Area Forecast Discussion
Issued by NWS Boston, MA

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947 FXUS61 KBOX 222104 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 404 PM EST Thu Feb 22 2018 .SYNOPSIS...
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A wave of low pressure tracking along a stalled frontal boundary to the south will bring a period of snow and sleet to much of the region late this afternoon and early evening, then drying overnight. Active weather pattern late Friday through the weekend as a series of frontal boundaries usher wintry weather across the interior. A near-seasonable, quiet pattern for early next week.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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400 PM update... Mid level Fgen precip continues to stream eastward across the region. 1040 mb high over Quebec continues to advect cold air southward with surface temperatures tumbling thru the 30s. Shallow warm layer around 700 mb combined with warm blyr resulting in ptype of mainly rain over RI and southeast MA. However as this mid level warm layer erodes and blyr cools from wet bulb process, rain and sleet changing over to snow across much of northern MA into the city of Boston. However impacts are limited with surface temps above freezing yielding just wet roadways. Strongest forcing for ascent occurs around 21z and then races eastward and offshore by 00z or shortly thereafter. Meanwhile northerly winds continue to advect shallow cold air southward. Thus very small window for accumulating snow. Still thinking main impacts will be the high terrain of western- central MA where temps will be AOB freezing during most of the event yielding snow accumulations from a coating to 2 inches with a low prob of 3 inches along the MA/NH border where strongest forcing occurs in the snow growth region yielding higher SLR. Farther south into the Hartford, Providence and Boston corridor, already snow and sleet in Greater Boston area and this trend will continue into Providence and Hartford. However with temps likely remaining AOA freezing most roadways will remain wet, limiting impacts to the late day commute. Thus have continued the Special Weather Statement to address this. As for model guidance, both 3km NAM and HREF handing ptype transition very well at the moment and followed this guidance closely. Overnight...later this evening dry weather overspreads the area however with light winds and subfreezing temps across much of the region, any standing water or slush will freeze. Thus icy spots possible overnight.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
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Friday... Dry and seasonably cool in the morning as 1042 mb high crest over Maine. Could have some leftover icy spots early that may impact the morning commute. Otherwise fast/progressive upper air pattern results in any breaks of morning sunshine quickly giving way to increasing clouds as next WAA pattern develops. Precip overspreads the region during mid to late afternoon from west east. Cold air is not as deep tomorrow, more shallow and will support a period of sleet and freezing rain across western-central MA. Remainder of the region will see a chilly rain. Friday Night... Weak wave crosses the area with evening rain but risk of freezing rain across western-central MA. Drying trend second half of the night as post frontal airmass overspreads the region. Not as chilly as tonight with lows Fri night in the 30s.
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&& .LONG TERM /SATURDAY THROUGH THURSDAY/...
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Highlights... * Dry and warm as Spring returns on Saturday * Widespread precip on Sunday with wintry mix in the interior * Dry weather trend for early next week * Pattern change for mid to late next week Overview... 12z guidance is in general agreement with the extended forecast. Just some minor differences related to thermals which provide different outcomes in p-type for Saturday night into Sunday. Trough across the western CONUS with southeast ridging deamplifying over the weekend. Last of the Pacific waves will move through Sunday with surface high pressure to follow into Monday. By early next week the pattern will begin to change as NAO turns negative. Mid-level ridge across the southeast into the Great Lakes with cut-off low off the Maritimes. Both the GEFS and EPS show anomolous high pressure building over Greenland which will influence the cut-off low to either retrograde towards SNE or remain across the North Atlantic. Depending on where this all stacks up, appears the trend is there that our pattern could turn more active late next week. Details... Saturday...Moderate confidence. Dry weather day will be on tap as surface high pressure will fill in behind the offshore front. Still seeing westerly to northwest flow as 850mb CAA moves into the region. However, appears that surface temps have the potential to warm into the mid 50s thank to longer daylight hours and mixing to 925mb which are cooler to cool. Some guidance even hints as low 60s. Still some uncertainty as how far south the front will sage will lead to cooler temps or if it slows down then warm temps are possible. Thus a low confidence forecast for temps on Saturday, but high confidence on mainly dry weather. Saturday night into Sunday...Increasing confidence. Potent wave will eject out of the desert southwest producing a surface low pressure system up into the northern Great Lakes. Stalled front to the south will begin to lift northward as a warm front. Along the front, triple point low will develop and strengthen as the system pulls away from the area Sunday night. Before the secondary low develops, another overrunning precip set-up as moisture from the south increase as warm front begins to lift northward. Surface high pressure will be set-up north to northeast of the region allowing for surface temps to fall thanks to cold air damming. Models are always to quick at warming temps above freezing, so trended towards cooler 2m temps and wet-bulb effects. P-type will once again be the issue as models are still struggling with the warm layer. EC trended to be the coolest of the guidance which could suggest more snow than freezing rain, whereas the NAM is the warmest. GFS and the CMC are in the middle of the ground and thus trended towards the ensembles guidance. Therefore, went a bit cooler with this forecast resulting in snow at the onset, as cold air looks to be deep enough north of the Pike and esp Route 2. In fact with good omega in the snow growth region, could see a few inches of snowfall. Once the mid-level warm front pushes into the region, a transition to more sleet/freezing rain will occur leading to slick travel across the interior. South of the Pike and especially across RI and SE MA conditions look to warm for any wintry weather, but still will have to watch as a change in just a few degrees will impact p-type. This system looks to the most roust resulting in more widespread precip, esp as we continue to tap into that Gulf moisture. Thus increases POPs as confidence is high that precip will occur. Lastly, with the surface warm front trying to push northward, may have a large gradient in surface temperatures somewhere across southern New England. While warm fronts tend to struggle to lift northward this time of year something to watch as we could see low 50s across the south coast and mid 30s across the interior. Sunday night into Wednesday...High confidence. Surface cold front will push through the region resulting in a drying trend for Sunday night. Strong high pressure over the Ohio Valley will begin to strengthen Monday into Tuesday as anomolous ridge sets-up over the Mississippi Valley. This high appears to stay in place into middle of next week before a quick moving wave from the southwest gets kick into the flow. Thanks to ridging and surface high, dry weather will prevail as temperatures turn slightly above average.
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&& .AVIATION /21Z THURSDAY THROUGH TUESDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...Moderate confidence. 18z TAF update ... Thru 00z... VFR trending toward MVFR 21z-00z in rain changing to PL/SN west to east. Snow accumulations of a coating to two inches possible western-central MA including KORH. Elsewhere trace to a coating possible. After 00z... MVFR in PL/SN ending 00z-03z from west to east then trending dry and VFR. Friday... VFR but trending MVFR late in rain with PL western- central MA. Friday night... low risk of LLWS with low level WSW jet. MVFR in evening rain improving to VFR and dry overnight. KBOS Terminal...Moderate confidence TAF. Low risk for period of steady SN/PL 22z-00z then improving. KBDL Terminal...Moderate confidence TAF. Low risk for a period of moderate SN/PL 21z-23z. Outlook /Saturday through Tuesday/...Moderate confidence. Saturday: VFR. Saturday Night: Mainly VFR, with local IFR possible. Breezy. RA likely, SN likely, slight chance FZRA across the interior. Sunday: MVFR/IFR conditions possible. Windy with gusts to 35 kt. RA, FZRA. Sunday Night: Mainly VFR, with local MVFR possible. Monday through Tuesday: VFR. Breezy.
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&& .MARINE...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...Moderate confidence. Tonight... SCA for southern RI waters and south of MVY/ACK with marginal NE winds up to 25 kt and seas up to 5 ft, then trending downward later tonight. Snow, sleet and rain limit vsby this evening but improving later this evening from west to east. Friday... 1041 mb high over Maine in the morning slides offshore with weak low pres moving across southern New England. Light NE winds become SE. Vsby lowers late in rain. Friday night... Weak low pres and attending cold front move offshore with WNW winds developing along with improving vsby from early rain showers. Outlook /Saturday through Tuesday/... Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Chance of rain. Sunday: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Rain. Sunday Night: Winds less than 25 kt. Areas of rough seas. Monday: Winds less than 25 kt. Seas up to 5 ft. Monday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 PM EST this evening for ANZ255- 256.
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&& $$ SYNOPSIS...Nocera/Dunten NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...Dunten AVIATION...Nocera/Dunten MARINE...Nocera/Dunten

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