Area Forecast Discussion
Issued by NWS Boston, MA

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540 FXUS61 KBOX 190242 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 942 PM EST Wed Jan 18 2017 .SYNOPSIS... Low pressure southeast of Nantucket will continue to push offshore this evening. 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 AM THURSDAY MORNING/... 10 pm update... Main take-aways are simply: 1) Temperatures are dropping in and around freezing and there is subsequently the threat of black ice. Already reports have been gathered of icing over N MA. 2) Freezing drizzle persists. The absence of ice nuclei aloft as observed via latest water vapor satellite is resulting in the low level moist profile under weak ascent to spit out drizzle (some of this not being detected by radar). This in addition to developing black ice with sub-freezing temperatures is adding to the icing across the region within the advisory and resulting hazardous travel conditions. 3) Freezing drizzle should change over to snow as mid to upper level moisture associated with a weak shortwave impulse on the back side of the broader trough over SE Canada (see the latest satellite) trails S/E across N/E portions of the NE CONUS. The introduction of ice nuclei will result in the change over. 4) Low clouds and abundant moisture will make for poor visibilities. Lowest conditions atop high terrain and over N/E MA where colder conditions pushing S with N winds is resulting in the lower moist atmospheric column to condense out. Visibilities on average around 3 to 4 miles, but there are some spot 1 to 2 mile visibilities. Overall...hazardous travel conditions will exist across much of the area except areas S of the Mass Pike. However do not want to rule out anything. Believe within areas S of the Mass Pike there will be isolated to spotty impacts to travel, mainly with respect to the development of black ice. Be prepared if having to travel. Give yourself extra time and take it slow if hitting the roads. These words of wisdom should be exercised during the Thursday morning commute when black ice will still be a threat. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Thursday... High pressure ridge builds across the region during the day. Drier air works in from the W, so clouds will push out allowing for sun to return during the afternoon over most of the region. Some clouds may linger along east coastal areas through most of the day. Temps will recover to the 40s, mildest across eastern Mass into N RI. Thursday night... High pressure sets up from Quebec to North Carolina during the night. Expect light N-NW winds in place. Some question as to whether lower clouds will back in off the ocean across E coastal Mass during the night. Temps will fall back through the 20s away from the coast, ranging to the 30s along the immediate shore. && .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 /03Z THURSDAY 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. 3z update... Tonight...Widespread MVFR/IFR with pockets of LIFR will continue tonight with gradual improvement toward daybreak. Patchy DZ / FZDZ up to around midnight changing over to SHSN, hopefully. N/NE winds diminishing, rather light otherwise. Thursday...CIGS improve to VFR from SW-NE during the morning. VSBYS improve to VFR, though may linger at IFR through mid morning across the higher inland terrain. Thursday night...Mainly VFR. May see area of MVFR CIGS move in along E coastal areas during the evening, then shifting W. KBOS TAF...Moderate confidence. Conditions gradually improving toward daybreak. Exact timing uncertain. KBDL TAF...Moderate confidence. Conditions gradually improving late tonight. Exact timing uncertain. 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. 10 pm update... At least by the 4 am update will convert small craft advisories to small craft advisories for hazardous seas. Will hold onto for the time being with some 25 kt N gusts persisting over the SE waters out around Nantucket. Thursday and Thursday night... Winds shift to N-NW and diminish to 10 kt or less. However, seas will linger AOA 5 ft mainly on the eastern open waters. Winds will gust up to 20 kt after midnight Thu night, which will keep the seas from subsiding. Visibility restrictions will linger on the eastern waters through mid-late Thu morning, then improve. Small craft advisories will linger, pretty much through the period on the eastern open 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>010-012-014-026. RI...None. MARINE...Small Craft Advisory until 6 AM EST Friday for ANZ250-251- 254>256. && $$ SYNOPSIS...KJC/EVT NEAR TERM...KJC/Sipprell/EVT SHORT TERM...EVT LONG TERM...KJC AVIATION...KJC/EVT/Sipprell MARINE...KJC/EVT/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.