Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 232302 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 702 PM EDT Fri Mar 23 2018 .SYNOPSIS... A series of upper level disturbances impact the region over the weekend 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. The chance for rain then increases towards the end of the next workweek. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 700 PM Update... Current forecast looks on track. Continued slight chance pop this evening for snow showers along the east slopes of the Berkshires and western Hartford County. Otherwise partly to mostly cloudy skies continue this evening. Late afternoon discussion follows... Broad trough over the Northeast USA and Maritimes. One shortwave from NY state sweeps through the trough tonight. This brings cold advection aloft which brings vertical lapse rates of 6 to 6.5C/Km. Total-totals are forecast to reach 50 near NYC with 40s over Southern New England. Cross sections of RH show high values from surface to 700 mb. This along with the decreased stability and increased lift suggest lots of clouds and at least widely scattered showers in Western Mass and Western CT where the southwave approaches. Clouds will restrict cooling, especially where winds keep the air stirred. Temperatures upstream early this morning were in the 20s. Min temps tonight should be similar, although we will show coastal lows around 30 or the low 30s due to effects of the ocean. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Upper trough remains over New England Saturday and Saturday night. The shortwave moving through that trough moves off to the east Saturday. But another shortwave from Hudsons Bay drops south tonight and moves across New England Saturday and Saturday night. A pocket of colder air aloft moves through with this second trough, and moisture cross sections continue to show high values to at least 700 mb. Lapse rates 925 to 700 mb are forecast to reach 7C/KM. Lift will also be stronger, roughly 20 mb/hr rather than the 10 mb/hr forecast for tonight. Best forcing looks to be along/east of the Worcester Hills, covering Eastern Mass and RI. Considering all of this, we will bring chance pops to these eastern areas and slight chance pops to Western Mass and Northern CT. Temperature profiles favor light rain during the day Saturday and snow for Saturday night. Snow accumulation potential looks around an inch, possibly two inches in spots. Best chance for accums would be over Northeast Mass, where the forcing is strongest. Mixing reaches to 850 mb with winds in that layer at about 15 knots and temperatures at the top of the layer -8C to -9C. Thus expect north winds to gust 15-20 mph and max sfc temps in the upper 30s to mid 40s. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Details... Sunday... Upper low centered over and to the east of southern New England moves southeastward, while strong 1040 mb high centered over Quebec shifts to the St. Lawrence River Valley to northern New England. The high pressure keeps a low level NE onshore flow in place over our area, which will keep temps well below seasonal normals. Thinking highs will be in the mid 30s to around 40. A tightening pressure gradient is expected as low pressure moves off the Carolina coastline and tracks eastward. So expect some increase in NE winds, with gusts up to 25-35 mph along E coastal areas as well as on the islands. Models indicate a short wave rotating into our area during the day. This should yield more clouds than sun and isolated to scattered snow/rain showers. Looking at Bufkit model soundings, the better chance for showery weather is over central and eastern MA/northeast CT/RI, where onshore flow keeps low levels moist. However further west into the CT Valley region of MA/CT, the chance for measurable precipitation is diminished and hence it should be drier there. Sunday night through Monday night... Cool Canadian high pressure sinks southward into New England, yielding dry conditions with a continuation of somewhat below normal temperatures thanks in part to a continued onshore breeze. Quite dry air in place thanks in part to subsidence inversion, so mostly clear/mostly sunny skies forecast for this timeframe. The exception to this will probably be over the Cape and Islands, where just enough low leve moisture may linger to keep skies partly to mostly cloudy at least into Sunday night. Lows in the 20s to around 30, and highs Monday in the mid 30s to mid 40s. Tuesday... Models show mid level ridging and surface high pressure over our area, yielding dry conditions with a warmup. This dry/warm region narrows during Tuesday night, with a low pressure system passing thru the Great Lakes Region, and an offshore low pressure retrograding somewhat to the west. Thus we may see an increase in clouds Tuesday night. Wednesday through Friday... Model spread brings uncertainties on timing of different features, but overall expecting an increase in moisture with the potential for rain and snow showers. For Wednesday low pressure lifting well northeast of the Great Lakes Region, associated cold front may bring showers to southern New England. However mid level ridging could persist and keep the day dry. At this time limiting pops slight chance. During Thursday-Friday, mid level ridging is replaced with a broad trough placing the east coastal US in SW flow. Should see an increase in southern stream moisture heading out way as a low pressure system moving towards and into our area, bringing a chance for rain. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... Tonight...High confidence. VFR, with ceilings 4000-7000 feet. Isolated rain/snow showers with little/no effect on aviation conditions. Saturday and Saturday night...High confidence. Mainly VFR during the day, with a period of MVFR cigs possible Cape/Islands. Small potential for MVFR cigs/vsbys elsewhere in rain showers Saturday. Cigs lowering to MVFR Saturday night with a chance for reduced vsbys in isold-sct snow showers. North winds during the day will turn out of the northeast later Saturday night. KBOS Terminal. High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Sunday through Wednesday/...Moderate Confidence. Sunday: Mainly VFR, with areas 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 Night: VFR. Breezy. Wednesday: Breezy. Slight chance SHRA. && .MARINE... Short Term /through Saturday Night/...High confidence. Tonight...High confidence. Quiet tonight with winds below 25 knots and seas below 5 feet. Saturday...High confidence. Quiet weather continues. Scattered rain showers in the afternoon/evening may lower vsbys along the Eastern Mass coastline. Saturday night...Moderate confidence. A wind shift moves south across the waters after midnight. Snow showers along this line may bring brief reductions in vsbys. The wind shift will turn winds from the northeast and bring building speeds. Gusts late at night will be between 25 and 30 knots. Seas will build after midnight, reaching heights of 5-7 feet on the eastern waters. Outlook /Sunday through Wednesday/...Moderate Confidence. 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. Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 15 ft. Wednesday: Winds less than 25 kt. Rough seas up to 12 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Watch from Sunday morning through late Sunday night for ANZ250-254>256. && $$ SYNOPSIS...WTB/NMB NEAR TERM...WTB/NMB SHORT TERM...WTB LONG TERM...NMB AVIATION...WTB/NMB MARINE...WTB/NMB is the U.S. government's official web portal to all federal, state and local government web resources and services.