Our recent stretch of hot weather has thankfully come to an end. Des Moines topped out at 104 degrees on Friday, which was the warmest temperature on record this late in the year. The featured chart displays the latest date of the year for a given high temperature. The 104 for Des Moines set the latest date for the 104, 103, and 102 temperature levels. The year of the temperature record is shown as well. Temperatures are expected back into the 90s next week with next to zero chances of precipitation.

The featured chart shows the evolution of aridity since mid April evaluated on a weekly basis for a trailing 14 day period. The weekly values are connected in time to show the trajectory of the change in aridity. This type of chart has a nice property of tending to make circles as temperature and precipitation departures tend to influence each other. The chart shows an interesting transition from 1 Jul to 8 Jul, where we went from above positive precipitation to negative and have not make it back. In fact, it it kind of interesting to see how we've gone from a cold and dry to a hot and dry situation with the precipitation departure holding close to negative one sigma. The line is nearly horizontal going from 19 August to 2 September. The forecast does not hold much hope for positive precipitation departures with more hot weather on the way next week.

The current drought situation in the state is very complex as shown by the featured chart displaying trailing departures for IEM computed statewide areal averaged precipitation. While the 365 day window is slightly above long term average, the 30 day and 90 day are well below. The wettest spring season on record for the state is what is currently allowing most crops in the state to hold on. The chart shows an interesting combination of the 365 day departures becoming less negative while the 30 and 90 day go the opposite direction. The reason being the wet spring replacing a much drier period during the previous year and that same period falling out of the 30 and 90 day windows. So while the dramatic drop in 90 day departure screams a flash drought, the long term departure is being bolstered by the wet spring.

Will 2013 be remembered for being a wet year or a dry year or both? The featured chart displays the statewide accumulated precipitation for this year, last year, 1993 (wettest year), and 1988 (driest year). The shading represents the range of values since 1893. Around the end of June, this year was the wettest to date on record, including 1993. The weather since then has been very dry and how the year to date accumulation is approaching merely average with 1993 some 15 inches higher! The forecast continues to hold little hope of widespread precipitation.

For the month to date, the average temperature for 1-8 September has been warmer than 1-8 July for Des Moines. The featured chart shows the frequency of this happening for the first of each month to the given date. For the 8th, about one in five years have seen this happen. The forecast has parts of Iowa near 100 degree today with cooler weather expected by the end of the week. If Des Moines were to hit 100 degrees today, it would be the latest into the year for such a warm temperature.

Monday was one of the warmest days in September on record for Iowa with a number of locations hitting triple digits. The featured map shows reported high temperatures along with the number of years since as warm a temperature was reported in September. Waterloo and Des Moines had the warmest temperature since 1939. Iowa City had the warmest temperature since 1901! Today looks to be another scorching hot day with highs only in the upper 90s!

While twelve years in the past, still seems like yesterday in our memories. May we never forget.

Only a week after the IEM seminar looking at the difficulties of answering the simple question of how much rain did Ames get, life imitates art. The featured map presents NOAA MRMS Project estimate precipitation and local point observations of total rainfall from Tuesday and Wednesday. The map is showing the Ames metro area. The airport reported 0.30" while most of the northwestern section appeared to get just sprinkles. The MRMS estimate shows remarkable cell to cell variability with totals ranging from 0.15" to 0.4" within a few street blocks of each other. So how much rain did Ames get from this event?

The United States Drought Monitor is a hand drawn analysis of weekly drought condition in the country based on quantitative and qualitative measures of drought. The featured map shows the most recent analysis valid on the 10th. The entire state except about two counties is shown in some level of drought condition. The plotted numbers are IEM computed counts of the number of recent periods having a precipitation departure of at least negative 1.2 standard deviations from average. Four periods are shown covering the recent 30, 60, 90, and 120 days. So if a location has a four plotted, each of those periods has a precipitation departure worse than negative 1.2 sigma. The negative 1.2 level is about when "D2" drought is to be analyzed. The numbers show the most consistently dry areas of the state are from central to southeastern Iowa. Northeastern and extreme western Iowa have not been as dry.

The featured map is an analysis of precipitation accumulation percentage of normal since 1 July. For the period, the driest part of the state has been the around the Rockwell City area with less than 20% of average amount received. All of the state is shown below 70% with the extreme northwestern portions faring the best. The forecast does have chances of rain this week along with a return of 80 degree temperatures.

The featured map presents the IEM computed percent of average number of tornado and severe thunderstorm warnings issued per NWS forecast office for the year to date. The average is computed for the same year to date period over the years 2003 to 2012. Most of the Iowa forecast offices are below average for warning counts, except LaCrosse. The largest positive departures are out west with some offices 200% of average. The southeastern US is generally below average with many offices below 50%.

For places like Des Moines and Ames, it finally rained again yesterday! The Des Moines Airport weather sensor reported measurable rainfall on seven different hours, which exceeded the total to date for all of September and August. The featured chart shows the monthly climatology of number of hours with reported precipitation and this year's total. It is interesting that the climatology for January is about the same as July! Of course the nature of storms for those two months is much different and intensities are much smaller in January. It is raining again this morning in some parts of Iowa and more rain looks to be on the way this week.

Severe weather looks to be a possibility today with the passage of a cold front. The featured chart presents the combination of 500 hPa flow as observed by the Omaha sounding on September days with severe thunderstorm and/or tornado warnings issued for Iowa. The polar chart shows the wind direction and speed with the size of the dots indicating the number of warnings issued that day. 500 hPa (same unit as millibar) flow is important to thunderstorms as changing wind speed and/or direction is necessary with increasing height to separate updrafts and downdrafts in long lived thunderstorms. During the summertime, wind shear is often weak and so a downdraft will often choke out the updraft of a storm and limit its lifetime. The chart shows that at least 10-20 knots of wind speed are typically needed for severe events. The forecasted wind at 500 hPa this evening for Omaha appears to be around 30-40 knots from the WSW.

The featured map displays the combination of estimated past seven day rainfall along with the most recent US Drought Monitor analysis. Much of the state received much needed rainfall in the past week, but some of the driest locations in southeastern and northwestern Iowa missed out on the heaviest totals. The rainfall on Thursday was accompanied by damaging winds with Des Moines and suburbs experiencing considerable tree damage. The forecast has the dry weather returning with pleasant fall conditions.

Very cool overnight temperatures visited the state this past weekend with lows approaching freezing over northwestern Iowa on Saturday morning. This very cool weather was close to the most recent full moon late last week. There is a saying in weather folklore that the first freeze of the fall season often happens on the full moon as having a full moon impacts the profile of water vapor which allows temperatures to more rapidly cool. The featured chart looks at the first fall sub 29 degree temperature for Ames and the proximity to the nearest full moon. As you can clearly see, these events happen at about any time during the lunar cycle. The reason that this folklore gets perpetuated is that the first frost is almost always on a clear sky night with limited water vapor in the air, so any moon will appear very bright and crisp.

Our first day of fall was wonderful with highs in the 70s, clear skies, and low humidity. The featured chart looks at the frequency of this afternoon weather combination. The chart shows two clear peaks around late spring and early fall. This type of weather becomes fleeting the further into October we go, so enjoy it now if you can!

The hope was going into this fall that we could extend the growing season as far as possible due to the late planting dates and cool summer. So far a killing frost has yet to happen in Iowa and the featured map shows that minimum temperatures this fall are on the warm side of climatology. Most of the state is above the 70th percentile, meaning this year has been warmer than at least 70% of the previous years since 1951. The forecast looks to continue the warm weather into at least the first of October.

It is always fun to look at MODIS satellite true color imagery when there are no clouds in the sky. The featured image compares the Aqua MODIS true color image from this year and last on 23 September. The difference in color is noticeable with 2012 much browner than this year. While both years experienced a drought, last year had record warmth and this year had a cooler summer with a late spring planting date. The effect is that many fields are still green this year as the crops have not reached maturity. This is why avoiding a freeze is so important as the crops need more time.

The featured chart displays average weekly river flows for the Squaw Creek in Ames at Lincoln Way (near ISU Campus). The dry summer has river flows at zero again and resembles what happened last year. Since 1990, there isn't a comparable period to what has happened in the past two years. Between 2006 through 2010, the chart shows significant flows occurring in the late fall thanks to late season rains. The Flood of 1993 shows up nicely on this chart as the high flows lasted into the next year. The forecast does have good chances of rain for this weekend, so perhaps some water can start flowing at the gauge site again!

The featured map displays the average September temperature rank for the long term climate sites in Iowa. For Des Moines, this September is currently the third warmest on record, but other sites in the state have ranks ranging from 7 to 28. The difference is probably due to the enhanced urban heat island effect for Des Moines. Temperatures will continue to be warm to finish off September today and it'll continue into October with highs in the 80s for much of Iowa.