The forecasting challenges continue with issues of inversions, convection and advection leading the difficulty ratings.
Cool air continues to invade the boundary layer on a persistent northeast flow that was enhanced by the low winding up south of New England last night and reinforced by the high drifting south over Eastern Canada and Maine. The gradient weakens, the high shifts off the coast and the wind changes direction tonight and tomorrow, and this sets warm advection into gear for New England. At the same time that warm advection kicks in Thursday night and heights build into the the northeast with the expanding ridge, a shortwave currently producing the thunderstorm complex over the Central Great Lakes moves east and brings rain and thunder into New England late Thursday night into Friday morning. This comes from the combination of dynamic forcing with isentropic lift across the region, assisted by speed and directional convergence associated with an 850 mb jet poking across Upstate NY at 30-40 kts. This will lead to convective development over Northern and Eastern NY but the mid-level mean flow is from NW to SE, and this means a SE propagation of the showers and thunder, making VT/NH/Ern MA and Southern ME the wettest locales early Friday morning. Again, this is associated with the combination of a shortwave and warm advection, so as the shortwave moves by and dynamic lift weakens, so too will go the bulk of the precipitation, but the continuing warm advection will combine with some subsidence in the anticyclonic vorticity advection to allow for decent warming where the sun comes out. 925 mb temps would mix out to either side of 80 with the exception of ME where clouds are likely to be more stubborn thanks to the heart of the cool air with the surface ridge being so slow to depart. A reinforcing shot of boundary layer warmth and moisture will push out of NY and into Western New England Friday afternoon and evening, and scattered severe convective cells would be possible with this warm frontal push.
Though convective activity would die Friday night, warm advection continues. The ridge builds in for Saturday, but is vulnerable to height falls on its northern periphery thanks to the active northern stream. With 850 temps supporting mid 90s and 925 temps supporting upper 80s, plus a downsloping flow, seems like 90s are a good bet for many areas both weekend days. The shortwaves in the northern stream that miss to the north still provide enough lift in a very unstable environment - on the order of 3000+ J/kg of CAPE - to instigate thunder and potentially severe thunder on Saturday, especially in Northern and Western New England, nearest to the dynamic lift. By Sunday, a better chance of thunderstorms will exist for more of New England with the passage of a surface trough with a shortwave moving across the Canadian border, but dragging the trough across most of New England. The continued barrage of norther stream shortwaves seems to encourage southward drift of a cold front on Monday, and though most areas will still be in the warm air, showers and thunder would become more numerous.
Matt
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Posted by: Ray Ban | Saturday, April 14, 2012 at 10:33 AM