Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 152022 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 322 PM EST Fri Dec 15 2017 .SYNOPSIS...
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Low pressure will deepen to our southeast late tonight, with some light to moderate snowfall this evening out ahead. Expect dry but cold conditions Saturday night into Sunday. Flurries changing over potentially to freezing drizzle before going to plain rain Sunday night into Monday, potentially lingering into Tuesday. Cold and blustery Wednesday, warm rebound Thursday. Quick moving system Friday into Saturday. An eye on the holiday break for a possible storm system.
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&& .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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Gradually lowering CIGs suggest a slow moistening of the lower lvls. However dwpt depressions at the sfc still running 10-15F at the time of this running. Still, column in the process of moistening to its peak PWATs of 0.5 inches by late this evening. Phasing N and S stream shortwaves still rotating into PA, but the precip shield associated with sfc cyclogenesis beneath a the poleward exit region of of a 170kt jet continues to blossom across PA/NJ and is just reaching S NY state with light SN already reported at NYC to central LI. The big issue through the evening, particularly 22Z (5PM) through 05Z (midnight) will be how far N this precip shield is able to shift as the low pres and inverted low-mid lvl trof develops through the late evening hours. Although the overnight runs were less progressive and more amplified than previous runs, 15.12Z guidance has split somewhat with the GFS now less amplified than the previous run, the NAM more amplified and the ECMWF/HREF in the middle. Will lean most closely on the latter two. The low lvl dry air will define the initial timing and how far N the precip shield ultimately reaches, but feel that guidance is likely too far suppressed with T-0.01 QPF, as this is similar to the event that occurred yesterday where light SN inched all the way up to along and N of the Mass Pike. Will highlight categorical and likely POPs to this point, dropping to chance and below further N. Peak QPF is along and just S of the S coastal region, in an area where there is modest low-mid lvl banding signature and peak (12-15 microbar/sec) within the -12 C to -18C dendrite growth regime and modest negative EPV. This could be enhanced by a little bit of southerly ocean enhancement, thanks to H85 temps cool to -10C while SSTs hover just below +10C. SLRs will likely be on the higher end within the anomalously cold airmass, looking at about 20:1. Therefore with QPF values in the 0.10-0.30 mark, could see an area of 2-4 inches along the Cape/Islands, with lower amounts across the mainland with a few hours of decent snowfall rates. Band of 1-3 will be featured across S and central CT, most of RI and interior SE MA, with a localized pocket near 4 inch along the immediate S coast. Areas N of this band will likely struggle to reach an inch if even a T is observed. Hoisting a Winter Wx Advisory for immediate S coastal locations and the Cape/Islands given the risk for some 3+ amounts. Will continue SPS elsewhere. The key with this will be timing, as SN will start during the later portion of the commute, and given cold road sfc temps, should begin to stick quickly where untreated. Given this is a very progressive system that deepens offshore. SN will quickly end after midnight, giving way to clearing skies through the AM hours such that most areas see the sunrise. Min temps will likely hold in the mid 20s, near the sfc wetbulb temps.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
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Upper lvl shortwave shifts into the Maritimes allowing for modest mid and upper lvl ridging to take its place through the early half of the weekend. Soundings gradually dry and give way to subsidence inversion, which may initially allow for the development of some CU under W flow during Sat afternoon. These dissipate diurnally. Otherwise, H92 temps hover near -8C through the Sat and Sat night timeframe, which, will allow highs to reach the mid-upper 30s by day. Some breezy conditions possible during the afternoon as better mixing could tap a nearly 30 kt LLJ. Sat night, good setup for radiational cooling with light snowpack and cresting 1025 hPa high pres. Will see several spots drop back into the single digits and low teens.
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&& .LONG TERM /SUNDAY THROUGH FRIDAY/...
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*/ Highlights... - Light wintry mix Sunday night into Monday, possibly into Tuesday - Ebb and flow for Wednesday into Thursday - Quick moving system Friday into Saturday - An eye on the holiday break, possible storm system */ Overview... Transitional pattern emerging with the increasing potential of mixed precipitation events. Whether the NW Pacific H5 ridge prevailing, or broken down by a series of mild, Pacific disturbances, accompanying jet streaks, the pump of milder air from the N Pacific to the Arctic persists all the way up through the stratosphere. Broad-scale higher heights building across the CONUS with time, cross polar flow and dominant region of low pressure / storm development push subtly E, aimed into E/NE N America however remaining stretched back W. The N Atlantic high building SW with higher heights, starting to see a La Nina type pattern emerge with dry and warm conditions building over the SE CONUS as cooler conditions are rocked back W, wrapping on the backside, sheared S by the polar jet through the preferred H5 trof pattern across N-Central America, however not as deep, more flat. Amplified, then break down, amplified, then break down, NW Pacific H5 ridge acts like a pump, broken down by a series of mid to upper level disturbances only to rebound due to the buckling traffic jam of weather, energy becoming sheared S, cutoff, a constant over the SW CONUS. Colder air sheared S in concert, cooler and wet pattern emerges over N-Central CONUS, but up against the prior mentioned SW building N Atlantic high, a SW to NE transitional zone emerges over the Central and E CONUS, with a sharp temperature gradient in the low to mid levels. It then becomes a focus on the depth / magnitude of individual waves, as to which side of the envelope S New England will reside with respect to mixed precipitation events as we go into the holiday week. N flow dominance of colder air is relaxing in the long term but not absent. Allowance of S warmer flow, over-running, threats / impacts associated with wintry mixes are forecast. Closely monitoring the Sunday night through Tuesday timeframe with a slow moving warm front N parent with weak ascent. Warm nose around H95 within moist low levels, a drizzly to light wintry mix of snow, sleet, freezing rain is possible. Could see similar impacts along a forecast warm front Thursday night into Friday. Threats and impacts discussed in detail, along with forecast thinking, below. */ Discussion... Sunday through Tuesday... Potential light wintry mix along a slow moving warm front lifting N. Broadly speaking over the forecast period, H95 warm-nose protrusion above which there`s indications of weak isentropic ascent along the 280-300K theta surfaces, possibly some weak convergent forcing along the nose of a H85-7 jet, all contributing to periods of weak roughly W-E frontogenetical banding around H7. Greatest forcing more than likely along the S/W facing slopes of high terrain with orographic tendencies. N/NE ageostrophic flow N/NE at first turning E maintains interior sub-freezing wet bulb temperatures especially over the high terrain. All throughout, BUFKIT soundings showing plenty of moisture within low-levels to act upon. ECens exhibits high probability of 0.01" in 24 hours from 0z Monday to 0z Wednesday prior to a sweeping cold front. Similar signals in GEFS/SREF plumes. More than likely the S/W slopes of high terrain under greater risk seeing a wintry mix of precipitation outcomes. Favoring the NAM trends with 2m wet bulb temperatures, retreating N to NE, leaning towards high-chance PoPs with light, drizzly outcomes with the wintry mix of initially flurries changing to freezing light rain / drizzle prior to rain Sunday night into roughly late Monday, the high terrain and NE MA seeing the changeover to plain late the latest. Snow to freezing rain subsequent of drying indications with drying occurring within the dendritic growth zone. Even with a trace of ice, WINTER WEATHER ADVISORIES required. Will note the possibility in the HAZARDOUS WEATHER OUTLOOK. Monitoring the strength and depth of the rather flat feature approaching late. If it can become more pronounced and exhibit a stronger S flow early on out ahead, could see more significant impacts, especially if it speeds up. Otherwise cold front sweeping through Tuesday night with some showery weather, cold air blasts right back in for midweek. Wednesday into Thursday... Ebb and flow. Cold blast Wednesday into Thursday morning with breezy NW winds. Rebound during Thursday with S flow. Dry throughout. Friday onward... Quick sweeping system. Warm front lifting on Friday with potential onset of wintry mix precipitation types. A mild rebound overnight prior to the cold front sweeping through Saturday. A dry forecast but the cold front stalling along the aforementioned SW building Atlantic high in the OVERVIEW section. Energy out of the SW CONUS acting upon, the potential for an ice storm over the SE CONUS and potentially on up the E coast exists as the cold front is slow to push out while taking on anafrontal characteristics. There`s still a lot of uncertainty, but can`t ignore the general synoptic pattern.
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&& .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
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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 Saturday Night/... Through 21Z...High confidence. VFR. Although CIGS will gradually lower to low end VFR by 12Z. Winds light out of the S-SW. 21Z this evening into 6Z tonight...Moderate confidence. Area of light to moderate SN moves over S New England, starting from the SW and lifting up to just N of the Mass Pike by 00Z. This SN then gradually shifts E 00Z through 05Z. Best chance for accumulation 2-4 inches will be Cape/Islands, with 1-3 possible across central CT, most of RI and interior SE MA. N of this line only a coating to an inch at best is possible. Best chance for MVFR/IFR conditions will be across central CT most of RI and SE MA. Winds shift to the W after snowfall ends. After 06Z tonight into tomorrow...High confidence. Trending toward VFR and clearing by early morning. Some lower VFR clouds tomorrow afternoon. Winds W, with gusts between 20-25 kt at times. Tomorrow night...High confidence. Mainly VFR, W winds recede. KBOS Terminal...High confidence. Although light SN may fall between 22Z and 04Z this evening, mainly VFR conditions are expected with only minimal accumulation on runway, less than an inch. KBDL Terminal...Light SN this evening with temporary MVFR vsbys possible. Some light accumulations around an inch on the runways. Outlook /Sunday through Wednesday/... Sunday: VFR. Sunday Night: Mainly VFR, with local MVFR possible. Chance SN, slight chance FZDZ. Monday: Mainly VFR, with areas MVFR possible. Chance SN, slight chance DZ, slight chance FZDZ. Monday Night: Mainly VFR, with local MVFR possible. Slight chance FZDZ. Tuesday: Mainly VFR, with local MVFR possible. Breezy. Chance DZ, slight chance FZDZ. Tuesday Night: VFR. Windy with local gusts to 30 kt. Chance RA, slight chance SN. Wednesday: VFR. Windy with gusts to 30 kt.
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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 Saturday Night/...High confidence. Tonight into tomorrow...High confidence. Low pressure will develop and pass just southeast of the waters late tonight into tomorrow. Once winds shift to the W late tonight, gusts to around 30 kt with a few gusts to 35 kt will develop and continue into the day Sat. This will allow seas to peak around 7-9 ft on the SE ocean waters. Small craft advisories will be issued. Tomorrow night... Winds and seas diminish, this will allow any lingering small craft advisories to be dropped through the overnight hours. Outlook /Sunday through Wednesday/... Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday Night through Monday: Winds less than 25 kt. Chance of drizzle, chance of snow, slight chance of freezing drizzle. Monday Night: Winds less than 25 kt. Slight chance of drizzle, slight chance of freezing drizzle. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Chance of drizzle, slight chance of freezing drizzle. Tuesday Night: Low risk for gale force winds with gusts up to 35 kt. Local rough seas. Chance of rain. Wednesday: Strong winds with gusts up to 35 kt. Areas of rough seas. Slight chance of rain.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Winter Weather Advisory until midnight EST tonight for MAZ020>024. RI...Winter Weather Advisory until midnight EST tonight for RIZ006>008. MARINE...Small Craft Advisory from 4 AM to 10 PM EST Saturday for ANZ231>234. Small Craft Advisory from 9 AM to 10 PM EST Saturday for ANZ236. Small Craft Advisory from 4 AM Saturday to 1 AM EST Sunday for ANZ235-237-250-254>256. Small Craft Advisory from 7 AM to 7 PM EST Saturday for ANZ251.
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&& $$ SYNOPSIS...Doody/Sipprell NEAR TERM...Doody SHORT TERM...Doody LONG TERM...Sipprell AVIATION...Doody/Sipprell MARINE...Doody/Sipprell

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