The featured plot presents a summary of two humidity metrics for the month of July for Des Moines. The top panel shows the average mixing ratio of water vapor to air and the bottom panel shows the highest dew point report for the month for each year since 1935. This year was on the dry side of both metrics and the driest since 2009, which was a cool July as well. Muggy air did return to the state to end July and looks to stick around for the start of August, air temperatures will not be that bad though in the lower 80s.

From a cold and extremely wet spring, to snow in May and the currently developing drought, the corn crop has had no shortage of stress thrown at it this year. While the recent period has been dry, it has also been cool during an important period in corn development after tasseling. The featured chart assumes that the corn crop was planted on 1 May each year and then plots the number of days between the VT and R2 corn development stage based on growing degree days. The bars are then colored based on whether the Story County (Ames) corn yield that year was above or below the trend line. Obviously, one metric is not a perfect predictor of yield, but it does appear having a longer period helps yield.

The featured chart is of one minute interval precipitation from the Philadelphia Airport for last Sunday. 8.02 inches fell on the calendar day making for a local record daily precipitation total. The chart shows that most of the 8 inches fell over a four hour period between 3 and 7 PM. The most intense rainfall came between 3:33 and 3:40 with almost an inch falling in that period.

For Ames, July ended up on the cool side of average for temperature. The featured chart presents the combination of August vs July average temperature for each year since 1893. A number of extreme years are labelled. While some recent feature plots have shown this year trending like 1947, the near term forecast does not indicate this August will be as warm as that. We could certainly use rain more than anything right now.

When the sun heats the ground, the ground heats the lower atmosphere which drives a mixing process with warm air rising and cold air sinking. This mixing process grows vertically creating a well mixed layer. If this mixing encounters colder air aloft, relatively more heating needs to occur to continue raising the temperature of the mixed layer. If this mixing encounters warmer air aloft, the heating process can accelerate as temperatures are already warm. Atmospheric soundings sample the atmosphere twice per day. The morning sounding provides a sampling of the air above our heads that may be mixed down during the day. A informal rule of forecasting in the plains is to add 13 Celsius to the 850 hPa (millibar) level temperature to provide an afternoon high temperature. For example, the morning sounding on Monday for Omaha had 16.2°C at 850 hPa and when you add 13°C to that reading it yields a high temperature of 85°F. The actual high was 87°F! The featured chart looks at the monthly distribution of change in afternoon high temperatures from the 850 hPa morning temperature. The green bar represents 13°C. The box plots provide detail on the temperature distributions and for the warm six months of the year, this relationship closely approximates the monthly mean. The cold months are dominated by horizontal advection processes and also have decreased solar heating, so this rule does not apply well then.

Outside of the very wet summer for northeastern Iowa, rainfall has been hit or miss for the rest of the state as shown by the featured map of the difference between the 1 June through 6 August precipitation total for 2013 vs 2012. Having the wettest spring on record for Iowa is currently helping the agricultural crops limp through this dry period of summer. There are more chances of rain in the forecast, but the heaviest totals look to be south of Iowa.

Growing Degree Days are commonly used to track crop development. For "105 day" corn, about 2600 GDD units are necessary to reach maturity. The cool and wet spring combined with a early May snowfall made it difficult for corn to get into the ground on time in Iowa. Planting corn deep into May and June puts the crop at risk of not accumulating enough GDDs prior to the fall freeze. The featured chart attempts to provide probabilities of accumulating GDDs based on the combination of growing season begin (planting / crop emergence) and end (first freeze) date. The left hand plot shows the overall frequencies based on yearly data for Ames since 1893. The right hand plot combines what has already happened this year with scenarios based on all previous years for 7 August and on. For example, for a corn crop emerging on 15 May, about 20% of the years got to 2600 GDDs by 15 September prior to the first fall freeze. Given this year's condition, for a corn crop emerging on 15 May, there is near no chance that 2600 GDDs can be reached by 15 September and only about 50% of all years will get us to 2600 GDDs at all. One mitigation is that as planting dates get later, farmers will plant hybrids with shorter maturities (less GDD requirements). Although, this seed change is not always logistically possible. The chart would indicate that we need to make it through much of September without a freeze to give the longer maturities a chance.

The featured map presents total precipitation since the first of July as estimated by NOAA's stage IV product. Much of western Iowa is shown in the less than an inch up to two inches for the period. You do not have to travel far to find extremely high totals in eastern Kansas and southern Missouri with amounts upwards of 20 inches! The forecast looks to continue this pattern, but there will be chances for rain in Iowa.

The featured chart shows the hourly precipitation totals for Des Moines from 1 June up until 12 AM 12 August for this year and the long term climatology. Only the 11 PM to midnight hour has seen a total above the long term average. A majority of our precipitation during the summer season comes at night and that has certainly been missing this year. This chart does not include the seven tenths that fell so far this Monday morning. Unfortunately the forecast looks dry for the next 7-10 days with the heaviest rainfall again off to our south.

Dew Point temperatures are back in the 50s this Tuesday morning allowing the air temperature to cool into the 50s as well. It seems our periods of very muggy weather this summer have been brief. The featured chart presents the percentage of hours since 1 June that the dew point temperature for Des Moines was at or above the given level. A climatological average is shown by the black line and two extreme years are shown as well. Indeed the amount of time at high dew points has been below average this year. The bottom chart shows the percentage of time that the dew point was at or above 50 degrees. Since 2001, each year has been greater than 90%.

Atmospheric soundings provide twice daily vertical profiles of the troposphere and lower stratosphere. The lapse rate is calculated from this profile as the rate of temperature change with height. The featured chart presents the monthly distribution of the lapse rate between 850 and 500 hPa (millibar) pressure levels for the Omaha sounding site. This is roughly from a typical cloud base to midway vertically through the cloud. When this lapse rate is below 6 °C per kilometer, the atmosphere is in a stable condition meaning vertical temperature gradients are not large enough to drive robust motions due to buoyancy. When values are between about 6 and 9.5, the atmosphere is conditionally unstable and vertical motions are likely to occur. Values above about 9.5 are said to be absolutely unstable and vertical mixing will happen quickly. The chart shows the strongest instabilities during the summer season, which is also the time of year for thunderstorms. Of course, there is a direct relationship between thunderstorms and lapse rates. Stability increases (lower lapse rate) in the winter time and so we see fewer thunderstorms. For comparison, recent lapse rates have been in the mid 5s, which is not high enough for robust storms.

The featured chart shows the daily climatology of growing degree days along with the actual values for 2013. The past three weeks have been a struggle with most days only accumulating 10-20 units. Given the late planting dates for crops in the state, having cool weather this time of year is actually the better of two evils. Having hot weather and little precipitation would have imposed heat stress on the crop. Instead the cool and dry weather has allowed the crop to limp along as it matures. While the near term benefits of cool weather are good, an early fall frost is of now of concern more than ever.

While up to three inches of rain fell on Thursday over a few counties in southwestern Iowa, the rest of the state, including Des Moines, got a big fat zero! It seems like Des Moines has missed out on the rainfall events this summer and the featured chart illustrates it. The chart shows the percentage of days during Jun, Jul, and Aug where some location in the state picked up an inch of rain and Des Moines reported over 0.5 inch as well on the same day. The percentage this year is the lowest since 1951! While heavy rainfall events have occurred this summer, some locations in the state, like Des Moines, have consistently missed out. Some might call this being jinxed.

The Iowa State Fair wrapped up on Sunday with ten days of near perfect weather. Humidity levels were low, temperatures were in the low 80s for most days, and a northerly breeze blew. The featured chart looks at the weather reported by the Des Moines weather station for the period of the Iowa State Fair back to 1880. This year was actually slightly warmer than last year for both high and low temperature! You may recall that after a brutally hot July in 2012, the period of the fair was very pleasant. The forecast has heat and humidity building back this week.

More summer like conditions have finally returned to Iowa after a number of weeks of very cool weather. The cool weather has provided a substantial deficit of growing degree days. The featured chart shows the number of days of given weather it would take to catch us back up to average for GDD accumulation since 1 May. The green bars are for a scenario whereby we accumulate 120% GDDs of average each day and under such a scenario we only get back to average by the first of October! The red bars assume daily record GDD accumulations based on long term data and this unrealistic scenario only gets us back to average by the first week of September. This deficit is not a terrible thing as the crop growth situation in the state is very complex at the moment. The cool weather was a good thing given the lack of rain for most of the state as it mitigated water stress. While the weather turning warmer will help GDDs and crop maturity, the lack of moisture will mean water stress should dramatically increase. The further complication is that very little corn was planted by 1 May, with most of it emerging by mid to late May.

Warmer and muggier weather is back in Iowa with dewpoint temperatures in the upper 60s and lower 70s. The increase in moisture means we are done with the overnight lows in the 50s as the dewpoint puts a floor in on how low the temperature can drop. The reason being that once the air reaches saturation, fog will form and temperatures will be stabilized as long wave radiation is absorbed and heat is released as water vapor condenses. The featured chart looks at the relationship between the 4 PM dewpoint temperature and then the morning low temperature for the next day. The top chart shows the percentage of days the low temperature is within 3 degrees of the 4 PM dewpoint. The middle chart shows the frequency of the low temperature being colder than the 4 PM dewpoint. The bottom chart shows the overall bias of the dewpoint and the next day low temperature. The reasons that this relationship does not always work include: winds can create mixing that prevents temperatures from cooling to the dewpoint, clouds prevent heat from escaping, and/or advection processes can transport more humid or drier air masses changing the dew point overnight.

Same plot for Ames and Cedar Rapids

Yesterday's feature looked at the afternoon dew point being an indicator of the next day's low temperature. The theory being that the air temperature would cool off to the dew point temperature and thus be the low temperature. Today's feature looks at the distribution of dewpoint depression at the time of the daily low temperature for each of the ASOS sites in the state. The dewpoint depression is simply the difference between the air and dewpoint temperature. Differences between sites are likely due to micro-climate effects as each ASOS has different surroundings which include soil types, vegetation, and amount of urban area / concrete around it. The chart shows a large difference between Ames and Des Moines with the median dewpoint depression two degrees less at Ames. The first guess explanation for this difference is the larger urban heat island effect at Des Moines, which would tend to keep the temperature higher and further away from the dewpoint than surrounding locations. Also, the Ames ASOS is located in a topographic depression which would promote cooler temperatures.

Additional plots: same plot for high temperatures, Ames weekly dewpoint depression, and weekly for Des Moines.

The updated US Drought Monitor released on Thursday shows most of west- central and central Iowa in moderate drought status. The featured map shows the drought monitor along with two day rainfall from our most recent event. The heaviest totals were an inch or two and mostly missed the D1 area. While this rainfall will not dent the drought as we need an inch or two every week just to keep pace with climatology, every little bit helps! The bad news is that our chances of widespread rain are over for the immediate future and the weather is about to get very hot, which is increase the stress on moisture starved plants.

The status of this year's corn and soybean crop is about to get very interesting. The featured chart contains model estimates of soil moisture within the top 1.5 meters of soil for a location near Rockwell City. The cool weather of the past few weeks and the lone rain shower earlier in August have bought some time for crops in this area, but the available soil moisture is about gone and the hottest weather of the year has arrived. Plants are not able to access all soil moisture found within their root's reach, so while the chart does not show zero for soil moisture it is practically that for the plant. The Rockwell City area has been one of the driest locations since the first of July with a total of only a half inch or so. Rainfall shows up in the chart as brief upticks in soil moisture. The wet year of 2010 is shown for comparison.

The featured chart shows NOAA estimated precipitation totals for August through the 26th. Much of the southern half of the state is shown well below an inch total for the month. The forecast has near zero hope of precipitation for the next week, so these totals look to be it for August. Today does look to be the hottest day of the period with temperatures nearing 100 degrees.

The heat is on for late August with high temperatures nearing 100 degrees in Iowa. This hot weather is thanks to a strong ridge of higher pressure centered just to our south. Ridge is a meteorological term that describes an area of higher pressure that tends to create sinking motions in the atmosphere and steer storm systems around it. The featured map presents a composite of 18 August events where the high temperature reached 100 degrees for Des Moines. The field plotted is geopotential height at 500 millibars. This is a measure of the depth of the atmosphere up to 500 mb. The composite field shows approximately our current weather situation with a strong ridge centered just to our south. The approximate geographic center of the ridge is plotted on the map as an x for each case. Our weather will remain hot and dry as long as the strong ridge remains in place.

The high temperature for Des Moines has reached 99 for each of the past three days. This ties the longest streak of 99 for the site set in July 1916. The featured chart presents the longest streak of consecutive days at a given high temperature. Five days is the maximum for any temperature. Hitting 99 again today is possible, but it would appear the temperature will fall short of it by a few degrees.

The featured chart displays the ratio of total statewide averaged precipitation in April and May to that in July and August of the same year. The ratio this year is well over four meaning that four times as much precipitation fell this April and May versus this July and August, which is by far the largest ratio shown since 1893. The long term average is close to one meaning these paired two months receive about the same total, but the year to year variability is high. The good news is that the heat looks to break in time for the start of September, but significant rainfall is not expected.