Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 291413 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1013 AM EDT Wed Mar 29 2017 .SYNOPSIS... Rain has ended in New England, although clouds lingered at sunrise. Expect a clearing trend through the day with dry weather continuing through Thursday. Low pressure from the Ohio Valley passes S of New England Saturday. This brings a mix of precipitation Friday and Saturday. High pressure brings dry weather Sunday and Monday, followed by another approaching storm on Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10 am update... Mix of sun and clouds over much of S New England with blustery NW winds. Evaluating, fair amount of moisture within the H925-85 layer as cyclonic flow prevails through which potent mid-level impulses traverse. Just looking at high-res GOES-16 data, subsidence behind the morning mid-level wave now moving S and offshore is allowing for some partial clearing, but can see N and upstream over the St Lawrence Valley the next wave coming S. The wave may skirt S New England. However given cold air advection and the anticipated boundary layer mixing of residual moisture beneath increasing high pressure and dry-subsidence inversion that we should see broken cloud decks lifting up around 5 kft agl. As the prior forecaster mentioned, cold air cloudiness. Plus the additional energy skirting S later today may touch off some light shower activity. Prevailed light rain accordingly to linger along the E coast. So all in all, am forecasting temperatures to be just a degree or to lower for highs still anticipated around the upper 40s to low 50s. Feeling a bit cooler. NW blustery winds gusting as high as 30 mph especially along the high terrain and over the E coast beneath the crux of heightened N/W winds within the low levels behind the departing low. Going to feel roughly around 5 degrees colder. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Overnight... Clearing occurs during the evening hours as diurnal/CAA cloudiness erodes. This will begin to allow some decoupling although it may take until after midnight for most places to drop to near calm as pres gradient remains relatively tight through about 06Z. This will limit timing for radiational cooling. Therefore, the floor for most overnight lows is in the mid 20s, although a few spots (mainly the most sheltered valleys) could drop a bit lower. Thursday... Another seasonable day for the most part although with generally weaker winds. Gusts mainly hold 15-20 mph. Highs per slight warming to an average of -4C at H85, look to remain in the upper 40s to around 50, although a few spots may do a bit better given more sunshine and less in the way of diurnally driven clouds. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Big Picture... Longwave flow is nearly west-east with a slight trough over the Northeast USA. Shortwave scale flow shows split northern and southern jet streams directing at least three and possibly four shortwaves through New England through early next week. Model mass fields are similar through the weekend, then diverge late in the forecast period with the approach of the next weather system Tuesday. Pattern confidence is moderate through the weekend, diminishing to low by Tuesday. Details... Thursday night... High pressure over the region shifts east overnight. The overall pattern suggests warm advection precip starting Friday morning. Even so, the main system return flow continues to focus on NY/PA/NJ through 12Z, even as the models move QPF across Southern New England during 06Z to 12Z. Model omega fields show weak lift over the region during late Thursday night, but RH fields show a dry layer below this lift. All of this leads to concern that the models are uniformly rushing pcpn in too soon. We have slowed the onset by a few hours, but continue to bring chance pops to the Western Hills, CT River Valley, and Central Hills overnight. Cold air damming signal in the pressure pattern over the interior, and light northerly low level ageostrophic flow. Within the context of chance pops, we expect enough cold air for a snow ptype in the interior and a rain/snow mix along the coastal plain. Friday through Saturday... Models agree on closed low over Illinois/Indiana Friday morning with surface low over the Midwest. The Midwest low is forecast to redevelop along the Mid Atlantic coast, likely in response to the cold air over interior New England. Strong low level southerly jet at 45-50 knots in the Ohio Valley into Western NY. Meanwhile southerly flow into New England is quite light Friday morning, then slowly increases through the afternoon. The stronger 25-45 knot onshore flow waits until late in the day and during Friday night as the coastal low develops. Stronger winds develop aloft and move over the South Coast and southern waters Friday night and Saturday morning. There is concern that the models are not fully accounting for the potential that this wind reaches the surface at that time. Expect greater confidence for precip Friday night and Saturday morning. We will feature increasing pops through the day Friday with highest values Friday night and Saturday as the coastal redevelopment passes offshore. Increasing low level ageostrophic flow from the north during the afternoon and night...reaching 25-35 knots at night. This should maintain and build low level cold air for Friday night. This builds confidence in a snow/ice event for the interior, while the onshore easterly wind keeps coastal areas above freezing. There is potential for several inches of accumulation inland, especially in northern/western Massachusetts. Sunday and Monday... Northern stream shortwave moves overhead and carries an appreciable cold pool aloft, especially over Northeast MA. Meanwhile the surface layer shows less cold advection than aloft. So with clearing skies mixing should bring temps into the 40s and near 50. This combination should destabilize the airmass and support diurnal cumulus during Sunday. It is possible we are underestimating the sky cover for Sunday...this will depend on the amount of lingering moisture for clouds. High pressure and lighter wind will move in for Monday with sunny skies. Mixing will support max sfc temps in the upper 40s and lower 50s. Tuesday... Low pressure ejecting from the Southwest USA is projected to approach our area on Tuesday. This system bears some similarity to the Friday-Saturday system, including a cold air damming signal and a coastal redevelopment. This could be another coastal rain/inland mix situation, but currently expected temperatures support all rain. Much too early to lock onto a solution for this. && .AVIATION /14Z 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/... 15z update... Today... IFR cigs lifting to low-end VFR SCT-BKN around 5 kft agl. With IFR cigs -DZ may accompanying. Increasing NW winds. Gusts up to 30 kts towards evening into the overnight as cigs erode. Thursday...High confidence. VFR. NW flow weaker than today. KBOS TAF...Improving cigs through the day with increasing NW flow. KBDL TAF...Improving cigs through the day with increasing NW flow. Outlook /Thursday Night through Sunday/... Thursday night through Saturday...Moderate confidence. VFR Thursday evening will lower to MVFR Friday and IFR/LIFR Friday night and Saturday. Inland areas will see a rain/snow mix Friday change to snow/sleet/freezing rain Friday night with several inches of snow/sleet possible in the Western and Central Hills. More likely rain in the coastal plain with a period of snow/rain mix Friday night. Precipitation tapers off west to east Saturday afternoon. Gusty easterly winds in the coastal plain Friday night and Saturday, becoming northerly Saturday night. Sunday...Moderate confidence. Conditions trend to VFR with at least partial clearing. Northwest winds with gusts 20 knots or less. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/... 10 am update...No major changes to the forecast. Today and tonight...High confidence. Winds will increase across the waters through the day, but will peak by late this afternoon and early in the overnight. NW gusts at peak will reach 25-30 mph with a few gusts just shy of gales. Seas increase with this wind as well, peaking early Thu morning at 8-10ft on the SE ocean waters. Small craft advisories remain as issued. Thu...High confidence. NW winds diminish and waves slacken through the day. Small craft advisories are likely to be dropped through the daylight hours. Outlook /Thursday Night through Sunday/... Thursday night... High confidence. Lingering 5 foot seas on the outer waters. Diminishing wind as high pressure builds in. A Small Craft Advisory is possible for leftover seas Thursday evening. Friday-Saturday...Moderate confidence. Midwest weather system redevelops along the Mid Atlantic coast Friday night and passes the waters on Saturday. Winds remain below 25 knots Friday, but become east and may increase to 30 knots Friday night and Saturday. Winds turn from the north as the system moves past. Expect seas to build Friday night and Saturday with heights 5 to 10 feet possible. Winds of 40-50 knots are possible aloft, about 2000-3000 feet above the surface. There is at least a low concern for 35 knot gusts Friday night and Saturday primarily over the southern waters. Sunday...Moderate confidence. Low pressure moves away from New England, with diminishing winds over the waters. Lingering 5-6 foot seas on the outer waters may require a Small Craft Advisory there. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 PM EDT Thursday for ANZ231>234- 251. Small Craft Advisory from 5 PM this afternoon to 8 AM EDT Thursday for ANZ230. Small Craft Advisory from 8 PM this evening to 8 AM EDT Thursday for ANZ235>237. Small Craft Advisory until 8 PM EDT Thursday for ANZ250-254. Small Craft Advisory until 2 PM EDT Thursday for ANZ255-256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody/Sipprell SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody/Sipprell MARINE...WTB/Doody/Sipprell

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