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ARE THE JET STREAMS SHIFTING? IF SO, WHAT DOES THAT MEAN?

This article is a bit old (released in April), but interesting, nonetheless, as an overview of research done at the Carnegie Institution for Science.  Cristina Archer and Ken Caldeira of the Carnegie Institution’s Department of Global Ecology went back over 23 years of records, and tracked the jet stream winds.  Those of you who read my weather discussions on the New England Weather Analysis Page know that I often refer to the jet stream as the fast river of air aloft (fast winds) that steer our storm systems and separate cool air to the north from warm air to the south.  A change in the jet stream wind position over time would indicate a change not only in the storm track, but also in the thermal gradient of the earth.  In this study, it's determined that the jet streams are migrating toward the poles.  Keep in mind as you read the study, that this is based on only a mere 23 years of data on an earth that is billions of years old, and there's no way to know if this is part of a recurring cycle that occurs on a scale greater than the time period examined.  Click here to read the summary, and click here to read a report from ScienCentral News, complete with a brief interview of one of the researchers.

AFTER WEDNESDAY'S SEVERE STORMS, DAMAGING SQUALL LINE POSSIBLE ON THURSDAY - NO REST FOR THE STORM-WEARY

With one severe convective event ongoing as I write this, another is never far away in this persistent Northeast U.S. troughing pattern that continues.  In the immediate term, there has to be some fairly significant street/urban flooding in the Boston area right now given the training thunderstorms along the stationary frontal boundary, which, incidentally, should get pushed gradually east tonight as the weak surface wave rippling along the front nudges the front offshore as it moves into Maine.  This also decreases the potential for areas of widespread fog, especially with a southwest gradient overnight that will keep temps up, with the exception of south-facing coastlines that will see pockets of marine layer fog in place.  Convection, on the other hand, is expected to die with the loss of diurnal heating and with the passage of the weak surface wave, and though there is positive theta-e advection moving east out of the Ohio Valley at 850 mb, it ends up diffuse enough and there's low enough relative humidity above and below that the overall effects should be negligible.

The same cannot be said for the shortwave producing damaging storms in the Upper and Central Great Lakes today that will continue east on Thursday and cross New England.  Deep southwest flow in advance of the shortwave will bring warm and moist advection through Thursday, and the low level jet only strengthens during the afternoon and evening immediately ahead of the disturbance and associated cold front, screaming at 40 to 50 knots at 850 mb by 00Z Fri.  Convective initiation shouldn't be difficult with the amount of cyclonic vorticity advection tangent to the southwest to northeast oriented vorticity maximum.  Morning insolation in New York State will provide some instability for convective growth ahead of the shortwave, but further destabilization will occur in New England where sun lasts longest.  There's likely to be some high altitude cirrus blowoff from today's Great Lakes convection, but strong sun angle should do its dirty work through this deck.  Dewpoints never really were beaten down all that much in Central and Eastern New England, and Western areas where dewpoints dropped into the 50s will rebound overnight Wednesday night and Thursday with the strengthening southwest boundary layer flow, reaching the mid 60s regionwide on Thursday afternoon.  Lifted indices drop to between -2 and -4 and CAPE values are 1000 to 1500 J/kg - neither is all that amazing, owing largely to mid-level warm air around 500 mb.  Kinematics should take over with the developing convection, however, that will develop as single cells and quickly morph to clusters over New York State, that will become a well established pre-frontal trough squall line by the time they march into New England from WNW to ESE Thursday afternoon and evening.  This trough will move quickly enough to get storms through before fireworks time Thursday night for those New England communities that are having them, but the exceptions may be in Southern CT and Eastern ME, where it's a closer call.  There's also the concern of scattered activity along the front itself, which accelerates over New England Thursday evening as the pre-frontal trough loses definition while moving offshore, but at this point it appears instability will be weakening by the time the front passes post-sundown, which means we may be looking at a situation where a second round of scattered convection deteriorates enough for most communities to get the show off OK.  Of course, a quick progression of a convective line also implies widespread damaging wind potential as we tap the unidirectional southwest flow to 40-50 kts at 850 mb, and this means downburst winds are of concern.  Hail threat exists, but warming mid-level temps decrease the large hail threat somewhat.

The front will be yet another stubborn, slowing, pesky front for New England that we'll have to contend with this weekend.  At this point, my thinking continues to be that dry air in the mid-levels will contribute to mostly dry conditions across New England on Friday, with deeper moisture suppressed south, though a few showers still can't be ruled out at the South Coast.  Being on the north side of this front isn't all that easy from a forecasting perspective, though, as a series of weak waves of low pressure will ripple along it associated with shortwaves aloft, and this will mean the potential for weak northeast wind flow at times through the period.  On Friday, the wave to our south is weak enough and suppressed far enough south that effects for most areas excepting the South Coast should be minimal, though I do think it'll be hard to clear out low level moisture and therefore lots of clouds may still linger for much of especially Southern New England, with a possible solid overcast in SE MA.  Meanwhile, with the high building over the Eastern Great Lakes and a bubble of this high breaking off over Northern Maine and migrating east, there will be cool and dry air available, which not only will bring a cool Friday night to Northern New England, but also will be available east of New England on Saturday as the next wave approaches.

Herein lay the issue for the weekend - just how strong will the Saturday wave be, how much overrunning precipitation will result, how far north would it extend and how much of that cool air will be tapped on Saturday?  Right now my best estimate, based on the composite of the guidance and the position of the front is enough easterly flow to keep Eastern New England cool on Saturday, with lots of low level moisture regionwide that will keep plenty of clouds in place.  I've cooled temperatures as a result in the Saturday forecast, but Northern New England should hold more sunshine and a terrific day with drier air in place.  I do think there is the potential for a period of significant rain in Southern New England after midnight Friday night into Saturday, heavily dependent upon the timing of the wave, but that's more based on the thought of overrunning than anything else and re-evaluation of the strength and timing of the surface wave on the stationary front south of New England will be necessary again tomorrow and Friday.  By Sunday, my thinking is that the front snaps north as a warm front - probably slow early on, then with increasing speed.  This may actually make for a worse day in the North, but a day of clouds breaking to increasingly warm and humid sun south.

That's all for today.  See you back here for the severe weather threat tomorrow - radar links on the right side of this page.

Matt

MORE THUNDERSTORMS LIKELY ON WEDNESDAY

Convection is firing today as a result of two forcing mechanisms - one is the surface cold front that has once again gone stationary, and the other is the vorticity maximum churning east across New York State associated with the eastward migrating longwave trough.  Radar imagery is linked on the right side of the page and you can track the thunderstorms by clicking on the thumbnail for your area.  The tendency has been to slow the trough's passage and this means we have to consider the possibility of convection in the forecast on Wednesday.  But, let's not put the cart before the horse!  Tuesday convection in the Boston Metro area fired quickly around midday, and was forecasted well by the higher resolution NMM, WRF and SUNYSB MM5, the product of increased instability along and ahead of the stationary front, in the 60+ dewpoint air that built CAPE values to over 500 J/kg with hardly any convective inhibition.  Those convective elements died down with drier air moving into the mid-levels, but while the convection firing up in the area of cyclonic vorticity advection across interior New England may have less surface moisture to work with as evidenced by lower surface dewpoints, the dynamic forcing is greater and the dry air isn't so obtrusive.  As the weak vorticity maximum shooting northeast out of PA moves across Central and Southern New England late this afternoon and into this evening, showers and thunder will shift slowly southeast into the Route 495 corridor.

Overnight, the stationary front will continue to waver over Eastern New England, and this will mean another night of dense fog in Eastern areas, particularly where the light southerly boundary layer flow interacts with cool ocean waters - on Cape Cod and along the coast of Maine.  Visibility was near zero in fog in both spots this morning, and that should be the case again.  A similar scenario will occur in a more patchy nature either side of the frontal boundary.  Meanwhile, the most significant lobe of vorticity will begin a process of ejecting/accelerating northeast, crossing New England on Wednesday.  With the surface front lingering across Eastern New England, a weak wave of low pressure - the surface feature serving as a reflection of the upper level disturbance - may instigate convection near the boundary.  The greatest cyclonic vorticity advection occurs during the morning, but the axis of the vorticity maximum moves overhead around 18Z in Southern New England - later in Northeastern New England.  When coupled with the instability forecast of -2 to -3 LI, and CAPE values building to 1000-2000 J/kg in Eastern New England at the time of shortwave passage, the dynamic forcing should be sufficient to initiate convection.  Winds are fairly light aloft, though 30-40 kts of SW flow is present at 500 mb, so a locally damaging storm would be possible given the available CAPE for downdraft creation.  There will be sea breeze interactions, as well, so there is the concern for storms to assume mesoscale rotation along the seabreeze boundary/frontal zone.  At this point, there doesn't seem to be enough instability to generate a sufficient updraft for anything more dramatic (tornadic), not to mention inflow off the Atlantic would introduce cool air to each system.

Matt