much drier weather began in late May and persisted across most of Iowa through mid-August.   Drought conditions began to appear in south central Iowa in mid-June, spread to west central and southeast Iowa in early July and reached its maximum extent in mid-August when 45% of the state (a diagonal area from northwest, to central, to southeast Iowa) was classified as in drought.   Portions of 13 south central and southeast Iowa counties were classified as ‘extreme’ drought in August and/or September.   Rain became much more frequent over western Iowa in mid-August with short-term dryness becoming most apparent in far eastern Iowa by September.   The first two weeks of October were very wet across most of the state and greatly slowed early harvest progress.   However, a very dry weather pattern prevailed statewide from mid-October through the end of the year with only about one-third of normal precipitation falling in this period.   Annual precipitation totals were more than ten inches below normal over much of south central and southeast Iowa, covering much the same area that was unusually dry in 2016.   Annual precipitation totals varied 47.21 inches at Fayette (9.48 inches above normal) to only 24.33 inches at Oskaloosa (13.89 inches less than normal and their lowest annual total since 1988).   Above normal annual precipitation totals were mostly confined to west central and northeast Iowa.   There were some very heavy rains in 2017 with the largest event coming on the night of July 21 when 9.52 inches of rain (nearly six inches of which fell in just two hours) fell overnight at Ionia in Chickasaw County.

Severe Weather.   Severe thunderstorm activity got off to a very early start with one short-lived tornado in Clinton County on the evening of February 28.   A March record of 18 tornadoes touched down on the evening of March 6 with greatest damage resulting from an EF-2 tornado at Seymour in Wayne County.   However, no strong tornadoes (EF-2 or stronger) occurred again in the state until June 28.   Overall the National Weather Service reports a preliminary annual tornado count of 55 storms, slightly more than the recent annual average of 47.

The following is the number of new daily records set at COOP sites based on data back to 1951.

                 _____2017_______________________________________
                 DEC  NOV  OCT  SEP  AUG  JUL  JUN  MAY  APR  MAR
Maximum High:     55   84    1  360    0    1   28   36    5   92
Minimum High:    250    9   96   15   41    0    8  189  122   22
Maximum Low:       7    3    2  129    0   12   95   13   43   43
Minimum Low:      95    5    4    3   23    1   59    5   12   42
Maximum Precip:   24    0  150   27   47   38   35   39   97   40

The following is a table summarizing how well the IEM daily data estimator is working in comparison to the quality controlled data.

                      2017                                   
                 DEC   NOV   OCT   SEP   AUG   JUL   JUN   MAY   APR
High Temp Bias  -1.2  -0.9  -0.4  -0.2  -0.4  -0.2   0.1  -0.2  -0.3
High Temp STD    M     M     M     M     M     M     M     M     M  
Low  Temp Bias  -0.6  -0.2   0.1   0.2  -0.3  -0.3   0.1  -0.2  -0.3
Low  Temp STD    M     M     M     M     M     M     M     M     M  
Precip    Bias  -0.21 -0.13  0.09 -0.05 -0.05  0.14  0.08  0.07  0.02
Precip    STD    0.25  0.21  0.64  0.64  0.70  0.75  0.66  0.42  0.49