Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 241150 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 650 AM EST Sat Feb 24 2018 .SYNOPSIS... Unseasonably mild today with a period of showers possible south of the Mass Pike late today and early this evening. A frontal system will bring more precipitation late tonight and Sunday with rain mixed with sleet at times in the coastal plain and snow sleet and freezing changing to rain in the interior. Near- seasonable, quiet and dry for the first half of next week, attention turns towards late week and an anticipated stormy pattern. && .NEAR TERM /THROUGH TONIGHT/... 7 am update... Low clouds continue to be a hassle this morning with light winds. While eroding over SE New England, becoming more problematic to the NW as moisture pools beneath a stout dry subsidence version around H9. Increasing confidence these clouds will erode with the narrow SW-NE band of clearing set up across the S/E half of S New England, only to fill back in later with increasing isentropic ascent ahead of an approaching warm front, showers developing over CT / RI / Southeast MA. PoPs increased for later today using a suite of high-res near-term forecast guidance including the HRRR-TLE. Highs topping out in the 50s, more likely towards midday into early afternoon before stabilizing under increasing clouds, dropping into evening with cooling and cold air advection proceeding ahead of wintry weather overnight. && .SHORT TERM /SUNDAY/... *** Mixed precipitation event for interior late tonight and Sunday with some snow and ice accumulations *** A few showers likely impacting south coast early evening, otherwise dry weather through midnight before main area of overrunning precip ahead of the warm front begins to overspread the region from the SW to NE late tonight into Sunday. Ptype will be biggest challenge with this event. Initially, the column is warm but all the guidance is indicating a good cold air damming signature developing overnight into Sunday as high pres builds over the Maritimes. In fact, notable cooling in the 925-950 mb layer 06-15z with temps dropping to -2 to -4C before warmer air moves in from the south. Warming temps above this layer will set up a sleet and freezing rain scenario in the interior where low level cold air will be difficult to dislodge given warm front remaining to the south and a sfc wave tracking along the boundary to lock in the cold air near the surface. Some snow is also likely late tonight and early Sunday, north of the Pike and especially near NH border where cold air just deep enough as precip moves in. In the coastal plain, mainly rain is expected, but given cold air below 850 mb, some sleet will likely mix in with the rain late tonight and Sunday morning even along the south coast. Expect 1-3 inches snow/sleet accum in the interior north of the Mass Pike with highest amounts near NH border. If warmer air aloft is delayed a few hours it is possible a few locations could approach 4-5 inches but this is a low probability. We are concerned with several hours of freezing rain and icy conditions across portions of the interior during Sunday, especially over the Berkshires, Worcester hills and portions of the CT valley. Ice accretion up to one tenth of an inch is possible with up to 0.25" over the Berkshires. Expect temps to gradually rise above freezing Sun afternoon in the interior as precip intensity diminishes. Winter weather advisories will be issued for all of central and western MA and northern CT. Some freezing rain is possible in NW RI and interior NE MA but confidence not yet high enough for an advisory in this area. Across NE MA may be dealing with more of a snow/sleet to rain scenario. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... */ Highlights... - Quiet, dry, near-seasonable early week period - Stormy pattern emerging late week for the beginning of March */ Overview... Blocky N Atlantic pattern, associated +3 standard deviation H5 high retrograding W into SE Canada, parent 1040+ surface high, pressing the thermal wind axis S through which N/S stream impulses merge, dug through a prevailing W CONUS H5 trof before ejecting and stretching E. Energy piling up, cutting off for a time given the block, an area of favorable storm development emerges off the SE Canadian coast, evolving S/W towards the NE CONUS per retrograding high, E ejecting energy. Airmass juxtaposition key as to outcomes towards the end of the week after a quiet start. Only confidence is persistence of NE winds of significant duration during high astro-tides, concern with respect to coastal flooding. Lower confidence otherwise, consensus forecast guidance heavily weighted with ensemble means. Hit targets of opportunity below. */ Discussion... Thursday into the weekend... While considerable ensemble member / deterministic guidance spread there`s cohesive signal of a Great Lakes warm occlusion transferring energy offshore beneath a more favorable region of storm development evolving towards the NE CONUS. Occluded front transitioning to an inverted trof, energy pinwheeling round a closed, stacking low well offshore. Questions surround frontal boundary magnitude buffeting the pressing N high, how far N/E outcomes slide into New England. NCEP noting poor run-to-run continuity with ejecting waves, EC, even the 24.0z, worse than GFS which appears to be locking in. Warm core low, absent Arctic air, anticipate precip-type issues, rain / snow, lean to occluded front / inverted trof hanging up into the NE CONUS as energy transfers, low rolls up offshore. Only confidence is the persistence of NE winds during high astro-tides, concern of coastal flooding. Only chance PoPs given spread. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...Moderate confidence. Today...MVFR/IFR eroding over S/SE coastal terminals, while BKN MVFR decks filter into W New England. With increasing sunshine should see decks erode, optimistically. However, towards later in the day, increasing low-mid level decks and -RA, especially for CT, RI, southeast MA. Light winds overall turning NW late. Tonight...VFR in the evening, lowering to MVFR from west to east late. Rain, sleet and freezing rain developing from SW to NE well after midnight with snow/sleet across northern MA. Mainly rain mixed with sleet coastal plain. Sunday...Widespread IFR/LIFR developing. Rain, mixed with sleet at times in the morning in the coastal plain. Snow, sleet and freezing rain changing to rain interior. KBOS Terminal...High confidence in trends. Conditions improving to VFR early this morning, but exact timing may be off a few hours. KBDL Terminal...High confidence in trends. Conditions improving to VFR early this morning, but exact timing may be off a few hours. Outlook /Sunday Night through Wednesday/...Moderate confidence. Sunday: Mainly IFR, with local MVFR possible. Windy with gusts to 30 kt. RA, PL, FZRA. Sunday Night: Mainly VFR, with local IFR possible. Slight chance RA. Monday through Wednesday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...Moderate confidence. Today...Diminishing winds becoming northerly late today. Seas below SCA. Tonight and Sunday...Increasing easterly winds late tonight and especially Sunday as low level jet moves across the waters. Given cold air above the boundary layer, mixing will likely be sufficient for a period of gale force gusts, especially over the south coastal waters during Sun. Otherwise, expect SCA conditions with gusts to 25-30 kt. Gale watches issued where highest confidence of gales. Outlook /Sunday Night through Wednesday/... Sunday: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 10 ft. Rain. Visibility 1 nm or less. Sunday Night: Winds less than 25 kt. Areas of rough seas. Slight chance of rain. Monday: Winds less than 25 kt. Seas up to 5 ft. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday Night: Winds less than 25 kt. Wednesday: Winds less than 25 kt. Seas up to 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Weather Advisory from 1 AM to 1 PM EST Sunday for CTZ002>004. MA...Winter Weather Advisory from 1 AM to 1 PM EST Sunday for MAZ002>004-008>012-026. RI...None. MARINE...Small Craft Advisory from 3 AM to 4 PM EST Sunday for ANZ232>234. Small Craft Advisory from 6 AM to 5 PM EST Sunday for ANZ230. Small Craft Advisory from 3 AM to 7 PM EST Sunday for ANZ231. Small Craft Advisory from 6 AM to 1 PM EST Sunday for ANZ236. Gale Watch from Sunday morning through Sunday afternoon for ANZ235-237. Small Craft Advisory from 6 AM to 7 PM EST Sunday for ANZ250- 251. Gale Watch from Sunday morning through Sunday afternoon for ANZ254>256. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Sipprell SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.