Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 292345 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 645 PM EST Tue Nov 29 2016 .SYNOPSIS...
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Heavy rain will impact the evening rush hour and some brief localized street flooding is possible. A short period of dry weather follows later this evening into at least part of Wednesday morning, before another soaking rain affects the region Wednesday afternoon and night. Near-seasonable conditions with breezy NW flow follows for the late week period into the weekend. Potential showery impacts across the high terrain and Outer Cape. Early next week, another wet-weather disturbance is possible with a potential wintry-mix with onset changing over to all rain.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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7 pm update... Heavy rain pushing offshore collocated with nose of higher convergent theta-E air. Already seeing the shift in the winds per vertical wind profiler as water vapor satellite is capturing the leading edge of drier air encroaching into S New England. The trend over the next few hours is for heavy rain to push offshore along with strong S winds as presently observed (45 mph gusts up at Isle of Shoals tower off the NH coast). Will keep with the present wind advisory along with special weather statement on heavy rain. Then looking at low clouds / fog development. A lot of moisture given antecedent rains and likely cooling overnight. Evaluating upstream, seeing a fair amount of low cloud decks but also visibility improving. Overnight temperatures should drop to the dewpoint as the dry-inversion builds overhead. Winds becoming light. Want to emphasize the risk of locally dense fog, especially within the sheltered valleys where it has been very cool today, such air not having the opportunity to scour out. But signals from the N American Rapid Refresh Ensemble are mixed, indicating more low clouds than fog. Greater risk of fog over the high terrain just given the low clouds. Am speculative and wish to echo similar sentiments as expressed by the previous forecaster in the discussion below. Previous Discussion... * Bands of heavy rain may result in brief localized street flooding during the evening rush hour especially south of the Mass turnpike * Rain ends later this evening but areas of fog expected which may become locally dense 1) Heavy Rain Continues into the evening hours... Convergent nose of low level theta-E fed by an impressive 50-60 mph S low level jet and pwats 2-3 standard deviations above normal are resulting in the continuation of a band of heavy rain pushing E over E/SE portions of S New England. Heaviest rainfall amounts are being tallied with the heaviest amounts showing up over S New England as expected along aforementioned convergent nose. Still remains the threat of localized street flooding with leaf-clogged drains. Ponding, drainage issues, typical nuisance. Special weather statement continues. 2) Strong Winds Across Cape/Islands... Wind advisory continues and there was discussion of expanding it across the S coast of New England as winds in Narragansett Bay were exceeding sustained thresholds of 30 mph. But too little too late as most stations have already peaked and winds are beginning to taper. Seeing the strongest winds collocated with heaviest rain no doubt due to precip-drag / mechanical mix-down to the surface, this despite an inversion below h925, above which the low-level jet blows. Warmer ocean likely contributing to better mixing than across the interior, thus lessening the limitation of the inversion. 3) Rain Ends Into Evening But Areas of Fog Develop Late... The rain should be over by late evening as shortwave/forcing moves east of the region. However, there is not a strong push of dry air to scour out the low level moisture. It is uncertain just how much partial clearing will occur, but expect areas of fog to develop which may become locally dense in some locations. Something later shifts will have to watch closely. 4) Temperature Trends... A good portion of southern New England should temporarily warm sector through late evening, allowing highs to spike well into the 50s to near 60 through midnight. However, portions of the interior especially north of Route 2 may remain in the cool sector with temps only rising into the 40s. By daybreak, temps should fall back into the 40s in most locales and even 30s in portions of W MA.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
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* Another soaking rain later Wednesday into Wednesday night Wednesday and Wednesday night... Areas of fog should burn off by mid morning on Wednesday. While it will start off dry, another shortwave will approach from the SW. Easterly winds at the surface coupled with moist SW flow aloft will increase the forcing for ascent. Light rain will develop later into the morning into the afternoon from west to east. The heavier rain will probably not arrive until late Wednesday afternoon/evening when another impressive low level jet of 45 to 55 knots joins with PWATS that remain 2+ standard deviations above normal. There will be pockets of impressive frontogenesis/omega that should lead to bands of locally heavy rainfall. We also can not rule out an isolated t-storm or two S of the MA Pike with some marginal elevated instability. High temps on Wednesday should be in the upper 40s to the middle 50s, with the mildest readings found south of the MA Turnpike. In a nutshell...rainfall amounts from this second system should average between 0.50 and 1.50 inches. The rain should come to an end toward daybreak on Thursday as shortwave energy moves east of the region. Low temps will be in the 40s.
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&& .LONG TERM /THURSDAY THROUGH TUESDAY/...
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*/ Highlights... - Near seasonable, dry pattern with breezy NW winds late week into the weekend - Initial wintry mix changing over to all rain possible early next week */ Overview... Perhaps an over-running onset of mixed wintry precipitation events, but otherwise a warm-wetter pattern continues. La Nina is seemingly the dominant influence upon the N Hemisphere as all other atmospheric teleconnections are near-neutral, relatively flat. A more buckled pattern than idealized across N America, with strong C CONUS cyclo- genesis drawing down colder air W beneath a favorable region of troughing while latching into tropical maritime air S. Would appear with prevailing E CONUS downstream ridging, deepening and occluding storm centers out of the C CONUS are stretched N/W of S New England, inside-runners, placing S New England in the warmer and wetter region of outcomes. A progressive pattern of initial over-running followed by a sweeping cold front before high pressure and colder air settles back in. Can not rule out a continuation of mixed wintry precipitation events with the onset of storm systems noting the potential for cold air damming and triple point lows. Will note in the details below. Would appear Arctic air presently is locked up round the polar low. Indications though that as we go towards mid-December the polar low and associated energy becomes displaced. Associated jet stream round the maritimes becomes buckled and more amplified. Potentially thus we could see shots of Arctic air being introduced as we go deeper into Winter. No doubt if the pattern continues we would see such air slide S across the Rockies into the C CONUS before sliding E, likely moderating. It`s early but worth noting of potential trends down the road, something to watch. */ Details... Thursday through the Weekend... NW flow and cold air advection initially through which mid-level energy and moisture continue to rotate round a dying low pressure center over the N/E Great Lakes Region. A reinforcing cold front inbetween around Saturday morning. Usual pattern: colder NW flow over the Lakes invokes broken to overcast cloud decks along with showery precipitation, downsloping over the high terrain N/W of our region, only scattered to broken decks remain with a slight chance of a shower, the greatest chance being over the Berkshires. Well- mixed lapse rates with diurnal heating, will see the mix-down of drier air and faster winds, but nothing that would appear to reach advisory level criteria. Cooler air over warmer waters, ocean-effect processes proceed, and depending on the direction of the wind, will more than likely impact the Outer Cape and Islands, especially on Saturday behind the reinforcing cold front. Will go with chance of showers at times for the aforementioned locales. Conditions overall will feel cooler given the breezy NW winds, but in actuality remain near-seasonable for this time of year. High pressure across the region over the weekend from the SW, will see a return S flow. Early next week... Consensus indications of wet-weather though the exact evolution and timing remains uncertain. With onset it is possible to see wintry mixed precipitation associated with over-running ahead of an area of low pressure emerging out of the S CONUS, especially if high pressure sets up over E Canada as some of the solutions indicate. Leaning towards an inside runner solution occluding into the E Great Lakes as a secondary low develops at the triple point and rapidly deepening offshore. Going with ensemble means but advertising low confidence. Chance PoPs for now through the remainder of the week.
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&& .AVIATION /23Z TUESDAY THROUGH SUNDAY/...
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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 Wednesday Night/... Tonight... With -RA/RA: gusty S winds upwards of 35 kts. MVFR-IFR with visibility restrictions due to rain, lowering cigs if not already IFR. LLWS over SE coastal terminals with 50 kt S winds 2 kft agl. Behind -RA/RA: cigs lowering to LIFR-IFR, possibly becoming BKN towards morning, low confidence of clearing. Vsby challenging. Confidence high terrain socked in clouds / fog, but lesser so at lower elevations. Anticipating to see a mix of either low clouds or fog into morning. If cigs become SCT, then the likelihood of fog increases. Wednesday and Wednesday night...Moderate confidence. Fog dissipating after sunrise. Improving MVFR to low-end VFR during the morning. -RA/RA moving in towards midday, lowering back down to MVFR-IFR afternoon into evening with heaviest RA. Improving towards daybreak Thursday as RA concludes. KBOS TAF...Confident that cigs will lower after the -RA/RA moves out. Not so confident with respect to fog as we go into morning. KBDL TAF...RA has moved out and now expect cigs to lower into the overnight hours as winds become light. Quite cool at the terminal, higher confidence of fog impacts given how wet and cool conditions are presently. Outlook /Thursday through Sunday/... SCT-BKN low-end VFR. NW winds, blustery at times, especially along the coast with gusts initially Thursday and Friday up to 30 kts, tapering downward with time. MVFR possible over the Outer Cape with chance -RA and a breezy N wind.
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&& .MARINE...
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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 Wednesday Night/... 7 pm update... Tonight...Moderate to high confidence. S gusts up to 40 kts expected with moderate to heavy rain which will push E of the waters later tonight prior to midnight. With gales, will see waves build 7 to 10 feet as they have already at buoy 44097. With heavy rain, expect reductions to visibility. Expect winds to diminish while turning out of the W towards Wednesday morning. Wednesday and Wednesday night... Winds should not be much of a factor into early Wednesday afternoon. However, east winds of 25 to 30 knots shifting to the south should develop later Wed afternoon and night. Brief marginal gale force wind gusts may develop, but inversion will keep those short-lived if they do occur. Seas once again should build back to between 7 and 10 feet across our southern waters. Outlook /Thursday through Sunday/... NW winds possibly gusting as high as 30 kts initially with a low risk of gale force, Thursday and Friday. Winds building with the wave stress. Likely to see the development of ocean-effect showers. Both wind and shower chances diminish as we go into the weekend.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Wind Advisory until midnight EST tonight for MAZ021>024. RI...None. MARINE...Gale Warning until 1 AM EST Wednesday for ANZ231>235-237-250- 254>256. Small Craft Advisory until 1 AM EST Wednesday for ANZ230-236- 251. && $$ SYNOPSIS...Frank/Sipprell NEAR TERM...Frank/Sipprell SHORT TERM...Frank LONG TERM...Sipprell AVIATION...Frank/Sipprell MARINE...Frank/Sipprell

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