Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221521 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1021 AM EST Sun Jan 22 2017 .SYNOPSIS... Onshore winds will result in cloudy skies with spotty light rain today. A significant storm will reach the East Coast 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. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1020 AM UPDATE... Areas of fog along the south coast including Cape Cod and the Islands as well as the CT river valley have thinned with vsbys now 2 miles or higher across this region. In fact weak departing short wave exiting eastern MA waters has resulted in subsidence and breaks in the overcast over Cape Cod, the islands and remainder of the south coast of MA into RI. Thus have cancelled the dense fog advisory and Special Weather Statement. SPC mesoanalysis shows much cooler/drier air poised over ME and northern New England with 925 and 850 mb temps below freezing. This leading edge of cooler air and a wind shift to the northeast is associated with a back door cold front moving southbound thru downeast ME and into the Seacoast of NH. Deeper into the cold air over ME dew pts drop into the 20s...this will be our airmass later tonight. Meanwhile for today...any breaks in the overcast will fill back in this afternoon as abundant low level moisture accompany the arrival of the backdoor cold front as it enters southern New England this afternoon. This combined with low level moisture and increasing onshore flow will yield spotty light rain/drizzle... which is already occurring over northwest MA. Temps not as mild as yesterday but still above normal for late January. Previous forecast captures these details nicely so no major changes with this update. Earlier discussion below. ==================================================================== Previous discussion... One other concern is for spotty light icing early this morning with temperatures bottoming out near freezing away from immediate coast. However, ground temperatures are a bit warmer thanks to recent stretch of mild weather, so this should not be a significant issue. Flow turns onshore today with high pressure over Quebec and low pressure off Mid Atlantic coast. Forecast soundings show plenty of low level moisture so a cloudy day is in store after fog dissipates. We may see some spotty light rain or drizzle, especially near east slopes of Berkshires and Worcester Hills which will provide some added lift. Temperatures will run a bit cooler than our preview of spring yesterday, but still above average for late January, generally in 40s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Downstream ridging aloft is present over southern New England as mid level trough closes off over Carolinas. As a result, models have slowed down onset of precipitation associated with developing coastal low, aside from spotty light rain/drizzle from persistent onshore flow. Since lower levels cool with high pressure ridging down from Quebec, we still cannot rule out a light wintry mix across much of region, especially from Berkshires to Worcester Hills. Temperatures are borderline for any accumulation near I-95 corridor between Providence and Boston, but it will be a little colder farther north and west where we could see light coating of snow or ice by daybreak. Lows will fall back into the low to mid 30s, with some upper 20s possible in NW MA. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... 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 Thursday, with a similar broad scale flow Friday-Saturday and differences in details. The GGEM is noticeably farther offshore with the Monday-Tuesday storm, taking the 500 mb center well south of New England while matching the surface low position of the other models. An overall model blend should work, especially with 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 and Thursday, but then the advancing trough from the Western USA will bring below normal heights for the latter part of the week. All of this suggests a mild period following the storm, then seasonable cold for Friday and Saturday. Details... Monday-Tuesday... Onshore east flow will maintain clouds over the region Monday morning. Winds will increase during the day and may supply enough low level lift for areas of sprinkles or drizzle. More organized precipitation will be difficult due to dry air at the nucleation zone aloft between -10C and -15C. This will moisten during the afternoon as deeper moisture advects in from the south, at which point precipitation will be more likely. Low pressure reaches the Carolinas Monday morning and model consensus brings the system up the coast Monday evening and night. Meanwhile, Quebec high pressure settles south into the Maritimes and Northern New England. The low brings plenty of moisture, with precip water values of 1.0 to 1.25 inches, while the high brings cold air. As the two collide, the pressure gradient will increase and generate strong winds both at the surface and for several thousand feet above the ground. Upper low becomes negatively tilted Monday, suggesting a slow movement of the system up the coast Monday night and Tuesday. The high will slowly move off, allowing the coastal low to move past New England. Model consensus brings the surface center across SE Mass near Buzzards Bay Tuesday afternoon/evening. Wind... As the pressure gradient increases on Monday, so will east winds. Low level east winds will start Monday focused on the Mid Atlantic coast, but will edge north to the New England South Coast during the afternoon/evening. Winds in the core at 2000 feet AGL are forecast at 65 knots. These winds will move across south coastal areas early Monday night and sweep north of the MA North Shore by Tuesday morning. This will produce strong wind gusts later Monday and Monday night, primarily over coastal areas including Cape Cod and the Islands. There is also a concern for east coastal MA which will be most open to the east winds off the ocean. Gusts to 60 knots are most possible here. Strong winds may reach farther inland, but at lesser speeds. Precipitation onset... Upper venting starts Monday focused over eastern PA and adjacent NY/NJ. Low level east flow starts the day south of New England aimed into the Mid Atlantic. Expect the precip shield to start the day in this area and then shift north/east as the upper venting and low level jet also shift. We estimate starting time around midday along the south coast and early evening along the NH border. Precipitation type... Cold air drainage is still indicated by the surface pressure pattern, but ageostrophic pattern still shows a northwest flow rather than the typical north flow for cold air drainage. Nevertheless, low level temperatures will be cold enough for a period of snow/sleet over parts of central and western MA, and possibly briefly in parts of Northern CT. Farther east and south the precip should be mainly rain. Rainfall amounts...Lots of precipitation moves in with the weathersystem, with precipitable water values as noted. Highest values should be near the storm center and a little to the left of its track. So expect highest rainfall amounts in RI and Eastern MA. Overall, amounts over Southern New England should range from 1 to 3 inches. This should not be a cause of any river flooding. But urban and poor drainage flooding may be a concern in any heavier downpours. A Flood watch continues in RI and Eastern MA where the heaviest rainfall is expected. The storm center moves off along the coast of Maine Tuesday night. Expect precipitation to diminish at that time, followed by slow clearing overnight. Wednesday-Thursday... 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. Friday-Saturday... No significant changes were made to this part of the forecast due to our focus on the Monday-Tuesday storm. Dry and cooler weather moves in behind the front for Friday and Saturday. && .AVIATION /15Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 1020 AM update... IFR cigs over northeast MA into southeast NH will overspread the remainder of MA/RI and CT this afternoon. This will be accompanied by a wind shift to the NE as a backdoor front sweeps across the area. Light spotty light rain/drizzle will become more widespread as the afternoon progresses. 12z TAFs capture these details so no major changes with next TAF update. Earlier discussion below. ===================================================================== Short Term /through Monday/... Moderate confidence. In Northeast and Central MA cloud bases are VFR while vsbys are mixed VFR and MVFR in fog. Remaining areas are IFR/LIFR in fog, including areas of dense fog. Winds are mostly light, and the fog should linger while that remains in place. A weak cold front is moving through the region this morning. Expect winds to increase from the Northeast, especially along Eastern MA, during the day. That will improve vsbys to P6SM or close to that. Ceilings will linger at MVFR with patches of IFR through the day. The area of higher cloud bases in NE Mass should fill in again by afternoon as northeast flow off the ocean takes place. Ceilings should be either IFR or low-end MVFR. Ceilings should improve to MVFR tonight with increasing E/NE flow. May see spotty light rain or drizzle near coast and light wintry mix farther inland, especially after midnight. Gusts to 30kt possible near immediate coast overnight. KBOS TAF...Moderate confidence in TAF. Greatest uncertainty concerns how fast skies will fill in again late this morning or early afternoon as winds increase from the NE. KBDL TAF...Moderate confidence in TAF. Areas of dense fog with LIFR cigs/vsbys will improve late morning but remain low-end MVFR. Outlook /Monday Night through Thursday/... Moderate confidence. Monday-Tuesday... MVFR cigs will slowly lower to IFR/LIFR as precipitation moves in from the south. Patchy MVFR VSBYs may occur in drizzle Monday morning. As the precipitation moves in from the south, this will become a more general MVFR with areas of IFR vsbys in fog and rain. Snow and sleet will be possible in the interior, with a period of freezing rain before it changes to rain. 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 65 knots are 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, then from the Northeast on Monday night and turn from the Northwest Tuesday. CIGS and Vsbys improve to VFR Tuesday night. Wednesday-Thursday... VFR. An approaching cold front may bring a period of MVFR cigs/vsbys in rain showers for parts of Northwest Mass. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 1020 AM update... Backdoor front over coastal NH will continue to slide southward thru the day and will be accompanied by a wind shift to the NE and speeds increasing 15 to 20 kt. Low clouds will increase along and behind this front and spotty drizzle/patchy fog may lower vsby to 3 miles or so. Previous forecast captures this well so no major changes expected with this update. Earlier discussion below. ===================================================================== High confidence through tonight. Increasing N/NE winds today with gusts reaching 25kt during afternoon. Winds should not reach Gale Force until later tonight, probably around midnight or so, but seas will steadily build today and tonight, reaching 10-15 ft offshore by daybreak Mon. Dense fog will gradually lift this morning, followed by spotty light rain or drizzle at times later today and tonight. Outlook /Monday Night through Thursday/... Moderate confidence. Monday-Tuesday... Strengthening low pressure off the Mid Atlantic coast will move up the coast Monday and cross near Cape Cod Tuesday afternoon/evening. Expect increasing winds 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 and night, diminishing Tuesday. 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 Warning continues from Monday afternoon through Tuesday morning. This covers all waters except Boston Harbor and Narragansett Bay. These waters will have a Gale Warning. 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-Thursday... 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. && .TIDES/COASTAL FLOODING... Still looking at potential for splashover or minor coastal flooding along E MA coast during high tide Mon evening and Tue morning. Despite low astronomical tides (8.2 ft at Boston Mon evening and 9.4 ft Tue morning), combination of persistent and very strong E winds and high seas up to 20-25 ft offshore may lead to splashover with Mon evening high tide. Minor coastal flooding could occur with Tue morning high tide since it`s a bit higher, although winds would have to still be from NE which is questionable. Overall, 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 Warning from 9 AM Monday to 6 AM EST Tuesday for MAZ022>024. Flood Watch from Monday afternoon through Tuesday morning for MAZ005>007-013>021. High Wind Watch from Monday afternoon through late Monday night for MAZ007-018-019-021. RI...High Wind Warning from 9 AM Monday to 6 AM EST Tuesday for RIZ008. Flood Watch from Monday afternoon through Tuesday morning for RIZ001>007. MARINE...Storm Warning from 1 PM Monday to 4 AM EST Tuesday for ANZ231- 232. Storm Warning from 1 PM Monday to 3 AM EST Tuesday for ANZ233- 234. Gale Warning from 7 AM Monday to 7 AM EST Tuesday for ANZ230. Gale Warning from 3 AM Monday to 4 AM EST Tuesday for ANZ236. Storm Warning from 7 AM Monday to 2 AM EST Tuesday for ANZ235- 237. Storm Warning from 11 AM Monday to 7 AM EST Tuesday for ANZ250- 254. Storm Warning from 2 PM Monday to 6 AM EST Tuesday for ANZ251. Storm Warning from 4 AM Monday to 4 AM EST Tuesday for ANZ255- 256. && $$ SYNOPSIS...WTB/JWD NEAR TERM...WTB/Nocera/JWD SHORT TERM...JWD LONG TERM...WTB AVIATION...WTB/Nocera/JWD MARINE...WTB/Nocera/JWD TIDES/COASTAL FLOODING... is the U.S. government's official web portal to all federal, state and local government web resources and services.