A Pleasant Autumn

           A WARMER AND DRIER AUTUMN MADE FOR AN OVERALL PLEASANT SEASON                                                                                                                                                                                                        Written by: William R. Deedler, Weather Historian NWS White Lake – DTX                                                   

                                                              December 15th, 2010

The Autumn of 2010 weather pattern over Southeast Lower Michigan was primarily mild and pleasant with mainly below average rainfall. The driest of weather remained in the northern areas of Southeast Lower Michigan around the Saginaw Valley and Thumb Region. The weather wasn’t all nice, by any means, as our predominantly pleasant fall contained some typical autumn storms, coming late October and increasing in frequency  by the second half of November (unlike November 2009, which was atypically calm).  October’s weather was the most pleasant relative to normal.  All regions saw periods of beautiful, warm Indian Summer weather in October with even a few days of notable warmth returning for an encore in November (See autumn statistics below, Table -1).

                                                                                         Table - 1

                                            

                                                                                          

Month by Month

September’s weather contained normal to below normal temperatures for the most part but a strong warm-up, ironically coming the first few days of fall (21st-24th), was mainly responsible for boosting monthly departures above normal.  The autumn rains actually started out around normal for two thirds of the region from the I-69 corridor (Flint to Port Huron) and southward to the Ohio border. North of this region, the Saginaw Valley and Thumb Region missed out on some of this rain activity with an inch or two deficit.

As mentioned, October was a classic nice autumn month is these parts with warm and dry weather prevailing across much of the land. In spite of the warmth, most of the region around the Saginaw Valley, Thumb Region and Flint did see killing frosts during the first week of October, and later around metro Detroit during the third week. Temperatures averaged 2 to 3 degrees above normal in October while rainfall was a good inch below normal at both Detroit and Saginaw and near normal at Flint. The best stretch of warm weather came from the 7th through the 11th when temperatures surged into the 70s and 80s, and then again from the 23rd into the 27th, when readings rose into the mid 60s to mid 70s. Temperature averaged around 10 to 15 degrees above normal during these warm spells. October was marred by an unusually intense storm center that moved into Upper Midwest on the 26th pivoting a strong cold front, accompanied by a band of heavy showers and gusty winds, through Southeast Lower Michigan. The strongest winds actually arrived in the wake of the front on the 27th, when winds gusted well into the 40s/50s mph, causing some power outages and tree damage.

November turned in a mixed performance with a little bit of everything. While once again temperatures averaged above normal, the path getting there was a bit more uneven. The first week of the month started out below normal, the second week above, both by about a 5-6 degree average.  A more typical late fall pattern change commenced in mid November, with storm centers pushing up through the western Great Lakes along with aparade of fronts traversing the region. Later in the month, a three-day warm spell just before Thanksgiving /21-23rd/ along with a warm up on the 30th was primarily responsible for the 1½-2 degree above normal monthly departure.  

Outlook Performance

While we’ve had some warm falls in these parts recently with 2001, 2004, 2005 and 2007 all making our top 20 warmest falls list at various locations, Autumn of 2010 failed to place in the top 20 lists. The Autumn Outlook for 2010 held up well for projecting a warmer, somewhat drier fall across Southeast Michigan. Not only did the warmer than average autumn forecast materialize but when the largest above normal departure would occur was also pegged.

From the AutumnOutlook regarding temperatures:

“Even though most years (analogue) did contain sharply colder polar air masses at times, they were more transitory in nature. An overwhelming number of analogue years displayed normal to above temperatures.” “It was October’s data set (and even with those two cold Octobers) that revealed October had the best chance to be above average and by the largest departure.” “Much of the time (in autumn) upper air ridging prevailed but with a change commencing later October or in November (this would also follow the ahead of schedule pattern seen since the spring). Of all the three months, while still averaging above normal, November does show more of a mixed pattern.”

Looking at Table-1, the warmest weather relative to normal did in fact come in October with all areas averaging up to 3 degrees above normal. All three sites averaged 1 ½ to 2 degrees above normal with Detroit’s average at 53.8 degrees, Flint’s 51.2 and Saginaw’s 50.9. The average this fall for Southeast Lower Michigan using the three sites came in at 52.0 degrees. The normal for Southeast Lower Michigan using all three sites is 50.3 degrees. Therefore, Southeast Lower Michigan averaged 1.7 above the autumn normal. The projected average temperature for Southeast Lower Michigan in the Autumn Outlook analogues using the three sites was 51.4 degrees /1.1 above normal/ and 0.6 of degree shy of the actual /52.0/ (Table-2).  One must remember, however, Detroit’s temperatures are now influenced by a heat island somewhat (the exact figure is unknown and would be variable) and are artificially a bit warmer.  This is not accounted for in the analogues since most analogue years were pre-heat island effects at Detroit Metro Airport and thus, the projection would be lower.

                                                                                                      Table– 2

 

 From the Autumn Outlook regarding precipitation:

“While the present above average temperature trend is expected to lag into autumn, many autumns actually reversed to a drier pattern from the dominant wetter pattern seen in the majority of locations since May. This pattern has already begun to emerge this August with drier weather observed across the entire region.”

“In this fall’s set of analogue data, a notable drop in rainfall was seen in the majority of years (eight out of 11) at Detroit, six out of ten at Saginaw and equal chances at Flint (the smallest data sample) with four out of eight.”

The rainfall across Southeast Lower Michigan averaged 7.25” or 1.42” below normal. Our analogue average for the three sites in Southeast Lower Michigan was not far off from the actual (only about a quarter inch /+.27/) and projected 7.52” or 1.15 below normal.

 

 



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