Area Forecast Discussion
Issued by NWS Boston, MA

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186 FXUS61 KBOX 251742 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1242 PM EST Sun Feb 25 2018 .SYNOPSIS... A fast moving low pressure system will bring rain today to the coastal plain and a mixture of sleet, snow and freezing rain changing to rain across interior Massachusetts. Near-seasonable, quiet and dry for the first half of next week, attention turns towards late week and a potential coastal storm that looks to just skirt S New England. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10am update... The most significant change with this mid morning update is to drop the E extent of the WWA. Temps/dwpts now rest comfortably in the mid and upper 30s in these areas, and even with the development of the meso-frontal low by mid day it is unlikely to fully mitigate the warm layer expansion sufficiently to re- introduce freezing temperatures at or just above the sfc. In the Higher terrain, and portions of W MA/CT this risk does continue, as mesowest/obs show temps much closer and even still below freezing in a few spots. Will continue the headlines there for the time being and reassess at mid day. Otherwise, temps are a blend of raw 2m temps and latest RAP/LAV data, which has handled the temps/dwpts best so far this morning. No significant changes to POPs as precip should rapidly end from W-E during the afternoon hours. */ Previous discussion... PWATs 2-3SD above normal combined with good low level isentropic ascent will bring a moderate QPF event with widespread precip today. Progressive system so much of the precip will be exiting the region by late afternoon or early evening. A mixed bag of wintry precip is expected given warm nose aloft and presence of low level cold air. In fact models are indicating temps in the 950-925 mb layer cooling to 0 to -2C near the south coast this morning and down to -5C near the NH border. With warming in the 850-700 mb layer advancing northward this will set up a good sleet signal this morning, especially across northern MA. In addition, with sfc warm front remaining south of New Eng and sfc wave tracking along the boundary, low level northerly winds will help to lock in cold air at the surface, especially higher terrain which will increase threat of icing after warming aloft changes precip to rain. North of the Mass Pike... Cold air is initially deep enough for some snow especially toward NH border and NE MA for at least a few hours after precip onset before changing to freezing rain and rain. Snow in NE MA may last until late morning as warmer air aloft coming up from the SW. An inch or 2 of snow accum is possible, mainly along and north of route 2 into Essex county with low risk for 3 inches in portions of northern Essex county. Some minor sleet accum is also possible down to the Mass Pike region. Regarding freezing rain, it appears greatest threat for appreciable icing will be over higher elevations in the Berkshires and Worcester hills where temps may hold at or below freezing much of the day. We relied heavily on 3km NAM and WRF-ARW which shows the cold air over higher terrain. Potential for localized 0.10-0.20" ice accretion over the Worcester hills and especially the Berkshires with less than 0.10" elsewhere. South of the Mass Pike... Precip will be mostly rain but will likely mix with sleet for a few hours this morning across northern CT, northern RI and along the I-95 corridor to BOS. Freezing rain is less of a threat south of the pike, but may still see a few hours of freezing rain over higher terrain in northern CT and central MA. Advisories largely unchanged but we dropped it for coastal Essex county and SE Middlesex county where temps expected to remain above freezing with just a mix of rain and sleet. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/... Precip will exit eastern New Eng by early evening, otherwise a dry night expected. Stratus and patchy fog in the evening may clear overnight, but any clearing may lead to more fog developing. Patchy black ice will also be a concern overnight with moisture near the ground and lows dropping to upper 20s to lower 30s away from the coast. && .LONG TERM /MONDAY THROUGH SATURDAY/... */ Highlights... - Quiet, dry, near-seasonable early week period - Possible stormy pattern late week, beginning of March */ Overview... Blocky N Atlantic pattern associated with an anomalous -NAO of 4 to 5 standard deviations (SD) below average, associated +3 SD H5 high retrograding W into SE Canada, parent 1040+ surface high, pressing the thermal wind axis S thru which N/S stream impulses merge having dug through a prevailing W CONUS H5 trof before ejecting, stretching E. Energy piling up, slowed, pressed S by high pressure building N, favorable storm development emerges off the E CONUS coast. Airmass juxtaposition key as to outcomes towards the end of the week after a quiet start. Only confidence is persistent NE winds of significant duration during high astro-tides, coastal flooding concerns. Lower confidence otherwise, consensus forecast guidance heavily weighted with ensemble means continues. Hit targets of opportunity below. */ Discussion... Thursday into the weekend... Battle of airmasses between an ejecting low out of the Central CONUS up against building high pressure into Canada. Slowed pattern, each building their own weight, juxtaposition between the two ultimately determining outcomes. Strong -NAO, -4 to -5 SD, would expect a cool, dry setup for NE CONUS, storm track lying further S per block. Mid- Atlantic getting crushed. Interrogating Washington DC top 10 snows versus the NAO time series after 1948, 5 out of 7 storms, 3 prior to 1948, were associated with a -NAO around -3. Recall February 2010? Washington DC "snowmageddon" of 17.8"? Monthly NAO averaged near -2. Cherry picking data? Perhaps. CIPS analogs? Interrogating where NAO indices <= -1, noting trend of sliding low S of New England, further so with a stronger -NAO. Ensemble members clustering off the Delmarva Peninsula while model deterministic solutions are hugging further N close to Long Island. Believe S trend is emerging per 25.0z EC. Echoing prior forecaster, strong -NAO, subsequent block, storms forced E, slowed, evolving into coastal storms, but storms could get suppressed S. It is all about timing. Simply wait and see with later forecasts. But per climatology, prefer ensemble clustering / 25.0z EC which are further S compared to all other guidance. Thursday into Friday lingering into Saturday, will continue with chance PoPs. Warm-core system, absent Arctic air, precip-type issues mainly as rain / snow first glance. Confident of persistent NE winds during high astro-tides, coastal flooding concerns. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/...
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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 Monday Night/... Through 21Z...High confidence. Mainly a mix between MVFR and upper end IFR with RA/fog and lowered CIGS. The RA/fog will dissipate initially late afternoon. 21Z this evening through 10Z Mon morning...Moderate confidence. Evening and early overnight, expect IFR/LIFR conditions in fog, especially as winds remain 100-040. These winds will be backing toward the NW during the early morning hours, which should allow improvement to MVFR/VFR although timing may be off a bit in TAF. Some sites, especially those typically prone to fog, may stay IFR/LIFR longer. After 10Z and into Mon night...High confidence. Winds settle NW around 10 kt. Lingering IFR/fog conditions dissipate through the early AM, with VFR then remaining through Mon night. KBOS Terminal...MVFR in RA/BR gives way to some IFR CIGS/Fog this evening. Once winds shift mainly to the N-NW between 04Z- 06Z, conditions should improve to VFR then remain. High confidence in these trends, but lower confidence in timing. KBDL Terminal...Moderate confidence. Lingering RA/BR keeps conditions mainly MVFR through 21Z. Then with winds shifting N, then NW late, lingering BR should dissipate, with a trend toward VFR after. Timing somewhat uncertain, as some lower ground fog cloud form as CIGS rise. Outlook /Monday through Thursday/... Sunday: Mainly IFR, with local MVFR possible. Windy with gusts to 35 kt. RA, PL, FZRA, patchy FG. Sunday Night: Mainly VFR, with areas IFR possible. Breezy. Patchy FG. Monday through Monday Night: VFR. Breezy. Tuesday through Wednesday Night: VFR. Thursday: Mainly VFR, with local MVFR possible. Breezy.
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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 Tonight/...Moderate confidence. Today...A period of marginal easterly gales likely across much of the open waters today as low level jet moves over the waters. Cold air above the boundary layer will promote decent mixing this morning but inversion will strengthen during the afternoon. Rain and fog will reduce vsbys. Tonight...Winds diminishing and becoming NW during the night. Speeds dropping below SCA but hazardous seas will continue over outer eastern waters. Outlook /Monday through Thursday/... Sunday: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Rain, patchy fog. Local visibility 1 nm or less. Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Patchy fog. Local visibility 1 to 3 nm. 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 locally approaching 5 ft. Wednesday Night: Winds less than 25 kt. Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. && .TIDES/COASTAL FLOODING... Thursday through Saturday... The potential for coastal flooding and beach erosion exists on several high tide cycles late next week. This will depend upon the development of a potential strong/long duration coastal storm. While it is too early to give more specific information...high astronomical tides and the potential for a strong coastal storm will need to be watched especially along the eastern MA coast given NE onshore flow and resultant swell. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Winter Weather Advisory until 4 PM EST this afternoon for MAZ002>004-008>010-026. RI...None. MARINE...Gale Warning until 3 PM EST this afternoon for ANZ231>234-255- 256. Small Craft Advisory until 6 PM EST this evening for ANZ230. Small Craft Advisory until 3 PM EST this afternoon for ANZ236. Gale Warning until 1 PM EST this afternoon for ANZ235-237. Gale Warning until 6 PM EST this evening for ANZ250-254. Small Craft Advisory until 5 AM EST Monday for ANZ251.
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&& $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Doody SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell TIDES/COASTAL FLOODING...

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