TROPICAL RAINS TO BRING MODERATE AND PERHAPS MAJOR FLOODING FOR SOME, SEVERE WEATHER FOR OTHERS, POSSIBLE ISOLATED TORNADO THROUGH THURSDAY

An overview of the weather pattern driving this remarkable mid-summer storm was laid out in this discussion yesterday, featured below or via the archives link at right.  The period through Thursday evening will be quite active, and I encourage one and all to make frequent use of the radar links at the right side of this page.  It may also be worthwhile to sign up with NationalWeatherOnline.com before the event to post images/observations/analysis on your own during the event.

The bottom line is that the expected confluence of ingredients for a flooding rain and potential severe weather event are coming together, and will be meeting over New England.  Water vapor satellite imagery depicts a well-developed moisture feed from the Tropical Atlantic into New England, and a steady feed of moisture from the Gulf of Mexico into New York State.  This is moisture-loading the atmosphere in advance of the strong vort moving east across New York - quite impressive on water vapor and infrared satellite imagery, and responsible for vigorous dynamic lift ahead of it.  This vorticity maximum is in the process of closing off as it interacts with the primary upper low that has begun to fill over the Quebec/Ontario Provincial line, and this southern energy center will become the new upper low of primary focus.  The dynamically unstable ridge discussed here yesterday that is upstream from the low has already begun to break down, but has done its job in amplifying the shortwave, and the subsidence in the wake of this shortwave is rather remarkable, evidence of the recent strengthening that's taken place with the vorticity maximum.  With the slowing of the upper level low has come a slowing of the surface front, of course, and the area of greatest lift has slowed dramatically over PA and NY, with multiple lines of convection, but the tropical feed and instability over Long Island and Southern New England have helped to feed new bands of convection, and the initial surge of warm and moist advection continues to produce mostly stratiform rains farther north.

The most pressing concern with this event is undoubtedly the threat for significant flooding in Western New England, and particularly in Vermont.  The NMM guidance computes a remarkable 9"-10" of precipitation in Western MA and VT ending Thursday night.  The 8"+ area is rather large, and implies even higher amounts, which would be epic for these areas.  Substantial amounts like this are not unprecedented - the Great May floods of 2006 were one example, when an amazing 17" of rain was recorded on the North Shore of Massachusetts.  Many of you will remember the catastrophic flooding that followed.  There are some similarities to such epic New England flood events in the evolution of this most current situation, namely the presence of both a Bahamas/Tropics Atlantic feed and a Gulf of Mexico feed of moisture.  The differences are also apparent, however, including weaker surface convergence, and a somewhat more progressive system.  Therefore, I have trouble believing in a widespread 8"+ amount for all of Western New England, though I do think such amounts will be realized in orographically favored regions and communities.  Where this happens, flash flooding will be significant, with multiple road washouts and smaller rivers rising rapidly above bankful.  Depending on where such amounts are recorded, some evacuations are certainly possible in Vermont and Western Massachusetts along smaller, quicker responding rivers, though I expect that to be the exception rather than the rule, as it normally is.  6"+ would be a reasonable assessment for average rainfall amounts from Litchfield County, CT, through the Berkshires, then again in favored terrain of the Green Mountains, including Rutland and Addison Counties.  A widespread 3"-6" is expected in Western New England, decreasing the farther east one is, and I expect 1"-3" across Eastern New England, with less rainfall possible on Cape Cod (where most of Thursday may stay dry) and Eastern Maine (same).  The reason for decreasing rainfall amounts is the increasing speed of the front and the upper low later in the period as the band of tropical rainfall moves east.  All of this having been said, there will be substantial deviations from the average amounts.  A town like Concord, MA - not far northwest of Boston - is a great example of recent variation, with observer Dave Cook picking up nearly 4" of rain in the last 72 hours!  Spots that see heavier tropical downpours that record one or two inches in only an hour or so will surely exceed expected amounts by a couple of inches.  The ground across New England is wet, but nowhere is it as wet as Northern Vermont, where greater than 3" of rain in a 6 hour period will cause flash flooding, and with tropical convective elements this is a certainty in many locales in the Green Mountain State.  It's not just Vermont that's seen wet conditions lately, however.  Almost the entire area forecasted to see 3"+ of rain in the Western half of New England, and across the North Country, has flash flood guidance values at both 6 and 12 hours of 3", so essentially I am forecasting flooding for most of Northern and Western New England.  Considering the precipitation forecast is based on sound scientific reasoning, there seems little reason to question the results of the rainfall, meaning a moderate flood event in most of VT, and minor to moderate for susceptible locales in Central, Western and Northern NH, and Western ME.  Smaller rivers will be the quickest to rise in most instances, and while NH rivers like the Swift and Pemigewasset may see minor or moderate flooding, even greater concern comes along the Otter Creek in Rutland, or the Mad River and Winooski River, where major flooding may be achieved.  Larger rivers like the Connecticut and the Housatonic, also are vulnerable to minor or moderate flooding, but take longer to rise.

It should go without saying that street and urban flooding will occur wherever prolonged tropical downpours are observed.

The next threat is severe weather, which is a more vague threat, but a real one, nonetheless.  Instability is limited owing to limited insolation with plenty of cloud cover, but the situation is one that lends itself to substantial situational instability - that is, a deep flow of tropical warmth and moisture in the lower levels of the atmosphere, while the slowly migrating upper low to our west brings falling heights and therefore steepening mid and upper level lapse rates.  The northward progression of the surface low ensures the low level flow will remain backed out of the south and southeast, and the increasing pressure gradient between a cool eastern Canada high and the low to our west also will serve to increase the wind.  The backed flow is essential to increasing low level helicity, and I think it's quite telling that as I write this, almost every cell is depicted as mesoscale rotation on the radar algorithm on our SkyTracker radar, and I actually just paused to investigate a TVS (tornadic vortex signature) that was showing up as a possible developing tornado on the south coast of Connecticut.  An examination of the radial velocity plot showed weak enough winds inside the cell that I'm not immediately concerned, but there is rotation showing up in many showers and thunderstorms.  This is not atypical for a tropical moisture influx, and brief, shallow tornadoes often are recorded in such situations, especially with the rainband associated with frontal passage (forecasted to occur from west to east on Thursday) as the rotation occurs within the cloud, and the tropical nature of the air creates a low lifted condensation level (cloud base) that means brief tornadoes can skip across especially hilly terrain, often not detected by radar and known only by the damage they cause.  Considering the low level jet will only increase Wednesday night and throughout the day Thursday, this means low level helicity will also only be on the rise, and I think the mention of an isolated tornado for New England is quite warranted given the setup, and I mentioned that potential several times in today's broadcasts.  The biggest motivator for severe weather potential Wednesday late afternoon and evening is the northward nudging warm front from the South Coast of New England that will reach for Southern NH by Wednesday late evening and night, and this influx of warmth and moisture will continue to produce intense isentropic lift in the backed wind environment through the overnight, so some severe storms will continue to be possible, especially the farther west one is, closer to the dynamic lift.  Thursday will dislodge the heavy rain band from the west and slowly mark time eastward, carrying the severe weather threat for straight-line winds and for aforementioned potential isolated tornadoes.

Of concern on Thursday will also be the strengthening low level jet.  A tropical airmass is well mixed, and even with a southeast flow off the ocean, such an airmass will allow for the mixed layer reaching to 880 mb to carry wind to the surface, with wind gusts to 50 mph possible during the midday and afternoon.  Strong winds are likely to begin even earlier across Vermont, where overnight Wednesday night may bring an onset to the strong wind, gusting 60 or 65+ mph for a time Thursday morning/midday just prior to the onset of the front, and downed trees/lines are possible as a result.  This steady wind will churn the seas, as well, and mariners will face dangerous conditions for pleasure craft boaters throughout the day on Thursday.

After frontal passage Thursday night, drier air will move across New England and my thinking into the weekend has not changed from yesterday's discussion, except that there's now more evidence to back it up, as the previously wet GFS for Saturday has jumped on board with a drier solution.  This means Friday should be a mostly dry day with the follow up vorticity maximum and associated cold pool sliding across the far North Country with a late shower possible in far Northern New England, but less likely farther south owing to ambient low relative humidity air.  The atmosphere still looks like it will try to reload in the lower levels on Saturday, but also still looks like it will hold onto dry air aloft, fighting the showers and thunder that move out of New York State in association with the next vort, and leaving New England with an increasingly breezy and warm day prefrontal, but then bringing greatest lift into only a marginally favorable New England sky for thunder with the diurnal max for heating passed by the time the greatest forcing arrives, meaning scattered late day and evening showers and thunder still looks like the way to go for mostly Western and Northern New england, with an evening scattered shower/storm in eastern New England, and probably nothing in Southeast MA.  By Sunday, one stronger vort moves across Northern New England early, and another drops into the lakes late, and this should result in more widespread convective activity.  Behind the vort, cooler and drier air is likely on Monday.

Should be quite a storm in the next 36 hours.  Noticing flood watches being hoisted by the NWS for more of New England (eastern) as I put this finishing sentence on - fasten your seat belts.

Matt