Some distinct changes in the weather (and challenges, especially for forecasts along the immediate coast late tonight and tomorrow) are on-deck for the next few days. We can’t forget that this is the final day of September, yet another very wet month across the Eastern Region. There’s been a substantial amount of rain THIS WEEK ALONE (locally greater than four inches) across northeastern Pennsylvania, as well as in parts of upstate New York and in northwestern New Jersey. The I-95 corridor between New York and Washington D.C., as well as most coastal communities, have only had about an inch or less. Lee’s tropical moisture, and the historic flooding it brought to the Susquehanna River Basin three weeks ago, definitely played a key role. This will officially be the wettest September ever in Allentown, Pa. (there was 12.99″ for the month as of midnight, and a couple of showers prior to midnight tonight might push this location past the 13-inch mark).
Today is getting off to a dry start, since we’re currently in between weather systems. The impulse of energy which led to yesterday’s showers and heavy thunderstorms has now pushed into eastern Maine. Therefore, the focus of our attention will be shifting to a very deep, upper-level low pressure system, which has been stuck over the Great Lakes for the past week or so. It will start to finally make its move to the east, and will be “finding a new home” in the Eastern U.S. this weekend. This energetic storm includes a large pool of unseasonably cold air aloft, and it will push a cold front across New York State and Pennsylvania this afternoon. Out ahead of the boundary, there will be some sunshine and a southwesterly wind today. Both will allow most temperatures to reach the upper-70s this afternoon. But later this afternoon and this evening, showers will start popping up along the coastal plain, and many of these will happen prior to 4 p.m. much farther inland.
Tonight, a large trough in the jet stream will slide to the east, and its axis will have a “negative tilt” — which typically means that any surface front will be slowed down to a crawl, and a new wave of low pressure will develop along it. Most of the global models are indicating that the wave will be forming off the Jersey Shore and south of Long Island. That is why we must be mindful that if there are some “showers” around this evening near the I-95 corridor and just west of it, this will probably evolve into a steadier rain after midnight, and primarily right along the coast. Temperatures should fall through the 60s and into the 50s in most places tonight as the much cooler air arrives, and then there will be very little recovery occurring tomorrow. The models are also implying that the “open trough” of low pressure tomorrow will once again develop more of a “closed circulation” — or, there will be a cut-off low creeping northeastward along the I-95 corridor between Washington, D.C. and Philadelphia. What this will mean is that our surface wave, located near the Jersey Shore tomorrow morning, will be pushing tonight’s shield of steady rain northward into New York and New England. However, beneath the pool of cold air aloft that’ll sneak in behind it from the south and west, instability generated by tomorrow’s daytime heating will cause some additional showers to pop up. There will be a brisk wind, averaging 12-25 mph with some higher gusts, and temperatures in the low-60s in most of the big cities will “feel like” they’re in the lower or middle-50s.
Perhaps the biggest forecast changes we’re implementing this morning pertain to Sunday and Monday. The consensus amongst the various forms of guidance is suggesting that this upper-level low pressure system isn’t going to move all that much either day. And, while this doesn’t mean there will be a steady, or persistent rain occurring, the chances for seeing bright sunshine, especially on Monday, aren’t going to be as great as we previously thought. And, whoever winds up directly beneath that upper-level low will also probably encounter a few widely-separated (and mainly afternoon) showers both days… Drier, milder and more tranquil weather will probably not return until Tuesday or Wednesday of next week.
Have a good weekend!!!