Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 182106 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 406 PM EST Wed Jan 18 2017 .SYNOPSIS... High pressure builds over the region Thursday through Saturday. A weak cold front slips through the region late Saturday, followed by Canadian high pressure and slightly colder temperatures for Sunday. A significant storm system may impact New England Monday into Tuesday with heavy rain and coastal wind but some interior snow and ice is possible. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 120 PM Update... Have kept the freezing drizzle advisories going for now. Temps in those areas (Worcester, interior Essex, and the east slopes of the Berkshires) are still hovering around the 32 degree mark, though it appears mainly across the higher terrain. Also see some spotty freezing rain, but precip is light so will likely see predominately drizzle or freezing drizzle through the afternoon. Have some scattered showers that have developed across E Mass into N RI over the last hour or so on KBOX 88D radar imagery, which will continue to move slowly E-SE as low pressure passes offshore well SE of Nantucket. Also note the cold air damming across NE/Central and W Mass on the 18Z surface map with high pressure across the Maritimes. However, appears this is rather shallow, hence the reported temps near or just below freezing were mainly across the higher terrain into interior NE Mass, somewhat closer to the better damming. Temps across the coastal plain mainly around 40, with readings in the mid and upper 30s across N CT. Will see temps start to slow fall over the next few hours as we approach sunset. Suspect more -FZDZ will develop as temps fall to freezing or below inland. Also see more scattered precip moving out of the Hudson Valley which will help in bringing a light wintry mix into the CT valley through the remainder of the afternoon. Have updated near term grids to bring conditions current and incorporated into the remainder of the afternoon forecast. && .SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY NIGHT/... Tonight... Continued ascent of low level moist profiles ahead of weak mid-level shortwave energy and associated positively tilted trough axis across the region by morning. Stronger signal of drier mid to upper levels and thus the lack of ice nuclei favorable towards snow development. But still a good onshore flow and maritime influence. Temperatures dropping overnight, there is the expectation over a good chunk of S New England of transitioning back over to wintry precipitation types. Precipitation still rather light given the weak ascent, will keep it simply and go with freezing drizzle OR light snow for those areas where temperatures are at or below-freezing. Confidence that the window of freezing drizzle will be from sundown to roughly around midnight. After midnight, another mid-level shortwave impulse should yield the right ingredients to make the dominant precipitation type as snow for any sub-freezing locales. Will see the light precipitation move offshore towards morning with winds reverting northwesterly. The drying process begins as cloud decks become broken W to E. Thursday... High pressure and drier air working into the region. Will see clouds become scattered W to E through the day allowing for sunshine prior to sundown over a good portion of S New England. Continued NW flow. Will see temperatures warm into the low 40s as heights rise across the region and warmer air moves in from the W. H85 temperatures nudge above freezing towards the end of the day. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Mainly dry with above normal temps Fri/Sat * Low risk of some spotty light rain or freezing rain Fri night * Heavy rainfall and strong coastal wind possible Mon into Tue with some snow/ice possible in the interior Overview... Mid level ridging will build north along the east coast into the weekend resulting in mainly dry conditions with above normal temps. However, piece of energy breaking off from central CONUS trof will weaken as it moves into New Eng Fri night and may bring some light precip. Attention will then focus on a significant southern stream system moving across southern and SE CONUS then lifting north along the eastern US. Still lots of uncertainty given the time range but potential exists for a significant QPF event with strong coastal winds sometime Mon/Tue timeframe. Ptype mainly rain along the coast but interior ice/snow is possible depending on the track of the sfc low and positioning of the downstream ridge. Details... Friday into Friday night... High pressure will bring dry weather and light winds on Fri. Model cross sections indicate enough low level moisture to support a decent amount of cloud cover and it is possible clouds could become trapped under a low level inversion as mixing will be limited. Temps slightly above normal. Then Fri night, a weakening shortwave will move into the ridge across New Eng and may bring some light precip to mainly SW New Eng as the column moistens. Low level temps will be critical and some light freezing rain is possible in the interior if there is precip around, otherwise ptype mainly rain. It appears best chance of any freezing rain will be across W MA into the hills in N CT where moisture is deepest and temps cold enough. Saturday into Sunday... High pressure remains in control. Mild day Sat with temps in the 40s then cooling Sun as Maritimes sfc high ridges south into New Eng. Cross sections indicate abundant low level moisture both days which suggests mostly cloudy skies. There is a low risk deeper overrunning moisture to the south could approach SNE late Sun but odds favor dry weather holding on through Sun. Light precip may move into the region Sun night and ptype would be an issue with sfc high to the N/NE providing a source of cold air with some snow and/or mixed wintry precip, especially interior. Monday into Tuesday... Low confidence forecast in details as there is considerable uncertainty with the track of southern stream storm and positioning of downstream sfc high. Highest confidence in a significant QPF and possible coastal wind event as 850 mb easterly jet anomalies near 3- 4SD and anomalous PWATs which is quite impressive at this time range. Also, GEFS ensembles and EPS both show modest probs of greater than 2 inches QPF with individual members far exceeding that. Also, high probs of strong coastal easterly winds. The greatest uncertainty is in ptype, especially interior, and timing of heaviest precip. Along the coast, it will be difficult to lock in deep cold air given long fetch of onshore flow so precip likely mainly rain. However, ptype more problematic in the interior as some of the deterministic and ensemble guidance showing potential for significant ice and/or snow. Variability in model solutions is likely until energy reaches the west coast this weekend so all scenarios remain in play. Astronomical tides are quite low early next week which will limit coastal flood potential. However, given potential duration and fetch of easterly flow, there could be enough surge and wave action for some minor coastal flooding if highest surge and waves can coincide with high tide. Wednesday... GFS/ECMWF indicating mainly dry weather but low confidence forecast. Timing of Mon/Tue event will determine sensible weather next Wed. && .AVIATION /21Z WEDNESDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/...Moderate Confidence. 1820Z update... Through 00Z... Mainly IFR-LIFR CIGS/VSBYS across central and eastern areas as well as the CT Valley in areas of fog, drizzle and patchy freezing drizzle, with some spotty -SN across interior NE Mass. Some improvement to MVFR across portions of N Central and W Central Mass. Light north flow through the CT River Valley into N central and NE Mass, while E-NE winds along the immediate E MA coast with gusts up to 25 kts, especially across the Cape and Islands. Tonight... Precipitation tapering off as DZ for most terminals, though FZDZ or -SN is forecast for interior terminals, again the focus being N of the Mass Pike and W of the I-495 beltway. Roughly speaking W and Central MA. Winds turning more N/NW and diminishing into the morning period. Will begin to see cigs become BKN W to E, lifting low-end VFR. But most of the overnight period is characterized as a mix of MVFR-IFR. Thursday... Conditions improving as cigs become BKN to SCT, lifting to low-end VFR. NW winds continue. Specific Terminals... KBOS TAF...will hold rain across the terminal throughout the duration of the TAF. With the E/NE onshore flow will keep cigs IFR down around 800 ft agl into Thursday morning. Can not rule out cigs lifting briefly at times up to 1500. KBDL TAF...RA/DZ for the most part. Will be watching the overnight period as to whether sub-freezing temperatures impact the terminal thereby creating a risk of icing with FZDZ. Low confidence at this time as the expectation is that NW winds will proceeding during the overnight period with drier air allowing conditions to begin to improve into morning. Outlook /Friday through Monday/... Moderate confidence. Friday...Moderate confidence. Mainly VFR, but areas of MVFR cigs possible. Friday night into Sunday...Moderate confidence. Areas of MVFR/IFR cigs possible at times. Monday...Moderate confidence. MVFR/IFR in coastal rain and mixed precip interior. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/...Moderate Confidence. 120 PM Update... This afternoon... Small craft advisories remain across the waters. Report from pilot boat at 1815Z reported seas of 7-10 ft in Mass Bay, with 3-6 feet on Nantucket Sound. Will continue to see E-NE winds gusting to 25-30 kt, with the higher seas across eastern open waters as well as the northern portion of Cape Cod Bay and eastern Nantucket Sound. Scattered showers and areas of fog, with some visibility reductions at times. Tonight into Thursday... Winds turn out of the NW and taper. Drizzle will linger into Thursday yielding some visibility restrictions out on the waters. Otherwise waves dampen as conditions improve going into Thursday. Will see see small craft advisories persist throughout the period over the outer waters. Outlook /Friday through Monday/...Moderate confidence. Friday into Saturday...High confidence. Light winds as high pres builds over the waters. Lingering 5 ft seas outer waters early Fri, then subsiding. Sunday...Moderate confidence. Increasing NE winds with gusts to 20 kt in the afternoon increasing to 25-30 kt Sun night. Vsbys lowering Sun night in rain. Monday...Low confidence. Potential for strong easterly gales. Poor vsbys in rain and fog. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Freezing Rain Advisory until midnight EST tonight for MAZ002>006-008>010-012-026. RI...None. MARINE...Small Craft Advisory until 1 AM EST Thursday for ANZ231-232- 235-237. Small Craft Advisory until 10 PM EST this evening for ANZ230. Small Craft Advisory until 7 PM EST this evening for ANZ236. Small Craft Advisory until 6 AM EST Friday for ANZ250-251- 254>256. && $$ SYNOPSIS...KJC NEAR TERM...EVT/GAF SHORT TERM...Sipprell LONG TERM...KJC AVIATION...KJC/EVT/GAF MARINE...KJC/EVT/GAF is the U.S. government's official web portal to all federal, state and local government web resources and services.