Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 272003 AAD AFDBOX Area Forecast Discussion...UPDATED National Weather Service Taunton MA 403 PM EDT Thu Jul 27 2017 .SYNOPSIS...
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A cold front moves into the region this evening and will be accompanied by scattered showers followed by a drying trend behind the front later tonight. The front weakens or washes out Friday giving way to mainly dry weather along with seasonably warm temperatures and humidity. Low pressure then tracks south of New England Saturday, likely bringing a windswept rain to the Cape and Islands but possibly dry weather for the remainder of the region. High pressure and dry weather builds southward into New England Sunday. Another coastal low may bring more wet weather and chilly onshore winds to the region early next week. By the middle of next week the ocean low moves farther out to sea with a drying and warming trend for New England.
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&& .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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4 PM update... Low amplitude mid level trough entering western NY per latest SPC mesoanalysis. Good moisture advection ahead of this feature with PWATs up to 1.6 inches across western CT/MA and eastern NY along with K indices in the mid 30s. This combined with some weak synoptic scale lift has been sufficient to generate scattered showers across the region this afternoon. This will continue into early this evening. Most of the shower activity will be light given the weak forcing and lack of instability (both SB & aloft). But given the abundant moisture can`t rule out isolated and brief downpour. Later this evening and especially overnight the short wave trough moves east of the region with its attending cold front sliding south and reaching the south coast by sunrise. Good drying through the column from top down as K indices fall from the mid 30s this evening to mid teens by 12z Fri. This will result in a drying trend from north to south tonight. However this mid level dry air will not reach the surface with dew pts remaining in the 60s regionwide overnight. So while showers will taper off fog will likely develop as the night progresses. Given the relatively high dew pt air it will be much milder tonight than previous nights with lows in the 60s.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
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4 pm Update... Friday... Behind departing short wave tonight mid level confluent flow develops across the area. This provides mid level subsidence over the region weakens or washes out the frontal boundary near the south coast. This mid level drying and associated subsidence provides a cap tomorrow and should yield a mainly dry day. Given dew pts will remain in the 60s there will be some SB instability present tomorrow afternoon along with some weak low level convergence from seabreezes. Thus can`t rule out an isolated low top shower during the afternoon. Given the mid level cap and limit instability (SB and aloft) decided not to include any thunder. So much of the day looks dry tomorrow along with seasonably warm temps in the upper 70s to low 80s and typical summer humidity with dew pts in the 60s. A combination of some mid level cloudiness along with diurnal clouds should allow for at least a few breaks of sunshine. Friday night... Other than an isolated low top shower the evening hours should be mainly dry as mid level cap and dry air aloft hold til about 03z or so. However thereafter closed mid level low near Pittsburgh moves east and backs the mid level flow along the eastern seaboard, advecting deeper moisture northward into southern New England, especially the south coast. This will result in the risk of overrunning rain /yeah, it really is late July/ to invade the south coast including Cape Cod and the Islands. The remainder of the region likely remains dry especially north of the MA Pike. Seasonably mild with lows in the 60s.
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&& .LONG TERM /SATURDAY THROUGH THURSDAY/... Highlights... * Risk continues for showers and a few thunderstorms possible Fri night and Sat, best chance S coast * Gusty E-NE winds possible along coastal areas, highest along S coast, Cape Cod and the islands Sat * Dry conditions return early next week, but timing uncertain Overview and model preferences... Per latest 00Z upper air obs, it`s somewhat difficult to believe that what is currently a very innocuous wave along the Canadian border with North Dakota/Minnesota will be one of the main components to the weekend forecast. This weak wave, deepening in response to upstream convection, looks to phase with a Hudson valley wave rotating into Quebec. What occurs after the phasing is where most significant disagreement begins. The N cutoff shifts E allowing the the remnant longwave trof along the E seaboard to meander. GFS has this longwave cutting off completely, and hence is the slowest to shift the pattern. ECMWF/NAM are more progressive with ensembles falling on either side. For this update, given GFS seems to be an outlier, will lean more on the more progressive ECMWF solution with this update. All-in-all with stalled front to the S, a slow frontal low pres associated with this wave will pivot, mainly SE of the 40/70 benchmark Sat, and slow as the system continues to dig. This brings about a prolonged period of NE flow off the Gulf of Maine and how long it last is based on the depth of the longwave trof. Using the ECMWF timing, will release it from the throes of this feature by early in the work week, but this remains somewhat uncertain. Beyond this timing, a period of ridging is likely, as the longwave trof weakens and the pattern reloads. Details... Fri night into Sun... Convection firing along a stalled warm front draped from the mid Atlantic, although there remains the uncertainty of convective feedback, will assist in generating a low pres wave which will traverse the front slowly late Fri night into Sat. This allows for the development of NE flow, drawing air across the Gulf of Maine where SSTs rest between the upper 50s and low 60s. The combination of cooling flow and modest increase in moisture should lead to increased cloud cover through the period, culminating in temps remaining below normal. PWATs, closely associated with the low pres hover near 2.00 inches and ensemble probabilities keep the core of this higher moisture mainly offshore. Combine this with the influence of high pres from the NW under anticyclonic flow, and expect a very tight gradient in QPF and therefore, POPs as well. Will be tightening the POP/QPF gradient somewhat with this update based on the ECMWF/NAM blending suggested above. However, along S coastal areas, some heavy rain is possible given the higher PWATs and conditionally unstable soundings. Low risk for 1.00+ inches of rain, but very uncertain on this axis as it could remain entirely offshore. Overall, a period of potentially gray/cool/damp weather, with the highest risk for heavy rain along the S coast. Temps as much as 5-10 degree below normal except for where there is a chance for breaks of sun, mainly NW MA. Raw 2m more supportive of this than MAV/MET data. Otherwise, risk for some gusty NE winds as LLJ is nearly 5 std deviations above normal, out of the E. Coastal areas show 40-50 kt E-NE LLJ. Mon and Tue... Gradual improvement as the longwave trof slides slowly E assuming no cutoff forms. The increased anticyclonic curvature and induction of drier air in the form of subsidence should allow for more breaks in clouds and a much lower risk in precipitation as the higher theta-e ridge/PWATs and strongest LLJ shifts further offshore. Filling low pres wave noted as well. Moderate NW flow as well, suggest a slight moderation in temps at least closer to (but still below) normal as the shallow cold air should give way to better mixing, reaching H85 where average temps run about +12C yielding low-mid 80s, however, given the uncertainty, this will be dependent on how quickly the low pres clears the SE waters. Wed into late week... As the longwave trof gives way to resurgence of upstream ridging, more zonally oriented flow should allow for a return of drier with temps near to potentially above normal given current timing. Still uncertain as front begins to parallel the mid- upper lvl flow, and this setup is strongly dependent on the timing of the phased wave system this weekend. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 18Z Update... Through 00z...high forecast confidence. VFR conditions will trend to MVFR as the day progresses. Scattered showers will continue to impact the region. After 00z...high confidence on trends but lower on exact timing. Showers and isolated T-storm will shift southward with a drying trend developing from north to south as the night progresses. IFR- MVFR this evening in showers and fog will trend toward VFR late especially away from the south coast. Friday...moderate confidence. Any morning IFR-MVFR along the south coast should lift to VFR by midday. Then marginal MVFR-VFR develops in the afternoon regionwide with isolated shower possible especially near the south coast. Friday night...moderate confidence. Marginal VFR-MVFR in isolated shower threat during the evening will trend toward IFR late especially south coast as risk of rain increases. KBOS Terminal...high confidence on trends but some uncertainty on exact timing of lowering cigs this evening then timing improvement overnight. KBDL Terminal...high confidence on trends but lower confidence on exact timing of changing categories. Outlook /Saturday through Tuesday/... Sat...Moderate confidence. Area of rain/fog and low clouds possible mainly close to the S coast. This could be coincident with periods of MVFR/IFR conditions. Meanwhile areas across NW MA and extreme W CT could stay mainly VFR. Otherwise, winds gusting 25-30 kt possible, especially across SE MA/Cape Islands and RI out of the NE. Sun and Mon...Moderate confidence. Some improvement possible as rain/fog and lower CIGS shift gradually E of the region, however exact timing uncertain. This improvement would see more widespread VFR conditions develop where not already observed. Otherwise, winds still gust 20-25 kt mainly across SE MA/RI.
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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 Night/...High confidence. 4 pm update... Tonight... Light SSW winds this evening becoming west toward daybreak. Showers and patchy fog will reduce visibility late this evening and overnight. Tranquil seas continue. Friday... Light SE winds and mainly dry weather. Vsby may be reduced in morning fog but improving by midday. Tranquil seas continue. Friday night... Winds become NE and increase to 20-25 kt by daybreak Sat. Seas increase rapidly given the long fetch. Chance of rain increases especially southern waters. Outlook /Saturday through Tuesday/... Sat into Sun...Moderate confidence. Low pres will slowly pass south of the waters, bringing a period of rain/fog and potential for occasional thunderstorms. NE winds, could gust to low end gales mainly Sat and Sat night with seas 7-9 ft offshore. Gale warnings may ultimately be needed, or at the very least, high end small craft advisories. Mon...Moderate confidence. N-NE winds continue, but gusts should generally stay around 25 kt with seas still 5-8 ft. Small craft advisories are likely to continue even as rainfall/fog improves slowly.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Gale Watch from Saturday morning through late Saturday night for ANZ231>235-237-254>256.
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&& $$ SYNOPSIS...Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...Doody AVIATION...Nocera/Doody MARINE...Nocera/Doody

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