A brief discussion today as I've been running behind...
Drier air is moving into New England and will take hold for the weekend but it's by no means an instant surge of drying. Instead, new fall air is coming in installments, one chunk at a time - the first of which moved from west to east across New England Friday morning and the next sparked thunderstorms over Upstate NY that will move east and northeast, maintaining the most integrity across Western and Northern New England with the potential for a few damaging wind bursts and large hailstones in the stronger storms. These storms will have a tendency to weaken as they move farther east in Southern New England, farther away from the upper level energetic disturbance that's resulting in their development.
Overnight Friday night we should see at least some clouds given the passage of the upper level disturbance overhead, though precipitation will diminish and a general clearing trend will take hold. The new airmass is cool and dry, and temperatures will fall into the 40s even with an active wind.
The drying trend will bring fantastic, cool, but dry fall air into
New England for the weekend, locking
sunshine in for a great weekend of fall festivals and fairs!
Saturday's high temperatures will rise to either side of 70 degrees -
warmest south - and an area of high altitude clouds will filter the
sunshine on Sunday but weather should continue to be pleasantly perfect
by autumn standards for the American
Liver Foundation's Football Walk in Boston that I'll be hosting (click
here to find out more about it, and to find out how you can join the
walk!). Temperatures are likely to be cooler on Sunday, however, thanks to one final installment of fall chill and a northeast wind off the ocean.
Next week is likely to bring yet another stretch of fair weather and moderating temperatures at the beginning of the week as high pressure builds over New England and then off our coastline, though there are two features to pay attention to. The first will be the potential for a southeast wind to bring low clouds or fog when the dry and pleasant air lets go and starts moving east, though that is contingent upon a number of factors and not at all certain, but something worth watching along the South Coast by late Tuesday. What seems more likely is the northward expansion of a tropical feed of moisture up the coastline that would leave the East Coast vulnerable to tropical systems and tropical moisture once high pressure lets go...allowing rain to move in by later Tuesday and Wednesday, though the axis of deepest tropical moisture will still need to be determined.
Have a great weekend!
Technical Discussion: None today.
Matt
MATT'S TECHNICAL DISCUSSION - Updated Wednesday, September 26 at 1:45 PM
Severe weather threat shaping up as discussed yesterday. Dewpoints have risen into the middle 60s, temps are 90-95 and this is rapidly increasing instability. Vort max already resulting in multiple cumulus lines across Nrn NewEng/Upstate NY as of this writing and that trend will continue as the cyclonic vorticity advection encounters instability with LIs to -4 and CAPE values of either side of 2000 J/kg. Mid-lvl speed max discussed yesterday is present and plenty of mid and upper level dry air is available to further increase downburst potential. Damaging downburst wind and frequent cloud to ground lightning to be biggest threat and I expect afternoon storms in Nrn/Wrn NewEng to initiate in orographically favored areas but then increase in cov'g and intensity in response to dynamic forcing. Organizing line in Upstate NY looks ready to evolve into broken squall line and bowing segments are likely along the line given intense unidirectional flow and sufficient speed shear. There will be some backing of the surface wind in north/south oriented valleys, but at this point it appears the lifted condensation level would remain high enough to avoid rotational problems in cells. Convection should weaken overnight and leave a quiet cold front to stall in Central NewEng.
Guidance is touting a quick return as warm front to the Canadian border on Thu - that seems unlikely given magnitude of surface cool air behind the front and I think we'll be surprised by the surface cold and resultant clouds in North Country, tho farther S - esp from RUT-CON-PWM southward - sun should break thru and temps will respond by coming either side of 80 with warmest temps south. Great agreement on dry air at multiple levels in Central/Srn NewEng on Thu and really don't think it looks like that bad of a day for most communities in these areas! Thunderstorms likely to refire either side of front in Central/Nrn NewEng later Thu and shortwave passage Thu night will expand coverage in these areas. Still a very difficult call on what to do with the tropical wave that will parallel the coastline Thu Ngt into Fri. Yesterday I felt pretty good about the NMM being an outlier and the 00Z NMM was farther east, making me feel better. Unfortunately, 12Z run is farther west than them all! GFS is progging a sfc low to move over Cape Cod like the NMM, but analysis of the 850 mb vorticity shows it moves east of the Cape, as does the 850 mb vorticity on the GGEM. For now, I continue to believe the NMM is too far west in error, and therefore continue to believe the shield of steady rain may move over SE MA and then perhaps into Ern ME Thu Ngt and Fri AM respectively, but I'm not excited about filling the rain in between the frontal precip and the tropical wave. Nonetheless, both areas should produce heavy precip rates with over an inch of QPF with locally over 2" in the northwest frontal band, but I'm not ready to jump on the 2-3" NMM amounts farther east yet.
Fri AM rain to give way to sun with drier air advecting in on west wind behind occlusion, but reinforcing cold front and assoc wind shift comes later Fri afternoon - swinging from Berks to Boston between 18Z and 00Z, for example - and likely to bring one final round of late Friday scattered convection before dry and cold advection sets in on an active northwest wind Fri Ngt.
Matt
Tuesday's Discussion:
Increasing gradient flow to advect warmth and moisture into NewEng boundary layer with Tds on the rise and the first uncomfortably sticky night in awhile for Srn and Wrn NewEng. Positive theta-e advection continues as SW wind continues on Wed, increasing instability regionwide as LIs drop to -2 Nrn and Wrn NewEng where dynamic lift moves in first assoc with vorticity lobe stretching across NY State and toward NewEng Wed afternoon. Dynamic lift to shift SE into Central NewEng by Wed eve when LIs in Srn VT/Wrn MA/Srn NH drop to as low as -4 to -5 and CAPE values rise to in excess of 2000 J/kg! Seems like it's been forever since we've seen this kind of instability given our recent stretch of fair weather, so let's be sure to take a good look at the potential for severe weather: 250 mb jet streak puts Central NewEng in particular into right rear quadrant of jet streak favoring large scale ascent. 500 mb winds are strongest across Nrn NewEng at 55-65 kts Wed afternoon but this is the core of the mid-lvl winds and speeds closer to 35-45 kts over Central NewEng, but that's plenty good for downburst creation and so are 700 mb winds on the order of 50+ kts north and 35-40 kts central. At 850 mb jet streak is also centered across Nrn NewEng where confluent flow into Ern ME trof will keep winds fast and, alas, confluent, and this directional confluence will be supplemented by increasing speed confluence over Central NewEng later Wed with 25-35 kt llvl jet. Dry air is available aloft and this should further enhance potential for damaging downbursts, which become increasingly likely as dynamic lift shifts south and east from RUT to MPV to IZG points southward into Wrn/Nrn MA. Lift is deep and frequent cloud to ground lightning also likely.
Of course, the fun won't stop here as frontal boundary slows/stalls with lack of strong height falls aloft and ridge holding strong over Western Atlantic. This persistent ridge will be problematic from two levels - first, it will hold the front over Central NewEng to focus occasionally heavy rainfall again on Thu, which may not be so impressive in its areal coverage as it will be for the locally intense nature of it, which should be situated over Central NewEng and governed largely by convective processes that will remain healthy given either side of this front will be an amazing temp difference as cool, dry air settling southward into the North Country will have moisture added and therefore cool substantially to support temps in the upper 50s to lower 60s for highs, while Srn NewEng remains on the warm side of the front and finds temps around 80 - perhaps higher in any areas that can find sunshine. Rain and thunder vicinity of the front are likely to be heaviest during the afternoon and evening not only because of diurnal contributions but also given passage of weak vort max.
Meanwhile, an important feature will be rounding the western periphery of the Western Atlantic ridge. This feature has been rather subtle in previous guidance cycles, but has been present throughout and will undoubtedly garner more attention given the 12Z NMM attempt to spin up a tropical cyclone along the Eastern Seaboard and bring a glancing blow to Cape Cod late Thursday night and early Friday morning. I should start by noting that this is a concern I've been keeping an eye on for several days, as this tropical wave has been very well modeled, but more than that it's the combination of an Ohio Valley trough trying to dig while the Western Atlantic ridge holds firm, and this potential was laid out in Tuesday, September 18's technical discussion, so it's something that's begged for attention for a long time. Now the question is much more finely tuned - how close does the tropical wave come to NewEng and how quickly does it intensify. Though the NMM paints the solution I've been most concerned about, this is a western and strong outlier of scenarios. I'm willing to entertain the potential for quicker strengthening given relatively warm waters sitting in the path of the wave and the dynamic interactions that will occur as it nears the height falls and quickening jet, but it's that fast jet that gives me pause on buying into a western solution, as confluent flow is very well pronounced through early Thu. It does relax as the wave rides northward and the strong shortwaves dig across the Great Lakes, but it appears as though the backing of the mid-level flow comes literally just about 12 hours too late to get the wave up over Cape Cod, and the speed of the wave very well may leave it as a transient open wave that carries a load of tropical moisture east of NewEng that becomes a bigger player upon merger with the northern stream energy in Nrn Canada, instead contributing to our Friday and Friday night wind. This definitely is still something to watch carefully, but my estimate right now is that the NMM western outlier solution is probably not going to verify.
Shortwaves combine to drive longwave trof thru NewEng Fri Ngt after occlusion carries drier air but not cold air into Northeast Fri afternoon after Fri AM rain, and though drying occurs behind this front there very well could be one more round of convective showers with the passage of the trough Fri eve or early Fri Ngt, after which the winds become northwest and quite active overnight as drier air moves in ahead of next high pressure bubble. Fair weather cell in charge this weekend as it follows in the footsteps of its predecessors, slowly shifting east and ready to settle in a position offshore Mid-Atlantic coast. This difference this time is the indication that there will be a bit more moisture return ready to come back at NewEng, and with the combination of cool and dry air relinquishing control while winds become southeast, low clouds and fog are a possibility on Mon before nicer wx settles in next week with only very gradually moderating temps unlike the current stretch of warmth, largely because the high center doesn't shift as far south and therefore there will be more ocean input to our air, though average NewEng temps will still be above normal after Monday.
In the longer range, it's waaaayyyy too early to try and speculate on whether newly formed Karen or the tropical wave by the Lesser Antilles will be contenders in NewEng weather, but one way to at least get a general idea is to examine the long wave pattern in the extended range. Any attempt by the wave near the Lesser Antilles, should it attempt a move toward NewEng, would come after the upcoming push of high pressure and cool air (which incidentally may help to fuel development of a system near the FL east coast, but that shouldn't have an opportunity to move toward Neweng and will be dealt with on the Tropical Meteorology Page) which means the Lesser Antilles wave, or future cyclone if it develops, would come at a time when an inverted trough *would* be near the Eastern Seaboard with lingering very weak troughing after departure of this weekend's Ern US trough, but it also looks as though the SE US ridge holds enough to keep this inverted trough offshore, thereby indicating a northward jog by any wave would stay east of Neweng. The situation with Karen, however, who is over the Central Atlantic is more interesting for the East Coast, if not determinable from this far out for NewEng or not. That is, the storm is currently forecasted to curve northwest and then most guidance takes the storm northward thereafter, which would result in a nearly certain recurve given the strengthening jet that far north. I believe this widely agreed upon solution is probably in error, though, given the strength of the Atlantic ridge and the hesitancy for this ridge to break down. This should keep Karen on a WNW course longer than anticipated, and once we reach the middle of next week the jet stream actually lifts north and by 10 days (end of next week) ridging tries to rebuild from the Western Atlantic to the Eastern Seaboard, indicating Karen would have very little opportunity for recurvature and given East Coast residents something to at least pay attention to. Again, too early to even guess, but not to early to guess on the longwave pattern which indicates Karen may pop up in a few more discussions!
Have a great Tuesday.
Matt
