Autumn 2013 Outlook For Southeast Lower Michigan

"Basically, autumn is a transition season as the thunderstorms and severe weather of the summer give way to a tamer, calmer weather period before the turbulence of the winter commences." I wrote that sentence back in 1996 when doing research on Indian Summers which was one of my most popular articles. Look for the *article at the bottom of this Outlook.

Autumn 2013 Outlook

Temperatures

Indications are temperatures this fall will be quite variable (even more than usual see Upper Wind Discussion below) but in the end, temperatures should average near normal to below (or -1.5 to +0.5 degrees of normal). The data in our neutral analogue falls remains quite mixed and looking at that, along with the upper wind projections, suggest end results will average near normal to below.

Precipitation

Precipitation over the area is projected to average around normal to above in keeping with our recent trend.

The ENSO pattern over the Pacific Ocean is expected to hold in the Neutral state through the fall. Since the ENSO state will hold Neutral, this leaves very little to go on for the fall period in regards to influencing patterns for the Great Lakes from the Pacific and not unlike our recent summer.

Upper Wind Discussion

Upper air general projections indicate normal ridging oscillating with a deepening troughing pattern early to mid fall. By mid to late fall more troughing will extend from mid and eastern Canada into the upper Midwest and Great Lakes. While this is in agreement with the normal or average season change; early indications are upper wind patterns including the Polar jet over the Arctic extending into Canada will become more aggressive than average and dip south. In addition, our Neutral Autumn analogues also suggest more below normal temperatures as the fall unfolds.

Latest model guidance intimates an average to slightly above average overall. This data lies more heavily on very recent trends.

On to the Analogues

Our most recent Autumn Analogue to this fall occurred in 2008 when neutral conditions persisted from the summer. While the summer of 2009 was also basically Neutral ENSO, the following autumn evolved into an El Nino...not expected this year. Looking at the data from the Autumn of 2008 gives us the following;

AUTUMN 2008 TEMPERATURES/DEPARTURES 

 LOCATION     SEPTEMBER    OCTOBER    NOVEMBER    FALL 

DETROIT	66.3 / +2.4	50.6 / -1.3	39.0 / -1.7	52.0 / -0.2
FLINT	62.7 / +2.0	49.2 / -1.5	37.3 / -0.8	49.7 /+0.4
SAGINAW	62.1 / +2.1	47.7 / -1.5	37.8 / -0.2	49.2 / -0.2

AUTUMN 2008 PRECIPITATION/DEPARTURES

LOCATION    SEPTEMBER  OCTOBER   NOVEMBER     FALL                                                                                                                

DETROIT	5.99/+2.72	1.15/- 1.08	3.31/+ 0.65	10.45/+2.29/20th wettest
FLINT	8.64/+4.88	1.26/-1.08	2.10/-0.55	12.00/+3.25/6th wettest
SAGINAW	4.55/+0.60	2.26/- 0.23	1.47/-1.18	8.28/-0.81

Looking at our earlier Neutral analogue fall statistical package (below) and comparing it to our most recent Neutral fall 2008 data (above) holds us in good stead and thus, continues to be useful. Data from all analogues was extremely mixed and basically, there are as many of warmer falls as cooler with some normal falls tossed in for good measure. In addition, the same can be said about the precipitation side of things with some dry falls, wets falls and near normal falls. This “equal-parts” variability is somewhat unusual and one of the most mixed season data set seen.

                                                                                                NEUTRAL ANALOGUE AUTUMNS 

                                                                                                    (Normals 1971-2000)

In this smorgasbord of weather are there even any subtle trends? Not really other than there were notable wet and dry falls, similar to preceding summers. Keeping one eye on our recent rainfall trends and the other on the 2008 period, I'm inclined to keep rainfall projections at least around normal if not above.

How about monthly trends? Temperatures in September and October were mixed with little in the way of prevalent trends. Novembers did show a bit more consensus toward a cooler than normal, but even that wasn’t a strong trend. Overall, the composite charts for the fall do indicate temperatures averaging a degree or so below normal for Southeast Lower Michigan while precipitation averages around normal. 

Frost and Freeze Trends:

Our guidance indicates temperatures are quite variable and most years our frosts and freezes were on schedule. Checking back on the years in our study reveals some common frost/freeze statistics. Using our stats from Detroit, (and estimates of frost when most areas saw overnight lows fall in the lower to mid 30s for the first time). Our analogue falls show the risk of frost and/or freeze the last week of September into the first week of October. This is right in there with the average frost dates. The more widespread freezes occurred by the third week of October.

Indian Summer this Fall?

Perusing our analogue autumns, the likelihood of a period or two of Indian Summer weather this fall looks promising. Note, most temperature patterns in October and November varied considerably in the analogue years, this is generally a prerequisite for Indian Summer falls. You want a progressive amplified upper wind pattern during the fall to deliver large cold high pressure systems. While these highs can bring sharp cold snaps (and attending solid frost/freezes, necessary for Indian Summers), after they move east, impressive strong warm-ups, sometimes lasting as long as a weeks bring Indian Summer weather…let’s hope.

Notable Autumnal Dates

Harvest Moon:  Thursday - Sep 19th, 2013 (Traditionally, this designation goes to

the full moon that occurs closest to the Autumnal (fall) Equinox. The Harvest Moon

usually comes in September, but  about every three or four years it will fall in early

October - closest full moon to the beginning of fall).

Autumn Officially Begins:       Sunday - September 22nd, 2013 at 444PM EDT

Average First Freeze Date:    October 21st (Detroit area), October 11^th (Flint and

Saginaw area)

Halloween:                       Thursday, October 31st, 2013

Thanksgiving:                  Thursday, November 28, 2013 (latest possible)  

Have a nice fall and look for my Winter 2013-14 Outlook early in November  

*Indian Summer
http://weatherhistorian.blogspot.com/2013/09/just-what-is-indian-summer-and-did.html

Making weather fun while we all learn,
Bill Deedler -SEMI_WeatherHistorian