Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 190801 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 401 AM EDT Wed Apr 19 2017 .SYNOPSIS...
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A weak low pressure area will approach New England this evening. A weak area of high pressure moves across New England on Thursday. Favorable wet weather chances around AM Friday in tandem with an area of low pressure lifting across the interior. Dry and quiet for Saturday. Skirting rain from the S around Sunday. Perhaps dry yet cooler going into next week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Today should be dry across most of S New England. A SE to S onshore flow will keep some clouds across the area and hold temperatures generally in the 50s except upper 40s near the immediate coast. The south wind may be a bit gusty this afternoon, especially in the CT River Valley. Scattered showers will spread into the east slopes of the Berkshires after 5 PM and near the CT River Valley by 6 PM.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
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Tonight... Flat upper shortwave trough moves across New England tonight. Impressive low level jet of 50 KT across central New York State weakens to about 30 KT in southern New England. Associated surface low weakens as it approaches New England. Have gone with likely POPs but believe QPF values will be generally less than .25 inches due to the weakening dynamics of this system. Temperatures will not drop much tonight given the southerly wind and cloud cover. Thursday... With the exception of a few lingering showers possible along the south coast in the morning, anticipate a weak short wave ridge to keep most of the region dry. Trapped moisture under a mid level inversion will likely result in considerable cloudiness lingering across the area, especially during the morning. Temperature guidance is fairly bullish on warming most locations even with considerable cloud cover. This seems reasonable over the interior given the warmer air mass across the area on Thursday but think sea breezes along both coasts will hold temperatures down some there. Suspect Boston and other coastal locations may experience a late morning high and then cool during the afternoon in response to the sea breeze coming off a water with sea surface temperatures in the mid to upper 40s.
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&& .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
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*/ Highlights... - Rain likely around Friday AM - A lull Saturday - Rain possible for Sunday, better chances S - Seemingly dry but cooler for early next week */ Overview... Persistent H5 low around the Aleutians influencing the Pacific jet while steering individual mid-level disturbances into the W CONUS / W Canada that ultimately undergo lee cyclogenesis E of the Rockies. This as a H5 low also persists over N Canada. Mid- latitude features coupled with N-stream energy, the challenge becomes interpreting out- comes across the NE CONUS which have swung considerably within the long-term deterministic guidance as of late. Quite the headache. Preference towards ensemble means, however an air of scrutiny. The GEFS stands out as having a more pronounced phase 7/8 of the MJO forecast for late April into early May compared to all other guidance, suggesting variable temperature and precip anomalies over the W and Central CONUS, however a cooler and drier than normal pattern over the NE. Perhaps some weight based on 19.0z 180-240 hour deterministic EC/GFS guidance via a cold air damming signal and onshore flow. What may seem like an active, progressive pattern when evaluating upstream may not entirely be the case. Feel there are some external factors gumming up the works. Hit highlights and targets of opportunity in the discussion below. */ Discussion... Thursday night into Friday night... Rain likely around Friday AM. Better shot over N/W portions of S New England per convergent low-level forcing/ frontogenesis beneath both favorable dynamics, diffluence aloft, especially along S/W facing slopes of higher terrain. Precipitable waters exceeding 1-inch along with weak elevated instability ahead of a leading mid-level impulse, expect widespread rain with embedded heavier showers and perhaps a rumble of thunder, however not worthy of a mention in the forecast though the better chances will be across CT / RI. Overall, a good front-end thump of rain associated with the lifting warm-front and mesoscale low centers. Clearing out late beneath both the dry slot and right front quadrant of the upper level jet. Earlier thinking of afternoon convection ahead of a sweeping cold front overnight is no longer considered as the environment will likely be capped with low clouds per lingering low-level moisture beneath warmer, drier air aloft. Would expect low clouds to linger for a time negating any opportunity for the environment to destabilize. Saturday... Better of the two days making up the weekend. Behind the cold front a cooler and drier airmass follows. Should clear out to some degree however a fair amount of moisture lingers ahead of the elongated positively tilted H5 trough axis. Anticipating scattered to broken cloud decks, the latter more so N/W. Sunday... Potential rain clipping the S-tier of S New England, with higher chances across CT / RI / SE MA. Follow-up Pacific energy is sheared equatorward per upstream ridge enhancement per strengthening SW flow and warming around H7. This as N-stream energy associated with the persistent low over N Canada cyclonically digs S across SE Canada into the NE CONUS with much colder air. The challenge here becomes the juxtaposition and strength of such features with respect to S New England, whether beneath regions of favorable isentropic moist ascent while undergoing forcing beneath mid-level synoptics and the right rear quadrant of the upper level jet. Evaluating mid to upper level features, lapse rates, and available moisture, favoring a non- GFS (19.0z) solution with rainfall outcomes mainly focused over the S-tier of S New England with a sharp N-S gradient. Between the two streams there needs to be a region of prevailing subsidence and that appears to be the case SW to NE across central portions of the NE CONUS. Early next week... Low confidence forecast. Will stick with ensemble means while noting both deterministic guidance and long-term teleconnections. This puts the NE CONUS beneath anomalous ridging, however, given the amplitude and proximity, surface high pressure should center N/E lending to an onshore flow and cooler conditions. Will keep it overall dry for now with coldest conditions Tuesday behind the cold front and cooler airmass aloft sweeping the region on Monday. A moderating temperature trend thereafter.
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&& .AVIATION /08Z WEDNESDAY 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 Thursday/...Moderate confidence. Today... Anticipate VFR conditions across the area but scattered MVFR cigs will persist at least through the morning. Wind will be veering from the east to the south. May see southerly winds gusting between 20 and 25 knots, especially along the CT River Valley and over exposed high terrain. Tonight... Much of the area will probably remain VFR but there could be scattered areas of MVFR ceilings in the 2 to 3 thousand foot range. We only have moderate confidence, since the model guidance seems to be giving a mixed signal. Thursday... A SE to S onshore flow will likely result in considerable cloudiness with areas of MVFR ceilings possible at least in the morning. Scattered showers may approach the western portion of the area late in the day. KBOS TAF...Moderate confidence. Anticipate VFR conditions most of today except for possibly a period or two MVFR ceilings this morning. Enough rain expected tonight to help saturate the air column and result in the formation of MVFR ceilings. Believe clouds will slowly lift Thursday morning. Wind become south today. Sea breeze (easterly wind) looks likely on Thursday. KBDL TAF...High confidence. Southerly winds may gust 20 to 25 knots this afternoon. VFR should prevail today but then MVFR cigs and perhaps briefly MVFR vsbys in showers tonight. Outlook /Thursday night - Sunday night/...Moderate confidence. Thursday night through Friday night... MVFR/IFR mix with -RA mainly around AM Friday. Drying out late as low cigs linger overnight, breaking towards morning. E winds shift late out of the N. Saturday... SCT-BKN low-end VFR cigs. Lower risk of MVFR, mainly over the high terrain. N flow. Saturday night through Sunday night... MVFR/IFR mix with -RA. N winds becoming E shifting back N again late. Improvement towards Monday morning.
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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 Thursday/...High confidence. Anticipate frequent gusts around 25 KT this afternoon across Ipswich Bay, waters about Cape Ann, Stellwagen Bank, and at least portions of Mass Bay this afternoon and evening in response to a tightening pressure gradient and partial mixing from a low level jet at the top of the boundary layer. With less confidence, believe enough risk for seas to rise to near 5 feet across the outer SE coastal waters to warrant a Small Craft Advisory for Hazardous Seas being issued there for a time later this morning and tonight. Elsewhere, anticipate conditions to remain below Small Craft Advisory conditions but will need to keep an eye on seas over the outer coastal waters south of New England as they may approach 5 feet later today. Also, there is a possibility of wind gusts close to the Small Craft Advisory threshold in Cape Cod Bay. Suspect will not mix sufficiently for those gusts but certainly not out of the question. Outlook /Thursday night - Sunday night/...Moderate confidence. Thursday night through Friday night... Low pressure sweeps across the interior. With a lifting warm front will see E winds become more S into Friday with gusts up to 20 kts. This prior to a cold front overnight behind which winds shift N/W. Waves building up 5-6 feet on the E/SE outer waters. SMALL CRAFT ADVISORIES (SCAs) would be required. Rain at times, especially around Friday AM. Saturday... Brief lull with vicinity high pressure. However N winds remain brisk and thus will keep 5 foot seas on the E/SE outer waters requiring the continuation of SCAs. Saturday night through Sunday night... Low pressure sweeping S of the waters. E flow reverting back N late. Breezy though not considerably strong, seas lower slight however 5 foot seas maintain over farthest E/SE outer waters. Rain at times with a better chance over the S/SE waters.
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&& .FIRE WEATHER...
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Some gusty winds between 20 and 25 mph are possible this afternoon across the interior, but the relative humidities will probably bottom out between 40 and 45 percent, not as low as in recent days. Will need to keep an eye on trends but at this time don`t think a SPS is justified.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 2 PM this afternoon to 8 AM EDT Thursday for ANZ250. Small Craft Advisory from 2 PM this afternoon to 2 AM EDT Thursday for ANZ251. Small Craft Advisory for hazardous seas from 11 AM this morning to 6 AM EDT Thursday for ANZ254-255.
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&& $$ SYNOPSIS...Sipprell/Thompson NEAR TERM...Thompson SHORT TERM...Thompson LONG TERM...Sipprell AVIATION...Sipprell/Thompson MARINE...Sipprell/Thompson FIRE WEATHER...WFO BOX Staff

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