Past IEM Features tagged: sep
The weather feels much more like fall now after a very muggy and summer-like previous week in September. The featured chart presents the number of days each September that the maximum hourly dew point was at or above 70 degrees. This year's total is similarly anomalous to last year's total with the long term average near 3 days. The year of 1978 comes up as the winner in this chart with thirteen days shown.
The coldest high and low temperature this month for Des Moines happened on the 11th for high and 12-13th for low. The featured chart presents the frequency of a given day in September being the coldest for that month during one year. Intuitively, the highest frequencies occur toward the end of the month as our average temperatures slide into fall. The forecast has warm temperatures up until next Wednesday, so there is a chance that these early September lows will be the coldest this year.
Temperatures warmed very nicely into the 80s on Monday thanks to the strong southerly wind. The featured chart displays the frequency of having a 10+ knot wind speed at a given air temperature in September. With an increasing temperature there is an increasing chance that we strong winds as well. There is a good reason for this relationship as it takes strong winds to transport warm air into the state and also indicates that vertical mixing is taking place which brings warm air from above to the surface. Without these processes, it is very difficult to support very warm temperatures in September.
September has been very warm so far with the temperature at the Des Moines Airport at or above 70 degrees for nearly the past 6 days. Based on hourly reports, this streak above 70 is the longest on record for the site in September as shown by the featured image. The previous longest streak was back in 2013 and only another streak back in 1938 was longer than five days. Cooler air is settling into the state today, but as of this posting the temperature remains above 70 for Des Moines, so the streak continues!
There is some hope that we'll reach 80 degrees later this week as our stretch of very nice fall weather continues. The featured wind rose presents the wind direction and speed frequencies when Ames September temperature is at or above 80. The dominate wind direction is clear with southerly winds the most common, which makes sense as it is our source of warmer air. The winds will be out of the south for much of the rest of the week, but speeds are not expected to be all that strong.
Tags: windrose sep
For the month to date, the average temperature for 1-8 September has been warmer than 1-8 July for Des Moines. The featured chart shows the frequency of this happening for the first of each month to the given date. For the 8th, about one in five years have seen this happen. The forecast has parts of Iowa near 100 degree today with cooler weather expected by the end of the week. If Des Moines were to hit 100 degrees today, it would be the latest into the year for such a warm temperature.
Tags: sep jul
Muggy weather returned over Labor Day weekend with humidity levels running more like what we would have expected in August. For Des Moines, the dew point on Monday was slightly higher than even the highest dew point for all of August. The featured chart shows the yearly combination of highest September versus August dew point. Points above the one to one line would represent years like this year with a higher dew point in September. This combination is somewhat rare and last happened in 1998.
Tags: aug sep
According to IEM estimates, the statewide precipitation departure since 1 May was around an inch for the period up until 1 September. The featured chart compares the departure from 1 May until 1 September and 1 October for years back to 1893. Seven out of 59 years whereby the departure was negative on 1 Sep was the September total able to push departure into positive territory.
The last five days have seen a remarkable stretch of cool weather with daily high temperatures in the 50s and lower 60s. The featured chart shows the past five days being the coldest on-average for any five day period prior to 19 September since 1903 for Ames! The forecast calls for a warm up to closer to normal high temperatures along with some more chances of rain.
Tags: sep11 sep