Area Forecast Discussion
Issued by NWS Boston, MA

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673 FXUS61 KBOX 170848 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 348 AM EST Sat Feb 17 2018 .SYNOPSIS... Fast moving low pressure will bring a plowable snow to southern New England tonight, with the heaviest amounts near and south of the Massachusetts turnpike. The snow will be over by daybreak Sunday with rapidly improving conditions thereafter as temperatures rise well above freezing. A warm front will then approach the region Monday afternoon and night with a period of rain showers likely. Near record high temperatures likely follow Tuesday and especially Wednesday before a cold front brings cooler temperatures by Thursday and Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10PM update... Working toward the SKC conditions, only remnant CI across outer Islands at the time of this writing and some CAA/Lake enhanced SC across the E slopes of the Berkshires/Litchfield Hills. However, monitoring pres rise/fall couplet, which is shifting orientation, suggests that winds should shift more to the W over the next few hours, cutting off the Lake enhancement. Therefore, trended sky grids toward SKC. In spite of the clearing the pres rise/fall couplet remains a player at least through the early AM, which will keep lower lvl winds in play, and not allow for radiational cooling to really get going until closer to sunrise. However, in a scenario like this, it would really only take an hour or two for temps to rapidly drop, especially as sfc dwpts continue to drop overnight. Therefore, did adjust temps/mins a bit, suggesting a quick dip around sunrise. Mins are now mainly in the low-mid teens in the normal decoupling spots of NW MA, with ranges from the mid teens to low 20s elsewhere. Typically chilly Feb night. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... *** Periods of heavy snow tonight *** Highlights... * Snow overspreads the region between 8PM and 7AM tonight. * Periods of heavy snow with rates of 1-2in/hr possible especially along the S tier. * Rain mixes in along the S coast. Overview... WV imagery suggests that the arctic wave with Northwest Territories origins is moving through WY/NE, and is phasing with very weak S stream stretched vort-max. Even though this phasing occurs, and the DT drops as low as 550mb, it continues to be embedded within the progressive zonal jet, hence why between 12Z and 00Z today it shifts from NE to the E coast and remains open. The wave`s approach to the mid-Atlantic yields rapid cyclogenesis and deepening offshore, as the resulting low pres deepens at a rate roughly exceeding 1mb/hr. As it deepens, models have settled on a track that takes the low pres generally about 50nm S of the 40/70 Benchmark. Given the wave remains open and embedded within the strong jet, it is very progressive, hence why most impacts will fall during the overnight hours. Given the feature has some S stream influence, it will carry with it a modest PWAT plume around 0.75 inches (1 std deviation above mid Feb climatological values). Also, with S flow, BL temps will be key, as some marine influence could yield a change to rain, and lower SLRs to the N, mainly along the S coast, Cape/Islands. The key will be the two potential banding setups: 1) mid lvl f-gen/deformation within the core of the open wave, yielding strong omega in the DGZ furthest N between H7-H8 and 2) lower lvl coastal front, as marginal, marine-influenced temps will try to shift inland. The former is likely to setup further inland while the other could feature banding along the S coast, and final location will help to pinpoint peak banding. Timing... With good synoptic scale agreement now, can generally agree that precip onset occurs as mainly light snow between 01Z-03Z (8pm-10pm), peaking mainly 03Z-09Z (10pm-4am), then tapering off between 09Z-11Z (4am-6am). Timing of any change/shift to RA will occur during the peak period. Snowfall/Amounts... With modest PWAT plume and potential for banding. Total QPF amounts between 0.30 mainly across NW away from peak omega, to +0.6. SLRs will not be as high as previous storms given the influence of warmer marine air before precip onset and some areas where lift is actually below the DGZ. In peak banding, which lines up from central CT-central RI-SE MA, will use SLRs 12-15, with generally 12 or less on either side. This suggests a risk for a narrow band of 6-8in in the region mentioned above with 4-6in just outside, dropping toward NW MA. Watches will be shifted to warnings as the areas where 6+ are forecast generally fall within the current watch regime, with advisories to buffer. Within this area where banding/omega signal is strongest, will also likely see 1-2in/hr snowfall rates. Lingering uncertainty... With the possibility of at least two banding setups, the overall area of 6+ inches could ultimately be slightly wider or even slightly narrower than current warnings suggest. This is especially true along the S coast, where marine influence will lead to lower SLRs and snowfall totals as rainfall mixes in. Impacts... Given the wetter, heavier nature of the snow, cannot rule out some isolated power outages especially along the S coast. Snowfall rates of 1-2in/hr will likely lead to hazardous road conditions when untreated. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Highlights... * Snow ends by daybreak Sun: Rapidly improvement thereafter * Period of rain showers likely Mon afternoon/night with a warm front * Near record warmth likely Tue and especially Wed * Turning cooler Thu/Fri but temps still above normal Details... Sunday... Rapidly departing southern stream shortwave will have allowed the snow to have come to an end by daybreak. Improvement occurs quickly Sunday morning and with plenty of sunshine temps will quickly rise above freezing. In fact..highs should reach well into the 40s by Sun afternoon. Therefore...a good portion of the snow that falls Saturday night will melt by the end of Sunday. Sunday night... A ridge of high pressure builds in from the west resulting in mainly clear skies/light winds. This will allow a good night of radiational cooling with low temps bottoming out in the upper teens and 20s across many locations. This might result in a bit of patchy black ice developing from the snow melt that occurred during the afternoon. Monday and Monday night... Dry weather anticipated Monday morning...but clouds will be on the increase ahead of a warm front. This warm front will likely bring a period of showers sometime Monday afternoon and/or night. High temps Monday should be in the 40s to around 50 as there should be an inversion in place. Temps Monday night will either remain steady or slowly rise given low clouds and increasing southwest flow. Tuesday and Wednesday... Impressive setup for near record warmth Tue/Wed as anomalous upper level ridge of 594 decameters sets up off the southeast coast. The southwest flow/high height fields result in 850T between +12C and +14C in SNE. Good mixing on southwest flow should result in high temperatures well into the 60s both days...away from the cooling marine influence of the south coast/Cape/Islands. In fact...70+ degree readings are possible in some locations on Wednesday based on support from most model guidance as long as enough sunshine is realized. The one fly in the ointment would be if the cold front moves through earlier than expected...which would result in cooler temps than our going forecast on Wed. If current guidance holds steady...many record highs will be broken on Wednesday. As for precipitation chances Tue/Wed...upper level ridging and lack of synoptic scale forcing will result in mainly dry weather. We can not rule out a few spot showers...but again appears much of this time will feature dry weather. Thursday and Friday... While the southeast upper level ridge will maintain itself...strong 1040 mb high pressure system will allow significantly cooler air to work back into the region. While temps will still average above normal...it will be noticeably cooler. && .AVIATION /09Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tonight/... Through 00Z this evening...High confidence. VFR. NW winds diminish this morning, then gradually shift to the E then SE through the day today, but remain around 10 kt. Overnight...Moderate confidence. A quick moving winter storm passes SE of New England with -SN developing between 00Z and 02Z across all of New England, then periods of +SN with 1-2in/hr snowfall rates continue from about 03Z-09Z especially across portions of CT/RI and SE MA. Accums on runway expected. The SN then tapers off between 09Z and 12Z from W-E. Peak uncertainty across Cape/Islands, where some rain may mix in. Winds out of the S with a few gusts to about 20 kt across coastal areas. KBOS Terminal...High confidence in TAF overall, timing of lower categories this evening may be off a bit with beginning of light SN this evening. Accumulations on runways can be expected. KBDL Terminal...High confidence in TAF overall, timing of lower categories this evening may be off a bit with beginning of light SN this evening. Accumulations on runways can be expected. Outlook /Sunday through Wednesday/...Moderate to High Confidence. Sunday: VFR. NW wind gusts up to 25 knots. Sunday Night: VFR. Washingtons Birthday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA. Monday Night: Mainly MVFR, with local IFR possible. Breezy. SHRA likely. Tuesday: Mainly VFR, with areas MVFR possible. Breezy. Tuesday Night: Mainly VFR, with local MVFR possible. Breezy. Wednesday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tonight/... Through this evening...High confidence. Lingering high seas dissipate through the morning, remaining above 5 ft longest on the ocean waters. Small Craft Advisories will be expiring through the morning, with quiet boating weather and winds shifting to the S-SE during the afternoon and evening. Overnight...Moderate confidence. A quick moving coastal storm will pass south of the waters overnight. This will lead to rain/fog and lowered vsbys. Also, as winds shift, gusts to around 25 kt are possible, but it will be close. Small Craft Advisories may be needed. Outlook /Sunday through Wednesday/...Moderate to High Sunday: High risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft. Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Washingtons Birthday: Risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Monday Night: Risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Rain showers likely. Tuesday: Risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain showers. Tuesday Night: Risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Wednesday: Risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Storm Warning from 7 PM this evening to 7 AM EST Sunday for CTZ002>004. MA...Winter Storm Warning from 7 PM this evening to 7 AM EST Sunday for MAZ011>013-016>021. Winter Weather Advisory from 7 PM this evening to 7 AM EST Sunday for MAZ002>010-014-015-023-026. Winter Storm Watch from this evening through Sunday morning for MAZ022. RI...Winter Storm Warning from 7 PM this evening to 7 AM EST Sunday for RIZ001>007. MARINE...Small Craft Advisory until 7 AM EST this morning for ANZ231- 235-237-251. Small Craft Advisory until 1 PM EST this afternoon for ANZ250- 254>256. && $$ SYNOPSIS...Frank/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...Frank AVIATION...Frank/Doody MARINE...Frank/Doody

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