Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 141451 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 951 AM EST Sun Jan 14 2018 .SYNOPSIS... Canadian high pressure will bring very cold weather into Monday with some ocean effect snow across parts of eastern MA and RI, mainly very late tonight into Monday. Low pressure may bring snow to the region from Tuesday into Wednesday night followed by moderating temperatures into the weekend. && .NEAR TERM /THROUGH TONIGHT/... 950 AM Update... Bands of light snow showers are pushing toward E MA coast per BOS TDWR and we have seen reports of flurries from Provincetown and Cape Ann so far this morning. This activity should graze coast into at least early afternoon as forecast soundings show persistent northerly low level flow and subsidence inversion remaining near 2-3KFT, before gradually lowering again. Updated PoPs to reflect this thinking. Otherwise forecast looks good with plenty of sunshine farther inland and highs mainly in 20s. && .SHORT TERM /MONDAY/... ***A few inches of ocean enhanced snow possible very late tonight into Monday from the south shore into Plymouth County and Cape Ann*** Tonight and Monday... 1045+ MB high will continue to shift east into Quebec tonight into Monday. The result will be a very cold night across much of the region with lots of single digits and even a few below zero readings under mainly clear skies. Low temps will be a bit milder along portions of the eastern MA coast, Cape and Islands. Surface winds will turn NNE overnight allowing ocean effect clouds and some snow showers to develop especially after midnight. The main focus on Monday will turn to the mesoscale and potential for a few inches of localized snow accums. Given the strong high shifting into eastern Canada an easterly 925 mb LLJ of 20 to 25 knots will develop. Meanwhile...a mesoscale coastal front will setup probably near the Plymouth county coast. Surface temps might be in the upper teens and lower 20s to the northwest of the coastal front...while portions of the Cape/ACK will likely see temps rise above freezing. This seems to be a very good setup for ocean effect snow showers with 925T between -8c and -12C along with added enhancement along the coastal front and help from the land/sea interface. While a coating to 1 inch of snow will be possible across eastern MA and even into portions of RI...the main focus will likely run from Boston/s south shore and into Plymouth county along the coastal front. Cape Ann may also be impacted by this mesoscale snow as well. Snow accumulations of 1 to 3 inches are certainly possible in this region with the low risk of very isolated 4-5 inch amounts. This could potentially impact both the Monday morning and afternoon commutes...especially on the highly traveled Route 3 connecting Boston to the south shore. While we are not confident enough in amounts to issue a winter weather advisory at this time...will go ahead and issue a special weather statement to highlight this potential. Meanwhile...not too far northwest of the BOS-PVD corridor probably will not see much more than flurries. High temps will mainly be in the lower to middle 20s...except near or above freezing near the Cape Cod Canal and Islands to the southeast of the coastal front. && .LONG TERM /MONDAY NIGHT 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 /15Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/... This Afternoon...High confidence. Bands of light snow showers or flurries on Cape Ann and Cape Cod will bring MVFR ceilings. Tonight and Monday...Moderate to high confidence. MVFR Cigs across the far southeast New England coast will push northwest overnight across eastern MA/RI and probably further back into the interior Monday. Scattered snow showers will likely develop after midnight across eastern MA...especially from Cape Ann and into Plymouth county where localized IFR to even LIFR conditions will be possible at times into Monday. Some of these scattered snow showers may reach into portions of RI. KBOS Terminal...High confidence in TAF into this evening...but then uncertainty after midnight and Monday on how ocean enhanced snow showers impact the terminal. Probably will see at least some snow showers, but duration and intensity is uncertain. KBDL Terminal...High confidence in TAF. Outlook /Monday Night through Thursday/... Martin Luther King Jr Day: Mainly VFR, with areas MVFR possible. Breezy. Chance SHSN. Monday Night: Mainly VFR, with local IFR possible. Breezy. 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/... Today through Monday...High confidence. North wind gusts of 20 knots today shift to the northeast tonight with gusts increasing to 25 knots on Monday. Small craft headlines pretty much posted for all open waters though Monday for seas and wind gust. Ocean effect snow showers will lower VSBYS at times mainly late tonight and Monday. Outlook /Monday Night through Thursday/... Martin Luther King Jr Day: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 8 ft. Slight chance of snow showers. Monday Night: Winds less than 25 kt. Areas of rough seas. Slight chance of snow showers. Local visibility 1 to 3 nm. 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... Heavy rainfall and complete melting of our snowpack resulted in significant rises on rivers and streams. Flood warnings remain in effect for several rivers (please refer to our Flood statements and warnings). For details on specific area rivers, including observed and forecast river stages, please visit the Advanced Hydrologic Prediction Service /AHPS/ graphs on our website. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 4 AM to 7 PM EST Monday for ANZ231>234. Small Craft Advisory until 7 PM EST Monday for ANZ235-237-250- 251-254>256. && $$ SYNOPSIS...Frank/Doody NEAR TERM...JWD SHORT TERM...Frank LONG TERM...Doody AVIATION...Frank/Doody/JWD MARINE...Frank/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.