Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 161400 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1000 AM EDT Sat Jun 16 2018 .SYNOPSIS... Dry weather continues this weekend with a warming trend each day. Dangerous heat and humidity impacts the area Monday along with record high temperatures. As a result, heat headlines will likely be needed. A cold may trigger isolated thunderstorms late Monday into Tuesday, otherwise dry weather prevails. Behind the departing cold front, not as hot and less humid from late Tuesday through Friday. && .NEAR TERM /THROUGH TONIGHT/... 1000 Am Update... Overall trend in the forecast remains on track for today. Northwest flow aloft due to building ridge to the west will allow for high level cirrus to push into the region today. In fact this cloud cover has already spilled into eastern MA resulting in filter sunshine. Still anticipate temps to warm into the 80s today despite the increase cloud cover. Previous Discussion... Plenty of sunshine as any lingering stratus is very shallow and will erode quickly. Only limiting factor will be the moisture associated with the outflow upstream which could lead to some SCT mid and high clouds through the day. It will do little to limit mixing however, which all soundings suggest exceed H85, where temps warm to near +13C. Therefore, looking at highs well into the low-mid 80s. The deep mixing will keep dwpts low however, mainly in the upper 40s and low 50s. && .SHORT TERM /SUNDAY/... Tonight... Although there is indications of convection firing near the OH valley late today, the steering flow given the ridge aloft is actually amplifying with time should force the bulk of the outflow to the SW of New England, leading to clear skies. The resulting weak pres gradient also supports light to calm flow. Therefore, another round radiational cooling likely. Afternoon crossover dwpts are in the upper 40s and low 50s. There may be some of these, however mins may be just a hair warmer as light return flow overnight will allow dwpts to gradually rise. Sun... Ridge continues to build with establishing warm front to the N. The continued amplification suggests some capping of the sounding, which current bufkit data supports. Therefore, think the risk for convection in S New England is limited as the cap will limit destabilization. This is supported by good sfc mixing again limiting ML dwpts and CAPE values. Otherwise, the continued warming in the low-mid lvls yield H85 temps closer to +15C. Mixing may be limited to this level given the increased subsidence inversion, but this will still allow for highs to reach the mid and upper 80s. A few low 90s not out of the question in the usual spots, such as the CT and Merrimack valleys thanks to downsloping. Weak gradient will limit near shore temps thanks to sea breezes. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Highlights... * Dangerous heat and record highs Monday * Not as hot and less humid late Tuesday thru Fri * Mainly dry w/low risk of isolated T-storm Mon & Tue Sunday night... Warm front moves across the region but with little fanfare as core of mid level ridge just to the west of southern New England. This yields sufficient subsidence to keep the region dry with any showers and T-storms likely rotating up and over the ridge across northern/central New England. Low level WAA via SW winds will result in warmer conditions than previous nights as surface dew pts climbing thru the 60s and likely yields areas of fog. Monday... Dangerous heat advects into southern New England with anomalously warm airmass as 850 temps increase to +20C/+21C per both ensembles and deterministic guidance. 925 mb temperature anomalies even more impressive with temps at this level warming to +28C Monday afternoon! Close inspection of model soundings suggest temperatures may overachieve with boundary layer mixing beyond 850 mb and winds within this layer from the west (250 degs). This suggest some downslope/adiabatic warming in the CT and Merrimack river valleys where hottest temps will likely be observed. All of this supports highs 95-100 Mon afternoon, cooler toward the south coast given some onshore wind component (240 degs). Thus some relief at south coastal beaches. This excessive heat combined with dew pts rising into the low 70s will likely yield heat indices of 100-104F. Thus a dangerous heat and will likely need heat headlines as the event nears. Tuesday through Friday... Northern stream short wave and attending cold front move through the region sometime Tue. Low risk for isolated T-storms then trending less humid in post frontal airmass later Tue. Not as hot behind the front but still warm with ensembles and deterministic guidance offering +14C/+15C airmass at 850 mb over southern New England Tue thru Fri. So highs well into the 80s but less humid than Mon. Will have to watch mid week when some of the guidance suggest a wave develops on the front south of New England, possibly yielding a risk for showers. However at this time range frontal waves offer very low predictability. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/... Today and tonight...High confidence. Mainly VFR. W-SW winds through the day, although more southerly thanks to sea breezes near the S coast. E coastal sea breezes are possible, but it would be late start, likely around or after 20Z if at all. VFR continues into tonight with light and variable winds. Sun...High confidence. VFR. Some daytime SCT CU, mainly 050-080. Winds shift to the S except sea breezes starting mid-late morning. KBOS Terminal...High confidence in TAF overall. Late day sea breeze possible, but may stay just offshore. KBDL Terminal...High confidence in TAF. Outlook /Sunday Night through Wednesday/... Sunday Night: VFR. Monday through Monday Night: Mainly VFR, with areas MVFR possible. Breezy. Slight chance SHRA, slight chance TSRA. Tuesday: Mainly VFR, with areas MVFR possible. Slight chance SHRA, slight chance TSRA. Tuesday Night: VFR. Wednesday: Mainly VFR, with local MVFR possible. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/... Through Sunday... Light flow and diminished seas through the period. Some fog with lower vsbys possible early Sat morning, mainly along the SE coastline. Outlook /Sunday Night through Wednesday/... Sunday Night: Winds less than 25 kt. Monday: Winds less than 25 kt. Local rough seas. Slight chance of rain showers, slight chance of thunderstorms. Monday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers, slight chance of thunderstorms. Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms. Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Wednesday: Winds less than 25 kt. Slight chance of rain showers. && .TIDES/COASTAL FLOODING... Final combined tide was 12.5 ft this morning in Boston. Early Sunday morning high tide in Boston is 11.9 ft. While this is not far from minor coastal flooding levels, offshore flow and higher pres will limit wave action over it. Therefore, at this time additional coastal flooding is not anticipated, but this will be evaluated through the day today. && .CLIMATE... Here are the record and forecast highs for Monday June 18... BOS ... 94 (1907 & 1929) ... Forecast High 96 BDL ... 95 (1957 & 1994) ... Forecast High 97 PVD ... 94 (1929) ... Forecast High 90 ORH ... 93 (1907 & 1929) ... Forecast High 91 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/Doody NEAR TERM...Doody/Dunten SHORT TERM...Doody LONG TERM...Nocera AVIATION...Nocera/Doody MARINE...Nocera/Doody TIDES/COASTAL FLOODING... CLIMATE...

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