Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 112318 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 718 PM EDT Fri Aug 11 2017 .SYNOPSIS...
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Low pressure from the Great Lakes moves E/NE tonight and Saturday. This will draw a warm front up across S New England by Saturday morning, then swing a cold front through Saturday night. With the remainder of the weekend dry, some shower chances emerge through the week, mainly Tuesday, but nothing of a wash out. Overall seasonable conditions with low humidity. However, long- range, may see warmer summertime conditions return.
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&& .NEAR TERM /THROUGH TONIGHT/...
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7 pm update... Turning wet around midnight. Fair amount of theta-E moisture associated with a mid-level vortmax pivoting round the broader troughing regime across the region. Precipitable waters rising especially along the S-coast with values up to 2-inches, some early indications simply given the more soupy airmass pushing into the region with lower clouds and reduced visibility. Have made some modifications to the forecast. A marginal setup given a strong influx via SW flow of higher theta-E within a conditionally unstable airmass. Mainly focused on the rain which has the potential of being heavy, the greater risk of which is along the S-coast in lock step with a H925-85 theta-E gradient along which low-level winds are to a degree convergently focused (though the stronger convergence, if the RAP is correct, remains mainly over the waters). The possibility of a rumble of thunder, mainly leaning higher categorical of rain with lesser threat of thunder. Prevailed with "heavy rain" wording in the forecast. The S offshore Islands at greatest risk, especially Nantucket, of heavy rain. The over-running setup, went with LIKELY to DEFINITE PoPs. A word of caution. A lot of energy and subsequent area of low pressure streaming into the interior NE CONUS parent with the mid to upper level low out of the Great Lakes Region. Though the lower heights are also pulling N the sub-tropical warm front just S/E offshore. Two neighboring areas of low pressure, as we`ve seen before in this situation before someone is going to see very little if anything subsequent of a subsidence wedge. Looking at a region SW to NE across S New England. Places such as Boston up the NH/ME coastline may only see wet-weather associated with the main vortmax sweeping through around midnight into the early morning hours on Saturday then very little after that. Something to monitor as we go through the next 24 hours. Mild night, soupy along the S-shoreline with a greater propensity of low clouds and fog. Lows in the 60s.
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&& .SHORT TERM /SATURDAY INTO SATURDAY NIGHT/... Saturday morning... The first shortwave moves past New England during the morning. This may be enough to move the warm front through our area and bring us a more humid airmass. Expect showers to continue during the morning, and then most will taper off as the warm front moves past. Saturday afternoon and night... The second shortwave approaches from the Midwest Saturday evening and crosses New England toward Sunday morning. The cold front associated with this shortwave will cross NY and PA during the afternoon and then cross New England Saturday night. Difficult to determine the presence of a pre-frontal surface trough ahead of the front, one that could be a focus for convection to fire. Model theta-e fields do show a theta-e ridge crossing the Northeast Corridor Saturday afternoon/evening. After a short break early Saturday afternoon, expect a second round of showers mid to late afternoon. Totals are forecast at 47-51 after 18Z, with lifted index sub-zero. So along with the showers, we should also have a few thunderstorms. Winds aloft are rather light for generating any damaging gusts, although 500 mb winds do increase to 50 knots around midnight. This is a little late for us regarding any severe weather. So at this time we are not thinking of any severe storms in our area. Total rainfall amounts through Saturday night should generally range between one-half and one inch, with the greatest amounts toward the south coast, including the Cape and islands. Once the cold front moves through, expect winds to shift out of the west and showers/storms will move off. && .LONG TERM /SUNDAY THROUGH FRIDAY/... */ Highlights... - Overall dry pattern, some shower chances, but nothing drenching - Seasonable conditions with low humidity overall - Long-range pattern change, could see warmer temperatures return */ Overview... A downstream cascade of outcomes. Within the mid- to upper-levels, progressive deep troughing over N-Central Asia acts to cut off an area of low pressure near the Kamchatka Peninsula. Both continuing E subsequently flip the dipole over the N Pacific with lower pressure building into the Aleutians and high pressure developing over the Gulf of Alaska. The subsequent breakdown seemingly has implications downstream with flow becoming more zonal across the CONUS with less amplification of preferred troughing over our region. Subsequently there are indications of a slight warm-up across the region at times interrupted by continued weak Pacific-origin impulses and chances of wet-weather. Overall though it would appear that the sub-tropical airmass and more sultry air is buffered offshore, thus conditions do look to remain comfortable. Will hit on specifics below. */ Details... Sunday into Monday... Lingering showers early Sunday, turning dry. Sunday morning cool frontal push along with a mid-level trough axis, much drier air builds into the region. Increasing confluence of mid to upper level flow aloft, surface high pressure builds across the region Monday, light winds contributing to onshore sea-breezes. Scattered to broken cloud decks associated with some mid-level energy through the confluent flow. Overall a dry period of weather with near-seasonable temperatures and low humidity. Tuesday into Wednesday... Possibly our next best chance of wet-weather. Confluent mid to upper level flow becoming lax, deeper troughing and attendant vortmax energy sweeping the region. Model forecast guidance mixed as to whether the offshore sub-tropical quasi-stationary boundary lifts back to the N or rather shower / thunderstorm activity developing over the N/W interior of the NE CONUS impacts our region. Could be a situation where S New England is wedged-out within subsidence being neighbored by two areas of lower pressure. Will keep the mention of chance PoPs on Tuesday given better moisture availability, also lean warmest conditions for the week. Wednesday looks to remain dry with the majority of forcing/lifting mechanisms swept through the region behind which cooler, drier air builds in. Thursday into Friday... Dry, near-seasonable, slight warm-up by day. High pressure building across the region yet again. Cooler airmass in place on Thursday which is quickly replaced Friday by warmer, S flow. Both days the winds have the potential of being light to allow onshore sea-breeze winds. Increasing clouds late on Friday with the chance of showers overnight ahead of a warm-frontal boundary. Weekend... Potentially wet on Saturday, dry Sunday. A warm front looks to lift through the region, hanging along the coast up against the cooler waters. S flow increasing as the mid to upper level pattern exhibits less troughing, looking at a possible slow build up of heat and humidity with the chances of showers and thunderstorms into early next week. Way out there, very early to say, so a low confidence forecast. But if the mid to upper level pattern is changing as was addressed in the overview, the pattern we`ve seen as of late of continued N flow of cooler and drier air may become relaxed allowing warmer Summer conditions to return as hinted by the Climate Prediction Center`s 6-10/8-14 day outlooks. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
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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 Saturday Night/... Tonight...Moderate confidence. VFR lowering towards MVFR into morning for most terminals. S-coastal terminals in a IFR-LIFR soup if not already. Light S winds. -RA moving in closer to 5-6z spreading SW to NE, could be RA/+RA along the S-coast by morning. Saturday...Moderate confidence. MVFR lifting back to VFR across the interior, but the IFR-LIFR soup continues to plague S-coastal terminals. ACK at the highest risk of remaining in low clouds all day. Increasing S winds. -RA and possible TSRA encroaching from the S/W late, potentially impacting the W terminals, sliding along the S-coast into evening. From roughly BDL-ORH and terminals N/E up through BOS may have no issues at all. Saturday night... VFR lowering back down to MVFR briefly across the interior before winds shift back out of the W. IFR-LIFR continues to plague S-coastal terminals, clearing out towards Sunday morning with the wind shift. KBOS Terminal... Will keep with VFR lowering to MVFR shortly after midnight with -RA. Feel the terminal may not experience much in the way of issues through the TAF period over the next 30 hours. KBDL Terminal... VFR lowering to MVFR with likely rain around midnight. There is the possibility of RA/+RA though feel the better risk will hold to the S of the terminal overnight. Though no mention, there is a risk of TSRA late Saturday from the W/SW. Outlook /Sunday through Wednesday/...Moderate confidence. Sunday... VFR. Lingering SHRA across S/E coastal terminals, otherwise dry. N/W winds breezy during the day, lax overnight. Monday... VFR. Light winds. Sea-breezes possible. SCT-BKN mid-high CIGs. Tuesday... VFR. S winds, breezy. Possible SHRA/TSRA, better chances N/W. Wednesday... VFR. Winds shifting N/W, initially breezy, becoming light. Sea- breezes possible.
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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 Saturday Night/...High confidence. A Small Craft advisory may be needed on the southern outer waters Saturday night. Southerly winds tonight and Saturday shift from the west later Saturday night as a cold front moves across the waters. Winds remain less than 20 knots through Saturday night. The sustained south flow prior to that may build 5 foot seas on the southern outer water by Saturday night. Showers and areas of fog will bring reduced visibility through much of Saturday night. Outlook /Sunday through Wednesday/...Moderate confidence. Showers clearing out over the waters early Sunday making for a nice weather the remainder of the day, though breezy out of the N/W. For the small boaters especially, can`t rule out gusts up to 20 kts. Will see winds diminish into Monday with high pressure. Monitoring for possible encroaching rain from the S on Tuesday with perhaps some breezy SW flow, otherwise Wednesday looks to be good boating weather with N/W winds behind a Tuesday night cool frontal passage.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Sipprell NEAR TERM...WTB/Sipprell SHORT TERM...WTB LONG TERM...Sipprell AVIATION...WTB/Sipprell MARINE...WTB/Sipprell

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