Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 232121 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 521 PM EDT Thu Mar 23 2017 .SYNOPSIS... High pressure along the eastern seaboard will move offshore later tonight bring windy and milder conditions Friday. A cold front will push south across the region Saturday, then stall south of New England early next week. Weak low pressure waves forecast to push along this front, with the potential for periods of rain and mixed precipitation. Another stronger front may approach around the middle of next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... Tonight... High pressure will move off the Mid Atlantic coast. Expect winds to become light and variable for a time as the ridgeline moves overhead. A light south-southwest flow is possible late at night as the ridge moves offshore. Some high thin clouds moving over Ontario will reach our area early tonight. But thicker mid level clouds should hold off until after midnight, allowing a period of light winds and fair skies. So we expect radiational cooling the first part of the night, shutting down later tonight as the clouds thicken. Dew points start in the single numbers below zero and climb into the teens by late night. We favored going a couple of degrees below guidance with a range from the high single numbers in the interior to near 20 along the coastline. Most areas would be in the teens. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Friday... High pressure offshore will provide a flow of milder air. Winds above the surface will climb to 30-40 knots by midday. That will provide a flow of moist air with precipitable water values reaching near an inch by Friday evening. Weak overrunning created by this southwest flow will create clouds and light precipitation starting in the morning. Temperatures will be cold enough at the start for snow, but quickly moderate to rain by afternoon if not sooner. The transition period may feature some sleet. The best chance for this precipitation will be in Northern and Western Mass. But frozen amounts will be light, with any snow/sleet accumulation less than an inch. Have high confidence in the overall trend, but only moderate confidence in the precipitation types. The strong southwest low level jet will also provide the momentum for gusty winds, especially during the afternoon when mixing is greatest. It still appears the greatest risk for gusts over 30 mph to be along the Boston-Providence corridor and southeast from there. Max temperatures should make a 5-10 degree rise from today, but values in the mixed layer Friday are supportive of highs in the low to mid 40s so still below March normals. Friday night... Clouds and a southwest flow of milder air will keep temperatures from falling too much, and most of that should be in the evening. Temps may rise a little overnight. We maintained a chance of showers in far Northern Mass, but low values for pops. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Highlights... * Seasonal temperatures on Saturday with periods of light rain * Unsettled weather at times Sunday-Tuesday with some rain, sleet and/or freezing rain possible during the overnight periods * Low pressure and a cold front push across around mid week, then improving conditions around next Thursday Overview... While the 12Z operational model suite and ensembles continue to signal an active weather pattern across the northeast U.S. during most of this timeframe, noting some improving conditions toward the end of the long term. Still noting a mainly amplified but progressive steering pattern through most of the long term period. Noting a fast flow aloft to start this weekend while cutoff H5 low sits across the central Mississippi valley. This will keep weak low pres waves moving across the northeast while a front wavers across southern New England. The cutoff low will lift NE while it opens up to a short wave over the weekend into early next week, while large high pressure builds southward out of Quebec across Maine and N NH. Some question as to whether the eastern extent of the stationary front over the region will shift south for a time during the weekend. At this point, the ridging may push a bit too far E to bring somewhat drier conditions. The cutoff low will bring another shot of lift and moisture and, with some marginally cooler air trying to work in from the passing ridge to the E, may see a wintry mix of snow, sleet and/or freezing rain Sun night into early Mon morning. This should pass quickly as yet another H5 short wave works E from the central U.S early next week. Another surface low and associated cold front should shift sometime in the Tue-Wed timeframe, though wide model solution spread lends to lower timing confidence. Details... Saturday...Moderate confidence. 850 mb temps Saturday morning will be close to 8C but drop through the day as cold front begins to slide southward. Surface temperatures will reach into the mid 50s, cannot rule out a few 60s if the mid- level ridge continues to build and slows the timing of the front. Along the front, there could be a few rain showers but not expecting widespread rainfall as heights continue to build overhead. Saturday night into Sunday...Moderate confidence. Cold front will pass through southern New England dropping surface temps below freezing. The timing of the cold air drainage from the high pressure over northern New England will determine if precip type remains all rain, or if snow and/or mixed precip develops. Mid- level ridging could keep a warm layer around 850 mb which would aid in the mixed p-type. This is something to watch in the coming days. For Sunday, cold front will stall somewhere between southern New England and the Mid-Atlantic as high pressure begins to move southeast into the Gulf of Maine. Depending on how close the high pressure gets, we could see a dry Sunday or a few showers. One thing to keep an eye on is the easterly flow, which could keep the low level moist resulting in drizzle. Still a lot of uncertainty with this time frame. Sunday night Monday into Wednesday...Low confidence. While guidance is in agreement synoptically, still a lot of details to work out for this portion of the forecast. Biggest uncertainties will be the placement of the upper level low and how quickly it will diffuse early next week. Right now, the stalled front looks to return back north as a warm front late Sunday into Monday, as surface low pressure moves into the Great Lakes. Although warmer air aloft should move easily back into the region, the low-level cold air may be a little tougher to dislodge this time due to the nearby surface high pressure. Believe this is the timeframe where we will see the heaviest precip due to passage of the open wave. Another open wave right behind this one will push through the flow developing around shot at precip on Tuesday. As mentioned before, still a lot of uncertainty as to the thermal profiles. Continue to believe that there could be a period where the region could see a mixed bag of precip with the highest confidence Sunday night into Monday and another shot Monday night into Tuesday due to surface temps falling overnight. Still a lot of uncertainty with this timeframe as thermal profiles will change. Will continue the mentioning of snow, sleet, ice and/or rain for the forecast until thermals come better in alignment. Lastly, weak ridging in between waves may allow for temps to moderate back to seasonable during the day light hours Monday and Tuesday. && .AVIATION /21Z THURSDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... Tonight...VFR. High confidence NW winds with gusts 20-25 kt this evening, becoming light and variable for a time tonight. Winds become light south to southwest late tonight. Friday and Friday night...VFR with some MVFR mainly north of the Mass Pike. Moderate confidence. Increasing and lowering clouds during the morning. Areas of MVFR cigs/vsbys in light snow or sleet, with best chance along or north of Boston and the Mass Pike. All frozen precip should change to rain during late morning/early afternoon. Southwest winds will gust 25-30 knots during the day, highest along the coast and in higher terrain. Winds diminish Friday night. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Saturday through Tuesday/... Saturday-Saturday night...Moderate confidence. Periods of light rain through Saturday with areas of MVFR CIGS and local MVFR- IFR VSBYS. May see brief period of mix of sleet/freezing rain overnight Sat night across higher terrain of Central and W Mass into N CT. NE wind gusts to around 20 kt Sat night across Cape Cod and the islands. Sunday-Monday...Low to moderate confidence. Areas of MVFR CIGS/VSBYS with local IFR conditions through most of the period. May briefly improve for a time during the daylight hours. Areas of rain, sleet and/or freezing rain possible late Sun night into Mon morning, which may linger most of Mon across the higher terrain. E-SE winds may gust to around 20 kt along the immediate E coast Monday. Tuesday...Low confidence. Areas of MVFR CIGS/VSBYS with local IFR at times. Should see mainly patchy light rain, though can not rule out brief freezing rain early Tue morning and again late Tue night with marginally cold temps. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... Tonight...High confidence. Diminishing NW winds becoming light SW overnight. Seas near 5 feet on the outer waters east of Massachusetts early, but subsiding during the night. Seas elsewhere less than 5 feet. A Small Craft Advisory lingers on those Eastern Outer Waters for a few more hours until seas subside. Friday...Moderate confidence. High pressure moves well offshore, bringing southwest winds to the waters. Potential for wind gusts 25-30 knots. This will build seas with 5 foot heights on many of the waters by the afternoon. Patchy light rain may bring brief visibility restrictions. Small Craft Advisories are being issued for most waters. An advisory for the eastern outer waters will be issued once we move clear of the existing headline. Friday night...Moderate confidence. Southwest winds 25 to 30 knots may become west toward morning. Seas of 5-7 feet will linger through the night on the outer waters and all southwest-exposed waters such as RI Sound. Small Craft Advisory will lower on Boston Harbor and Narragansett Bay, but linger on the remaining waters. Outlook /Saturday through Tuesday/... Saturday...High confidence. NW winds gusting to around 25 kt Sat morning, then diminish and veer to NE by Sat night. Gusts may approach 25 kt on the southern outer waters Sat night. Seas 5-6 ft on the southern waters early then subside. Sunday...Moderate confidence. E winds gusting to around 25 kt, become SE Sun night. Small Crafts may be needed. Seas build to around 5 ft over the outer waters E and S of Cape Cod and the southern outer waters. Monday-Tuesday...Low to moderate confidence. E-NE winds gusting to 25 kt. Seas building to 4-7 ft over the eastern open waters Mon night-Tue. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 10 AM Friday to 6 AM EDT Saturday for ANZ232>235-237-255-256. Small Craft Advisory from noon to 8 PM EDT Friday for ANZ230. Small Craft Advisory from 10 AM Friday to 3 AM EDT Saturday for ANZ231-251. Small Craft Advisory from 10 AM to 10 PM EDT Friday for ANZ236. Small Craft Advisory until 2 AM EDT Friday for ANZ250-254. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB SHORT TERM...WTB LONG TERM...EVT AVIATION...WTB/EVT MARINE...WTB/EVT

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