Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 162227 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 527 PM EST Mon Jan 16 2017 .SYNOPSIS... Low pressure will move from the Great Lakes to New York state with secondary low pressure forming near Long Island Tuesday and passing east of Nantucket by Wednesday. A mixed bag of wintry weather will impact much of southern New England from Tuesday afternoon into Wednesday. Could see leftover spotty showers along the shore late this week. High pressure will build along the eastern seaboard with dry, milder conditions into next weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 450 PM update... A high pressure ridge will remain over New England tonight with mainly clear skies to start, then increasing mid and high level cloudiness from southwest to northeast overnight. Low temperatures should bottom out in the upper teens in northwest MA, ranging to the mid 20s elsewhere, except lower 30s on the Islands. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Low pressure moving eastward from the Great Lakes to New York state will spawn a secondary low pressure area near Long Island Tuesday morning, which will head east toward Nantucket late Tuesday night. It will cause a wide variety of messy weather across our region. All models are coming into good agreement on the timing. Precipitation is expected to break out in western MA and northern CT in the afternoon and spread eastward toward evening. The most difficult part of this forecast is the precipitation types. For this forecast package, we have expanded the Winter Weather Advisory to include central Middlesex and western Essex Counties in northwest MA. We have upped snowfall totals significantly over interior and especially northern MA. Although the typical 1540m value for 850-700 mb partial thickness will be to our northeast through the bulk of the event implying warm air aloft, cold air will remain in the low layers of the atmosphere. The 1000-850 mb partial thickness values remain below 1310m over a large part of MA through Tue. night. In fact, the ECMWF and GFS show a pronounced lowering of the 1540m line down to 1535m in northeast MA as heavier precipitation moves in Tuesday night and evaporational/dynamic cooling takes place. All model snowfall algorithms and the NCEP WPC forecast have at least 3 inches of snow, with perhaps as much as 6 inches occurring in the Route 2 corridor of northern MA. Sleet and freezing rain will be the predominant type of precipitation in northern CT and in western and central MA south of the MA pike, before likely changing to all rain. Up to one-tenth inch of ice accretion is expected, although we cannot rule out local two- tenths amounts, especially in the slopes of the Berkshires. In northern RI and interior eastern MA, an inch or two of snow is possible before turning to rain. All rain is expected in southern RI and southeast MA. A lot could change with this situation. A small change in temperatures, both at the surface and aloft, could result in big changes of precipitation type and corresponding snowfall amounts. A thin coating of ice on untreated roadways can be dangerous. Please slow down and use caution if driving late Tuesday and Tuesday night. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Highlights... * Precipitation may linger across Wed into Wed night, with change back to light snow or wintry mix across the interior * Low pressure passing off the mid Atlantic coast may push light precipitation into the region Fri night into Sat * Another low may push slowly up the coast next Monday Details... Wednesday-Wednesday night...Moderate confidence. Low pressure will pass S of New England during Wednesday, while high pressure remains across the Canadian Maritimes. This will keep colder air across inland areas where a mix of rain, sleet and snow will continue for most of the day. However, expecting only light amounts of melted precip, generally around 0.15 inches or less. With temps close to freezing, will see low snow to rain ratios, so may receive an additional dusting to 0.5 inches of snow, with the best shot across northern Mass along and N of the Route 2 corridor as well as across the higher terrain of northern Worcester county along with the E slopes of the Berkshires. With deep low level easterly winds, could see spotty light rain and/or snow linger through Wed night across E Mass into RI. Should see precip tapering off over central and western areas Wed evening. Highs on Wed will range from the mid 30s well inland to lower-mid 40s along the immediate coasts. Readings Wed night will fall back to the mid and upper 20s inland, ranging to the mid to upper 30s along the coastline. Thursday...Moderate confidence. May see a few lingering rain and/or snow showers across eastern areas early Thu, then precip should end by mid to late morning. However, with continued N-NE winds, clouds will linger through most of the day before starting to break across the lower CT valley toward sunset. Some question as to weak low pressure that may rotate SE out of northern New England across NE Mass around midday or so. The light onshore wind flow may enhance any precip and possibly bring a brief period of rain showers across Cape Ann. Will not last too long, though. Expect temps to top off in the upper 30s across the E slopes of the Berkshires to the lower 40s over the coastal plain. Friday...Low to moderate confidence. With a relatively progressive steering flow continuing across the lower 48, will see another low develop across the south central U.S. With a general W-SW wind flow aloft, this low will work toward the region. Still some model solution spread as this low, along with its H5 short wave, shifts mainly E toward the mid Atlantic coast. The 12Z GFS remains on the fast and further N end of the model suite. Have kept only slight chance POPs in the forecast for most areas, but best shot will be along the immediate S coast and the islands. Then, as winds shift to E Fri night, could see some spotty precip lingering across eastern areas as well as along the S coast. Saturday-Monday...Low confidence. For now, looks like the low should pass offshore during Saturday, but some precip may again linger during the morning across central and eastern areas before heading eastward. High pressure looks to build across the region Sat night through Sun, so should see mainly dry conditions. Models and ensembles signaling development of cutoff H5 low pressure developing Sat/Sat night across the lower Mississippi Valley, then will shift E-NE late Sun and Mon. At this point, have brought in CHC POPs late Sun night through Mon. With the heart of winter in place, may see PTYPE issues as well. Still way too early to tell exactly when, if any, wintry precip will take place and what effect this could have on the region. Still quite a bit of time to see the evolution of this system. && .AVIATION /22Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High Confidence. 2150Z update... Tonight...VFR. Tuesday...VFR to start. MVFR developing from W to E in the afternoon. Wintry mix of SN, IP, FZRA, and RA spreading east during the afternoon. Conditions then becoming IFR throughout the region. Snow may be locally heavy reducing visibility in the higher terrain areas of northern and western MA. Tuesday night...IFR in SN, IP, FZRA interior, changing to rain and in RA in RI and southeast MA. Easterly winds gusting to 20-30 kt near the coast late at night. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Wednesday through Saturday/... Wednesday-Wednesday night...Moderate confidence. IFR-LIFR CIGS and MVFR-IFR VSBYS in patchy RA across the coastal plain and mixed RA/SN/IP inland. Precip tapers off late Wed/Wed night. Locally reduced visibilities during Wed. E winds gusting to 25-30 kt on outer Cape Cod and Nantucket Wed, diminishing as winds shift to NE Wed night. Thursday...Moderate confidence. MVFR-IFR CIGS early, improving to mainly VFR. May see local MVFR CIGS lingering across higher inland terrain during the day. Friday-Saturday...Low confidence, mainly for exact timing of lower conditions. VFR early Fri, then conditions may lower to MVFR in patchy RA/SN/IP, mixed inland Fri afternoon through Sat. May improve inland during Sat. Light N-NE winds Fri shift to E Fri night, then back to N-NE later Sat. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High Confidence. 450 PM update... Tonight...Have dropped Small Craft Advisories for this evening, as wind gusts have diminished to below 25 kts. Dry weather and good visibility. Tuesday...Winds becoming easterly and increasing to 10 to 15 kt. A chance of rain late in the afternoon, especially along the RI coast. Tuesday night...Low pressure will be passing near or just south of Nantucket. Small Craft Advisories have been issued for the overnight hours into Wednesday for the eastern outer waters. Seas will be increasing to 3 to 6 ft as easterly winds persist...and gusts increase to 20-30 kt over the eastern outer waters. Outlook /Wednesday through Saturday/... Wednesday-Wednesday night...Moderate confidence. Strong E winds gusting up to 25-30 kt on the eastern waters. Seas build up to 7-10 ft on the eastern waters, then increase to 5-8 ft on the southern outer waters Wed night. Low risk of gale force gusts. Locally reduced visibilities in light rain Wed, then in scattered rain and/or snow showers Wed evening across the eastern waters. Winds and seas begin to diminish Wed night as winds back to NE. Thursday...Moderate confidence. N-NE winds gusting to 20 kt early, then becoming W-NW around 10 kt. Seas remain above 5 ft on the outer waters through Thu, then slowly subside Thu night but remain around 5 ft on the eastern outer waters. May see briefly reduced visibilities in isolated showers on the waters E of Cape Ann late Thu morning into the afternoon. Friday-Saturday...Low to moderate confidence. Light NE winds shift to E Fri night. May see gusts to 20 kt Sat on the waters from Cape Cod southward Sat. Seas may approach 5 ft during Sat. Visibilities may be locally reduced in patchy light rain Fri afternoon through Sat. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Weather Advisory from 1 PM Tuesday to 7 AM EST Wednesday for CTZ002>004. MA...Winter Weather Advisory from 1 PM Tuesday to 7 AM EST Wednesday for MAZ003>005-010>012-026. Winter Weather Advisory from noon Tuesday to 7 AM EST Wednesday for MAZ002-008-009. Winter Weather Advisory from 4 PM Tuesday to 7 AM EST Wednesday for MAZ006. RI...None. MARINE...Small Craft Advisory from 1 AM to 5 PM EST Wednesday for ANZ250-251-254-255. && $$ SYNOPSIS...EVT/GAF NEAR TERM...GAF SHORT TERM...GAF LONG TERM...EVT AVIATION...EVT/GAF MARINE...EVT/GAF is the U.S. government's official web portal to all federal, state and local government web resources and services.