Probably the most persistent climatological item the media has been trumping-up lately is our strong El Nino that is, by all accounts, projected to even grow stronger during the fall months. This presently strong El Nino could very well rival the two most recent very strong El Nino's; 1997 and back further, 1982. Thus far, there are some striking similarities between the 1997 El Nino, including the overall, Pacific water temperatures and this season's El Nino. Widespread, above normal water temperatures dominated in 1997 over the Pacific and now again, in 2015.
Therefore; does it mean the upcoming fall and winter will be similar to that of 1997's - if only it were that easy. There are many other variables to research to get a most likely scenario for both the fall and winter, therefore let's deal with the fall first.
AUTUMN 2015 OUTLOOK
I look for temperatures to average a degree or two below normal this autumn.
Analogues (below) and history tells us while September's temperatures have started off above normal and may very well, average above for the month; that is not the main trend of the entire El Nino fall. Generally, El Nino fall temperatures are normal to below with chances of below normal temperatures increasing as the fall unfolds (see analogue discussion). One of our best analogues as far as El Nino strength and above normal Pacific water temperatures is 1997. That autumn averaged over two degrees below normal (in line with the analogues) but there are other just as important variables such as the NAO, EPO and PNA that dictated the below average temperatures along with El Nino. Recent upper air trends across Canada suggest an aggressive upper low and resulting jet stream with time.
All data suggests rainfall to average normal to below normal rainfall with only one out ten falls wet. Analogues were quite variable as far as rainfall trends and amounts but with a slightly drier overall trend. Generally drier than average conditions develop into the winter of El Nino years.
Below are the August water temperature anomalies across the Pacific. A one can see there are a lot of above normal water temperatures in both the central and southern Pacific. Note the two distinct above normal water temperature bands extending west from North America and South America ( El Nino). El Nino is highlighted in the white rectangle with specific descriptions, below. I find many people still don't have an understanding of what "El Nino" constitutes and where it is over the Pacific.
Description of above map :
Sea-surface temperature anomalies (degrees Celsius) in the Pacific Ocean on Aug. 13, 2015. The area highlighted by the white rectangle shows the warmer-than-average waters of the equatorial Pacific Ocean. Significant warm anomalies also were present in the eastern Pacific Ocean west of California and Mexico's Baja peninsula, while cool anomalies were seen in parts of the equatorial western Pacific Ocean.
Areas of interest in ENSO
All computer trajectories increase El Nino's strength this fall, before decreasing (and some rather dramatically) El Nino's strength by late winter into spring. On a side note; the timing and actual decrease later this winter may set up some interesting and plausible scenarios for our late winter-springtime time period.
AUTUMN 2015 ANALOGUES
Cooler than Normal Autumn?The most notable take from this set of data reflects a cooler than average fall. In addition, the chances for below normal temperatures increase as the fall wears on. Why the ten Septembers were mixed and averaged a degree below normal, there were still four where the temperature averaged normal to above which could very well happen again this fall. October's tended to average normal to below with just two above normal out of ten. Moving into November; shows the weather held mainly cooler than normal with seven below normal, two normal and just one warmer than average and that was only a degree and a half.
Scanning records from all nine of these falls showed two distinct periods where a killing frost or freeze took hold of the region. Using the mid 30s for the threshold at Detroit proper, most other areas would likely dip into at least the lower 30s and experience a killing frost/freeze. The most common period came mid to late October, or about the normal time. However, there was a notable subset of frost/freeze dates occurred earlier as early as mid to late September!
All data suggests rainfall to average normal to below normal rainfall with only one out ten falls wet. Analogues were quite variable as far as rainfall trends and amounts. Generally drier than average conditions develop into the winter of El Nino years.
Five out of the nine years /1876 is N/A/ had near to below normal snowfall through November during the autumn. However, there was a quite notable exception of the remaining four: 9.8" in 1925; 9.1" in 1940; 7.1" in 1972 and 4.7" in 1997 / This is too much of a trend to ignore and tells me with the dominance of colder than average Novembers; there is the risk of a significant snow in November IF the storm track and cooler than average temperatures align as in the past, something to watch for. Normal snowfall at Detroit is just 1.5" at Detroit/, with a inch or two more further north into Flint and Saginaw.
Important Dates This Fall
Autumn begins: Wednesday morn - September 23, 4:21 A.M.EDT
Harvest moon: Sunday eve-night - September 27, 2015. And it’s not just any Harvest Moon. It’s also a supermoon that’s staging a Blood Moon eclipse.
Halloween: Saturday - October 31st 2015
Thanksgiving: Thursday - November 26th, 2015
More on the rare super/blood moon
The last time a super moon and blood moon occurred at the same time dates as far back as 1982 — and the next isn’t expected to occur until 2033. Interesting it occured the fall of our strong El Nino and analogue year.
Enjoy the fall colors if we get the chance!
Bill Deedler -SEMI_WeatherHistorian