Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 202053 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 353 PM EST Fri Jan 20 2017 .SYNOPSIS... A weakening weather system may produce some spotty light freezing rain across northern Massachusetts tonight. Otherwise, tranquil weather is expected Saturday into Sunday. Then the area will likely be impacted by a significant coastal storm with heavy amounts of rain/ice and/or snow along with strong to damaging winds on the coast Monday into Tuesday. This will also be a dangerous storm for mariners with the potential for 50 knot wind gusts and seas building to 20 or more feet. High pressure brings dry weather for Wednesday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Weakening short wave trough is getting stretched as it runs into a quasi-stationary upper ridge across New England. Areas of light rain may intrude into southwestern zones where temperatures are expected to remain above freezing. However, cannot rule out a few spotty areas of light freezing rain across Franklin, northern Worcester, and far northwest Middlesex County tonight, generally after 10 PM per a consensus of high resolution models. As of 3 PM mesonet temperatures were mostly in the mid 30s with a few readings of 33 and 34 across northern Massachusetts. It is a fairly low probability event for any one spot but even a light amount of freezing rain on a bare surface can be quite treacherous. Hence, plan to issue a Special Weather Statement for this relatively limited area of northern Massachusetts where even under clouds may see temperatures slip a few degrees after dark. Evaporative cooling if precipitation makes it into the region could also help drop the temperature a degree or two tonight. QPF where rain/freezing rain occurs should generally be under .05 inches. Have gone with the NERFC quantitative precipitation forecast. Otherwise, anticipate clouds to persist through the night with some areas of fog developing. Do not think the fog will be as significant as indicated by much of the guidance. The clouds will also result in a narrow diurnal range with most temperatures remaining in the mid to upper 30s overnight except for some lower 30s across northern Massachusetts. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Saturday...Main question is amount of lingering cloudiness during the day. For now have gone with partial clearing. The air mass is warmer than normal with 850 mb temperatures generally +2C to +4C across the area. With modest mixing and light west to southwest flow, anticipate high temperatures reaching the upper 40s to lower 50s across most of the area. Saturday night...With elevated dewpoints and light wind flow through a deep column, model guidance suggests areas of low clouds and fog forming. The air mass remains considerably milder than normal through the night, although colder air begins to sag into northern New England very late at night. Anticipate most areas will have low temperatures above freezing in the mid to upper 30s with just a few spots near freezing along the interior northern Massachusetts border. && .LONG TERM /SUNDAY THROUGH THURSDAY/... Big Picture... Storm off the Pacific NW coast moves ashore this afternoon and evening, then moves across the Southern Tier of the USA during the weekend, arriving at the Southeast USA coast by Monday morning. The storm then moves up the coast Monday and Tuesday. Upper ridge then moves across on Wednesday. Meanwhile the Wednesday-Thursday upper flow shows a ridge along the Pacific Coast and positive tilt trough from eastern Canada to the Desert Southwest. A shortwave moves through this progressive flow and across New England Wednesday night or Thursday. Model mass fields show fair agreement through Sunday night, then differences in the positioning of the Monday-Tuesday storm as it moves up the coast. Mass field also show some agreement on the general pattern Wednesday and Thursday, but again with differences in details over the Midwest. As noted earlier, the shortwave to be associated with the Monday- Tuesday storm is still over the Pacific Ocean and outside of the better sampling over North America. This reduces confidence in what has otherwise been a consistant model solution. Expect that confidence will increase later today and tonight as the shortwave moves ashore in the West. This assumes there are no significant changes in the data as the storm moves ashore. Details... Sunday... Weak cold front moves south through New England early Sunday. Limited moisture with the front, enough for some clouds but not precipitation. North to Northeast winds behind the front will bring colder air into our region during Sunday. Lots of moisture below 800 mb, but limited lift. Generally light low level east wind expected over Srn New England during the day with a stronger east wind of 25 to 50 knots focussed on the Mid Atlantic coast and possibly to Long Island late. So not a lot of low level lift expected. Based on this, sky cover should be mostly clouds through the day. The weak east flow may create some drizzle or sprinkles, especially across RI and Eastern/Central MA. We will be trimming model POPs to less than 25 pct. Sunday night-Monday-Tuesday... Our most significant weather system of the forecast period takes place during this time. As noted above, a weather system off the Pacific Northwest moves ashore later today and then moves along with the southern jet stream across the southern tier of the country. Thedeveloping surface low reaches the Carolinas Sunday night and slowly moves up the coast Monday and Tuesday. The fast movement across the nation followed by slow movement up the coast is reasonable based on the developing negative tilt to the upper trough as it reaches the East Coast. The low will tap moisture from the southern stream, with precipitable water values of 1 to 1.25 inches reaching Cape Cod during Monday night. Meanwhile, cold air will be draining south due to the building of Quebec high pressure over New England along with a pronounced cold air damming signal in the pressure pattern. Low level ageostropic flow remains a question mark with a strong NW flow rather than the usual N drainage flow. Model data shows very high POPs for Monday and Tuesday. As noted earlier, the system that will give rise to the storm is still over the Pacific Ocean. While it may appear to be a slam-dunk forecast, there are enough scenarios still in play that could change that forecast. As the system moves ashore and becomes better sampled...and shows no changes in the forecast...our confidence will grow to match the model confidence. Meanwhile, we have undercut POPs to strong-likely. Timing... Onset of snow remains uncertain, but likely during Sunday night as persistant lift is expected to move into the region. Onset of the heaviest precipitation should be late Monday or Monday night as the strongest southerly winds move up over equally strong east winds closer to the surface. Precip types and amounts... Cold air is initially deep enough to support a period of snow in Massachusetts, especially areas north and west of the Mass Pike and I-495. Projected storm center tracks over or just outside of Cape Cod support this notion. Amounts of 1 to 3 inches before changeover are reasonable in northern MA and possibly an inch or two more in some spots. Winds... Forecast data is advertising a strong low level jet. An east jet of 50-70 knots at 925-950 mb moves over the region Monday night and early Tuesday. BUFKIT shows east winds of 50 knots within 1000 feet of the surface Monday night. While not all of it may reach the surface due to temperatures and time of year, it still suggests potential strong wind gusts and possible damaging wind gusts. Best chance of this would be along the coastal plain in MA and RI. Model data shows some differences in timing the end of the storm, but the surface low passes Tuesday evening. Would expect winds to shift from the northwest and any pcpn to diminish at that time. Wednesday-Thursday... High pressure builds over the region with sunshine and drier air. A shortwave moving across NY and Northern New England may come close enough to spread some light mixed pcpn Thrusday, but low confidence in this happening. && .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... Rest of this afternoon... High confidence. Will continue with a persistence forecast with a continuation of high end MVFR or low end VFR ceilings. Light winds. Tonight... Moderate confidence. MVFR with spotty IFR conditions anticipated. Have leaned toward the higher end of the guidance envelope. Very light rain or sprinkles possible western MA into CT. Spotty surface icing is possible across Franklin, northern Worcester and far northwest Middlesex County. Saturday... Moderate confidence. MVFR likely in the morning with spotty IFR possible. Anticipate gradual improvement to VFR by afternoon in most locations. Saturday night... Moderate confidence. Elevated dewpoints will help set the stage for areas of fog and low cloud development. Forecast guidance is fairly emphatic in lowering conditions to widespread MVFR ceilings and visibilities with areas of IFR conditions. Our forecast once again trended toward the more optimistic end of the guidance envelope. KBOS TAF...Moderate to high confidence in TAF. KBDL TAF...Moderate to high confidence in TAF. Outlook /Sunday through Tuesday/... Moderate confidence. Sunday... A developing NE wind Sunday may keep MVFR cigs over Eastern MA and RI, along with light drizzle. Sunday night through Tuesday... MVFR cigs and vsbys all areas Sunday night with rain/ice/snow developing during the night. Conditions will lower to IFR/LIFR Monday, especially Monday night and Early Tuesday as the heaviest precip moves over the region. Increasing east winds aloft with speeds at 1000-2000 feet reaching 50-70 knots over Eastern MA and possibly RI. This will mean low level wind shear, along with some strong gusts at the surface all areas. Winds diminish Tuesday, eventually shifting from the Northwest by Tuesday evening. Conditions improve to VFR Tuesday night. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... Tonight through Saturday Night...High Confidence. Light winds and seas under 5 feet expected to persist through Saturday night. A 2 to 3 foot swell is expected to dominate during this period. Sunday... Winds shift Sunday to be from the ENE with speeds near 20 knots. Seas remain 4 feet or less. Sunday night through Tuesday... Rain moves up from the south as a deepening coastal storm approaches and moves up against high pressure over the Maritimes. This will bring increasing winds from the east. Winds at 1000 to 2000 feet above the surface will reach 50-70 knots, bringing likely east gales at the surface and a chance of Storm Force gusts at the height of the storm. Strongest winds should occur Monday afternoon and night. The wind will build seas, with heights reaching 15 to 20 feet. Rain and fog will bring occasional poor vsbys on the waters. Based on expected winds, a gale watch/warning will eventually be needed for this time period, and a storm watch/warning may be possible if forecast data trends stronger. This is potentially life- threatening for mariners. && .TIDES/COASTAL FLOODING... Given potent easterly LLJ and period of strong wind gusts, expect seas to probably build to 20 feet or higher across the eastern waters late Mon into Tue. Fortunately, astronomical high tides are about as low as they get, and the strongest winds Monday evening will coincide with an astro high tide of only 8.2 feet in Boston. Thus the risk of any significant coastal flooding is low. In fact, it is hard to imagine a scenario of much worse than some splashover for the Monday evening high tide, thanks to the lowness of the astro high tide. The Tuesday morning high tide is higher at 9.4 feet. If winds are still blowing strong out of the northeast along with 20+ foot seas just offshore, minor coastal flooding could occur along the eastern MA coast. It would take a storm surge of nearly 3.5 feet and waves greater than 20 feet to even approach a moderate level of coastal flooding. Unless this system progresses much more slowly than the current consensus of models indicate, it is unlikely that we will experience anything worse than minor coastal flooding and some beach erosion for the Tuesday morning high tide in spite of such dangerous marine conditions just offshore. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...EVT/Thompson NEAR TERM...Thompson SHORT TERM...Thompson LONG TERM...WTB AVIATION...WTB/Thompson MARINE...WTB/Thompson TIDES/COASTAL FLOODING...Thompson

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