Area Forecast Discussion
Issued by NWS Boston, MA

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671 FXUS61 KBOX 211811 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 111 PM EST Wed Feb 21 2018 .SYNOPSIS... Record warmth this afternoon, then a cold front this evening will be followed by dramatically colder weather Thursday. A wave of low pressure tracking to the south will bring some accumulating snow and sleet north of the Pike with rain and sleet to the south. An active wet weather pattern Friday through the weekend during which there is the possibility of mixed precipitation, freezing rain late Friday into Friday night and again around Saturday night into Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Dense fog and record high temps are the headlines this afternoon. Dense fog continues along the South Coast of RI, along Cape Cod, and across the Islands. The overall pattern of moist air moving over cold water and then ashore should continue through mid afternoon. We will extend the Dense Fog Advisory in these areas through at least 3 PM for coastal RI and Cape Cod and the Islands. Morning soundings showed warm air in place at low levels just off the surface. Yesterday the mixing, per Amdar soundings, reached about 935 mb. All soundings show the inversion top at this level, so projecting the same mixing this afternoon brings 850-mb equivilent temps of 10-11C at ALB and GYX, 8C at OKX, and 5.5C at CHH. This supports mostly l-m70s inland, and should be achievable where there is sun and a land wind. As such, we have bumped max temp forecast up a couple of degrees with a range in the upper 60s to mid 70s inland, and 50s along the cloudy south coast. As per the ALB and GYX soundings, if full mixing were to be achieved this would suggest high temperatures 77-79F, with the best chance in northeast Mass and the CT Valley. Soundings show 25-30 knots in the projected mixed layer, possibly a few spots approching 35 knots. Cold front will be moving into western New Eng late in the day and may be accompanied by a few showers in western MA. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Tonight... Cold front moves south of the coast this evening and may be accompanied by a few showers south of the Pike. Areas of stratus and fog may linger near the south coast in the evening, otherwise improving conditions with much colder air will drain south across the region as strong high pres builds to the north. Low temps by morning will range from near 30 in NW MA to around 40 near the south coast. Thursday... Dramatically colder weather with afternoon temps 30-40 degrees colder than today. Strong high pres (~1040 mb) to the north will provide the cold air with falling temps through the 30s during the afternoon with temps falling to freezing or below over higher elevations by late day. Fast moving shortwave will result in a flat wave tracking along the old frontal boundary south of New Eng. Underrunning situation setting up with low level moist and colder air undercutting somewhat milder air aloft. Result will be an area of deep moisture and mid level lift along mid level frontal boundary with area of precip overspreading SNE, especially in the afternoon. Ptype will be a challenge as low level cold air pushes south across the region while the cooling process aloft will be a bit slower. This will set up potential for snow, sleet and freezing rain across the interior. Model differences in the thermal profile will result in lower confidence pytpe forecast. At this time, we blended GFS/ECMWF thermals which would suggest more in the way of snow and sleet north of the Pike with rain mixing with sleet to the south. Some freezing rain is also possible near or south of the Pike across higher terrain in northern CT and central and SW MA. Best chance for minor snow accumulations will be across higher elevations of northern MA where 1-3 inches possible. Less than an inch on colder surfaces for remainder of interior. Forecast remains uncertain and a warmer solution could result in more ice and less snow for northern MA. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... */ Highlights... - Active weather pattern Friday through the weekend - Watching for potential mixed precipitation, freezing rain - N/W interior out towards the Berkshires at greatest risk - Quieter, seasonable pattern into the following week */ Overview... Persistent SE CONUS sub-tropical H5 ridge maintained by parked N- Central Pacific split flow and downstream H5 trof over the W CONUS generating a warm-moist atmospheric river from the Central Pacific NE to SE Canada. But nothing lasts forever. Breakdown of upstream pattern, thermal wind flattens through which impulses stretch, dip southward. A brief midweek warm-up is followed by an active weather pattern late week into the following week. Along a parent frontal / baroclinic zone, sulked S, stretching across the NE CONUS, thermal profiles in question as intriguingly ensembles signal an evolving blocky N Atlantic pattern per forecast strong -NAO into March. Will hit on targets of opportunity below. Ensemble guidance preferred. */ Discussion... Thursday night... Hold with an influential near 1040 high and hold off on any weather. With that, light winds, partial clearing, favor some radiational cooling and will lean towards cooler 2m temperature guidance. Friday into Friday night... Freezing rain forecast late Friday into early morning Saturday over the N/W interior. Warm frontal isentropic ascent aided by convergent forcing per SW low-level jet beneath stretching H5-7 vortmax. Focus on thermal profiles below the warm nose up around H8, 2m wet bulb temperatures. Over-running setup, frontogenetical forcing within low- mid levels upon precipitable waters 0.75 to 1.00 inch, especially along the W-slopes of high terrain. GFS warmest, NAM coldest, while EC/Canadian split between the two, though closer to the NAM. Worst case scenario of around one-quarter of an inch of ice accretion over N/W MA, upwards of a tenth for surrounding areas away from the coast. CIPS analogs continue to signal the potential for a 6-hour freezing rain event over N/W areas of S New England. Some hesitation given the surface high is more E, ageostrophic flow off the waters rather than N. However not out of the question if you can get enough precip outcomes prior to colder air eroding. Going with one to two tenths of ice accretion mainly out around the Berkshires. Possible sleet / snow mix before the changeover. Otherwise rain, warming temperatures, non-diurnal trend during the overnight period. Saturday into Sunday... Classic warm occlusion over the Central CONUS with the leading warm front with secondary low development over the NE kinked by cold air damming. Once again, focus upon the low-level thermal profile below H9 and the warm nose, 2m wet-bulb temperatures. Cold front sweeping through Saturday, near 1040 surface high building NE Saturday night, perfect setup for N-funneling ageostrophic flow into New England, a better cold air damming setup. How well does cold air return and entrench ahead of precipitation expected Saturday night into Sunday remains a big question. Timing of outcomes as well. To early to say on specific details, a low confidence forecast, but attention is again focused out across N/W MA and Berkshires. Otherwise rain prior to the cold front and dry slot, clearing out as late as Monday morn. Early next week... Quiet, more seasonable outcomes as the pattern bottles up across the N Atlantic. The favorable region of storm development over SE Canada puts us in a cooler, drier regime with W/NW flow prevailing. Can`t rule out some weak disturbances across the region with time. A low confidence forecast. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
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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 Thursday/...Moderate confidence. This afternoon... Fog is slowly shifting south. Expect low clouds and vsbys to continue through the afternoon over Cape Cod and Islands, including Block Island. Southern RI around WST and UUU is questionable, but with shifting winds later afternoon those areas should see improvement toward 4-5 PM. Gusty southwest winds near 25 knots especially at ORH and along the coastal plain. Tonight... IFR fog lingers over Cape Cod and Islands in the evening, but should lift early in the night as winds shift west and northwest. VFR conditions remainder of Southern New England. Winds shifting northwest and then north overnight. Thursday... VFR in the morning. Winds turn from the northeast. Clouds lower and thicken as an area of precipitation moves in from the west. Afternoon cigs/vsbys lower to MVFR in snow/sleet north and light rain south. Light accumulation possible north of the Mass Pike. Thursday night... Rain/sleet/snow moves offshore along with the low clouds. Most of the night should be VFR with continued northeast winds. KBOS Terminal...High confidence TAF. VFR through Thursday morning. Cigs/vsbys lower to MVFR in the afternoon in rain, possible sleet toward evening. KBDL Terminal...High confidence in TAF. VFR through Thursday morning. Cigs/vsbys lower to MVFR in the afternoon in rain, possible mixing with sleet. Outlook /Friday through Sunday/...Moderate confidence. Friday: Mainly VFR, with local IFR possible. RA likely, chance FZRA. Friday Night: Mainly VFR, with areas IFR possible. RA likely, FZRA likely. Saturday: VFR. Chance RA. Saturday Night: Mainly VFR, with local IFR possible. Breezy. RA likely, FZRA likely. Sunday: Mainly VFR, with areas IFR possible. Breezy. RA, FZRA.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. This afternoon...Southwest flow continues today with gusts to 20-25 kt at times. SCA remains for most of the waters for combination for wind and seas. Dense fog will result in vsbys less than 1/2 mile, especially south coastal waters. A SCA has been added for Narragansett Bay through evening. Tonight...Winds shift to the north later tonight with gusts to 20 kt. Improving vsbys overnight. Thursday...Northerly winds with gusts to 20 kt at times. Vsbys lowering in rain in the afternoon, mixing with snow NE MA waters and sleet south coastal waters. Outlook /Thursday Night through Sunday/... Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain likely. Areas of visibility 1 to 3 nm. Thursday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain. Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Rain likely, chance of freezing rain. Friday Night: Winds less than 25 kt. Rain likely. Saturday: Winds less than 25 kt. Slight chance of rain. Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain likely, chance of freezing rain. Sunday: Winds less than 25 kt. Areas of rough seas. Rain, freezing rain likely. && .CLIMATE... Record Highest Observed Temperature for February... (since records began) Boston.........73 (2/24/2017) Hartford.......73 (2/24/1985) Providence.....72 (2/24/1985) Worcester......69 (2/24/2017) Record High Temperatures... February.......Today 2/21 Boston.........63 (1906) Hartford.......63 (1930) Providence.....63 (1930) Worcester......59 (1930) Record Warmest Min Temperature... February.......Today 2/21 Boston.........45 (1994) Hartford.......49 (1981) Providence.....50 (1981) Worcester......43 (2002) Extreme High Dew Points... Dew Point forecast has values in the 50s Today and Wednesday. February.......Today 2/21 Boston.........56 (1953) Hartford.......55 (1989) Providence.....56 (1989) Worcester......54 (1953) && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Dense Fog Advisory until 5 PM EST this afternoon for MAZ022>024. RI...Dense Fog Advisory until 5 PM EST this afternoon for RIZ006>008. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ231>234. Small Craft Advisory until 6 PM EST this evening for ANZ230- 236. Small Craft Advisory until 9 PM EST this evening for ANZ235- 237. Small Craft Advisory until 1 AM EST Thursday for ANZ250- 254>256. && $$ SYNOPSIS...WTB/KJC/Sipprell NEAR TERM...WTB/KJC/Sipprell SHORT TERM...KJC LONG TERM...Sipprell AVIATION...WTB/KJC/Sipprell MARINE...WTB/KJC/Sipprell CLIMATE...

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