While Labor Day is the unofficial end of summer, the end of August is the official end of meteorological summer (the months of June, July, and August). The average temperature this year was the warmest since 1988, but only slightly warmer than last year. The featured chart presents the average summer temperature for Iowa for each year since 1893. September has gotten off to a rather warm start with highs in the 90s for most in the state.

Muggy weather returned over Labor Day weekend with humidity levels running more like what we would have expected in August. For Des Moines, the dew point on Monday was slightly higher than even the highest dew point for all of August. The featured chart shows the yearly combination of highest September versus August dew point. Points above the one to one line would represent years like this year with a higher dew point in September. This combination is somewhat rare and last happened in 1998.

Tuesday was yet another very warm day this year and humidity came along for the ride this time pushing heat indices over 100 for much of Iowa. This is somewhat rare to occur in September with the last occurrence for Des Moines being back in the year 2000. The featured map is a simple analysis of computed heat indices from the primary automated weather stations in the state. All of this heat and humidity helped to fire numerous rounds of severe thunderstorms over night and into this morning.

With one of the warmest summers on record, one would not expect there to be a lack of growing degree days. Sure enough, the featured image shows that full season departures have been well above normal and as measured by mid July days (right hand axis) we are about 10 days ahead of normal this year. The black line shows the maximum departure on record since 1893 for the period from the given date to 5 September.

Our recent weather has seen a stormy change when compared with the drought filled summer. One reflection of this is shown by the featured chart of accumulated severe thunderstorm and tornado warnings for each of the past five years. The top chart shows the nationwide accumulation while the bottom chart is for warnings issued by the NWS offices with Iowa coverage (includes some warnings slightly outside of Iowa). A logical correlation with drought conditions is the lack of heavy rainfall associated with severe thunderstorms. After being ahead of each year in late May for Iowa (purple line), the lack of many summertime storms now has 2012 as being the least of the past five years in early September.

The updated United States Drought Monitor last week included the worst category of D4 (Exceptional) for a very small portion of the state near Sioux City. This is the first D4 designation in Iowa since at least the year 2000. The featured chart presents the weekly areal coverage of the drought categories since 2000. The small D4 percentage is not legible on this chart and hopefully it does not increase in size going forward! The D3 (Extreme) category is also at the largest percentage for Iowa since 2000.

Eleven years ago seems just like yesterday. May we never forget!

Tuesday was yet another very warm day in Iowa with highs pushing 90 degrees for much of the state. For Des Moines, Tuesday tied the previous record for most days with high temperature at or above 82 and 83 degrees. The featured chart presents the maximum number of days per year at or above a given temperature threshold. The current maximum number of days is shown in gold and the 2012 total so far is shown in cardinal (Go Cyclones!). High temperatures today will come close to breaking the tie at the 82 and 83 thresholds. Keep in mind that we still have 3.5 months left to potentially break some of these other thresholds.

Yesterday's feature compared 2012 total number of days above a given high temperature threshold for Des Moines with the maximum for any year. Today's feature is a slight modification showing the number of days above a given threshold for every year for the period up till 11 September. For temperatures above 100, 1934 and 1936 stand apart from any other year. But for lower temperature thresholds, this year shows a remarkable departure from other years. Much of this was thanks to one of the most extremely warm months on record, March 2012. The forecast for the next week has our weather turning much more fall like and very comfortable!

Des Moines experienced its first day with a high temperature below 70 degrees on Thursday since the first of July. The featured chart presents the first day each year after 1 July that the high temperature was below 70. The latest date for this event was 24 September 1931. Temperatures will warm back into the 70s today and near perfect weather around 80 degrees this weekend.

This weekend saw very pleasant temperatures with highs in the upper 70s and 80s. A cold front is set to bring a regime change to our weather today. Will this be the last we see of the 80s? The featured chart looks at the warmest high temperature after having a high temperature at or above a given threshold for the period including the rest of the calendar year. The maximum, average, and minimum of these yearly values is presented. The point of the chart is that typically these temperature step-downs in the fall happen in about 3-5 degree increments. The minimum values show that some years can see dramatic changes with highs in the low 70s and then not broaching 50 degrees for the rest of the year. These events happen much later in the year than September.

Monday was one of the rare occasions this year to have a high temperature below normal, which started a streak of one day in a row below normal! The featured chart presents the streaks of above or below (negative numbers) daily high temperatures for Des Moines. This year is presented along with 3 famously warm years of the past. This year has yet to see a streak over 10 days below normal, whereas each of the other years shown have seen at least one. The longest of these streaks below average tend to occur in the winter time with significant snow cover. Our current streak looks to end on Wednesday with highs back near 80.

Monday morning saw our coolest temperatures of the season so far with some very limited areas seeing temperatures at or just below freezing. The featured chart presents an IEM computed analysis of the areal fraction of the state that has experienced a temperature below 32 degrees prior to a given date. 2012 has yet to appear on this chart as the freeze area was negligible yesterday. The longer term averages are computed over two periods for comparison. The two hot and dry summers of 1988 and 1936 are shown as well for comparison. It is certainly the time of year to start seeing freezing temperatures in the state.

This chart has been featured a number of times this summer season as it shows the chances that this year will become the warmest on record for Des Moines. The bottom charts shows that while we faltered some with some cool weather in August, the warm weather in September has boosted us back up to the best chances yet that this year will beat 1931 (the current warmest). The brown spaghetti lines represent all previous years appended onto this year until the end of the year and so represents scenarios if we have any reasonable repeat of previous years. Using these scenarios, 86% of previous years appended onto this one would result in 2012 being the warmest on record. These lines would also suggest that 2012 will remain warmer than 1931 until at least the first week of December.

A reinforcing shot of cold air is set to arrive this weekend and some weather models are predicting very cold temperatures on Sunday morning. The featured map is from the NAM hi-res conus nest model showing forecasted near surface air temperatures on Sunday morning at 7 AM. The resolution of this model is sufficient to attempt to resolve subtle topographic effects on temperature in Iowa. You will also notice the heat island of Des Moines visible as well. The warmer areas in eastern Iowa are probably under some clouds, which help to support the temperature at night time.

Temperatures on Sunday morning dropped below freezing for much of the state. On the other hand, soil temperatures remain quite warm with the average temperature at 4 inch depth around 60 degrees. The sun has worked very hard all summer long to warm the soils and unlike the air, it takes a bit longer to lose that heat to the fall season. These warm soils help to create very pleasant fall days when the sun shines as will be the case this week. The sun has a head start warming the top soil in the morning and can quickly warm the dry fall air near the surface. A southerly breeze will also help the process out with highs near 80 on Tuesday.

After temperatures set or tied record low temperatures on Monday morning over portions of eastern Iowa, sunshine and warmer air pushed temperatures well into the 70s for the afternoon. Most of eastern Iowa saw a temperature increase by over 40 degrees for the day. The featured map displays the difference between the high and low temperature on Monday along with the year of the previous date with an as large temperature difference. A number of locations have not seen such a strong warm up since 2008. Forest City is shown with the largest change of 49 degrees, which is the locally largest since 2000.

Tuesday was yet another very warm day in Iowa this year. The high temperature hit 85 degrees in Des Moines and set the local record for most number of days at or above 80 degrees in a year. The featured chart presents the maximum number of days per year at or above the given threshold. 2012 is on pace to rewrite this metric for temperatures in the 70s and 80s. Days in the 70s and 80s remain in the immediate term forecast along with dry weather.

While daily high temperatures have certainly been on the warm side of normal, overnight low temperatures have been very cool and the net result is that this month looks to be the second in a row that is slightly below long term average for monthly average temperature. The featured chart presents monthly average temperature departures from current monthly climatology since 2000. March 2012 and December 2000 certainly stand out in this plot for being the most extreme.

The weekly update to the US Drought Monitor is out and there is no significant improvement to report in the state. Of course, it would need to rain first for things to improve and rain has been difficult to come by this September. All of the state remains in at least the D2 (Severe) categorical drought. The forecast has little hope of rain and temperatures look to warm into next week. The dry and warm weather has allowed the crops to be harvested at a record rate. The somewhat early frost has even helped to speed this process up some more!