Now that we finished July, we have two below normal summer months under our belt…June and July, so what does August look like? Before getting into that, I just wanted to explain one of the main reasons for the cool summer… a negative NAO. Generally I refer to the North Atlantic Oscillation /NAO/ (which includes the Arctic Oscillation /AO/) in just the Winter Outlooks, where they are most relevant. Recapping from the Winter Outlooks; when the NAO (or AO) is in a negative phase, troughing over Eastern Canada and the US is encouraged which in turn, brings predominantly northwesterly winds out of northern Canada resulting in colder than normal temperatures
The persistence and magnitude of the below normal temperatures this summer is in fact, well matched by the observed trace of the NAO (and AO). Just look at the Arctic Oscillation trend seen since summer (Jun 1st), it’s been exclusively negative!
In the last week or so (the red trace above) are the ensemble member projections into early August. This intimates a moderation of the cool weather experienced recently as the AO becomes more neutral (closer to normal temperatures) for the first few days of August anyway. But what of the whole month of August? What are the chances of making it three below normal months in a row? Actually, a little better than one may think when checking just the analogues used for the Summer Outlook. Checking our analogue summers, using just two groups (which were the most prevalent, anyway) our normal to below June and July and narrowing further, the below normal June and Julys.
*note; update final July 2009 ave to 68.9 and summer to 68.4
It’s interesting to note that when using the cooler side of the equation in the broader sense (normal to below), out of those eight Augusts, six turned out to be cool, or 75%. One August was within a degree of normal /71.8/ with 72.5 /1963/, while the other was cooking at 74.8 /1918/ - much warmer than any of them.
Narrowing it down further (and more like this summer) when both June and July were in the below normal category, two out of three were cool or 67%. Again that sizzling August of 1918 with an average of 74.8 completely turned things around and helped warm up the cool summer of 1918. Can it happen again, of course - but when looking just at the cooler analogues chosen from our past, the odds are against us. I’d settle for a normal August like in 1963 and in our initial Outlook, I was looking for the second half of the summer to be warmer than the first.