Area Forecast Discussion
Issued by NWS Boston, MA

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445 FXUS61 KBOX 141938 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 238 PM EST Sun Jan 14 2018 .SYNOPSIS... 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 may bring snow to the region from Tuesday into Wednesday night followed by moderating temperatures into the weekend. && .NEAR TERM /THROUGH TONIGHT/... 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 /MONDAY 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 SATURDAY/... Highlights... - Light snow across SE MA gradually ends Mon night. - Moderate snow event possible Tue into Wed. - Moderating temperatures late week into the weekend. Overview and model preferences... Strong rex block and resulting height anomalies look to phase with the higher height anomalies associated with a cutoff high pres settling across the central Atlantic. The result is a block an overall blocking pattern, that forces an arctic PV lobe S into the CONUS, which gradually transitions from a cutoff to an open wave as the strongly positive PNA/AO/NAO breaks down. As this PV anomaly shifts E, it looks to lead to a snow event mid week. The uncertainty plaguing the forecast of this event stems from the fact that this PV anomaly remains trapped in the Baffin Island vortex. This should become better sampled later tomorrow or tomorrow night. Interesting, the operational GFS has trended toward the operational ECMWF somewhat of late, while the ECMWF is less amplified. In any case, will continue to add the bulk of the weight to ensemble means as this continues to account for both the most robust and modest snowfall solutions, and accounts for uncertainty in thermal profiles near the coast. After this, with a breakdown of the wave, the remnants of the Rex Block to the west will yield rising heights into the weekend, suggesting another moderation to follow. Details... Mon night... The last vestiges of high pres yield a mainly dry night Mon night. Initially soundings are dry enough for modest radiational cooling, but mid lvl warm advection and increasing clouds will mitigate cooling during the early morning. This will only allow temps to drop into the upper teens to mid and upper 20s, mile compared to previous nights. Tue into Wed... A slight convergence in deterministic guidance will also be noted with this update, but will continue with means. The initial, moisture starved clipper with MSLP near or above 1025hPa looks to lose even more as the attendant wave, the PV anomaly mentioned above opens and gradually shifts toward near neutral status. This looks to yield offshore cyclogenesis along the weak stalled frontal boundary. This secondary low pres track will determine final snowfall totals, as PWATs increase to near 0.4 inches, which, although above normal, is generally less than 1 std deviation above normal. Lift is weak, and not always focused within the DGZ, even as the precip shield in advance of this secondary low overspread S New England. Operational total QPF values rang from about 0.25 to 0.5, and ensemble means support this. So may need to watch for an advisory lvl snowfall late Tue into early Wed assuming the secondary low falls near or inside the 40/70 Benchmark (ensemble clustering is close). However, still enough uncertainty that thermal profiles could warm enough along portions of the SE coastal plain to limit snowfall, or that the track could be far enough shore to also limit snowfall. These will continue to be better resolved. Temps run closer to normal, but given the uncertainty in weak low pres passage, there is enough uncertainty that temps may change a bit especially across the SE, where warmer low lvl thermal profiles are most pronounced. Thu... The wave continues to open and shift E, orienting the trof-base jet streak poleward exit region offshore of the mid-Atlantic such that yet another, stronger coastal wave looks to develop. Given the spread in the initial wave, there is even more so with this wave. ECMWF is still the most amplified, with enough W draw to influence S New England yet again on Thu, but several ensemble members from both the GEFs and ECENS are offshore. Will need to watch the how the PV lobe is handled as it is better sampled, as this could prolong the period of wintry precip. This also marks the peak of the cold dome as the opening wave begins to lift to the ENE. Fri and the weekend... The pattern breakdown allows the remnants of ridging to yield rising heights and more influence from the S stream through the weekend. A shift toward temps above seasonal normals are expected. && .AVIATION /19Z SUNDAY THROUGH FRIDAY/... 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 Thursday/... Monday Night: Mainly VFR, with areas MVFR possible. Breezy. Chance SHSN. Tuesday: MVFR/IFR conditions possible. Chance SN. Tuesday Night: Mainly IFR, with areas MVFR possible. SN likely. Wednesday: Mainly VFR, with areas IFR possible. Chance SN. Wednesday Night: Mainly VFR, with local IFR possible. Slight chance SN. Thursday: VFR. Breezy. && .MARINE... 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 Thursday/... Monday Night: Winds less than 25 kt. Areas of rough seas. Chance of snow showers. Tuesday: Winds less than 25 kt. Areas of rough seas. Slight chance of snow. Areas of visibility 1 to 3 nm. Tuesday Night: Winds less than 25 kt. Seas up to 5 ft. Snow likely, rain likely. Visibility 1 to 3 nm. Wednesday: Winds less than 25 kt. Seas up to 5 ft. Chance of snow. Visibility 1 to 3 nm. Wednesday Night: Winds less than 25 kt. Areas of rough seas. Chance of snow. Visibility 1 to 3 nm. Thursday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Local visibility 1 to 3 nm. && .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... The Taunton, MA WSR-88D (KBOX) is out of service due to a necessary replacement part. We hope to have the radar back in service sometime Monday. && .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 is the U.S. government's official web portal to all federal, state and local government web resources and services.