The quality controlled Iowa COOP data for May 2017 has been uploaded to the IEM. These observations are kindly provided by Harry Hillaker, our state climatologist, who passes along these notes on the month:

General Summary. Iowa temperatures averaged 58.6° or 1.5° below normal while precipitation totaled 4.85 inches or 0.29 inches above normal.   This ranks as the 69th coolest and 44th wettest May among 145 years of records.   This was the coolest May since 2008 and wettest since 2013.

Temperatures. May began with unseasonably cold weather for the first four days of the month.   Cherokee reported the lowest temperature of the month with a 27° reading on the morning of the 2nd.   No freezes were recorded anywhere in Iowa after the 4th.  Above normal temperatures prevailed each day from the 5th through the 17th.   Temperatures climbed into the nineties on the 8th, 15th and 16th over portions of the southwest one-half of the state.   Highest temperatures were 94° readings at Little Sioux on the 8th and Shenandoah on the 15th.   May finished with mostly cooler than usual weather for the final two weeks of the month.   The coldest late-month weather came on the 19th, 20th and 21st when daytime high temperatures were only in the forties over portions of northern Iowa.

Heating Degree Day Totals. Home heating requirements, as estimated by heating degree day totals, were 8% greater than last May and 16% greater than normal.   Heating degree day totals so far this season (since July 1, 2016) are running 2% less than last season at this time and 15% less than normal.

Precipitation. Widespread rain which prevailed over the final three days of April continued into the first day of May with the rain changing to snow over far northwestern Iowa on the night of April 30 and into the morning of May 1st.   The snow briefly accumulated northwest of a Le Mars to Spirit Lake line with a maximum amount of two inches reported at Lester.   A prolonged period of warmer, and mostly dry weather prevailed statewide from the 2nd into the 15th and allowed a large amount of planting progress to be made, although moderate to locally heavy rain on the 10th interrupted field work for a few days over the southeast two-thirds of Iowa.   Frequent widespread moderate to heavy rain from the 16th through the 20th, along with a turn to much colder weather, put a stop to planting activity until fields slowly dried out later in the month.   Despite the weather delays at mid-month the percentage of corn and beans planted by month’s end was near normal for late May.   However, planting was most delayed in south central Iowa where both April, and the second week of May, was wetter than elsewhere in the state.   Overall, May rain totals were well above normal across west central Iowa and least over the far southeast.   Monthly rain totals varied from 2.51 inches at Fort Madison to 8.24 inches near Anita.

Severe Weather. There were numerous dates with isolated severe storms during May on the 7th, 8th, 10th, 14th, 19th, 22nd, 27th and 28th.   However, one of the most active severe thunderstorm periods in the past few years came at mid-month on three consecutive nights beginning on the 15th.   The first event brought reports of high winds and/or large hail from 19 counties across the north one-third of the state.   The next night a band of severe storms were focused across west central into north central Iowa with large hail and/or high winds reported from 28 counties.   However, the largest severe weather outbreak by far came on the evening of the 17th when high winds and/or large hail was reported from 57 counties, mostly along and east of Interstate Highway 35.   The three nights combined to bring severe weather reports from 75 of Iowa’s 99 counties.

Spring Summary. Temperatures for the three spring months of March, April and May averaged 48.8° or 0.5° above normal while precipitation totaled 11.28 inches or 1.06 inches above normal.   This ranks as the 29th warmest and 32nd wettest spring among 145 years of records.

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

                 2017____________________________2016____________
                 MAY  APR  MAR  FEB  JAN  DEC  NOV  OCT  SEP  AUG
Maximum High:     36    5   92  546   43   40  226  242  225   32
Minimum High:    189  122   22    0   36   44   21    8   15   53
Maximum Low:      13   43   43  154  367   70  125  184  338   83
Minimum Low:       5   12   42    0   23   99   15    6    4   14
Maximum Precip:   39   97   40   55  219  167   85  118  295  196

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

                 2017                             2016             
                 MAY   APR   MAR   FEB   JAN   DEC   NOV   OCT   SEP
High Temp Bias  -0.2  -0.3  -1.0  -0.9   0.2   0.3   0.2   0.1   0.1
High Temp STD    M     M     M     M     2.0   2.0   2.1   2.2   2.0
Low  Temp Bias  -0.2  -0.3  -0.4  -0.2   0.0   0.1   0.0   0.2   0.1
Low  Temp STD    M     M     M     M     2.3   2.3   2.8   2.5   2.3
Precip    Bias   0.07  0.02  0.05  0.05  0.01 -0.02  0.00  0.00 -0.01
Precip    STD    0.42  0.49  0.39  0.33  0.07  0.07  0.09  0.12  0.27

The standard deviations are missing this month due to some new code that is not behaving as I expected. Just haven't allocated time to figure out why yet, sorry.