It's been quite some time since my last post...thanks for checking in. My wedding is Saturday, August 16, so I'm down to counting days, then it's off to Quebec City for a week, back to operational forecasting Monday, August 25. Updates the rest of this week will come only if necessary, and next week I hope to remain away from internet access (though I always have weather data on my cell phone, as I'm sure many weather nuts can attest to) on the honeymoon.
Let me start with where I think we stand in this pattern. What has now become a much talked about summer pattern of repetitive damaging weather is certainly unusual, and also has been record setting. My guess is that Caleb Boulter's (northconwayweather.com) realization that this has been the wettest summer in 50 years of record keeping for North Conway, New Hampshire, will be only the first of many similar declarations to come at the end of meteorological summer at the end of August. If you haven't seen the comparison pictures of last year to this year from the Swift River in New Hampshire, on the New England Photos Page, you may want to check them out - a rather amazing contrast.
Of course, the longwave pattern has featured a persistent trough across the Northeastern United States and Eastern Canada, and the Atlantic ridge that was discussed here a couple of weeks ago has only expanded over the course of time, now bridged from Western Africa all the way to the Bahamas, northward across most of the North Atlantic. This is a rather amazing subtropical ridge that began anomalously far north, but has simply expanded on the southern periphery in the last two weeks. Meanwhile, heat has continued to hold in the Southwestern United States, resulting in a returning ridge that amplifies and deamplifies dependent upon the somewhat flexible Eastern Pacific trough that occasionally drifts over the Pacific Northwest when strong enough shortwave energy digs into the longwave trough. The longwave, hemispheric pattern in the days to come will feature a weak bridging of the ridge all the way into the Tropical Pacific by early this weekend, but that bridge will degrade to gaps once again as the troughs - in the Eastern Pacific and the Eastern United States - return by the end of this weekend. The difference as we head into next week and beyond is the retrogression of the Northeast U.S. trough toward a position over the Eastern Great Lakes and Ohio Valley. This is an important shift in the longwave pattern that competes a cycle of placing New England into a "weather sink" that we've essentially been in throughout the year. A couple of weeks ago, those of you who read regularly will remember that I laid out the developing pattern by which a number of surface and low level fronts have been converging upon New England and wasting away on top of the region, leaving surface and low level convergent zones beneath a series of upper level vorticity maximums and associated cool pools. Of course, we know the result has been a series of thunderstorm events throughout the summer and the flash flood that New Englanders have been contending with in the past week. Now we're talking about a change to the longwave jet stream pattern that will bring the upper lows southward just slightly farther west, and while that's a small change meteorologically, it has significant ramifications, as the cyclonic flow to the west of New England will induce southerly flow, and will afford the opportunity for stronger shortwaves to lower heights at the base of the trough and just ahead of the trough axis. This type of setup is ideal for pulling the tropics to the Northeastern United States, and represents an open highway to New England, wtih one noteable gate.
The gate that will stop tropical activity from reaching New England is the aforementioned subtropical ridge, which will pulse westward, then retract east. Considering it will pulse more often than retract, this keeps the gate up the East Coast effectively closed much more often than it opens. When the gate actually does open - in other words, when a weakness in the ridge emerges - it will come courtesy of lowering heights associated with northern stream shortwaves, which therefore means it's the same delicate balance of timing that needs to occur to pull a tropical cyclone northward at the correct time and attitude to deliver a blow to New England. Then there's always the type of setup where the gate opens frequently, but there's little activity to tap. I doubt that will be the case this time around, as the tropics are becoming more active with many waves upstream and a simmering Atlantic in the Western Atlantic, though the central and eastern stretch of the ocean features a cool tongue that has likely been playing a role in retarding development over those areas.
In the short term, the retrogression of the longwave trough does certainly provide a favorable northern stream setup to open the tropics, but in the midst of summer with a bursting and northerly displaced subtropical ridge, all this does is make room for the Atlantic ridge to expand over the Mid-Atlantic and Southeastern United States, effectively strengthening the southern "gate" to tropical activity, and ensuring that storms will have limited if any window to actually run north. This favors a track into Florida and the Gulf of Mexico until at least August 25-27 for anything substantial that comes out of the normal storm breeding areas. I make this caveat because, as northern stream shortwaves move across New England, each drives a front southward along the East Coast. With warm ocean temperatures and enhanced low level convergence, the base of dying fronts over warm water can often lead to spin ups of tropical systems, and that will remain possible through the period, especially as fronts grow stale off the coast.
Once we reach August 25-27, things begin to change in the southern stream ridging, with a weakness in the height field over the Southern Plains expanding east and diminishing the ridge, thereby weaknening the gate for tropical activity, and increasing the potential for the Eastern Seaboard as we head into the beginning of September. It's during this time period - the first half of September - that I'm most intrigued by and think we stand our greatest chance for impacts from tropical cyclones. Not only is that the climatological maximum in seasonal storm frequency, but also is a special time climatologically for New England, when we see warm sea surface temperatures, and have a history of healthy tropical cyclone strikes.
Enjoy the next week and a half. See you on the flip side.
Matt
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