Last Build Date: Mon, 24 Apr 2017 08:04:15 -0500
While the cold periods this past winter season have been limited, the coldest wind chill reported for Des Moines was as cold as we have seen this past decade at about -30. Thankfully, there is no chance of having a colder temperature this spring. The top panel displays an accumulated histogram for the coldest temperature. The -30F this year is at about 70th percentile, so roughly seven out of ten years have had a colder minimum wind chill temperature.
As the month of May rapidly approaches, one starts to wonder if freezing temperatures are going to happen again this spring. The featured chart shows a time series from Ames for this month. Only one day had temperatures briefly below freezing and we have been comfortably above since. It will be interesting to see how cold temperatures get this weekend with some clear skies expected and good overnight cooling conditions.
With the late morning update yesterday, the Storm Prediction Center had a 10% tornado risk analyzed over parts of Iowa for Wednesday. Thankfully, only a few brief tornadoes were reported near Carroll. The 10% risk threshold only happens a few times each year as highlighted by the featured plot. The map presents the average yearly frequency of having a 10% risk threshold issued. The values for Iowa are in the 1 to 3 days per year range. Kind of interesting to see the highest frequencies show up in the eastern portions of the traditional "Tornado Alley".
Since October 2007, the National Weather Service's official Tornado Warning was geographically represented by a single polygon. These polygons are also associated with the counties they occupy and often the counties are what is reported. Anyway, just considering the polygons the featured map displays the year of last tornado warning based on the polygon coverage. While the polygons were not official prior to 2007, they were issued experimentally and are included in this analysis. The potpourri style map shows that some areas of the state have missed all the tornado warnings since 2003! There is a tornado threat in the state today, so hopefully this map does not change (meaning no new warnings) if you generate it tomorrow via the website. You can also generate this map for other states and also isolated to single NWS Forecast Offices.
The River Forecast Centers of the NWS issue Flash Flood Guidance for estimated rainfall totals over a given amount of time needed to begin flash flooding. These totals are not appropriate for steeped nor urban areas. The featured map presents the county/forecast zone estimates from last evening. Most of Iowa is in the 1.5 to 2 inch range. Our recent wet weather and upcoming storms have some concern for more flash flooding to occur.
A rainy Saturday gave way to a very mild and pleasant Easter Sunday. The featured chart displays the daily high temperature reports for Easter for Des Moines. The high of 72 for Des Moines was well above the long term average. Of course, with a holiday like Easter that can vary by a number of weeks, having a late Easter this year helped in the chances of it being warm.
The Storm Prediction Center issues risk categories and probabilities for severe weather for days (24 hour windows of time) 1 through 8. The featured chart looks at the frequency of having the slight risk category issued for "Day 1" from SPC's morning issued outlook. Interestingly enough and according to this unofficial IEM analysis, the highest frequencies in the USA can be found here in Iowa and parts of extreme eastern Nebraska. The SPC outlooks cover threats from large hail, damaging winds, and tornadoes. So while Iowa does not see the most tornadoes in the country, it does see its fair share of all severe weather threats. Iowa is effectively in a sweet spot for severe weather, including night time thunderstorms that bring hail and wind fueled by the low level jet and Mesoscale Convective Complex activity. The values on the plot are average number of slight risk events per year. Such a event is expected on Saturday with a "Day 2" slight risk currently in effect.
The featured map is of forecasted seven day total precipitation from the NWS Weather Prediction Center. It depicts a bullseye of heavy precipitation for much of eastern Iowa. This is the time of year for heavy rainfall events in the state and presents a catch 22 for farmers. While much of this region has been on the dry side of average this cold season and could use the rain, wet conditions create delays for planting. Showers and thunderstorms overnight have gotten Iowa off to a good start to meet these forecasted values over the next seven days.
Recent sunshine and warmer air temperatures have helped boost soil temperatures in Iowa. The featured chart is of daily ranges between high and low four inch depth soil temperature for the ISU Soil Moisture Network station at the ISU Horticulture Farm. The key level of 50 degrees is highlighted as it is often used as a threshold to start planting corn. The past three days we have mostly held this threshold for the location. It is still early in the planting season to keep temperatures always above this level, but the hope with early planting is that the periods below 50 are brief (not persisting for consecutive days).
The growing season is tantalizingly close and computed variables like growing degree days become important for tracking plant growth and development. The featured chart displays the combination of daily GDD totals for this year, the range between the 5th, 95th percentile and average of daily values based on period of record data. This year has seen a number of days with very warm temperatures and you can see these showing dots well above the 95th percentile. Of course, even with the warm weather, there is still plenty of time for freezing temperatures.
Very warm weather arrived just in time for this past weekend. The high temperature for the Des Moines Airport breached 80 degrees for the first time this year. The featured chart displays the year to date maximum daily high temperature. This year is shown along with the climatological bounds each day. Kind of interesting to see how the epic warmth back in February was only eclipsed by one degree during March. The weather looks slightly cooler this week, but warmer than much of the weather for this April prior to the past weekend.
After an extended stretch of cool weather, warmer air is set to return this weekend with highs in the 70s for much of the state. How common are 70+ temperatures? The featured chart displays hourly frequencies of having such warm temperatures by week of the year for Des Moines. The calendar says April now and frequencies are on the increase each week. For the first week of April, the frequencies peak out at about 25% late afternoon. So such warm temperatures are not common yet, but nothing out of the ordinary.
Hope springs eternal that our recent extended stretch of gloomy and cloudy weather will break for a few days with a triumphant return of full sunshine starting today. The featured chart displays recent daily solar radiation totals from the Ames ISU Soil Moisture Network station. The daily bars represent the energy received from the sun and the line represents a theoretical maximum assuming low humidities and clear skies. You can see that recently, we have been nowhere near theoretical maxes. The implication is that soil temperatures have not done much recently either with the limited solar inputs. With the recent rains, a few days of warm temperatures and sunshine will help green our landscape up even further!
The Storm Prediction Center has a large portion of the SE United States under a moderate risk of severe weather for today. The featured map presents an IEM analysis of the most recent year of having a moderate risk issued for map location. For most of Iowa, you have to go back to the year 2014 since the last issuance. This year has seen a number of moderate risks issued over parts of the country as the very active start to the severe weather season continues.
Recently, the clouds have been difficult to get rid of. For Des Moines, it has been mostly cloudy for the past 12 days or so. To pick an arbitrary comparison point with the long term archive of cloud coverage at Des Moines, the featured chart presents the longest streaks of having the 10 AM CDT observation indicate overcast. The current streak of 12 days is the longest since 2010 and only a few days shorter than the longest back in late 1979 and into 1980. When will our current streak end? At the moment, it would appear Thursday would have the earliest chance with a more likely chance on Friday. Needless to say, there will be plenty of happy folks to see full sunshine when it returns!
Snowfall totals this winter season have been a mixed bag over the Midwestern US. The featured map displays departures from average for the primary climate stations maintained by the NWS. Negative values in red indicate below average accumulations. The departure for Des Moines is approaching 20 inches, while Mason City, to the north, has seen above average snowfall approaching 10 inches in surplus. In general, the big snowfall events have been confined to the northern half of the Midwest. While the calendar now says April, there is still some time to get some big snowfall events.
With March winding to a close, it is a good idea to check in on monthly stats. For average low temperature at Des Moines, this month continues a streak of having above average daily low temperatures. The featured chart displays this metric since 2010 along with the monthly El Nino index. The climatology used in this plot is the present day period of record data. February 2015 was the last such month below average. Kind of interesting to remind ourselves how extreme March 2012 was as it really sticks out in this chart.
Continuing with the theme of Severe Weather Awareness Week in Iowa this week, yesterday was the statewide tornado drill with a test watch and warning issued for the entire state. Today's featured chart looks at the conversion of counties under a tornado watch into a tornado warning. For example, if a tornado watch was issued for 60 counties and 30 of those counties received a tornado warning, that would count with the 50% bin for the plot. About 20% of all tornado watches receive no subsequent warning. Most watches have less than 30% of the counties included eventually get a warning. Thankfully, tornadoes typically are not widespread and limited to a couple of high end storms during a typical watch. Hopefully that is the case today with a significant tornado threat expected over the southern US.
Continuing with the theme of this week being Severe Weather Awareness Week in Iowa, today's featured plot looks at an analysis of convective watch issuance verification based on probabilities included in Mesoscale Convective Discussions (MCD) issued by the Storm Prediction Center (SPC). The SPC issues the MCD product to provide a short term update and analysis of the storm scale environment during active weather. Prior to most tornado or severe thunderstorm watch issuance, the SPC typically issues a MCD to highlight an area of interest and provides a confidence percentage on if a new watch may be soon issued. The association of an issued watch and the MCD product is not exact, so a simple algorithm was used with reasonable defaults to provide a verification of these percentages. The algorithm being the new watch needed to be issued with 2.5 hours and at least 50% of the watch polygon had to overlap with the MCD polygon. What is the moral of the story? Well, the high confidence MCDs typically get an issued watch, the lower confidence ones typically do not. The next time you read a MCD product from the SPC, check out the confidence noted and see if a subsequent watch is issued!
This is Severe Weather Awareness Week in Iowa, which features a different aspect of severe weather each day. On Monday, the focus was Severe Thunderstorms and the primary product the National Weather Service issues to alert of these storms is the Severe Thunderstorm Warning. These warnings contain tags identifying the magnitude of threats associated with the storm. The featured chart looks at the combination of wind speed and hail size tags included in warnings issued by forecast offices with Iowa coverage. The most common combination is of one inch hail and 60 MPH winds, which are the minimum thresholds for severe storms (strictly speaking, only one of the two is necessary and the wind threshold is 58 MPH). Thankfully for us, the frequencies of storms predicted to have giant hail and/or hurricane force winds (75+ mph) are rather small. One of the reasons these tags exist are to provide the NWS with an additional information point to alert the public to when these extreme severe thunderstorm events are predicted.