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Preview: Iowa Environmental Mesonet Daily Feature

Iowa Environmental Mesonet Daily Feature



Iowa Environmental Mesonet Daily Feature



Last Build Date: Tue, 30 May 2017 01:53:02 -0500

 



Most since 2006
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Unlike the past two years, the amount of severe weather we have seen in Iowa so far this year has been more active. The featured chart displays the number of Severe Thunderstorm Warnings issued by NWS Des Moines for the year to date period each year. This year's totals is the largest since 2006. A big caveat is that the mechanism for issuing warnings has changed over the years. Setting that aside, there are some limited chances of severe weather in Iowa over the holiday weekend, but the most significant storms look to stay to our south, especially on Saturday.

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Opposite Effect
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The clouds have been somewhat difficult to keep away these past few days thanks to a large storm system spinning now off to our south and east. What sunshine we have seen often goes into self-destructing sunshine mode as heating quickly builds clouds and blocks the sunshine again. The impact on temperatures during the day light hours is clear as clouds tend to suppress temperatures as shown by the featured chart for Des Moines. The chart compares hourly temperatures by week of the year between the case of having mostly cloudy conditions vs all cloud conditions. Values in green indicate cooler temperatures with clouds and red indicates warmer. During the overnight hours, the effect is the opposite than during the daytime. Clouds act as a blanket during the night time by absorbing long wave radiation and emitting it back to the ground.

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Cold Air Above Us
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Tuesday was a return to chilly weather that has plagued our recent stretch of weather. Even with these chilly temperatures, convective clouds were able to form and bring brief shots of rain to the state and even a few reports of funnel clouds. The reason is that the air above us was relatively cool as well, so any bit of heating that the sunshine was able to produce resulted in rising parcels quickly reaching their lifted condensation levels and forming clouds. This rapid vertical motion with low cloud bases also promotes funnel clouds. Anyway, the featured plot shows mandatory sounding level temperature percentiles for the sounding Tuesday evening from Omaha. The lower atmosphere profile is dominated by cold percentiles around 10% and less. Interestingly, this reverses at levels above the tropopause.

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23 Years of RADAR Data
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Showers and a few thunderstorms returned to the state Monday evening. Of course having such storms is certain what one may expect for this time of year. The featured chart displays a NEXRAD composite for each of the past 23 years at 7 PM over Iowa. It is amazing to think we have such a long term record of NEXRAD data built-up now. RADAR data did exist prior to 1995, but the availability, formats, and legacy platform make it somewhat difficult to utilize today.

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Chilly Four Days
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After a stretch of very warm weather last week, the past four days have been a struggle. The featured chart displays the average high temperature for Des Moines between the 18 through 21 May period each year on record for the site. This year's value of 63 degrees is near the coldest shown on the chart. The mostly chilly weather looks to stick around this week with more chances of rain.

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Severe Thunderstorm Timing
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Late Wednesday afternoon, severe thunderstorms raced through central Iowa prompting the NWS to issue a number of Severe Thunderstorm Warnings. These warnings were issued during the time of day that has approximately the highest temporal frequencies as shown by the featured chart for Story County. The plot presents frequencies on a minute- by-minute basis. It is kind of interesting to see no events at around 8 AM. Our severe weather typically develops to our west during peak afternoon solar heating and then trucks it into our area late evening. The overnight storms are a combination of these storms and those forced by something called the Low Level Jet, which pumps muggy air into the state fueling elevated thunderstorms with associated wind and hail threats.

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Yesterday's Severe Warnings
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While the high end tornado threat did not pan out, there was no shortage of severe weather in the state on Wednesday. The featured map displays a heat map of storm based Severe Thunderstorm Warnings issued between 7 AM and midnight. Some isolated areas got five Severe Warnings (orange color) yesterday! The storms produced many reports of wind damage and even some large hail. There are additional chances of more storms this week, but nothing as severe as we saw yesterday.

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Reaching 90+ in May
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The high temperature for Des Moines on Monday reached 91 degrees and just barely missed 90 on Tuesday with a high of 89. Such warm temperatures are somewhat rare for May as shown by the featured chart depicting monthly percentiles and return intervals for having a daily high temperature 90+. The return interval of 1.3 years for May is approximately three total days over a four year period. Both 2015 and 2016 failed to yield a 90+ temperature for Des Moines in May.

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Decreasing Variability
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The featured chart displays three metrics assessing the daily high temperatures for Ames. The top panel shows the daily high temperature standard deviation and the other line shows the day to day variability in high temperature. The bottom panel shows the ratio between these two metrics. The annual cycle is most noticeable with both variances decreasing into the summer season. The daily climatology variance is shown to be higher than the day to day variance. This is sort of a back handed way to show that the day to day change in our weather is less variable than overall climatology for a given day. Rewording, our high temperature tomorrow is more likely to be close to today's than for the date last year. The downward trend into the summer season is the impact of increased humidity, which acts to dampen the magnitude of the daily temperature swings.

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Warm Mothers Day
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The warmth you expressed to your mother hopefully matched the warm temperature present for Iowa on Sunday. The featured chart displays high temperatures on Mothers Day for Des Moines. The high of 85 yesterday was the warmest high since 1991. It was also well warmer than the long term average of 70 degrees. More warmth and a return of thunderstorms are in the forecast for this week.

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Precipitation since 10 April
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Estimated precipitation totals from the MRMS project for the past month are shown on the featured map. Iowa is depicted in-between the extreme totals over southern Missouri and dry conditions to our northwest. Totals for Iowa are generally in the 3 to 8 inches range. Wet and cool conditions have made for a struggle to get the agricultural crops planted. The current near term forecast keeps the heaviest totals off to our north and south for the next week.

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Watches
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NWS Des Moines issued two watches yesterday, a Severe Thunderstorm Watch and a Flash Flood Watch. The eventual storms yesterday did yield a few Severe Thunderstorm Warnings over the watch area and even a few tornadoes further east within Davenport NWS's area, but no Flash Flood Warnings. The featured chart looks at some IEM computed metrics on the conversion of watches into warnings. The red bars represent the overall frequency of converting an individual watch event into a warning (meaning getting at least one warning anywhere within the watch). The blue bars represent the individual county/zone conversion of a watch into a warning. For example, there is a 59% chance of getting at least one warning within the Flash Flood Watch yesterday and 19% frequency of an individual county within the watch getting one warning. The moral of this story is that not all watches are created equal. The actual size of the event the watch is for creates large differences in the frequencies. One item to note is that this plot is for a straight conversion and does not consider the situation of the eventual warning being of a different type. For example, some Winter Storm Watches become Blizzard Warnings.




Cold Side of Chart
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The featured chart presents the progression of temperature and precipitation departures, expressed in sigma terms, since the beginning of February for Ames. For every seventh day, the trailing fourteen day departure is plotted. The chart is physically bounded, so the fourteen day window of averaging tends to yield a cyclical pattern. The most two recent points on the plot show the trend to colder departures. The recent few days of warmer weather have helped somewhat to dampen that trend, but for the fourteen day window the departure is still over 1 standard deviation from average.

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mid 60s to lower 90s
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High temperatures on Monday exhibited quite the contrast.as shown by the featured map of automated airport weather station reports. A warm front ushered in hot and somewhat muggy air into the state, but did not make it through all of eastern Iowa. The result was for highs in the mid 60s there and some low 90s in western Iowa. The current forecast does not have 90s return for any parts of the state for the near term.

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Limited 75 Plus
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The weather this past weekend was spectacular with mild temperatures, clear skies, and light breezes. Southerly winds did help to bring even warmer temperatures into western Iowa on Sunday. The high at Des Moines was able to reach 80 degrees. Temperatures this warm have been a bit below average so far this year as shown by the featured chart. The blue dots represent the average number of hourly temperature reports at or above the given temperature. The red dots are so far for 2017. For levels above 75 degrees, this year has been below average while for levels below 75, this year has been well above average. So we have enjoyed mild conditions so far this year without seeing significant amounts of really warm temperatures.

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Within Two Degrees
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The high temperature for Ames on Thursday was 69 degrees, which was within two degrees of the current climatology for the date. How common is it for our high temperature to be that close to average. The featured chart presents the frequency of having the daily high and low within two degrees of average for Ames. The frequencies shown are generally around 20%, which is one in five days. So we are typically outside of that range. This plot sort of shows how the simple daily average temperature can be a misleading metric as the actual value is typically more than 2 degrees away from it.

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Farewell Jefferson Webcam
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The saying is that you'll never forget your first and within the context of IEM archived webcam imagery, the KCCI-TV webcam in Jefferson was it. Back in 2003 an unique partnership was forged for the IEM to maintain an archive of the webcam imagery and about 1.5 million images later, the running archive for the site will come to an end. This webcam was replaced by an alternative solution. This location has been the most reliable in the network and provided some of the best lapses captured over the years. Cheers!




Sunshine Return Helps
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Bright sunshine and clear skies were a welcome sight for much of Iowa on Tuesday. The appearance of the sun broke a recent streak of cloudy and chilly conditions that had driven soil temperatures well below temperatures levels needed to sustain seed germination. The three panel plot presents data from the ISU Soil Moisture Network station at the ISU Ag Farm west of Ames. The bottom panel really tells the story of recent days with limited solar radiation and chilly temperatures keeping four inch depth soil temperatures at chilly levels. Full sunshine on Tuesday was able to rocket soil temperatures back to levels above 50 degrees. The current near term forecast looks very good for Iowa to get back on track with warmer and drier conditions.

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Undular Bore
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The powerful storm system that brought tornadoes, heavy snowfall, and flooding rains to the US this past weekend also produced an undular bore over Iowa on Sunday evening. These events are not atypical of strongly forced storm systems and typically are harmless only producing neat cloud features and shifts in wind speed, direction and air pressure. The featured plot presents 1 second interval data from the "Hamilton County" ISU Tall Towers site whose data will be featured on this website more prominently soon. The data shown is for the 10 meter above ground sensor and shows the jump in pressure associated with the bore and then oscillation afterwards. The drop in wind speed and shift to southwesterly is interesting to see.

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April Daily Winds
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April is in the books and is certainly went out on a cold, wet, and windy foot. The featured chart displays daily averaged wind speed and direction for Ames. The little arrow at the top of each bar represent the vector averaged wind direction. The averaged directions show a neat cyclic pattern as weather systems transverse the state during the month and wind directions respond in kind. It is also interesting to see that most of the windiest days came during the last half of April.

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