Area Forecast Discussion
Issued by NWS Boston, MA

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396 FXUS61 KBOX 160240 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 940 PM EST Fri Dec 15 2017 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 930 PM update... Mainly light snow falling across eastern CT, RI and SE MA. Back edge of snow already moving through moving western CT, but radar showing some enhanced returns and heavier snow just to the east across coastal CT and eastern LI. This assocd with area of low and mid level frontogenesis and steeper mid level lapse rates pushing 6.5 C/km. As this area moves east, expect a brief period of heavier snow to impact the south coast from west to east between 10 pm and midnight, and possibly lingering a bit longer outer Cape and ACK. No changes to current advisories although amounts will likely fall short. Will let them expire at midnight. Still looking at 1-3 inches along the south coast with amounts dropping off to the north. System exits shortly after midnight, followed by gradual clearing moving in from the west. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... 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. && .LONG TERM /SUNDAY THROUGH FRIDAY/... */ 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. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... 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... Widespread -SN/SN, heaviest S along the coast with IFR/LIFR impacts with 1/2 to 2SM. VFR/MVFR elsewhere. Through roughly 5-6z. Accumulations of around 1-3 inches along the S-coast and across the Cape and Islands. Coating to 1 inch elsewhere N of HVN-PVD-PYM. Improving quickly with winds shifting out of the W and becoming gusty up around 20 to 25 kts towards morning. Saturday into Saturday night... VFR. W winds. Blustery with gusts upwards of 25 to 30 kts along the coast, diminishing into the overnight hours. KBOS Terminal...VFR with a few flurries next 1-2 hrs. KBDL Terminal...Improving to VFR tonight. 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. && .MARINE... 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 Saturday...High confidence. Low pressure sweeping S/E of the waters into Saturday, deepening out to sea. Winds shifting W behind the system, strengthening to near gale force Saturday into evening. Seas peaking 7 to 9 feet on the SE ocean waters. SMALL CRAFT ADVISORIES issued. Saturday 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. && .BOX WATCHES/WARNINGS/ADVISORIES... 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. && $$ SYNOPSIS...Doody/Sipprell NEAR TERM...KJC SHORT TERM...Doody LONG TERM...Sipprell AVIATION...KJC/Doody/Sipprell MARINE...Doody/Sipprell

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