As we enter the final week of September, with seasonable temperatures finally returning for the moment, I'm awestruck with just how warm it's been up to this point. 14 out of 22 days thus far in September have been above average, with just three falling below average.
Why has it been so warm?
We can attribute the bulk of this extended and persistent heat to a strong area of blocking high pressure situated several hundred miles off of the Virginia Coast. This weather setup gets its name simply because it prevents, or blocks, any other type of weather system from moving into the area that it is occupying. High pressure systems of this magnitude are usually only displaced by major shifts in the jet stream, which can overpower and push the high pressure away from the area that it had been occupying. The rotation of the high pressure system essentially acts like a fan. Because the high pressure is situated to the southeast of Central New York, and high pressure systems spin in a clockwise direction, we're right in the path of warm humid air from the Gulf of Mexico to be whisked up our way. This diagram from last week showcases the setup we found ourselves in for most of September.
While this type of high pressure setup in it of itself is not rare, especially during the summer months, the duration of the high pressure is. It's longevity can be attributed to the absence of any jet stream shifts. Up until now, the jet stream has been parked well north of our area, exiting North America as far north as Newfoundland. Going into the end of September, the jet stream will finally creep south, and displace the blocking high. This will usher in an extended period of seasonable temperatures.
If you don't like the heat, were there still any positives to this setup?
Due to the moist nature of the airmass, the Holland Patent area experienced frequent but scattered rain showers throughout the past few weeks. This helped the Holland Patent area recover from the minor drought (2.5" below average rainfall over the summer) that it found itself in at the beginning of the month.
What can we expect going forward?
As we close out September, and head into the first week of October, cool and unsettled weather will be the rule. As the Northern Hemisphere as a whole begins to cool down as it receives less and less solar exposure, the jet stream will continue to slide south. This movement is not uniform. In other words, the jet stream will bounce back and forth as it inches southward. Because storm systems travel along the jet stream, we can expect an increase in cloudy and unsettled days as the jet stream will be spending a lot of time in the vicinity of Central New York. There's still no signs of a prolonged period of below average temperatures over the next few weeks, however it's fair to say that this time of above-average temperatures has come to a close.