Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 261951 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 351 PM EDT Wed Jul 26 2017 .SYNOPSIS...
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High pressure remains over the region tonight providing dry cool weather. The high moves offshore Thursday giving way to slightly more humid conditions along with the risk for a few showers and an isolated thunderstorm, but by no means a washout. A drying trend likely develops Thu night into Fri as a cold front moves through the region and then stalls near the south coast Fri. A coastal storm will emerge off the Mid Atlc coast Friday night and is expected to track south of New England Saturday possibly bringing a chilly windswept rain to portions of the region, especially south coast, Cape Cod and islands. Dry and seasonable conditions return later Sunday into early next week.
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&& .NEAR TERM /UNTIL 8 PM THIS EVENING/...
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4 PM Update... 1020 mb high over the region late this afternoon providing splendid weather with temps mainly 75-80 along with comfortable humidity. Stable/cool marine airmass has come onshore via seabreeze circulation and has resulted in a lack of diurnal clouds along the immediate coast per latest GOES-16 Imagery. All of these attributes will set the stage for very pleasant weather this evening. The only issue will be with dry airmass /dew pts in the 50s - lowest inland/ in place coupled with light winds, temps will fall quickly with sunset. So expect cool conditions with sunset. Otherwise tranquil weather prevails. Later tonight mid and high clouds will overspread the region from NW to SE and this will stabilize temps from falling much more. However given the ideal radiational cooling conditions the first half of the night sided with the cooler MOS temps for mins. Thus another cool night with lows in the 50s except around 60 in the urban areas.
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&& .SHORT TERM /8 PM THIS EVENING THROUGH THURSDAY NIGHT/...
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4 PM Update... Thursday... Increasing clouds ahead of next approaching short wave. Weak low level WAA will result in surface dew pts climbing into the low and mid 60s, so it will feel more humid than today. Airmass is slightly warmer than today but with increasing clouds highs tomorrow will be close to values of today, 75-80. It will be slightly warmer along the eastern MA coast/beaches than today given SSW winds tomorrow. Overall a typical summer day, perhaps a few degs below average. As for the risk of showers/T-storms, approaching short wave provides modest mid level moisture advection (K index low 30s) along with weak forcing/height falls for ascent. This should be sufficient for isolated to widely scattered showers. However T-storms especially strong to severe appears very low as NW flow precludes true warm sector airmass from advecting northeast into southern New England. This is evident on all model guidance offering less than 500 j/kg of MUcape. This combined with weak mid level lapse rates (H5 temps only cool from -9C to -10C) will result in weak updrafts and stronger storms likely remaining in the warm sector across NYC/NJ/eastern PA. After collaboration with SPC the slight risk was removed from southwest CT for Thu. So isolated to widely scattered showers and T-storms Thu afternoon, however many hours of dry weather too, so by no means a wash out. Thu night... Good drying thru the column per decreasing K index values as short wave trough moves east of the area and mid level anticyclonic flow and associated subsidence take over. This will result in a drying trend from north to south. So any evening showers/T-storms will shift south and then offshore. However front will not reach the south coast until Fri morning. So expect warm and humid conditions to persist much of the night. This moist low level airmass will increase the risk of patchy fog as well.
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&& .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Showers and occasional thunderstorms Thu evening especially * Risk for late Fri - Sat being cool/rainy, but this may be suppressed to the south * Dry and seasonable weather likely Sunday into Monday Overview and model preferences... While 26.12Z model guidance is beginning to settle toward a solution for the late week strong shortwave and gradual longwave trof production it is somewhat different from previous runs, particularly with the placement of the frontal boundary and the development of a frontal wave. Note that while there does seem to be a trend occurring here, there remain significant differences in the development of low pres along the stalled boundary mainly in the Fri-Sat time frame. So for now, will begin to shift toward a more suppressed solution Thu night into Fri, and a more unsettled/wet solution for late Fri into the early weekend. This does not favor any particular model or ensemble and gives room for better model convergence down the road. At odds appears to be the development of a weak wave associated with convection firing in the Canadian prairies and Great Lakes region late tonight into tomorrow. In spite of this initial uncertainty, there does continue to be reasonably good agreement thereafter that as the longwave trof and cutoff associated with the initial wave slide E, drier air and some ridging will bring about a quieter stretch to follow early next week. Details... Thu night... Ongoing convective risk thanks as shortwave and attendant weak frontal boundary slide across the region late Thu into early Fri night. Could see this activity actually continue a bit later than typical late afternoon/evening timing thanks in part to a slight increase in mid lvl lapse rates above 6.0C/km (keeping MU CAPE values elevated near 1000j/kg) and modest shear at about 30-35 kt. Therefore, can`t rule out a risk for SHRA/isolated TSRA most of the overnight hours, although given the typical diurnal minimum, risk drops especially during the early morning hours. Gradual POP reduction. Mins remain in the mid 60s thanks to a rise in dwpts through the day on Thu in response to deepening moisture. Fri into Sat... As mentioned above, there remains enough uncertainty here that anything described below is subject to change as we approach as there will be a tricky interaction between a dry high pres attempting to nose in from the NW and the front as it stalls from near the mid-Atlantic toward Nantucket. By daytime Fri, a second diurnal round of convection can be expected, which may remain mostly S of New England given the current mean for frontal placement. However, this second round of convection will also be the focus for a low pres wave development which would then traverse the front late Fri night into Sat. Noting definite convective feedback issues here, which will need to be refined to better tell how close of a passage this low pres will take late Fri night into Sat, which will have major implications for the axis of rain/clouds. The northern solution of this pattern though, with low pres passing near the 40/70 benchmark would be another round of damp/cool conditions beginning late Fri and continuing into much of the day on Sat. Brisk E flow given negative-u component of the LLJ is nearly 2-3 std deviations above normal would bring in cool marine air off of the Gulf of Maine. A secondary spike in PWATS to near 2.00 inches along with cool temps aloft could bring about another period where mod- heavy rain is possible. All-in-all, not a good solution for Saturday should the front remain close enough to New England. Temperatures Fri still near to below normal as some breaks of sunshine are possible (highs mainly in the upper 70s). Sat would depend on whether the wave impacts S New England or remains well S. If impacts are felt, temps will struggle to reach into the mid 60s, warmer if the latter occurs. Sun through Tue... One good result of the stronger solution implied most recently by operational guidance and some ensembles, is that it is the catalyst in shifting the longwave trof axis and cutoff in the Maritimes somewhat E. This allows drier air, and more anticyclonic motion into New England, yielding a 1020+ high pres. Therefore, will continue to trend toward a drier pattern for the late weekend, early next-week timeframe. In spite of the longwave trof, mid lvl temps will moderate such that temps could be close to seasonal normals. Mid week... Secondary shortwave approaches, finally phasing the mean jet with the longwave trof hanging to the S. && .AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 18z/2pm update... Thru 00z...VFR with any bkn cigs 4-5kft. Light S-SE winds continue. After 00z...VFR with winds becoming light and variable, then SSW toward morning. Thursday...VFR with bkn040-050 cigs in the afternoon along with isolated to widely scattered T-storms. Thursday night...VFR but trending toward MVFR especially across CT/RI and southeast MA including Cape Cod and the Islands where patchy fog will form. Scattered showers and isolated T-storm possible in the evening hours. KBOS Terminal...High confidence. Isolated to widely scattered showers/T-storms Thu afternoon/early evening. Strongest storms will likely remain well southwest of the region across NYC/NJ/PA area. KBDL Terminal...High confidence. Isolated to widely scattered showers/T-storms Thu afternoon/early evening. Strongest storms will likely remain well southwest of the region across NYC/NJ/PA area. Outlook /Friday through Monday/... Fri...High confidence. A few showers and thundershowers, mainly Thu evening. Then mainly near the S coast on Fri. Otherwise, predominantly VFR outside of some localized ground fog. Fri night into Sat night...Moderate confidence. Potential for a mix of IFR/MVFR in E-NE flow which could gust 25-35 kt mainly near the E coast on Sat. Rain, especially along S coast may be heavy at times, but all of this is contingent on how close low pres passes S of New England, which is somewhat uncertain. Sun...High confidence. Improvement, mainly VFR.
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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. 4 PM Update... Short Term /through Thursday night/...High confidence on wind and waves but lower confidence on probability of showers/T-storms. Tonight...light onshore winds continue. Small east swell of 2-4 ft continues across the ocean waters. Otherwise tranquil waves near shore. Dry weather and good vsby prevail. Thursday...SSW winds 10-15 kt possibly increasing to 20 kt in gusts late in the day. Isolated shower/T-storm possible. Good vsby continues. Thursday night...SSW winds 10-15 kt shifting to WNW toward daybreak Friday. Isolated shower/T-storm however fog may become fairly widespread but improving toward morning across the northern waters given the wind shift from SSW to WNW. Outlook /Friday through Monday/... Thu night into Fri...High confidence. Mainly quiet boating weather, SW winds will shift from SW to to the E-NE on Fri, but gusts should remain below 25 kt until Fri night. Seas generally at or below 4 ft. Fri night into Sat night...Moderate confidence. Strengthening low pres will pass S of New England. How close it ultimately passes remains somewhat uncertain and will determine final outcomes. However, NE winds could gust close to gale force Sat, but at least high end small craft thresholds. Seas also could reach as high as 7-9 ft on the open ocean waters. Still some time to sort this out. Otherwise, rain/fog mainly across S waters. Sun...High confidence. Improvement expected, remnant small craft advisory conditions gradually give way to quiet boating weathers late.
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&& .EQUIPMENT... KBOX radar is currently down. Technicians are assessing the problem and it is uncertain when it will return to service. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...Doody AVIATION...Nocera/Doody MARINE...Nocera/Doody EQUIPMENT...BELK

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