Area Forecast Discussion
Issued by NWS Boston, MA

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643 FXUS61 KBOX 201157 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 657 AM EST Fri Jan 20 2017 .SYNOPSIS...
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High pressure will result in rather mild and tranquil weather today and Saturday, but there is a low risk for some spotty light freezing rain across northern Massachusetts late tonight. Otherwise, the main concern is the potential for 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+ feet. High pressure brings dry weather for Wednesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Clouds linger into the morning in Southern New England. Just one break across Central MA and much of RI. High pressure is parked over New England with plenty of subsidence. The anticyclonic flow should erode some of the sky cover, but the subsidence will also trap the low level moisture which will work against any clearing trend. Mixing is forecast to reach 950 mb, where temps are 0.5C to 1.0C. Mixing this to the surface would support max sfc temps in the lower 40s. If there is a sufficient period of breaking sky cover, the sun could warm temps a little higher...into the mid 40s. This is in line with the existing forecast, so high temps from the 4 am forecast continue the same.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... updated 130 am... Short wave trough currently moving across the OH valley will enter southern New England tonight. However given the short wavelength between the ridge over the area today and the approaching trough tonight...this short wave becomes sheared and deamplifies. Thus the highest chance of light rain/sprinkles tonight will be across CT and then diminishing across RI and eastern MA...where mainly dry weather may prevail. However as previous discussions have mentioned we will have to watch for any rain entering northern MA before drying up. Temperatures across northern MA especially northern Worcester county will be at or just below freezing. Therefore there is a low risk (less than 30 percent) for a brief period of light freezing rain across northern MA tonight. Keep in the mind given temperatures will be marginal along with little if any precip across northern MA the threat for freezing rain is low. Thus no headlines at the moment. Saturday... Some morning cloudiness associated with deamplifying short wave trough. Thus the trend will be for morning clouds giving way to afternoon sunshine. Airmass slightly milder than previous days along with light westerly pgradient to promote more of an offshore wind. This will support highs Sat in the mid to upper 40s with a low probability of few locations briefly hitting 50! So another very nice day by late Jan standards with partial afternoon sunshine, light winds and temps 45 to 50! Enjoy. && .LONG TERM /SATURDAY NIGHT 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... Saturday night-Sunday... Weak cold front moves south through New England Saturday night and 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 /12Z FRIDAY 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 Saturday/... Today... Ceilings at 3000 feet plus or minus a few hundred. Daytime warming might nudge these levels a little higher, even bring a brief period of scattered clouds. So trend will be for mixed high-end MVFR and low-end VFR cigs, trending to VFR. Light winds. Tonight... MVFR with low risk of IFR. Very light rain or sprinkles possible western MA into CT. Very low risk of brief light icing across northern Worcester county into northern Franklin county. Saturday... MVFR likely in the morning with spotty IFR possible. The improving VFR by afternoon except in the higher terrain MVFR cigs will linger. KBOS TAF...Moderate to high confidence in TAF. KBDL TAF...Moderate to high confidence in TAF. Outlook /Saturday Night through Tuesday/... Moderate confidence. Saturday night-Sunday... VFR Saturday night. A developing NE wind Sunday may bring 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.
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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 Saturday/... Today... Light winds, dry weather and good vsby. Tonight... Brief period of light rain or sprinkles possible. Otherwise light winds trending SW late. Saturday... Light winds, dry weather and good vsby. Outlook /Saturday Night through Tuesday/... Moderate confidence. Saturday night-Sunday... Variable wind becomes WSW but with speeds less than 20 knots. Winds shift Sunday 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.
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&& .TIDES/COASTAL FLOODING... Given potent easterly LLJ and period of strong wind gusts, expect seas to probably build to near 20 feet or higher across the eastern waters late Mon into Tue. Fortunately, astronomical high tides are about as low as they get. The strongest winds Monday evening will occur with an astro high tide of only 8.2 feet in Boston, so the risk of coastal flooding is low. However, Tuesday morning the high tide is 9.4 feet. If winds are still blowing strong out of the northeast along with 20+ foot seas just offshore, minor coastal flooding will occur along the eastern MA coast with even a small chance of a low-end moderate coastal flooding. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 1 PM EST this afternoon for ANZ250-254-255. Small Craft Advisory for hazardous seas until 10 AM EST this morning for ANZ251.
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&& $$ SYNOPSIS...WTB/Nocera NEAR TERM...WTB/Nocera SHORT TERM...Nocera LONG TERM...WTB AVIATION...WTB/Nocera MARINE...WTB/Nocera TIDES/COASTAL FLOODING...

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