Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 212100 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 400 PM EST Sat Jan 21 2017 .SYNOPSIS...
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Weak high pressure will bring tranquil weather through tonight. Developing onshore winds Sunday will result in an increase in clouds as well as spotty light rain or drizzle as the day progresses. A significant storm will reach the East Coast Sunday night and Monday and move up the coast Monday night and Tuesday. This will bring a mix of rain and/or snow along with a period of icing well inland. The storm may also bring damaging winds to the coast and the coastal waters. High pressure then brings dry and mild weather Wednesday into Thursday. A cold front crosses New England late week followed by seasonably cool temperatures and blustery winds.
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&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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4 PM UPDATE... Quite mild late this afternoon with temps in the mid 50s across much of the region. Not record breaking (60s) but nonetheless impressive for late Jan. Mostly clear to partly cloudy skies will prevail this evening and along with light winds temps will fall fairly quickly with sunset. However with dew pts in the low 40s (well above normal for late Jan) this will support at least the formation of patchy fog. This becomes tricky later tonight as mid/high clouds overspread the area. This may result in vsbys improving toward morning. Nonetheless temperatures above normal tonight with just about all locations above freezing. Low risk of some patchy freezing fog across the higher terrain but this is a very low prob.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
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4 PM update... Sunday... High pressure over Quebec will combine with weakening low pres east of DE to yield an increasing moist onshore flow into southern New England. Thus any dim sunshine thru mid/high clouds at sunrise will give way to overcast conditions. This increasing onshore flow will result in lowering clouds and spotty light rain/drizzle especially across the east slopes of the Worcester Hills and Berkshires. Given the clouds and onshore winds temps will be much cooler than today with highs in the 40s...still above normal for late Jan. Sunday night... Tricky forecast as northern stream short wave exits southeast Quebec into New Brunswick. This advects a cooler/drier airmass into the region. This erodes the leading edge of the precip shield as it approaches from the south. However at the same time it provides lowering wet bulb temps increasing the risk for wintry precip...that is if precip makes it this far north. Despite the synoptic scale lift and deep layer moisture possibly remaining south of New England thru 12z Monday...should be enough low level moisture given onshore flow and combined with upslope component will likely see spotty light rain. The problem will be across the higher terrain where the hi res guidance lowers temps into the upper 20s and lower 30s. Thus at least a moderate risk (30-60%) of freezing rain/drizzle across the high terrain of the Worcester Hills and east slopes of the Berks...possibly into northern CT as well. Given the shallow nature of the cold air followed the colder 4 km NAM temps for Sunday night.
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&& .LONG TERM /MONDAY THROUGH SATURDAY/...
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Big Picture... Deep trough digs over the Western USA while a ridge builds over the West Atlantic. In between the two, a Pacific shortwave rides the flow and deepens to a closed low as it crosses the mid-South. By this point, the developing storm taps moisture from the Gulf of Mexico and draws it north. The storm runs into the West Atlantic ridge and slows, taking on a negative tilt as it moves across the Eastern States. The Pacific trough then shifts east across the nation, reaching the Northeast USA by late week. The GFS and ECMWF mass fields are similar until early Wednesday, and agree on the broad scale pattern later in the week while differing on details. The GGEM shows some differences such as moving the coastal storm farther offshore, but bears some similarity to the other two solutions. Temperature fields at 850 mb look similar through the long term period. The resulting forecast now that the parent shortwave is moving ashore from the Pacific is sufficiently similar to previous solutions to improve confidence in the forecast. An overall model blend should work, especially with some focus on the GFS and ECMWF. The Monday-Tuesday storm will lower 500 mb heights to near normal over the region Monday-Tuesday. Height may recover to above normal Wednesday, but then the advancing trough from the West will bring below normal heights for the latter part of the week. Details... Monday-Tuesday... Low pressure moves up the coast with the focus of venting and moisture inflow shifting over New England by the afternoon. This will mean an increase in precipitation. Temperatures in the coastal plain will be warm enough for mainly rain, while the interior will be cold enough for a mix of snow/sleet/freezing rain. There is a mix of cold air damming signals...the surface pressure pattern indicates damming while the low level ageostropic flow is more northwest than the typical north wind for drainage. Locations north of the Mass Pike may be able to hold onto the cold air much of Monday into Monday night creating a messy mix that could make travel hazardous. Meanwhile, the Maritime High and coastal low move closer to each other and build the pressure gradient, leading to strengthening wind surface and low level aloft. Winds at 950 mb increase from 60 kts aimed at New Jersey in the morning to 70 knots aimed at the Cape and Islands Monday evening. This core of winds shifts north across our area from later Monday afternoon through much of Monday night, then moves north of us Tuesday morning. In anticipation of this, we have issued a High Wind Watch for Cape Cod and all the islands. This headline could be expanded later to much of the East Mass coast. Most areas that aren/t in the watch will likely have a wind advisory as we get closer to the event. Winds should diminish Tuesday as the coastal low moves past. Fair amount of moisture moves north with this coastal system with precipitable water values above an inch over RI/SE Mass. Continue to expect between 1 and 3 inches of water over the two day period. The storm center is expected to cross Cape Cod Tuesday afternoon and move off toward Maine late in the day. Expect precipitation to diminish at that time, followed by slow clearing overnight. Wednesday through Friday... Generally dry weather through this period. Weak high pressure builds over the region Wednesday. A weak cold front moves across on Thursday, but starved of enough moisture for precip most places. There remains a chance of patchy light rain or snow in Northwest MA. Dry and cooler weather moves in behind the front for Friday.
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&& .AVIATION /21Z SATURDAY THROUGH THURSDAY/...
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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 Sunday Night/... 18z update... This evening and overnight...moderate confidence. Trending toward VFR. This will lead to the formation of fog later this evening...likely lowering vsbys into the MVFR category. However with mid and high clouds arriving toward morning this may result in vsbys improving toward daybreak. Dry weather prevails. Sunday...moderate confidence. Mainly VFR to start but lowering to MVFR as the day progresses along with spotty light rain/drizzle and patchy fog. NE winds increasing up to 20 kt along the coast by days end. Sunday night...moderate confidence... MVFR likely in spotty light rain/drizzle. Concerned about freezing rain/drizzle across the high terrain including KORH. KBOS TAF...Moderate confidence in TAF. KBDL TAF...Moderate confidence in TAF. Outlook /Monday through Thursday/... Moderate confidence. Monday-Tuesday... MVFR cigs and vsbys in rain and snow Monday will lower to IFR/LIFR cigs late in the day or Monday night, especially in RI and Eastern MA as well as parts of the Worcester Hills. Best chance for snow and sleet/freezing rain will be north of the Mass Pike, although Northern CT has a smaller chance. Rain and fog farther south and east. Increasing winds above the surface Monday, with speeds reaching their maximum Monday night and then diminishing Tuesday. Winds at 1000-2000 feet should reach 40 to 60 knots Monday afternoon and continue Monday night. East winds of 70 knots possible over Cape Cod and Islands. Expect low level wind shear during this time along with strong wind gusts at the surface. Winds then diminish during Tuesday as the low level jet shifts north into Maine. Surface winds start from the east Monday, shift from the Northeast Monday night, and then from the Northwest on Tuesday. CIGS and Vsbys improve to VFR Tuesday night. Wednesday and Thursday... VFR most likely. An approaching cold front may bring MVFR cigs/vsbys Thu in isolated rain/snow showers for parts of Northwest Mass during the afternoon/evening.
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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. 4 pm update... Tonight...tranquil weather this evening with light winds. Vsby may lower to 1-3 miles in patchy fog. Sunday...Increasing NE winds, especially in the afternoon with gusts to 20 kt developing. G25 kt over southern waters by evening. Vsby may lower in patchy drizzle/rain. Sunday night...northeast winds continue to increase to gale force toward daybreak Monday. Seas increase rapidly to 10-15 ft by 12z Mon across the southern RI/MA waters. Vsby may lower in rain and fog esp southern waters. Outlook /Monday through Thursday/... Moderate confidence. Monday through Tuesday... Strengthening low pressure off the Mid Atlantic coast will move up the coast Monday and cross near Cape Cod Tuesday afternoon. Expect increasing winds Sunday night and Monday as the low runs up against Maritime high pressure creating an increased pressure gradient over the New England waters, as well as 1000-4000 feet above the surface. The strongest winds will move across the waters from Monday afternoon to Tuesday morning. East winds above the surface will reach 60 to 70 knots during this time, and have the potential to produce storm force gusts at the surface. The exception may be Boston Harbor and Narragansett Bay, where somewhat lesser east gales may be the rule. The Storm Watch has been expanded to include Mass & Ipswich Bays as well as Buzzards Bay and Vineyard Sound. Only Boston harbor and Narragansett Bay remain in a Gale Watch. These east winds will build seas and push them toward the eastern shoreline. Expect highest values of 15 to 20 feet over the most exposed waters during Monday evening and night. Winds will shift on Tuesday, becoming North during the afternoon and Northwest Tuesday evening as the storm center moves past. Wednesday and Thursday ... improving conditions as weak high pres builds over the waters Wed. Then increasing WSW winds ahead of an approaching cold front Thu with its passage late Thu/Thu night.
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&& .TIDES/COASTAL FLOODING...
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Given potent easterly LLJ and period of strong wind gusts, expect seas to probably build to 20-25 feet 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.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Wind Watch from Monday morning through late Monday night for MAZ022>024. RI...High Wind Watch from Monday morning through late Monday night for RIZ008. MARINE...Storm Watch from Monday morning through late Monday night for ANZ232. Storm Watch from Monday afternoon through late Monday night for ANZ233-234. Gale Watch from Monday morning through late Monday night for ANZ230. Storm Watch from Monday afternoon through late Monday night for ANZ231-251. Gale Watch from late Sunday night through late Monday night for ANZ236. Storm Watch from Monday morning through late Monday night for ANZ235-237. Storm Watch from Monday morning through late Monday night for ANZ250-254. Storm Watch from late Sunday night through late Monday night for ANZ255-256. && $$ SYNOPSIS...Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...WTB/Nocera AVIATION...WTB/Nocera MARINE...WTB/KJC/Nocera TIDES/COASTAL FLOODING...staff

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