Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 240722 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 322 AM EDT Thu May 24 2018 .SYNOPSIS...
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Quiet and dry though morning, some spotty lower conditions associated with low clouds and fog where it previously rain and along the low coast. Dry, warm and humid conditions then return through Saturday. A cold front approaches from northern New England Saturday afternoon, bringing scattered showers and thunderstorms. The front stalls south of the region Sunday, with cool east winds and patchy showers lingering through the remainder of the holiday weekend. High pressure moves in with drier weather for the midweek.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... 130 am update... Remaining quiet. Quick glance, back door cold front slowing, stalling against W flow, convergent zone setting up over E MA. Dry air advection occurring, however over E/SE MA, higher dew- point air combined with antecedent rains, can`t rule out spotty dense fog as temperatures cool beneath mostly clear conditions. Meanwhile, several features linger offshore as discerned via GOES-16 night-time microphysics. Potential for low cloud features to expand as they progress SW, impacting Nantucket early on and potential E MA prior to sunrise. Low clouds either pushed off by mean wind or eroding into morning with daytime heating. Today... Again, back door cold front having come ashore, stalled over E MA up against W flow. With diurnal heating, convergence zone becomes well-defined, maintained by sea-breeze influences, push back into the interior. Airmass differential, likely mixing, scattered cloud decks atop the boundary layer. Gusty E winds more likely per mesoscale isallobaric wind response, whereas gusty W winds per mix-down of faster low-level winds. In both cases, potential for gusts up around 20 mph. Considerably drier W with 30-degree dewpoints, relative humidity down to 20 percent. Highs warming into the mid to upper 70s, whereas cooler, more moist E with onshore flow. Plenty of sunshine. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight... Clear conditions, return light S flow, opportunity for radiational cooling with boundary layer decoupling early on before higher dewpoint air pushes N. Perhaps some low cloud and/or fog issues along the immediate S-shoreline of New England, otherwise leaning with coldest forecast guidance with lows upper 40s to low 50s. Friday... Mild day on tap with increasing humidity. High pressure SE, W/SW flow prevails. Warm air advection within low-mid levels, ridge builds. Up against N-stream cyclonic flow / Baffin Bay low, in the confluence of which low-level SW winds increase. Daytime heating, boundary layer mixing, mix-down of drier air, faster winds. Looking at highs around the mid to upper 80s, the warm spot over the Merrimack River Valley and Boston-metro where we`ll likely see some locations top out over 90. The deep mixing, can`t rule out SW gusts upwards of 30 mph. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
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Big Picture... Zonal flow with embedded shortwaves in place across Canada and the Northern tier of the USA. Upper flow also features closed lows over the Gulf of Mexico and the Western USA. The Western low opens and feeds into the zonal stream early next week. The Gulf low opens into the zonal flow the middle of next week. One shortwave embedded in the zonal flow will move across New England early next week. Upper contour fields are in general agreement through Monday, while the MSLP fields show differences in details starting Sunday. Thermal fields also start showing differences starting Sunday. Overall forecast confidence is moderate-high through Sunday, moderate for Monday-Tuesday. Agreement on a building ridge Wednesday suggests higher confidence, but it is a day 7 forecast which limits such confidence. Contour fields from Saturday onward remain above normal through the forecast period. Thermal fields are also above normal, although fall to near normal Wednesday. Low level flow turns from the east and northeast Saturday night, then remains that way for Sunday and much of Monday. This will affect the surface layer with cooler temperatures while the deeper layer remains above normal. Concerns... Friday night and Saturday... High pressure over the West Atlantic maintains dry weather over Southern New England Friday night and Saturday morning. Low pressure passing across Northern Maine combines with the high to create a pressure gradient favoring west-southwest winds in our area. This flow brings in higher moisture air, with precipitable water values around 1.3 inches...more than 1 std dev above normal but less than 2. Dew points should climb through the 50s into the lower 60s. Temps aloft in the mixed layer favor max sfc temps in the mid to upper 80s inland and cooler along the South Coast. As the Northern Maine low pressure moves off through the Martimes, it pushes a cold front south through our area during the late afternoon and early night. PW values continue to rise, reaching 1.5 to 1.7 inches which would be near 2 std dev. Stability parameters become more favorable for convection, with CAPE 500-1000 J/Kg and Totals around 50. Mid-level lapse rates are forecast at 6-6.5C/Km. Theta-e ridge is indicated from Southern NY through CT and Nrn RI. All of this supports showers and scattered t-storms with potential for local downpours. Most favored area would be CT and Nrn RI, but all areas would have a chance. Winds aloft ahead of the front are rather weak, so not favoring strong wind gusts at this time. The cold front moves south through our area Saturday night. Increasing east winds behind the front may lead to gusty winds especially over eastern Mass. Sunday-Wednesday... Cold front stalls south of New England, aligned with the zonal flow. Shortwave trough moves east through the upper flow, passing New England around Monday. This may generate a weak wave along the front that moves past us on Monday. Thus we expect continued unsettled conditions Sunday and Monday with a chance of showers each day. Another cold front pushes SE out of central Canada during Tuesday. This May generate a few showers before pushing the moisture offshore. High pressure builds over the region Wednesday, bringing dry weather.
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&& .AVIATION /07Z THURSDAY THROUGH MONDAY/...
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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 Friday/... Rest of tonight... Watching low cloud features move SW with NE flow. Concern over E MA of IFR CIGs (ACK 8-9z, E MA around 930z). E flow over E/SE MA whereas W flow elsewhere. Thursday... IFR CIGs may be an issue over E/SE MA coastal terminals early on but quickly eroding. VFR with interior W flow up against surging E sea-breeze flow. Gusts up around 20 kts for both sides of the wind shift. Will see sea-breezes push towards the interior, S flow prevailing late. Thursday night... Hold VFR but concern there could be lingering issues for S-coastal terminals of low clouds, possible fog. S/SW winds with gusts up around 20 kts for E/SE terminals towards Friday morning. Friday... VFR. S/SW winds with gusts upward of 30 kts. Possible IFR CIG conditions for S-coastal terminals early on. KBOS Terminal... E onshore flow persisting, turning S late Thursday. Some concern of lower CIG conditions overnight around 930z. KBDL Terminal... VFR. N winds much of today, gradually turning S late. Outlook /Friday Night through Monday/...Moderate confidence. Friday Night: VFR. Breezy. Saturday: Mainly VFR, with local MVFR possible in afternoon showers/scattered thunderstorms. Breezy. Saturday Night: Mainly MVFR, with local IFR possible. Chance SHRA, slight chance TSRA. Sunday: Mainly MVFR, with areas IFR possible. Breezy. Chance SHRA. Sunday Night: Mainly IFR, with areas MVFR possible. Chance SHRA, patchy BR. Memorial Day: Mainly MVFR, with areas IFR possible. Chance SHRA.
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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 Friday/...High confidence. Tonight and Thursday... Back door cold front sweeping across the waters as of 10p will keep flow mainly NE over the waters through late Thursday. Gusts up to 20 kts possible;. Thursday night... High pressure moving off the coast will result in SW winds developing. Enough of a low level jet may result in marginal small craft conditions with southwest wind gusts of 20 to 25 knots and marginal 3 to 5 foot seas across the outer-waters. Outlook /Friday Night through Monday/...Moderate confidence. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, slight chance of thunderstorms. Sunday: Winds less than 25 kt. Local rough seas. Chance of rain showers, slight chance of thunderstorms. Sunday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, slight chance of thunderstorms. Memorial Day: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for winds from 11 AM to 6 PM EDT Friday for ANZ230-231-251. Small Craft Advisory from 2 PM to 8 PM EDT Friday for ANZ235- 237-250. Small Craft Advisory from 8 AM to 8 PM EDT Friday for ANZ254>256.
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&& $$ SYNOPSIS...WTB/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell MARINE...WTB/Frank

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