Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 142017 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 317 PM EST Sun Jan 14 2018 .SYNOPSIS...
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Canadian high pressure will bring very cold weather into Monday with some ocean effect snow across eastern MA late tonight into Monday evening. Low pressure drives east from the Great Lakes Tuesday, then redevelops off the Mid Atlantic coast. This will bring snow later Tuesday and Wednesday. Colder air follows for Wednesday night and Thursday. Atlantic high pressure then builds into New England with dry weather and milder temperatures by next weekend.
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&& .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... Bands of ocean effect snow showers persist on Cape Cod and off E MA coast. This activity has begun to diminish in response to lowering inversion as seen on model soundings, and clearing is already pushing south from Gulf of Maine. Strong high pressure to our north will maintain flow of very cold air into SNE on northerly winds tonight. Under clear skies with diminishing winds, temperatures should have no problem falling into single numbers across much of area, with some readings below zero across parts of interior MA and northern CT, with milder teens/around 20 along immediate MA coastline as well as Cape Cod and Islands. Later tonight, we will begin to see effects of a developing coastal front as clouds spread inland from Cape Cod and Islands. Models are a little at odds as to how quickly snow showers will develop, with some high-res members showing snow as early as midnight while others hold off until daybreak. We think the truth is somewhere in between, mainly between 4 and 6 am, and rather light at onset, perhaps just flurries. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... * Ocean Enhanced Snow Near E MA Coast * Winter Weather Advisory South of Boston PLEASE NOTE: Because of the possibility of a more widespread snowfall Tue into Wed (See Long Term below), our snowfall maps on the web (including probabilities) will focus on that since it potentially will affect more of SNE. We will post the expected snowfall map for late tonight into Monday evening on social media. Confidence is increasing on receiving accumulating snow near E MA coast Monday into Monday evening. This is not a classic "ocean effect" setup but rather "ocean enhancement" due to NNE flow over coastal front. If you look at forecast soundings, they certainly are not very impressive with very shallow mixing layer (less than 2,000 ft) and not much in way of ocean-induced CAPE. Focus for activity is coastal front which will move onshore overnight and meander just inland Monday into Monday evening. It is this boundary which is the key to the forecast - including temperatures (teens on west side and 20s/lower 30s to east), location of bands of snow showers, and where they become more persistent (and bring higher snowfall totals). Since this is purely a mesoscale event we need to rely heavily on higher resolution models and ensembles as opposed to coarser global models. We largely used a combination of SREF, WRF-ARW, and WRF-NMM to better predict PoPs/QPF/Snow and to depict the tight gradient that is likely to occur. Of course this means that we could be off by 5-10 miles which could make a big difference, but as we get into realm of more high-res models (HRRR, RAP) we can further refine the forecast. Good news is these high-res models and ensembles are very consistent with their positions of coastal front, extending from Cape Ann to Boston Harbor and into Plymouth County. It is this area where we should see 1-3" of snow, although due to banded nature some areas may only get a dusting while locations a few miles away get more. Western extent should be Route 128 and I-195 corridors which may only see flurries. Focus for more persistent snowfall will be just to west of this boundary, but especially just south of Boston into Plymouth County which will have added benefit of enhancement/longer fetch from NNE (020-030) low level winds. We do not see much in way of strong lift in dendritic growth zone although there are a few hours Monday morning where lift intersects with -8 to -10C (which is favorable with salt nuclei). For these reasons, we will show a maximum snowfall of 2-4" in these areas and issue a Winter Weather Advisory. It`s certainly possible a few locations end up receiving a bit more, like 5 or even 6 inches, but those seem to be outliers at this point. We did note a few 12Z HREF members showing potential for >5 inches of snow. For Boston itself, it`s a tough call as city will be on edge of better activity to south and tends not to be favored with 020-030 low level winds (we would like to see more NE winds of at least 040-050) so we are thinking an inch or two of accumulation right now. Meanwhile, not to ignore the rest of SNE, we do think clouds will expand inland but nothing more than flurries. Model 2-m temperatures suggest highs in lower to mid 20s, but again milder to the east of coastal front with highs in lower 30s on Cape Cod and Islands. Clearing but not quite as cold Monday night with lows in teens and lower 20s. && .LONG TERM /TUESDAY THROUGH SUNDAY/...
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Big Picture... Longwave pattern shows a trough over the Eastern USA and a ridge over the Western USA through Friday. The flow then trends toward zonal flow for the weekend. Short scale pattern shows a trough over the Great Lakes sweeping east Tuesday but maintaining a progressive axis as it moves across New England Wednesday. Upper ridge then builds over the Eastern USA and West Atlantic waters Friday through Sunday. Upper height fields are similar through Wednesday, with differences appearing after that. The GFS is fastest in moving the upper trough past New England and also is more amplified with upper shortwave moving over the area Friday night-Saturday. The ECMWF is slowest. All models show building heights through the weekend. MSLP fields are now similar in developing low pressure east of New Jersey Tuesday evening and moving it past Cape Cod Wednesday. They also show surface high pressure building over the region Friday through Sunday. Contour heights suggest cooler temps midweek, then milder values over next weekend. Confidence in the forecast is moderate for Tuesday-Wednesday, building to moderate-high for Friday through Sunday. Details... Tuesday-Wednesday... The 12Z models have moved toward a consensus on the midweek weather system. Low pressure over the Great Lakes moves east Tuesday, with precip moving into our area during the afternoon/evening. Upper heights are higher/milder than average, but eastern Quebec high pressure maintains a cold air damming signal south through much of New England. So we expect the precip, when it arrives, to be snow. As the supporting upper jet shifts east Tuesday evening, its right entrance region moves over the Mid-Atlantic coast, supporting a coastal redevelopment which then moves up the coast near 40N/70W. This should maintain snow over our area Tuesday night and early Wednesday. Depending on the exact track, there may be some mix of rain and snow over Nantucket and parts of Cape Cod. Snow tapers off Wednesday as the coastal low races off to Nova Scotia. QPF ranges from 0.2 to 0.4 inches, with snow-water ratios either side of 15 to 1, except closer to 12 to 1 along the South Coast. That suggests an advisory-level snowfall for much of our area. There is a low chance for slightly higher QPF and corresponding snowfall, and this will need to be monitored. Wednesday night-Thursday... Upper trough moves overhead Thursday morning and then off to the northeast. Wednesday night and Thursday will be the period of coldest air. The air will be dry, therfore a fair and cold day. The air is dry enough for Wednesday night min temps to reach 10 to 20. Mixing to 925 mb supports Thursday max sfc temps mid 20s to around 30. Friday-Saturday-Sunday... A weak shortwave trough moves across New England on Friday. This may bring a period of clouds but not enough moisture to consider precip. Otherwise, southern high pressure builds and should provide a spell of fair weather. Temps aloft trend higher each day, with support for 30s Friday, 40s Saturday, and possibly near 50 Sunday.
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&& .AVIATION /20Z SUNDAY THROUGH FRIDAY/...
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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 Monday Night/... High confidence through early tonight as bands of snow showers off E MA coast bring MVFR and local IFR conditions to Cape Ann and Cape Cod through late afternoon, otherwise VFR into this evening with N/NW winds. Confidence decreases to Moderate later tonight as MVFR ceilings off Cape Cod push back inland overnight across RI and eastern MA, and farther back into interior SNE Monday. Snow showers are expected after midnight in eastern MA, mainly from Cape Ann to Plymouth County with local IFR conditions at times. Activity will persist through Monday evening, again focused along immediate coast and especially Plymouth County, with little or no snow farther inland, before improving to VFR areawide. KBOS Terminal...Moderate confidence in TAF. It`s possible conditions lower to IFR at times in snow showers Monday, but lower conditions are expected to remain just offshore and south of terminal area. Snowfall forecast is for an inch or two. KBDL Terminal...High confidence in TAF. Outlook /Tuesday through Friday/... Tuesday: MVFR/IFR conditions possible. Chance SN. Tuesday Night: Mainly IFR, with areas MVFR possible. SN likely. Wednesday: MVFR, with areas IFR possible. Conditions improving in the afternoon. Breezy. Chance SN. Wednesday Night: VFR. Breezy. Thursday through Friday: VFR. Breezy.
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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 Monday Night/... High confidence. Small Craft Advisory winds/seas expected on all but Boston Harbor and Narragansett Bay. High pressure to our north combined with low pressure passing well SE will maintain strong N/NE flow through Monday with building seas offshore, before winds diminish Monday night. Bands of ocean effect snow with reduced visibility on E MA waters will diminish tonight, but more activity is expected overnight through Monday evening. Outlook /Tuesday through Friday/... Tuesday: Winds less than 25 kt. Areas of rough seas. Slight chance of snow. Local visibility 1 to 3 nm. Tuesday Night through Wednesday: Winds less than 25 kt. Seas up to 5 ft. Snow likely, chance of rain. Visibility 1 to 3 nm. Wednesday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of snow. Visibility 1 to 3 nm. Thursday through Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Friday: Low risk for Small Craft Advisory winds with gusts up to 20 kt. Local rough seas.
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&& .HYDROLOGY... Minor flooding continues along Taunton, Pawcatuck, and Wood Rivers where Flood Warnings remain posted. Colder weather will persist through midweek which will limit additional runoff, although we should see some building of snow pack from bands of ocean effect snow in eastern MA Monday and for much of region Tuesday into Wednesday. For details on specific area rivers, including observed and forecast river stages, please visit the Advanced Hydrologic Prediction Service /AHPS/ graphs on our website. && .EQUIPMENT...
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The Taunton, MA WSR-88D (KBOX) is out of service due to a part that needs replacing. We hope to have the radar back in service sometime Monday.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Winter Weather Advisory from 4 AM to 10 PM EST Monday for MAZ016-018-019. RI...None. MARINE...Small Craft Advisory from 4 AM to 7 PM EST Monday for ANZ231>234. Small Craft Advisory until 6 AM EST Tuesday for ANZ235-237-250- 251-254>256. && $$ SYNOPSIS...WTB/JWD NEAR TERM...JWD SHORT TERM...JWD LONG TERM...WTB AVIATION...WTB/JWD MARINE...WTB/JWD HYDROLOGY... EQUIPMENT...

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