Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211457 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 957 AM EST Sat Jan 21 2017 .SYNOPSIS...
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Weak high pressure will bring tranquil weather Today and Sunday. 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.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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10 AM UPDATE... Only change to previous forecast is to be more aggressive with clouds lifting and thinning this afternoon...giving way to at least partial sunshine from west to east. Reason for the optimismlatest visible satellite imagery already showing large breaks in the overcast across the lower Hudson valley of NY and western CT. Expecting this trend to spread eastward into MA and RI as deep layer ridge west of New England advects eastward. This will promote more anticyclonic flow and associated subsidence to erode low level moisture. In fact southwest surface winds this morning should veer to the west this afternoon, providing some downslope component helping to erode low level moisture. Current VWP already showing winds just off the deck at 1KFT and 2KFT from the WNW. Once sun breaks out these WNW winds aloft will mix to the surface and help the clearing trend. In addition new 12z NAM low level RH and ceiling grids support this clearing trend. Thus have trended the forecast toward decreasing clouds/increasing sunshine this afternoon. Other issue will be how high temps climb today. Mild start with temps already in the low to mid 40s many locations. 925 mb temps on SPC mesoanalysis has +5c to +7c already over MA/RI and CT. Given these temps and WNW winds just off the deck low to mid 50s very achievable. Previous forecast captures this well. However if enough sunshine develops this afternoon and blyr can deepen to 925 mb could see high into the upper 50s! Earlier discussion below. ====================================================================== Previous discussion... Weak mid level shortwave moves to the east today as weak high pres builds in from the west. Main forecast challenge is timing of any clearing this afternoon as low level moisture may remain trapped below shallow inversion. Expect low clouds to linger through at least the morning and it could be later, but we indicated some partial clearing developing in the afternoon from west to east as westerly flow becomes established. We have highs in the upper 40s to lower 50s.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Tonight... Weak high pres will be over the region with light to near calm winds. Model cross sections indicate enough low level moisture for patchy low clouds and fog to develop. Not certain of the areal extent of low clouds but best chance will likely be along the south coast. Lows will be mostly in the 30s. Sunday... Weak cold front will sag south across SNE during the day as surface ridging noses down from northern New Eng with increasing NE flow developing in the afternoon. The NE flow and increasing low level moisture will result in widespread lower clouds developing across SNE and can`t rule out some spotty light rain or drizzle in the afternoon. High will range from lower 40s north to upper 40s south but cooling from the north in the afternoon with temps likely falling into the 30s north of the Pike. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
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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... Sunday night... High pressure builds south into Northern New England. This will create an ENE wind coming off the Gulf of Maine into our area. Dry air aloft in the -10C to -15C nucleation zone suggests any precip before midnight will be limited to drizzle or freezing drizzle and perhaps some light rain that is just above trace level. The moisture starts filling in aloft during the night, which will bring a better chance of measurable pcpn after midnight. Even so, the focus of upper venting and lower level transport of moisture will remain to our southwest along the Mid Atlantic coast through the night with New England on the fringe. 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 /15Z SATURDAY THROUGH WEDNESDAY/...
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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/... 15z update... Only change from previous TAFs is to speed up clearing trend as currently seen on latest observations. Earlier discussion below. ================================================================= Today... Moderate confidence. MVFR/IFR cigs in the morning with patchy fog, improving to VFR in the afternoon. Tonight... Moderate confidence in patchy MVFR/IFR fog developing, but low confidence in potential for IFR cigs. Guidance is ranging from VFR to IFR/LIFR. If IFR cigs develop, highest prob likely along the south coast. Sunday... Moderate confidence. Widespread MVFR cigs developing with potential for IFR in the afternoon. Patchy light rain or drizzle possible in the afternoon. Increasing NE winds along the coast in the afternoon with gusts to 20 kt developing. KBOS TAF...Moderate confidence in TAF. KBDL TAF...Moderate confidence in TAF. Outlook /Sunday Night through Wednesday/... Moderate confidence. Sunday night...MVFR cigs through the night. Some drizzle possible in the evening but vsbys should remain no lower than MVFR for that. Light rain and snow develop after midnight but with vsbys roughly 5 miles or greater. Increasing ENE winds along the coasts with gusts 30 to 35 knots, but lighter inland. 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... VFR. An approaching cold front may bring MVFR cigs/vsbys in rain 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. 10 am update... Tranquil weather by late Jan standards. Only issue will be patchy fog this morning limiting vsby to 1-3 miles but improving this afternoon as winds shift from SW to W. Earlier discussion below. ==================================================================== Short Term /through Sunday/...High confidence. Today...Southerly winds becoming W/SW in the afternoon with gusts 15-20 kt at times. Easterly swell of 4 to 5 ft developing over the outer waters. Tonight...W winds gradually becoming N after midnight as weak cold front moves into the waters from the north. Some lingering 5 ft swell over the eastern waters. Sunday...Increasing NE winds, especially in the afternoon with gusts to 20 kt developing. G25 kt over southern waters by evening. Outlook /Sunday Night through Wednesday/... Moderate confidence. Sunday night 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... Winds diminish and back from the WSW. Lingering gusts to 25 knots. Seas will subside through the day, but with lingering 5 to 6 foot seas on the exposed waters.
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&& .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...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...WTB/KJC NEAR TERM...WTB/KJC/Nocera SHORT TERM...KJC/Thompson LONG TERM...WTB AVIATION...WTB/KJC/Nocera MARINE...WTB/KJC/Nocera TIDES/COASTAL FLOODING...

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