Past IEM Features tagged: april
This April continued the recent trend of having Aprils with above average precipitation. The featured chart shows the monthly precipitation departures for the past 22 years for Ames. Each colored cell represents a past month and year. The highlighted cells illustrate our recent history being having a wet April and a dry summer. The forecast for the first week of May looks mostly dry with better chances of rain arriving next week. At least it is not snowing like it was this time last year!
Temperatures warmed nicely on Wednesday thanks to a dry atmosphere and plenty of wind helping to mix warmer air. The featured chart compares the combination of 3 PM air temperature departure from long term average at 3 PM and the wind speed reported. The shaded areas represent relative frequency with the darkest shades being the most frequent. A simple fit of the data is provided by the red line and yesterday's value is indicated by the brown dot. The chart illustrates that as temperatures get more extreme in the cold or warm direction, wind speeds increase. Of course, correlation is not causation, but there are interconnected physical processes that drive temperatures and wind.
Today has the chance to be the coldest high temperature so far for the month of April for Ames. The featured chart presents the frequency of a day in April having the coldest or warmest high temperature for the April of each year. So having today have the coldest high for the month has happened twice in the past. The chart indicates that the coldest high is typically during the first 6 days of the month. Of course, our increasing daily climatology helps guide this chart and you can see the most number of warmest highs occurs on the last day of the month.
The featured graph displays the number of days during this and past Aprils that the low temperature was over 10 degrees above average for Ames. This year has seen 8 such days already doubling the long term average of just 4 days. Average lows are in the lower 40s and the forecast has some more 10+ degree warmer than average lows for the end of the week along with some rain.
The NWS has issued a wind advisory today for forecasted southerly winds around 30 mph. The featured chart displays directional frequency of reported 30+ mph wind speeds during April from the Des Moines Airport. Southerly winds are not the most favored for high winds, since warming is generally occuring which leads to upward motion near the surface acting to keep the strongest winds aloft from reaching the surface. The situation is reverse with northwest winds, since there is often cooling and decending motion which acts to bring stronger winds to the surface and keep them there.
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Des Moines April wind rose
April is a transition month for the major air masses with surface weather systems pulling warm air north and cold air south. The featured image is a wind rose for April which indicates frequency and magnitudes of the wind. You can see the primary directions from the NW and SE. Today will be one of those days with winds from the SE and thus will be warm.