No Extended Warm or Cold Spells; Shorter-Lived Pattern Changes ContinueSeptember 16th, 2013 at 9:45 pm by Don Paul under 4 Warn Weather
There are still no signs of a truly extended period of abnormally cold or warm weather over the next couple of weeks. Our extended range guidance still favors–in the mean–more ups and downs. At this time, we’re not looking at heat & humidity matching the 2 days last week, or many days as chilly as was today/Monday the 16th. But the ups and downs are fairly common as we get into autumn. This week’s warmup will bring a few days of 70s (maybe close to 80 on Friday) followed by passage of another cold front on Saturday. Cooler–but not truly cold– temperatures will return for Sunday and early next week. The Sct Tstorms reaching our region by Friday afternoon or evening will be out ahead of a cold front with enough upper air dynamics to possibly strengthen some of these cells. The showers & tstorms should exit our region by Saturday afternoon, giving way to a pleasantly cool and dry Sunday.
What’s interesting now is that the ensemble forecasts for our 3 most-discussed oscillations–the AO, NAO & PNA–are all showing tendencies which would eventually favor colder weather in the NE & Great Lakes. This trend (noticed by Think Snow last week) has now been showing up for a number of consecutive days. At this time of the year, the influence of these indices is less than it would be in the cold weather months, to be sure. But it is not a nonexistent or totally insignificant effect in late September, which is when these tendencies are forecast to take hold. I’m not yet seeing a reflection of any such impacts at the ends of the ensemble means. Frankly, however, if the indices continue in this direction (positive PNA, Negative NAO & AO), I would expect some reflection to begin showing up in the ensemble means soon. It’s possible there’s more of a lag than I recall between the indices and their reflection in the atmosphere of eastern North America. Or there may be other variables, such as the currently active MJO, which are dampening any such impacts. By the way, CPC says the 2 week MJO forecast favors suppressed tropical cyclone development in the Atlantic basin. This is in the middle of the climatological peak for that basin. I believe the more active trend will continue in comparison to mid-summer, but the activity will remain below average for this time of year through much of the remainder of the month.