Scattered convection is firing this afternoon with warm and humid airmass providing plenty of surface based instability. What's lacking is a focused trigger mechanism, with a weak vorticity maximum drifting overhead. An upper level cool pool migrating over Central/Southern New England was the focus for scattered convection in NY/PA yesterday, and today is aiding convective processes in New England. Terrain is also aiding in upward vertical motion, as evidenced in the Green Mountains. Weather enthusiasts have seen quite a severe weather season already, and with the Northeast U.S. jet stream trough expected to continue into the end of July, we should continue to find repeated thunderstorm days, though a bit of a break is in sight by the end of this week.
For now, the greatest concerns associated with Tuesday afternoon convection will be heavy rain and lightning, though most spots won't see these features. A wet microburst isn't impossible but also isn't all that likely within the storms given the relatively weak wind field aloft, and part of the reason I'm not real jazzed up about storm potential for growth - aside from the weak wind field - is the presence of very dry air in the mid and upper levels. This dry air is evident on water vapor imagery and was well progged by the guidance. Of course, where the cap can be broken a storm could strengthen and tap that very same dry air for a downburst, but I actually think it's so overwhelming that it will instead hold pulsing convective storms to a threshold of development that will keep them from becoming much more than downpours scattered across New England, though limited coverage in Maine where not so much surface based instability is available.
Wednesday brings the approach of the active cold front from the Great Lakes and Southern Canada, but the entirety of the day will be spend warm sectored. The southwest gradient flow strengthens ahead of the front with surface gusts to 35 mph possible, but the loading of dewpoints around 70 with temps rising to around or perhaps just over 90 will mean surface based CAPE values near or greater than 2000 J/kg in much of Southern and Central New England northeast through Maine. Very little indication of cloud pollution, so hearty insolation should be attainable. Mid-level lapse rates leave something to be desired, but LIs still creep down to -2 to -5 Wednesday afternoon. Perhaps the most precarious of locations in New England is late Wednesday afternoon to Wednesday evening in Central Maine, where backing winds ahead of a strengthening surface wave of low pressure moving just south of the Canadian border will increase low level helicity values dramatically. Farther south, helicity values are less impressive but still some veering with height in mostly unidirectional southwest flow aloft but southerly surface winds. 850 mb winds 30 to 40 kts combined to a core of 30-40 kt 700 mb wind will mean a straight-line damaging wind threat would be the biggest concern in most of the Southern half of New England. Very warm boundary layer and lower atmosphere will limit hail potential significantly.
Though the slow moving front takes its time to clear Southeastern New England Wednesday night into early Thursday morning, it should clear the coast early Thursday and allow the anticyclone responsible for great early week air in the Upper Midwest and Upper Great Lakes to spread that refreshment into New England on Thursday. Though the air will remain much less humid, temps will rebound on Friday without much surface cold available, and by the weekend a southwest wind returns to not only keep temps up but also reload humidity - which may only gradually rise Saturday, but should be high again on Sunday with thunderstorms back into the forecast.
As mentioned at the start of the discussion, there's good agreement among the ensemble members on holding the Northeast US trough pattern - at least weakly - into the end of July.
Matt
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