Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 172304 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 704 PM EDT Tue Apr 17 2018 .SYNOPSIS... An upper level system moving across New Eng will bring a period of scattered showers tonight, followed by dry but cool weather Wednesday. A low pressure area 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 /UNTIL 6 AM WEDNESDAY MORNING/... 7 pm update... Not much to add as the prior forecaster hit the nail on the head. Monitoring mPing, reports of rain mixed with sleet. Believe this to be the leading edge of cold air through the moist boundary layer however short of the -12 to -18C requirement for ice nucleiation. Yet with further cold air advection, nearly immediate, we`re seeing a mix of rain and/or snow being reported. The bulk of the wave and attendant forced ascent sweeping S, some visibility restrictions may arise, but feel the majority will remain S of New England. Previous Discussion... Rather potent mid level shortwave moving SE from Lake Erie expected to track just south of New Eng this evening. This shortwave accompanied by an area of steep lapse rates 7-8C/km in the 925-700mb layer which moves along the south coast. Deep moisture plume combined with low level instability and modest synoptic forcing will likely bring a round of scattered showers, mainly south of the Pike for the first half of tonight. After the shortwave moves through, improving conditions with partial clearing will develop overnight. Low temps will range from the upper 20s to mid 30s. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday... Mid level low to the north with a series of shortwaves rotating through the flow. One shortwave will move across the region in the morning with a second on approaching from the NW in the afternoon. The column is drier so expect mainly dry weather with just a low risk for a few showers in the Berkshires. Still a fair amount of cloud cover given lingering cold air aloft, but expect a bit more sunshine than today, especially in the coastal plain. Deep layer mixing through 850 mb supports another day of gusty west winds to 25-30 mph. Slight moderation occurs at 850 mb so high temps will be a few degrees higher than today, but still running several degrees below normal. Wednesday night... Another amplified mid level trough and shortwave approaches from the west late Wed night. Attending sfc low moves east from the Ohio valley with redevelopment south of the coast by Thursday morning. Decent shot of overrunning of frontal boundary to the south with deepening moisture will result in precip overspreading the region after midnight. Thermal profile warm enough for mostly rain, but marginally cold air with isothermal profile close to zero across northern MA may result in some wet snow/sleet near the NH border, especially higher terrain. If temps aloft are a bit warmer, some freezing rain is possible as sfc temps may hold near freezing in cold air damming pattern. && .LONG TERM /THURSDAY 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 into Thursday night... Morning wet weather followed by evening snows. Surface cyclogenesis emerging beneath cyclonically digging, closing off H5 low. Slowed by downstream traffic, maturation opportunity. Initial front-end thump along 290-310K isentropes Thursday. Ascent of anti-cyclonic warm, moist conveyor belt, strongest thermal packing / frontogenesis H8-9, light to moderate outcomes, especially SE New England, but watching closely N/W MA with N cold air drainage, sub-freezing wet-bulbs. H85 low track crucial, low-level warm air intrusions. Wintry mix remains possible for N/W high terrain, otherwise rain. 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 /22Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... 0z update... No major changes. Previous Discussion... Through Tonight...High confidence. Mainly VFR cigs, but a brief period of MVFR cigs and scattered showers possible for a few hours tonight south of the Pike. Improving after 06z. W gusts to 25 kt this afternoon diminishing tonight. Wednesday...High confidence. VFR. West wind gusts to 25 kt. Wednesday night...Moderate confidence. Conditions lowering to MVFR late as rain overspreads the region. Some snow possible over northern MA high terrain. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Thursday through Sunday/...Moderate confidence. Thursday: Mainly IFR, with areas MVFR possible. RA likely, FZRA likely. 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 Wednesday Night/...High confidence. Tonight...West gusts to 25-30 kts linger across south coastal waters into this evening before diminishing overnight. Rough seas continue outer waters. Wednesday...Another surge of 20-30 kt gusts develop over the waters, strongest over south coastal waters. SCA conditions continue. Wednesday night. Diminishing winds becoming light. Subsiding seas. Outlook /Thursday through Sunday/...Moderate Thursday: Winds less than 25 kt. Local rough seas. Chance of rain. Local visibility 1 nm or less. 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 Wednesday for ANZ232-235- 237-250. Small Craft Advisory until 8 PM EDT Wednesday for ANZ233-234. Small Craft Advisory until 8 PM EDT this evening for ANZ231. Small Craft Advisory until 6 PM EDT Wednesday for ANZ251. Small Craft Advisory until 2 AM EDT Thursday for ANZ254>256. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Sipprell SHORT TERM...KJC LONG TERM...KJC/Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell

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