Area Forecast Discussion
Issued by NWS Boston, MA

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342 FXUS61 KBOX 180657 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 257 AM EDT Wed Apr 18 2018 .SYNOPSIS... An upper level system moving into the Maritimes will bring dry but cool weather today. A low pressure area from the Midwest will produce a period of chilly rain potentially mixing with wet snow during Thursday. Lingering rain showers Thursday night may mix with or change to wet snow across the interior. The weekend looks to be dry but with more below normal temperatures with milder weather early next week. && .NEAR TERM /TODAY/... Upper low pressure moves from Maine into the Maritimes, but with several shortwaves moving through the cyclonic flow. The most significant shortwave moves off through the Gulf of Maine this morning, with drier air moving in behind the system. Deep moisture from overnight diminishes, and skies did clear over much of Southern New England during the early morning. Sunshine and lingering cold air aloft will allow clouds to redevelop later this morning, but expect more sunshine than Tuesday. Can`t rule out a stray shower, but the diminished moisture aloft will work against that. Mixing extends through 850 mb, which will bring gusts of 20-25 knots to the surface. Temperatures at that level -3C to -5C support max sfc temps in the low to mid 50s...except mid to upper 40s in the Worcester Hills and Berkshire East Slopes. && .SHORT TERM /TONIGHT THROUGH 6 PM THURSDAY/... Tonight and Thursday... Another shortwave moves across the northern tier of the USA, crossing the Great Lakes Wednesday and approaching New England early Thursday. This system redevelops south of New England Thursday morning, and with a progressive axis to the upper system we expect the surface system to steadily move past. The upper system will be supported by a 100 knot jet with upper divergence reaching New England between 06Z and 12Z. This will enhance lift over our region, supporting precip through the day. Strongest lift should focus around midday and early afternoon. As to precip type: thermal profiles mostly suggest rain in our area. Profiles are more marginal in northern Mass and over the Berkshires, so a wet snow or rain/snow mix is more likely in these areas. Once the surface system moves past late Thursday and winds turn more from the northwest, colder air may move in and change the rain to snow especially in Western/Central Mass and Northern CT. Snow accumulations would mainly be in the Berkshire East slopes where 1-3 inches is possible in spots near the Berkshire County line. The northern Worcester Hills would be another area of concern. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... */ Highlights... - Potential N/W snows late Thursday into Thursday night - Lingering showers for Friday - Gradual warm-up weekend into early next week, dry weather - Early to midweek, watching for any sea-breezes, dry weather */ Overview... Slowed, blocky, amplified pattern as of late, opportunity for storm systems to mature, draw S colder air, NE CONUS wintry outcomes. Deep N Pacific pattern one culprit, sub-tropic phase 2/3 MJO connection, wave train amplified. But perhaps relaxing, progressive, MJO becomes subdued, into late April. Model forecast N Pacific persistence of an Aleutian low, deepening, cutting off downstream SW CONUS energy as remaining energy shears E. An active W-E weather pattern into CONUS. So the downstream pattern, N Atlantic, traffic jam or progressive? NAO / AO sign important. Consequential storm maturation, whether storms are slowed, deepen, have longevity in our vicinity beneath preferred H5 trof pattern aloft, pulling colder air S over recent deep snowpack over N CONUS / S Canada, wintry outcome opportunities. Feel near-term winter maintains its grip, then relaxes through the late April remainder, an Spring-time ebb and flow pattern returns. However, if upstream pattern unchanging and N Atlantic traffic re- emerges, can`t rule out a return of cooler than average conditions. Indications early May per EPS H85 temperature anomalies. Long way off, however hardly any signal of H5 ridge / Bermuda high dominance going forward. Targets of opportunity in the discussion below. */ Discussion... Thursday night... Better snow chances Thursday evening / Thursday night. Wrap around recent storm remnant moisture, colder air pulled into deepening H5 vortmax. Steepening lapse rates, increased ascent, orographic lift, moisture with precipitable waters up around 0.5 inches squeezed out. Reinforcement by way surface cold front along leading edge of colder airmass pushing S/E. Enough kick through the dendritic growth zone and enough cooling through the moist layer throughout, can`t rule out initial wet snow becoming fluffy thereby increasing accumulation efficiency. Watching closely deformation along trailing cold frontal boundary for evening / overnight period, omega / forcing associated with H85 low track. So big question: how much squeeze on available moisture as cold air pours in? Synoptic forcing not appearing robust as forecast guidance wobbling on thermal fields, precipitation outcomes. Leaning light to moderate event, highest confidence N/W high terrain of a 1-3" event with trace accumulation S/E towards Worcester / Tolland Hills. Can`t rule out snow being observed to the coastline with storm departure. 17.12z NAM slightly colder, 17.12z EC wettest. Friday... Cyclonic curvature of lingering moisture, colder low-levels, steep lapse rates, daytime heating, breezy NW winds, likely showers, more so along leading edges of rotating vortlobes. Minimal impact, dry air working in, threat diminishing towards late. Early to Midweek... High pressure, warmer temperatures, closely watching potential sea- breeze activity. Yielding temperature forecast headaches per tight thermal gradients between the interior and coast, dependence on how strong any W/SW flow may be. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/... Today... High confidence. VFR. West winds gusting to 25 knots, possibly near 30 knots around Nantucket. Tonight... High confidence through midnight, moderate confidence after midnight. VFR through 5 AM with increasing/lowering sky cover. MVFR cigs/vsbys develop in Western/Central Mass and Northern CT as the morning rush starts. Rain develops, with snow possible over the Berkshires. Thursday... Moderate confidence. MVFR cigs lowering to IFR in the morning. Vsbys lowering to IFR in rain and snow. Some freezing rain is possible in Northwest Mass early Thursday morning. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Thursday Night through Sunday/...Moderate confidence. Thursday Night: MVFR/IFR conditions possible. Breezy. RA likely, chance SN. Friday: Mainly VFR, with areas MVFR possible. Breezy. Slight chance RA. Friday Night: VFR. Breezy. Saturday through Sunday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. Today...West winds gusting 25-30 knots from mid morning through evening. Seas 5 to 8 feet, mainly over the outer waters and RI Sound. Small Craft Advisory remains in place for all except Boston Harbor, Narragansett Bay, and Cape Cod Bay. Tonight...Diminishing winds and seas. Lingering rough seas over the southern and southeast outer waters. Small Craft Advisory lingers in those areas. Thursday... A coastal weather system brings rain to the waters, especially the southern waters, with visibility of 1 to 3 miles. Winds remain less than 25 knots. Seas remain at 5 feet or higher along the southern outer waters, but lower values farther north. Outlook /Thursday Night through Sunday/...Moderate Thursday Night: Winds less than 25 kt. Areas of rough seas. Chance of rain. Friday: Winds less than 25 kt. Local rough seas. Slight chance of rain. Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Saturday through Sunday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ232>235-237-250. Small Craft Advisory until 6 AM EDT Thursday for ANZ254>256. && $$ SYNOPSIS...WTB/Sipprell NEAR TERM...WTB/Sipprell SHORT TERM...WTB LONG TERM...Sipprell AVIATION...WTB/Sipprell MARINE...WTB/Sipprell

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