Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 231133 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 733 AM EDT Fri Mar 23 2018 .SYNOPSIS... A series of upper level disturbances impact the region into Sunday with occasional rain and snow showers, but these will be hit and miss. Conditions improve with high pressure on Monday, then a warming trend is expected through the middle of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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715 AM Update... Batch of mid and high clouds has developed from the Hudson Valley through into central and western areas over the last few hours as seen on latest GOES-East IR satellite loop, while skies were mostly clear across E coastal Mass. These clouds will continue to shift E and thicken through the remainder of the morning. Temps ranged through the 20s across most areas, with a few spots across N central and NE Mass as well as the lower CT valley held around freezing, and from 30-35 along the immediate coast. Readings were several degrees colder than forecast, so have updated to bring temps/dewpts current and incorporated into near term trends. Remainder of forecast in pretty good shape. Kept timing/trend of development of widely sct precip from late morning through the afternoon intact for now. Previous Discussion... Throughout the morning, an acute shortwave and associated vort max will shift E across the region this afternoon and early evening. Initially, soundings are mainly dry, however noting some moisture pooling mainly in the H85-H7 layer, at the top of and above the mixed layer. Speaking of which, continued cold advection aloft with this approach, but mild sfc temps should yield steep lapse rates. Combine these factors with a modest sfc trof in lower lvl mass fields, and most factors suggest the possibility of a spot SHRA/SHSN (mainly RA as temps should be rising into the low-mid 40s by the time the lower lvl convergence peaks). Will cap risk at isolated, as the moisture is relatively limited to that one layer most of the day today. Speaking of temps, H85 temps average about -6C by peak heating, however as noted, thanks to the CAA implied by the wave, mixing may exceed this level. Therefore, should see temps rise into the low-mid 40s, especially in the CT valley where less snowcover has been observed for some time now. This is in spite of a fair amount of mid day-evening cloud cover.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Overnight... Typical diurnal ending for temps this evening will be assisted by implied AVA with the passage of the shortwave/vort max. This should also lead to some breaks in the lower-mid lvl cloud cover. However, noting that upper lvl moisture increases, so even as low-mid clouds disperse somewhat CI is likely to fill in the gaps. This should limit cooling, especially given sfc pres gradient remains strong enough to maintain a slight N-NW wind overnight. Mins similar to this morning, mid 20s in coldest spots to the low 30s elsewhere. Sat... Given very little change in the downstream pattern (mean trof to the E but with enough in place across New England to maintain impact), expect another round similar to Fri as a second shortwave shifts out of the Boreal regions of Quebec. This more arctic influenced wave may in fact cutoff as it makes its approach late Sat, yielding a sfc response even stronger (better f-gen and convergence than Fri`s passage). It also carries with it generally higher moisture through the column, especially as winds shift toward and E component. Although lower lvl lapse rates may not be as steep with a colder airmass in place, the other dynamics look to make up the difference in this case. Therefore, expecting more widespread snow/rain showers than Fri, and have POPs at SCT category as a result. Lower lvl temps/particularly wet-bulbs are near freezing, so some of these could see light-mod snowfall. Sfc temps are mild, so any accums should be limited even on grassy surfaces. High temps a bit cooler thanks to reinforcing shot of cold air. Mainly low to upper 30s rather than widespread 40s expected today. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Overview... 00Z model suite continues to signal an overall amplified mid level steering pattern across North America and beyond through the majority of the long term period. H5 long wave troughing and disturbances in the N-NE flow at the surface and aloft will keep an unsettled pattern across the region through at least this weekend. As an H5 long wave trough slowly moves into the western states early next week, the east coast trough will slowly shift offshore, with ridging building from Hudson Bay to the lower Mississippi valley. This will be slow to shift east through the remainder of next week as cutoff mid level low pressure and surface system lingers across the western Atlantic. Will likely keep a cool onshore flow continue into early next week, then should start to see temperatures slowly moderate as the ridge builds to the west, bringing winds around to S-SW by around mid to late next week. Details... Saturday night and Sunday... H5 cutoff low pres moves across the region out of eastern Quebec and the Maritimes, bringing scattered rain and/or snow showers. Have carried CHC POPs across most areas, except only slight CHC POPs across the mid and lower CT valley into W RI Sat night into early Sun. As the low rotates S off the coast by Sunday morning, will see the chance for precip dwindle from W-E, with the best chance for precip across E coastal areas. Will also see NE winds increase, with gusts up to 25-35 mph along E coastal areas as well as on the islands. With the stiff onshore wind, temps will run well below seasonal normals. Expect highs on Sunday from the mid to upper 30s across most areas, except around 40 across the CT valley. Sunday night through Monday night... As high pressure slowly shifts E during this timeframe, will see dry conditions but continued cool temperatures as NE winds will persist as the high center remains over Quebec. Highs Monday will be a bit milder, but will still remain in the mid- upper 30s along E coastal areas. Tuesday through Thursday... Some model solution spread noted during this timeframe. A leading warm front pushes NE, which may bring a few rain and/or snow showers reach as far E as central MA/NE CT. Expect to see temps slowly moderate, reaching close to seasonal levels by Thu, while the cold front hangs up around the eastern Great Lakes. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/...
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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/... Through Tonight...High confidence. Mainly VFR. SCT CIGS become BKN-OVC between 040-060 from mid to late this morning into the evening, along with isolated SHSN/SHRA with little to no accumulation. Showers dissipate overnight with CIGS lifting closer to 10kft. W-NW winds, mainly around or less than 10kt. Saturday...High confidence. Mainly VFR again, but occasional MVFR CIGS/VSBYS in more widespread SHRA/SHSN than Fri. Winds shift from NNW to NE through the day. KBOS Terminal. High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Saturday Night through Tuesday/...Moderate Confidence. Saturday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHSN. Sunday: Mainly VFR, with local MVFR possible. Windy with gusts to 30 kt. Slight chance SHSN. Sunday Night: VFR. Windy with areas gusts to 30 kt. Monday: VFR. Windy with local gusts to 30 kt. Monday Night through Tuesday: VFR. Breezy.
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&& .MARINE... Short Term /through Saturday/...High confidence. Today... Winds continue to diminish, and should remain below small craft thresholds out of the NW from now on. However, a lingering swell tied to offshore low pres will keep mainly E waters above 5 ft through the afternoon. Small Craft Advisories will remain up until these diminish. Overnight into Sat... Overall quiet boating weather is expected through this period. Winds will gradually shift toward the N then NE by late Sat. Gusts to 20 kt at peak, and while this may lead to a few wind waves near 5 ft, these should be very limited. No headlines expected. Outlook /Saturday Night through Tuesday/...Moderate Confidence. Saturday Night: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Seas locally approaching 5 ft. Chance of snow showers, chance of rain showers. Sunday: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Slight chance of snow showers. Sunday Night: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 12 ft. Monday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Rough seas up to 11 ft. Monday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 11 ft. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 15 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 11 AM EDT this morning for ANZ231- 251. Gale Watch from Sunday morning through Sunday evening for ANZ250-254>256. Small Craft Advisory until 2 PM EDT this afternoon for ANZ250- 254-255. && $$ SYNOPSIS...Doody/EVT NEAR TERM...Doody/EVT SHORT TERM...Doody LONG TERM...EVT AVIATION...Doody/EVT MARINE...Doody/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.