September temps above average as current warm period nears 45 days

September goes down in the record books as above average as the warm pattern started in mid August continued and neared 45 days in length. Rainfall was near average, ranging from slightly above in Kansas City to slightly below in St. Louis.

September marks the transition into the cool season. Weather systems begin tracking further to the south into the central U.S. and rainfall transitions from the thunderstorm clusters of summer to the larger and more organized storm systems and cold fronts of the cool season.  Although hot weather can continue well into the month and both cities have recorded temperatures in the 100s on numerous occasions, cool outbreaks begin to be felt more frequently.  Frost in September is rare, as the growing season typically lasts into the third week of October.

Daylight begins a sharp decrease and 1 hour and 13 minutes are lost from September 1 to September 30th. The graphic below shows the changes in daylight on the 21st of each month. Note the rapid decline from August 21 to September 21. Click on the image to make it more readable.

This is a plot of the length of daylight over a period of 14 months and shows the length of the day on the 21st of each month.  The 21st is used as that is near the date of the Spring and Fall Equinox and the Summer and Winter Solstice.

This is a plot of the length of daylight over a period of 14 months and shows the length of the day on the 21st of each month. The 21st is used as that is near the date of the Spring and Fall Equinox and the Summer and Winter Solstice.

Here are the September averages for both Kansas City and St. Louis based upon the 1981-2010 climate period:

Average statistics for September in Kansas City and St, Louis

Average statistics for September in Kansas City and St, Louis

For September 2013:

In Kansas City:  The average high was 83.1, 4.1 degrees above average.  The average low 59.9 was 2.6 degrees above average.  The overall temperature was 71.5 which was 3.3 degrees above average.   Kansas City’s highest temperature was 100 on the 8th, which was also the highest temperature of the year.  Kansas City’s downtown airport reached 102. The coldest reading was 46 degrees on the 29th.  There were 8 days when temperatures reached 90 or higher and 1 date at 100.

In St. Louis:  The average high was 84.3, 4.1 degrees above average.  The average low 63.5 was 2.9 degrees above average.  The overall temperature was 73.9 which was 3.5 degrees above average.  The highest temperature was 99 on the 9th. The coldest reading was 51 degrees on the 22nd.  There were 6 days when temperatures reached 90 or higher.

The charts below show the month’s temperature trends with respect to above or below average. For both cities a majority of the month was warm with just a few cool days interspersed.  The warmest period with respect to average was in the opening days of the month with near record heat.

Daily temperature plot for Kansas City and St. Louis showing daily temperatures above or below average.

Daily temperature plot for Kansas City and St. Louis showing daily temperatures above or below average.

Taking a wider look at the North American continent, warmth stands out over central and western Canada where it is strongest, and extends southward through the center of the U.S.  Monsoon clouds and rain contributed to cooling over the Desert Southwest and elsewhere Alaska through northern and eastern Canada and the eastern U.S were also on the cool side.

North American temperature difference from average for September,

North American temperature difference from average for September,

Our warmth is the exception compared to the rest of the globe (except the polar areas) Across the northern hemisphere, the warmth over parts of North America really stands out. Aside from smaller areas of the Middle East and west central Russia, a majority of the hemisphere has experienced a cool month (blue shading).

Global temperature differences from average for September

Global temperature differences from average for September

We’ve been alternating extended periods of cool vs warm over the past year.  Those periods were 100-120 days from mid Fall 2012 to early Summer 2013 but have shortened since.

Temperature trends for the last 365 days are shown here.  There have been extended periods of alternating warmth and chill. Since June, these alternating periods have grown shorter.

Temperature trends for the last 365 days are shown here. There have been extended periods of alternating warmth and chill. Since June, these alternating periods have grown shorter.

The Climate Prediction Center’s September forecast issued in mid August indicated an equal chance for above, near or below average temperatures for September.

Rainfall for September was slightly above average in Kansas City and slightly below average in St. Louis.

Torrential rains on September 19th in Kansas City dumped 2.58″ of rain which was a record for that date.  Flooding was reported in several areas of the city with a band of strong to severe thunderstorms . That band of storms made all the difference. Take away the rain from that one event and September would have finished 2.36″ below average. September is now the first month in Kansas City with above average rain since May and is now only the 5th month in the last 21 to record above average precipitation. The 2.58″ of rain observed was also the most rain at one time observed since 3.15″ fell on August 17, 2009, 4 years 1 month and 2 days prior.

Average rainfall in September ranges from 4 to 5″ over central and western Missouri, and 3-4″ over NW, SE and Eastern Missouri and southern Illinois.  A few areas of 2-3″ rains are found in central and southeastern Illinois. The chart below shows average September rain over S Illinois, Missouri and E Kansas.

Average September rainfall across eastern Kansas, Missouri and southern Illinois.

Average September rainfall across eastern Kansas, Missouri and southern Illinois.

Rainfall totaled 4.84″ which was 0.22″ above average in Kansas City.  In St. Louis, rainfall totaled 2.74″ which was 0.39″ below average.

The charts below show the 30-day rainfall trends at Kansas City and St. Louis. Both cities experienced long periods without significant rain.

Daily rainfall plot for Kansas City and St. Louis showing rain events and running monthly total relative to average,

Daily rainfall plot for Kansas City and St. Louis showing rain events and running monthly total relative to average,

The map below is a combination of radar and surface observations of September’s rain across eastern Kansas, Missouri and Southern Illinois. What immediately stands out are the light green colors centered over S Illinois and adjacent SE Missouri.  These represent widespread monthly rain totals of between a half an inch and 1″ which has contributed to developing dryness and drought in this area.  The blue speck NE of Cape Girardeau represents an area of Union County, Illinois which only received a quarter to half an inch of rain all month. The heavy rains of September 19th show up nicely around Kansas City in the reds, representing small areas of 5″ monthly rain totals.

Observed and radar estimated September rainfall totals.

Observed and radar estimated September rainfall totals.

This map compares observed rain with average September rain.  The yellow colors represent a deficit with greens a surplus. Grey colors show areas that were within a half inch of average.  Most of the region had sub-par rains in September, with the exceptions over Kansas City and just NW and SW, a small area of NE Missouri and a small area of SW Missouri.

September rain difference from the 1981-2010 monthly average.

September rain difference from the 1981-2010 monthly average.

The NWS Climate Prediction Center’s September Outlook issued in mid August had called for above average rainfall for eastern Missouri and Illinois (low confidence) which was adjusted to equal chances for above/near/below average on the update issued August 31st, Equal chances for above/near/below average was predicted for the Kansas City area in the mid August forecast and in the end of the month update.

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