TWO DAYS OF SHOWERS, THUNDER AND HAILSTONES BEFORE IMPROVING HOLIDAY WEEKEND WEATHER
FINE WEATHER REGIONWIDE JUST IN TIME FOR THE HOLIDAY WEEKEND

UPPER LEVEL STORM MIGRATES OVER NEW ENGLAND AS IT PREPARES TO MAKE AN EXIT

The upper level storm that's been so pesky, and a regular part of the forecast for days, is finally on the move.  As it migrates east, a significant lobe of vorticity is swinging through New England and will continue to carry the upper level cold pool with it, setting up the temperature difference from surface to high altitudes that we call instability - and these favorable conditions for vertical cloud growth and convective processes are already resulting in plenty of showers, evident on radar imagery.  Throughout the day, you'll be able to monitor developing showers and even some thunderstorms on the radar imagery linked through the small thumbnail images on the right side of the screen - clicking on them brings up a larger window with your local radar.  CAPE values climb to between 100 and 300 J/kg, which, given the weak winds through deep layers of the atmosphere under the upper low, isn't enough for severe thunderstorm gusts and I have no concerns for potential damage.  Hailstones, however, seem fairly likely with an extremely low wet bulb zero, meaning saturation of the air will cool temperatures to or below the freezing mark only a few thousand feet up in the sky, and this allows hailstones to develop in heavier updrafts, so multiple reports of hail seem possible this afternoon.  Some hail may reach warning criteria of 3/4 of an inch, but I'd venture that very little damage potential exists with no crops growing yet that could be flattened, and no damage to vehicles or other structures from marginal hail.

The combination of diminished diurnal instability after sunset, along with anticyclonic vorticity advection and drier air moving in, will mean convective activity dies rather quickly this evening.  Cool and dry advection will continue overnight, and this will mean skies gradually clear during the overnight.  An active breeze for most spots out of the northwest means we don't decouple in many areas, but the air moving through is plenty cool and has a history of producing lows in the 30s over Southern Canada when the skies have partially cleared, so I'd expect our cooler locales in Northern and Western New England to bottom out in the 30s while the rest of the area hangs in the 40s.  In the cooler spots, some fog is likely thanks to the ample boundary layer moisture trapped in valleys where dry advection isn't so active.

The upper level cold pool will be moving east on Friday, meaning the threat for showers diminishes.  Nonetheless, one more vorticity maximum will swing from north to south during the day, moving across Maine and Eastern MA and providing enough dynamic forcing for scattered showers and a potential thunderstorm in ME...and maybe a shower in far Eastern MA, too, though the northwest wind will be downsloping and fighting against that.  All the while, the pattern evolution discussed here yesterday and earlier this week will be underway, meaning ridging will be building east and that will bring warming both aloft and at the surface.  The boundary layer warming, though, will be rather indirect, with a north and northwest flow prevailing.  This simply means rather than blasting warmth into New England, it's a slow progression of warmer air, mostly coming in aloft first, but making it to the ground thanks to deep mixing in a dry atmosphere.  Some cirrus debris seems likely this weekend with the northwest flow aloft still bringing a series of weak vorticity maximums overhead, and these shortwave disturbances may touch off a few rounds of thunder in Northern ME Saturday and Sunday nights as the disturbances aloft interact with the leading edge of the deeper warmth.  By Memorial Day, a stronger one of these shortwaves spawns a surface low pressure center in Southern Quebec that will drag a cold front southward across New England with afternoon and late day thunderstorms for at least some or perhaps most of New England, and cooler temperatures settling into at least Northern New England.

Matt

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