The weather map across New England is becoming increasingly complicated this week, even if we're seeing a stretch of quiet and fantastic conditions. First, we've seen the slow moving Tropical Storm Bertha, now nudging away from Bermuda, generating swell for our coastal waters. This swell is diminishing today, but may increase again on Friday as four to five footers may return on an easterly/southeasterly trajectory. Second, the front that moved into New England this past weekend and only slowly nudged through us on Monday is still sitting offshore. Though there's been a decreasing airmass difference (frontolysis - the death of a front) either side of this boundary, a wind shift and slight airmass difference still exists, so I still analyze this as a stationary front running all the way from the west coast of Florida, over the Outer Banks, just east of Cape Cod, and then into Eastern Maine. Third, a shortwave riding just north of the Canadian border yesterday and last evening pushed a cold front south into New England, and this front has also been slowing over our region. Becoming a west to east stationary front, the slight enhancement of surface convergence along the front raises the potential for an isolated shower or thunderstorm this afternoon. Additionally, sea breeze convergence will team with a weak trough extending from Southern New England to New Jersey...another weak but present feature that will persist into Friday.
What all of this complexity means is yet to be determined by nature, but it would seem we have a series of boundaries to contend with that could serve as convective initiators. Of course, to make anything of that, we need both instability and lift. With the jet stream lifting north, dynamic lift is somewhat limited, but a series of shortwaves will still pass close enough to, and occasionally over, New England to result in sufficient lift for some convection in the coming days. Thursday's shortwave moves across Northern New England, but in these Northern communities the dewpoint has lowered thanks to the slightly cooler and drier air behind the stalling cold front. Close to the front as it becomes stationary, however, enough surface moisture lingers for CAPE to climb near or just over 1000 J/kg (lower than progged by the overly moist NMM), and with LIs running below zero in many spots, the enhanced surface convergence associated with the stationary front may touch off an isolated shower or thunderstorm in Central New England - somewhere between Central VT/NH and Northern MA into Southern ME. Overall coverage will be quite limited given the isolated nature of any convection that manages to develop. Most of this action will diminish after sundown with the departure of the vorticity maximum and waning daytime heating. There's a decent chance that showers and perhaps some embedded thunder will fire up again Thursday overnight, though - especially after midnight - as another shortwave crosses New England. This shortwave moves through in a warm air advection regime, which means isentropic lift will team with dynamic lift, and the most likely region for sufficient lift to generate precipitation will be in Northern New England, where clouds and a few showers may linger into early Friday. Drier air moves in along with anticyclonic vorticity advection Friday morning, however, and this brings sun back out in almost all areas with a new, high theta-e airmass in place. Dewpoints will run near 70 while temperatures soar into the 90s, creating heat index values in the middle to upper 90s Friday afternoon! Of course, with such heat and humidity will come increased instability, with CAPE values climbing to between 1000 and as high as 2500 J/kg, very little convective inhibition, and lifted indices dropping to between -3 and -6 Friday afternoon. There will be at least one shortwave embedded in the zonal westerlies along the Canadian border, and though the vorticity maximum straddles the border, so much instability will be present that it won't take much of a trigger to generate storms farther south. This is actually something I've given more thought to while writing this discussion than I did when making my morning on-air forecasts, so I may end up hitting the convective potential a bit harder in the midday shows on NECN for Friday afternoon. At issue here is that, while there is a lack of trigger in Southern New England on Friday, there are all the boundaries I laid out at the start of this discussion. That's an awful lot of surface convergence to simply turn a blind eye to and dispel the threat for thunder from the forecast outside of the area of cyclonic vorticity advection, so I think it's wisest to keep the best chance for scattered thunder in Northern New England, but maintain a chance for scattered convection in Central and Southern New England during the afternoon, too.
The weekend: The overall weather pattern this weekend doesn't change much, keeping a fast and relatively zonal jet stream flow over the Northeastern U.S. and Southern Canada. Timing of shortwaves will be essential to the forecast, but I've been trying to convey the fact that the weekend, overall, should be decent for most spots. Saturday in particular looks good, though I think it would be foolish to keep showers and thunder out of the forecast. Fast flow aloft will carry at least week vorticity maximums overhead, including one during the afternoon, and while it is weak, the dynamic lift it provides may be just enough to take the 1000+ J/kg CAPE and turn it into scattered thunder along leftover boundaries both mentioned at the start of this discussion, and also resulting from any Friday convection. The next essential question on shortwave timing comes as to how quickly a follow-up shortwave ripples across New England Saturday night/Sunday. With such fast and active flow, the truth is that there are likely to be a number of vorticity maximums and it seems silly to try and nail down exactly when each one will move through and what it will bring, except to say that moisture continues to load into the atmosphere throughout Saturday, Saturday night and Sunday across Central and Southern New England, and with a slowing/stalling front in Northern New England this weekend, this will allow for additional pooling of moisture and increased dewpoints, meaning that by Sunday we should be able to trigger convection very easily, so more numerous thunderstorms seem likely. Still, as is often the case in a tropically enhanced atmosphere, the day will not be a loss, and most of it should be salvageable.
Monday and beyond: We have an interesting conundrum here because we're working on a number of assumptions that I've made, and it's important to keep that in mind. One or more incorrect assumptions makes a lot of the forecast wrong. But, consider one of the assumptions made is that we still have a ghost of a stationary front off the Eastern Seaboard right now that I contend still IS a stationary front, and even has been focusing convection farther south along the front the last couple of days. Does that front ever really disappear? It will continue to experience frontolysis, but the convergence band very well may linger. This is important because we're looking at a surface low sitting off the Georgia coastline that continues to show excellent closed circulation, but has thus far failed to develop deep convection, especially after its trek across the Florida peninsula yesterday. That system is over warm waters off the Southeast Coast, now, and will be approaching the Gulf Stream as it drifts northeast. If the surface convergent zone still exists through the weekend, there is the potential for these two to link up. Bottom line here, though, from a forecasting perspective is that even if this thing doesn't bodily come north - which is still a distinct possibility - the moisture associated with it seems destined to. With conditions aloft only marginally favorable for development presently, it may never really gets deep convection that can persist, it'll really just be a low-level circulation and tropical moisture infusion that will touch off relatively low-topped tropical convection as it meanders north, but this still can serve as input for New England tropical convective clusters by Monday, so I'd expect numerous downpours and thunder amidst breaks of sun on Monday as an early estimate in an uncertain pattern.
The pattern thereafter is really somewhat monotonous, but exciting nonetheless. That is, with a lingering trough for the Northeast, but rising heights overall through the nation (and here in the northeast - the trough just remains as relatively low heights compared to the rest of the Lower 48), there will be more warmth and moisture to work with. Meanwhile, the central Atlantic seasonal subtropical high will be flexing, and doing so a bit farther north than normal. In fact, the northward displacement of the subtropical high may prove very important to New England for a long time to come. In the one to two week range - through the start of August - this means a southerly low level flow continues while numerous northern stream shortwaves continue to maintain a trough in the Northeast. The result should be numerous severe convective events in the coming weeks for New England - especially Northern New England - with plenty of heat and humidity between them, even if beaten down for a day or two at a time behind the stronger of the shortwaves.
In the longer term, though, there are real implications for a pattern like this and it should give one great pause to think with regard for the hurricane threat along the Eastern Seaboard this season. Consider what we saw in the last week on a big picture scale - a storm named Bertha that had developing just off the African coastline made a relatively early-season jog WNW and then recurved over the ocean, passing near Bermuda. That all made sense and fit the atmospheric pattern, right? When Bertha emerged off the African coast, here in New England we were immersed in a trough and a fast flow aloft that showed no sign of weakening considerably, and a run to New England would have required some tremendous timing that was relatively impossible with the fast flow and numerous shortwaves digging across the East Coast. So, Bertha reaches a weakness in the subtropical high that was induced by the strong shortwaves and recurves toward Bermuda. What happened next posed tremendous challenges for hurricane forecasters and drove up hurricane forecast error significantly as Bertha meandered near Bermuda. But what was going on with Bertha, I contend, is that she was feeling the first effects of an important atmospheric shift. The jet stream retreated northward, the subtropical ridge flexed, and Bertha was stuck in a no-man's land. What started as a clear path to recurvature rapidly changed with the de-amplifying trough and the expanding ridge. Of course, the upper level low east of Bertha also was wedged in this pattern, but served to keep Bertha in check and prohibited an eastward drift, keeping her sitting off the coast of Bermuda for days. So now we apply that to the big picture and to a forecast that pulls the westerlies farther north over the Atlantic, weakens them considerably, but keeps a trough defined in the Northeast. Additionally, we see the expansion of a ridge across the Central and Western United States, leaving a weakness in the height field across the East Coast, with a relative sink over the Northeast. Finally, with the subtropical high setting up anomalously farther north that we'd expect, both surface and aloft, this has to give one pause to stop and ask if the implications here are that New England will serve as a sink for numerous Atlantic systems this season. Not only does the described scenario favor Bertha look-alikes running WNW before recurving - but farther west as the subtropical ridge expands - but also favors a continuation of the very type of situation we're seeing played out in real time as I write this - not model forecasted, but actually observed - and that's a situation of frontolysis in the Northeast and along the eastern seaboard. When you stop to think about the pattern, an area of frontolysis is caused by a number of boundaries and airmasses converging to one place where they enter a virtual no-man's-land of week steering flow and/or shifting wind patterns with height. This sink so far has caught a front last weekend that still sits off of our coastline, a new front that came in last night and will die over us, a trough from New England to New Jersey that is the remnant of yet another long-lost frontal boundary, yet another cold front that will drop in this weekend before stalling and washing out, and perhaps a low level tropical origin circulation traveling up the first front out of the Southeast US waters. If this is the pattern for the season to come, New England may be looking at not one, but a number of tropical interests. It's a pattern that I will remain quite alert and to and aware of in the weeks to come.
Matt
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