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