Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 230803 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 403 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/... GOES16 Night-microphysics channels suggest bulk of the lower clouds now offshore with mostly clear skies across S New England as the CI band has centered around stretched vortex across NY/PA. Noting that this is likely assisted by mid-upper lvl ridging in advance of a series of shortwave rotating through the longwave cutoff/trof across the Maritimes and extending into New England. The slow decline in overnight dwpts outside of W MA/CT has slowed the morning temp trend, and kept mins so far mainly in the upper 20s and low 30s. Will likely see a drop of a couple of more degrees before sunrise thanks to this most recent clearing and area OBS suggesting at least modest decoupling. Throughout the morning, an acute shortwave and associated vortmax, currently located near Lake Huron will rotate E toward New England, shifting 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. && .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/... High pressure system builds down from Quebec this weekend advecting cold air over the region, keeping temperatures well below normal through early next week. Global models are hinting to a low pressure system that forms off the coast of the mid- Atlantic and quickly intensifies. At the moment, models suggest that the strong high over Quebec will keep the low pushed out to sea. Regardless, if the low remains in close proximity to our coast, winds and swell will increase and likely remain that way until another system pushes it further east. Otherwise, the high pressure will continue to build over the region through Wednesday, before giving way to an approaching cold front Thursday. && .AVIATION /07Z FRIDAY THROUGH TUESDAY/... 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 predominantly 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. Sat...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: Mainly VFR, with local MVFR possible. Slight chance SHRA. Saturday Night: Mainly VFR, with local MVFR possible. Windy with local gusts to 30 kt. Chance SHSN. Sunday: Mainly VFR, with areas MVFR possible. Windy with gusts to 30 kt. Slight chance SHSN. Sunday Night: VFR. Windy with gusts to 30 kt. Slight chance SHSN. Monday through Monday Night: VFR. Windy with gusts to 30 kt. Tuesday: Windy with gusts to 30 kt. Slight chance SHRA. && .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: Winds less than 25 kt. Slight chance of rain showers. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas locally approaching 5 ft. Chance of rain showers, slight chance of snow showers. Sunday: Strong winds with gusts up to 30 kt. Areas of rough seas. Chance of snow showers, chance of rain showers. Sunday Night: Low risk for gale force winds with gusts up to 30 kt. Rough seas up to 10 ft. Chance of snow showers. Monday through Monday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Slight chance of snow showers. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Slight chance of rain showers. && .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 NEAR TERM...Doody SHORT TERM...Doody LONG TERM...Correia AVIATION...KJC/Doody MARINE...KJC/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.