Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 222358 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 758 PM EDT Thu Oct 22 2020 .SYNOPSIS... Weak high provides cooler weather tonight and Friday, however still above normal for late October. Dry weather prevails Friday but patchy fog and drizzle may impact the region tonight. Mild weather continues Saturday ahead of an approaching cold front, which will cross the region later Saturday followed by much cooler weather for Sunday. An unsettled weather pattern develops early next week as a series of fronts move across the region. This will bring the chance of showers at times Monday into Wednesday with perhaps the greatest risk for a period of steady rain late Tuesday night into Wednesday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... 745 PM Update... * Areas of locally dense fog to develop overnight. The areal extent will be monitored closely. Previous forecast remains largely on track. Did bring up the dew points by a couple of degrees based on obs. Parts of Eastern MA and Southern RI are experiencing locally dense fog, with vsby down to one-half mile at times so have gone ahead and put out a Special Weather Statement for locally dense fog. A quiet night of weather except for development of locally dense fog. There are some questions how widespread the dense fog will be. With light winds and dew points of low 60s being advected northwards, dew point depressions are expected to fall to near zero and that would argue for development of widespread dense fog. Also, the fog bank over Eastern MA is expected to advect westward, albeit subject to upslope effect. Somewhat drier air (dew pt in the mid 50s will also advect westward overnight thanks to a back door cold front, so areas of dense fog development may ultimately be limited to Worcester point west. Later shifts will continue to monitor the fog situation but if out and about tonight, be sure to slow down if encountering areas of locally dense fog. PREVIOUS DISCUSSION... 415 PM Update... Most of the region is enjoying beautiful late Oct weather at 4 pm with temps in the 70s and at least partial sunshine. However across coastal eastern MA, northeast flow associated with a backdoor cold front has advected a fog bank into this region. For example, at Logan airport, temp is down to 58 degs in 1/2 mile dense fog! This fog bank will continue advancing westward but the leading edge will likely erode as it traverses the warm land. However with sunset as the boundary layer cools, expecting this low level moisture to eventually overspread the entire region from east to west. The more difficult question is the duration, as dry air aloft is descending to the surface behind this fog bank over the Gulf of Maine. So overnight and especially toward Fri morning, clearing will occur from east to west. Until then, much of the model guidance suggesting spotty light rain/drizzle for a time overnight as this low level moisture moves across the area. This seems reasonable especially across the upslope regions of the Worcester Hills and the Berks. Given the NE flow cooler tonight than last night, but still above normal for late Oct with lows only in the 50s. Avg low is in the 40s. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... 415 pm update... Friday... Low clouds, spotty light rain/drizzle possible especially across CT into western-central MA trapped beneath subsidence inversion. Although low clouds will slowly lift along with clearing approaching from the east in response to drier air eastward well behind backdoor front. Thus expecting at least partial afternoon sunshine with more clouds inland. Not as warm as today given change in airmass with 1030 mb high over the maritimes. Highs in the 60s, more seasonable for this time of year. It will be pleasant given light winds Friday night... Ridge of high pressure holds on so quiet weather prevails. Low clouds may redevelop but with high pressure weakening, subsidence inversion likely also weakens, thus less of a trapping mechanism for low level moisture. Mostly clear conditions, at least initially combined with light winds and relatively dry airmass will promote temps to fall into the upper 40s and low 50s. && .LONG TERM /SATURDAY THROUGH THURSDAY/... 415 pm update... Highlights... * Mostly dry and quite mild on Saturday * Dry and becoming much cooler Saturday night & Sunday * Unsettled weather possible at times Mon through Thu with the greatest potential for a period of steady rain late Tue into Wed night Details... Saturday... The weekend starts out mild before becoming quite cool for the second half. This unseasonable warmth comes courtesy of SW flow lingering behind a warm front overnight. Ahead of the cold front 850 mb temps rebound briefly to 13C+ so, with decent sunshine after some morning clouds, we should see highs approaching 70 degrees. Dewpoints will be higher too (upper 50s), at least relative to what`s coming behind the front (dew points in the 20s and 30s). The cold front swings through during the afternoon and early evening hours. Not expecting much in the way of showers from it given the lack of deep moisture and dynamics. Saturday night and Sunday... The main sensible change during this period will be the much cooler and drier airmass behind the front, and the breezy post frontal winds overnight. Much lower dewpoints overnight together with post frontal cold air advection will allow for lows in the low 40s toward the coast, approaching freezing the the far NW interior. High pressure builds in at the surface, expanding SE from Canada. The placement of the high will also keep flow onshore (from the NE) during the day on Sunday. Highs won`t make it out of the low 50s, 5 to 10 degrees below normal for late October. In addition to the cooler airmass, winds will be breezy as a weak N/NW low level jet is mixed down. These winds will pick up as early as late Saturday afternoon when the front passes, lasting through the night. The strongest wind gusts though, 30 to 35 mph, will be over the outer waters. Monday through Thursday... By Monday a warm front lifts north associated with a wave of low pressure passing northwest of New England. This brings the threat of rain back into the forecast as well as rebounding temperatures, potentially back into the mid 60s along the south coast. Still thinking that the cold front dips through Monday night into Tuesday bringing a marginal cool down before a more substantial frontal system with heavier rain passes in our vicinity around Tuesday night into at least mid week. Plenty of uncertainty with regard to timing and details but overall next week looks more unsettled and cooler. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 00z TAF Update: Main concern is IFR cigs coming onshore to Cape Ann and outer Cape Cod this afternoon in response to NE flow. Timing is difficult. Warm land and sunshine will delay and may erode leading edge of low clouds, but as sunset approaches and temps begin to fall these low clouds will come racing onshore. So IFR cigs and MVFR vsbys expected to impact eastern MA after 19z and especially after 21z from NE to SW. Elsewhere, VFR cigs BKN040 at times along with light and variable winds. Tonight...high confidence on trends but lower on exact timing. IFR cigs and MVFR vsbys over eastern MA at 00z will spread westward into RI, CT and western-central MA. Mainly dry weather but can`t rule out spotty drizzle. Light east wind. Friday...High confidence on trends but uncertainty on exact timing. MVFR-IFR in morning low clouds will lift to VFR during the afternoon. Other than spotty light drizzle in the morning, dry weather prevails. Light ESE winds. Friday night...low confidence. VFR likely at 00z but MVFR-IFR possibly moving back onshore. Light south winds. Low prob of spotty light drizzle, otherwise mainly dry. KBOS Terminal...IFR cigs and MVFR vsbys in low clouds coming onshore about 19z-1930z. Uncertain on duration but will impact late day push. KBDL Terminal...High confidence in TAF. Outlook /Saturday through Tuesday/... Saturday: Mainly VFR, with local IFR possible. Breezy. Saturday Night: VFR. Windy with local gusts up to 30 kt. Sunday: VFR. Breezy. Sunday Night: VFR. Breezy. Chance FZRA, slight chance SHRA. Monday: Mainly VFR, with local IFR possible. Breezy. Chance SHRA. Monday Night through Tuesday: Mainly VFR, with areas IFR possible. Breezy. Chance SHRA. && .MARINE... 415 pm update... Tonight...east winds 10-15 kt but the bigger story is fog bank impacting eastern MA waters this evening, with fog bank south of Block Island coming onshore later this evening. Thus areas of dense fog and light rain/drizzle overnight. SE swells from Hurricane Epsilon will increase to 3-6ft. Friday...any leftover fog/drizzle improves by midday or earlier. SE swells increase 4-7 ft. Otherwise high pressure over the Maritimes provides light winds. Friday night...ridge of high pressure remains overhead but weakens. Thus light winds. Low clouds and fog may redevelop but confidence is low. SE swells from Hurricane Epsilon continue 4-7 ft. Outlook /Saturday through Tuesday/... Saturday: Winds less than 25 kt. Rough seas up to 9 ft. Local visibility 1 to 3 nm. Saturday Night: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Rough seas up to 9 ft. Sunday: Winds less than 25 kt. Areas of rough seas. Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Monday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. Monday Night: Low risk for Small Craft Advisory winds with gusts up to 20 kt. Seas up to 5 ft. Chance of rain showers. Tuesday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT Saturday for ANZ250- 254>256. && $$ SYNOPSIS...Nocera/BW NEAR TERM...Nocera/Chai SHORT TERM...Nocera LONG TERM...BW AVIATION...Nocera/BW MARINE...Nocera/BW

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