Autumn Outlook For Southeast Lower Michigan

Neutral conditions exist in the Pacific and with that; little downwind effects on the Great Lakes weather is anticipated into early fall. However; weak El Nino conditions will surface in the Pacific in the September to October time-frame as Pacific water temperatures continue to warm gradually in all sectors. Latest observed water temperature departures (sea surface temperatures /SST/) anomalies are displayed below /Fig - 1/.  Note; area 3.4 is our main concern in determining El Nino conditions in the Pacific but not necessarily the downwind weather effects in our neck of the woods over Southeast Lower Michigan. Weak to even moderate El Nino conditions along with a NAO-/AO- in the winter are notorious for active weather patterns across the country. One of my earlier studies presented such a case (Winter of 2002-03; a moderate El Nino). A new set of analogues are already being decided upon and watched for similarities to this autumn into winter pattern projected. In regards to a weak El Nino winter; one possible candidate of many is the Winter of 76-77 but it is much too early to let those "cats" out of the analogue bag just yet. One must see how the "birth" of our new El Nino plays out this fall and how he interacts with the phase(s) of the NAO/AO. Stay tuned.

                                                              /Fig - 1/

 Maps of SST's areas referenced above /Fig - 2/ and observed data:

                                                               /Fig - 2/

Past ENSO patterns (La Nina - /El Nino +)  /Fig - 3 / back to 1950.

        
                                                              /Fig - 3/

 

Model SST's anomaly projections /Fig - 4/ into Spring of 2013 with mean model averages highlighted.

                                                                    /Fig - 4/



The North Atlantic & Arctic Oscillation patterns of the past several months /Fig -5/ are shown here:


                                                                                  Fig - 5

I'lll discuss these more when we get closer to the Winter Outlook publish date (Nov). As you can see the North Atlantic Oscillation /NAO/ has been weakly negative much of the summer while the Arctic Oscillation /AO/ has been fairly neutral. Why such a hot summer with the NAO being negative? Just because the NAO was negative doesn't mean the implied cooler heights and surface air was delivered to the Great Lakes. Remember though; we did have an impressive Canadian Upper Low that forced somewhat cooler air at times into the region. That upper low was one of the polar upper air systems that was indeed reflective of the negative /NAO-/ at times this past summer. Meaning; in response to the NAO-, the cold upper low was able to meander far enough south and thus; infiltrate the Great Lakes and Northeast with refreshingly cooler, drier "Polar" oriented air. More about this soon in my summer review in September.

Back to the Autumn;
There are really no good indicators for trends this fall with basically; a downwind Neutral ENSO effect
across the Great Lakes and variable NAO/AO. With this kind of pattern we remain at the mercy of the
weakening summer ridge and the cold upper low (and thus; NAO/AO) in Canada; leading to an amplified and changeable pattern. Current model predictions of the NAO/AO are lousy. For example; just take a look at this model predictability of the NAO/PNA /Fig - 6/ during the Winters of 2000-05 for up to 15 days ahead.This is from an interesting paper by Ake Johansson in the Journal of Climate published in May of 2007.


                                                                               Fig - 6



The skill in predicting the index of (left) the NAO and (right) the PNA as measured by the correlation coefficient (1) is shown with thick lines for the NCEP (black) and ECMWF (gray) models as a function of forecast lead time. The regular traditional centered anomaly correlation (AC) for both models are shown as thin lines with corresponding color shades in both panels. The skill of a persistence forecast of the two indices is shown as thin dotted lines. The data are from the extended winter months (1 November–31 March) of 2000–05. Correlations are multiplied by 100.

Autumn Temperatures:

Looking at recent upper air trends, NAO/AO patterns along with neutral to weak El Nino patterns look for temperatures to average somewhat above normal for the Autumn. Since the downwind effects of El Nino really shouldn't be going for at least a month or two; little of the cooler and wetter autumn pattern normally associated with El Nino is anticipated till later this fall...especially since it will be weak.

Autumn Precipitation:

Normal to below normal precipitation is expected this fall based again, on the same reasoning as above.

Frost/Freezes

Guidance indicates while temperatures may average above normal; most years our frosts and freezes were on schedule to a bit later than average with the more widespread freezes occurring by the second-third week of October. 

Indian Summer

Perusing our summer to autumn analogues, the likelihood of a period or two of normal Indian Summer weather this fall looks promising based on neutral conditions resulting in more amplified pattern.

Snow Chances 

In these Neutral to weak El Nino autumns;  average or normal snow risks were found but with a few years containing early October snowfalls.

Updates and notable weather events will be continued to be reported on during the autumn. Enjoy the color change.

Some dates of interest this autumn:

Autumn begins on Saturday; September 22, 10:49 AM EDT
The crest of the full Harvest Moon will illuminate the sky on Saturday September 29 at 11:19 PM EDT 
Halloween falls on a Wednesday this fall; October 31st, 2012

Thanksgiving is the earliest possible this fall; Thursday November 22nd, 2012.

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