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Preview: Iowa Environmental Mesonet Daily Feature

Iowa Environmental Mesonet Daily Feature



Iowa Environmental Mesonet Daily Feature



Last Build Date: Thu, 21 Sep 2017 21:36:02 -0500

 



Drought Monitor Changes
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The weekly update to the US Drought Monitor was released this morning. The featured chart looks at the weekly change in drought coverage over Iowa since the product archive began in 2000. The height of the curves are a bit complex to explain and the y-axis units are being conflated with a value of 1 representing the area of the state improving or degrading one drought category. The bottom series represents this year and you can see that most of this summer has featured degrading conditions with only some improvement in August with some much needed rains falling over western Iowa.

Generate This Chart on IEM Website




85+ in September
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Temperatures were warm on Tuesday along with some rather muggy conditions for this time of the year. The high temperature for Des Moines reached 85 degrees after a morning low of 62. The featured chart looks at the hourly temperature distributions on days in September with the high temperature at least 85 degrees for the site. The boxes represent the 25th and 75th percentile and the whiskers are the 5th and 95th. It is kind of interesting to note the tight temperature ranges overnight on these days. Having temperatures in the 60s to start off the day are the most common. In some cases, we can warm out of starts in the upper 40s, but it is rare. The forecast looks to continue today's trend of 60s for lows and 80s for highs as it will feel like summer for a bit longer.

Generate This Chart on IEM Website




Many warm, few very hot days
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The featured chart looks at the number of days this year with the daily high temperature at or above a given threshold. The blue bars and labels represent the largest number of days and year at that level. For Des Moines, 2017 has seen plenty of warm days with values in the 70 to 80 degree range rather close to the largest values on record. But for warmer temperatures at 85 degrees and higher, this year has not even been close to previous warmest years. The forecast does have us adding days at these warmer temperatures for the rest of the week and even some low 90s are possible!

Generate This Chart on IEM Website




September warmer than August
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September has seen its share of warm weather, more so than what we experienced for much of August. There is a chance that this September will end up with a warmer average temperature than August. The featured chart looks at the frequency of having one month's average temperature warmer than another month for that same year. The most recent year that this happened is included for the lowest frequency cases. For the combination of interest for this daily feature, the last occurrence was in 2004 and it has happened three times overall for Ames. So this situation is somewhat rare, but not unheard of.

Generate This Chart on IEM Website




Ottumwa Precip Ranks
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Most of south central Iowa has seen very little rainfall this growing season. The featured chart looks at the precipitation statistics for Ottumwa (SE of Des Moines). The chart shows the accumulations from yesterday back to the given date, so the chart can be interrupted from right to left. The blue line represents the rank for that period with 1 being the wettest and when the line touches the dashed line, it is the driest on record. The chart shows that the entire period has seen dry conditions with accumulations since June being the driest on record. Chances of rain are at least in the forecast again, but it remains to be seen how much will fall over the driest areas of the state.

Generate This Chart on IEM Website




Days to get two inches
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The featured map displays the number of days you have to go back in time before accumulating two inches of rainfall since yesterday. The map shows a very localized area between Des Moines and Ottumwa that have been extremely dry this summer. The small area in black would be over 80 days (more than 2 months!). There are plenty of other dry areas analyzed in the state as well. Having to go 50+ days during the growing season with two inches or less of rain is very dry as well.

Generate This Chart on IEM Website




Flash Drought
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The drought situation in the state continues to be very complex and difficult to analyze. A case in point is the featured image for Elkader (far NE Iowa) showing trailing 31, 91 and 365 day precipitation departures. Check out what has happened with the 31 day departure, it went from approximately +10 inches to now -3 inches in just over a month's time. That is what happens after an extremely wet period is followed up extreme dryness. The 91 and 365 departures remain well above average, so what is its current drought status? It will be interesting to see the weekly US Drought Monitor release on Thursday to see what is done in this area.

Generate This Chart on IEM Website




Dry Two Weeks
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The featured map presents an analysis yesterday of the number of days since the location observed measurable rain. The map shows most of Iowa in the two week range of values with those zeros in northwestern Iowa only amounting to a few hundredths reported on Sunday. Sometimes tropical storms are able to work their way north into Iowa, but we won't have such luck this year with neither Harvey nor Irma. About our only chance of rain in the near term is on Saturday evening, but it doesn't look all that exciting either. Our boring stretch of weather looks to continue!

Generate This Chart on IEM Website




Remembering September 11th
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May we never forget the day and the lives that were lost that fateful day back in 2001.




Irma over Iowa
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As was done a few weeks ago for Hurricane Harvey, today's featured image transposes Key West's RADAR data to Iowa to give some size perspective to Hurricane Irma. The image nicely shows extent of the eye wall, which would be bringing 100+ MPH winds all the way from Pella to Des Moines.




Setting new coolest lows
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Iowa has seen its coolest weather of the season to date this week with crisp over night lows in the mid 40s and even some upper 30s. These days with a cooler low temperature set for the season could be considered step downs for the year. The featured chart looks at the number of step downs each year in low temperature, the temperature at which these step downs occur and the day of the year. Yes, a lot to ingest! The middle chart nicely shows the inertia to break through the freezing level as frequencies decrease as temperatures cool to near freezing. The reason being that heat is released as water freezes, so it takes extra cooling to get further below the freezing level. The bottom chart shows that this part of September is when the cooler temperatures start to show up more commonly than in August.

Generate This Chart on IEM Website




Corn Denting Progress
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The fall harvest season is soon to get in full swing. The corn crop is running somewhat behind this year. The featured chart displays yearly USDA NASS estimates of the percentage of the corn crop that is in 'dented' stage. This is one of the last development stages before harvest. The most recent analysis places 60% of Iowa's crop in this dented status and the 'x's on the plot denote the date during previous years that were at 60% as well. So we are a bit behind of 2016, but in line with the previous two years to that. It is kind of interesting to see how far 2012 sticks out. Of course, 2012 featured a drought and epic warmth, which got the crop quickly to maturity.

Generate This Chart on IEM Website




Low August GDDs
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The featured chart presents the combination of August Growing Degree Days and Precipitation departures expressed in terms of standard deviations from long term averages. The values are a statewide average and the 2017 totals are on the very low end of the scale for GDDs. Only three other years since 1893 come in with a lower total than 2017 according to this analysis. It will be interesting to see if heat is able to return in any force this September or if our current chilly weather will continue.

Generate This Chart on IEM Website




Yellow Labor Day
(image)

Significant amounts of smoke from fires burning to the west of Iowa created for an interesting Labor Day Monday. Between the smoke and some clouds, temperatures were rather pleasant and a nice northerly breeze helped keep things cool as well. The smoke also made for some interesting optics as parts of the day had a very noticeable yellow tint as shown by the featured webcam image from late Monday afternoon. If you could see the sun, it appeared as a plain orange disc. A cold front and upper level system should help to sweep the smoke out of the state for Tuesday.

Generate This Chart on IEM Website




Harvey Rain Rates
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The featured table presents an analysis of available one minute interval precipitation observations from ASOS sites near Hurricane Harvey in Texas. While the total column simply presents the six day precipitation total, the other seven columns show the computed hourly precipitation rate based on actual totals over the given period. The chart shows some mind blowing rates experienced of over ten inches per hour at the one minute timescale and even over four inches per hour on the hourly time scale.




Precipitable Water
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This week the daily features have been looking at precipitation climatologies for Iowa. In general, there is a northwest to southeast pattern with intensities increasing the further south you travel. One of the reasons for this gradient is the featured map for today. It shows a climatology from one of the main weather forecast models in this country for a variable known as preciptable water. This term is simply the depth of liquid water in the atmosphere above our heads if all of the water was condensed. The amount of water in the atmosphere is important to consider when looking at rainfall rates. The water coming as rain has to come from somewhere. So while the differences shown on this plot may not look large, on average a storm in southern Iowa will have about 10% more water (primarily water vapor) to work with than a storm over northern Iowa. These difference add up over the course of a year and help explain the differences in amount of rain. The observant reader may wonder why then there is not a more NW to SE gradient? Generally, that is likely more to do with the common storm track paths, which is a proxy to jet stream orientation.




Largest Yearly Totals
(image)

To finish off the series looking at Iowa precipitation climatology, the featured map presents the largest calendar year precipitation totals for long term climate sites tracked by the IEM in the state. As with the previous maps, there are storm totals from Harvey in Texas that are larger than even the largest yearly total in Iowa. This map does show a rather large northwest to southeast gradient in the state. Tomorrow's daily feature will look into why just a large gradient exists.




Largest Monthly Totals
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As with yesterday's featured map showing 24 hour precipitation maximums for Iowa, today's map shows maximum one month accumulations. Again, these totals pale in comparison with what was dumped over SE Texas from Harvey. Some reports in the area are in the 30 to 50 inch range, which are double of our wettest month in Iowa.




Max 24 Hour Precipitation
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The hard to fathom amounts of accumulated precipitation continued overnight for SE Texas from Tropical Storm Harvey. Many locations in the Houston area have reported 24 hour totals well above 10 and even 15 inches. Putting aside that some of these same locations have reported such amounts for multiple days in a row, has Iowa ever seen anything close to those amounts? The featured map presents 24 hour max accumulations for long term climate sites that the IEM tracks. Almost all locations on the map have seen maximum amounts well below even 10 inches. The 13.18 report was from Atlantic back in June 1998. How can such a large difference exist between here and Texas? Iowa does not see slow moving tropical systems with a direct and immediate fetch of extremely moist air from the Gulf of Mexico. Our heavy rain makers tend to be progressive (they move along), don't last long enough to pile up 10+ inches of rain (sources of rich moisture are transient), and not as efficient of rain producers as tropical storm systems (cloud ice and hail reduce rainfall rates).




Harvey Tornado Warnings
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Tropical Storm Harvey continues to absolutely pound SE Texas. While the mind blowing amounts of rain have, rightly so, garnered the vast amount of public attention, Harvey has also produced an extreme number of storms warranting tornado warnings. The NWS Houston office has been at the epicenter of this and has issued a prodigious amount of warnings over the past three days. The featured chart displays IEM computed maximum number of tornado warnings issued over a three day period. The chart also displays the maximum number for the local Iowa offices between 1996 and 2017 for comparison. Even just considering the 73 warnings issued on Sunday by Houston, this total is substantially higher than anything here in Iowa over all three days combined.