The featured chart presents an IEM estimate of the areal coverage of Iowa that has experienced its first fall season sub-freezing temperature. The blue line is the simple average of the previous 60 years with the red and green line being the previous two years. The first two weeks of October are typical for having the first frost of the year with the warmer parts of the state lasting longer into October. It appears likely that the rest of the state yet to experience a freeze will see it in the coming days with very cold air settling into the state.

Voting:
Good = 21
Bad = 1

Tags:   freeze  

Very chilly air for this time of year settled into Iowa over night and morning low temperatures were below freezing for most of those in the northern half of the state. Our bout with cold air will be brief as chances of rain are in the forecast along with lows well above freezing.

Voting:
Good = 10
Bad = 7

Tags:   freeze   2011  

The forecast holds a great deal of optimism for a streak of at least a week of above freezing daily high temperatures this would follow a period of 12 days with highs at or below (AOB) freezing. The featured chart looks at the longest streaks of having daily highs AOB freezing and then having them above freezing. There are two dots presented for each year denoting the longest streak on each side of the regime change. One of the interesting outliers in this plot is what happened last spring. After a period of 37 days below freezing, temperatures warmed on 2 March and we had 265 straight days of high temperatures above freezing. So the longest streak of 37 days was followed by a streak of 265 days. The crosses represent the longest second part of the streak and what the streak was below freezing before that streak. Kind of confusing, so please comment if this does not make any sense!

Voting:
Good = 32
Bad = 15

Tags:   high   freeze  

Nearly all of Iowa has probably not seen sub freezing temperatures yet this fall. The featured chart displays the date of the first sub freezing temperature for Ames along with the number of days afterwards until the first sub 29 degree temperature (typically considered the killing frost). A value of zero days would imply that the first day of sub freezing temperatures also dipped below 29. A final delineation is also provided with the size of the dots representing Jun/Jul/Aug precipitation with red dots being those years above 18 inches, like this year. The earliest freeze shown on this chart was in 1902, even after a wet summer. As for this year, our first good chance for a freeze is expected to arrive this weekend.

Voting:
Good = 14
Bad = 12

Tags:   freeze  

Clear skies and calm winds helped temperatures to drop to frosty levels Sunday morning as shown by the featured IEM analysis of low temperatures. The full impacts of this cold event will take a bit to be determined, but the corn crop was probably not far enough developed to be significantly impacted. Trees and flowers took the brunt of this cold weather with more cool weather expected this week. The big story will be the heavy rainfall along with highs only in the 50s for the next few days.

Voting:
Good = 29
Bad = 2

Tags:   may10   freeze  

Temperatures in some locations this morning in eastern Iowa are hovering around freezing, but the rest of the state looks to miss out thanks to a slight southerly breeze and higher dew points. The featured chart presents a composite of air temperature and dewpoint for the 96 hours around the final sub 32F temperature measured at the Des Moines Airport. The plot perhaps indicates the importance of having low dew points for the period up until the cold temperature. The dry air helps the atmosphere to cool off efficiently, allowing temperatures to drop in the early morning hours (-6 to 0 hour). The moisture increase thereafter is probably showing the effect of having a high pressure system pass to the east and southerly flow returning.

Voting:
Good = 24
Bad = 12

Tags:   freeze  

Low temperatures this Wednesday morning dipped into the 30s over parts of the state. The featured map displays the frequency of a long term climate site having the first fall sub freezing temperature (ties are shared). The map displays three preferred locations for the first fall freeze: Northwest Iowa, Northeast Iowa, and Westcentral Iowa in the Nishnabotna Valley (Atlantic is the site at 15).

Voting:
Good = 33
Bad = 3

Tags:   freeze  

The featured chart shows the frequency of low temperature thresholds for the fall period before the first freezing temperature. For instance, on average we would expect to see around 10 days with a low temperature at or below 45 degrees before the first freeze (blue dots). The maximum and minimum values are shown as well along with the current observation this fall. The moral of the story is that we probably should see a few more cool mornings before the first freeze with a day or two in the 30s and then finally that first fall freeze.

Voting:
Good = 25
Bad = 7

Tags:   freeze   climate  

It is commonly said that the first fall season freeze occurs during a full moon in late September or October. The featured plot shows that this simply is not the case in Iowa. The horizontal axis contains the number of days from the nearest full moon and the vertical axis is the number of days that observation was from the 6th of October (the median first freeze date). The contours indicate the relative frequency of the data. The important portion of the plot is the histogram in red at the top of the page clearly showing no full moon date dependence. Here is an Excel file with this data.

Voting:
Good = 20
Bad = 7

Tags:   climate   moon   freeze  

The featured chart presents the frequency of having a daily high temperature above freezing with a low temperature below freezing for Ames. The chart shows two seasonal maximums in late fall and early spring. Our weather this week looks very warm to start with values returning to near average by the end of the week.

Voting:
Good = 17
Bad = 7

Tags:   freeze   thaw  

The featured plot is from the Humbolt SchoolNet for Thursday morning showing the flat lining in temperature around freezing before more cooling occurs just before sunrise. You will often see this as special processes occur when water begins to freeze, which releases heat and therefore slows the cooling process some. Having the dew point near 32 degrees gives an indication that frost formation was a necessary process for the temperature to cool below freezing.

Voting:
Good = 39
Bad = 21

Tags:   freeze  

The featured chart shows the percentage of years since 1893 that a certain temperature threshold was exceeded by a certain date in the fall for the Ames climate site. For instance, the chance (observational frequency) of a sub-33 temperature by this time of year is roughly 50%. This means half of the years prior to this one have seen a sub-33 temperature by October 4th.

Voting:
Good = 42
Bad = 24

Tags:   freeze   climate  

The featured chart is a temperature composite of the time period before and after the first sub 32 degree temperature in the fall from the Des Moines Airport. The three lines represent the temperature range and average of the observations 3 days before thru 1 day after the event. The 12 hours before the first event show a rapid cooling which is often the case with clear skies during the night time hours. Those clear skies are then typically around for the next day when temperatures quickly recover.

Voting:
Good = 23
Bad = 4

Tags:   fall   freeze