Area Forecast Discussion
Issued by NWS Boston, MA

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892 FXUS61 KBOX 072031 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 331 PM EST Wed Dec 7 2016 .SYNOPSIS...
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Drier but cloudy weather continues into the overnight. An arctic front crosses the region late tomorrow. Mainly dry but unseasonably cold temperatures are on tap for the region Friday into Saturday. Low pressure will likely bring at least some rain/snow and/or even ice sometime Sunday into Monday. A shot of arctic air is then expected by mid to late next week.
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&& .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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Although strong ridging linked to high pres well to the SW continues to build across the region per latest mass fields, the resulting subsidence inversion began drying from H5 downward, trapping much of the moisture leftover from the weak inverted trof from this morning. This trapped moisture, although daytime mixing has ended precip/fog for the most part continues to appear in the form of low clouds streaming out of the N/NW. Temps have under- achieved once again. This trend will continue through the evening hours. The trapped moisture will take more time to erode given upstream clouds are still apparent per latest satellite imagery. Noting trapped moisture between H9 and H7 that may take until near midnight to begin to give way to lowering dwpts. Therefore, will maintain this cloud cover for much of the overnight hours. This will keep min temps from dropping too much also, given H92 temps only fall between 0C and -2C and the cloud cover remains. Looking for mins in the upper 20s and low 30s (mid 30s in the urban areas).
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
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Tomorrow... Unsettled conditions as arctic front approaches from the NW, arriving by evening across the N tier. W flow will allow for some influence of Great Lake enhanced moisture into the region. This combine with continued cold advection through the day could allow for a few isolates flurries to impact the region, mainly out of a mid deck of virga as the dwpt depressions continue to increase. Accums will likely be little to none, as even the BTV squall parameter is not too high given the overall lack of moisture through the lower column, and relatively weak W winds until the FROPA. Highs tomorrow once again remain mostly in the upper 30s and low 40s despite a few breaks of sun, mid lvl temps will be cooling with the approaching arctic front. The primary caveat could be the E slopes of the Berkshires, although shadowed under W flow, the enhanced upslope on the windward side of the Hudson valley could work with enough of this moisture to promote light, actual snowfall of around an inch or less. As one descends into the CT valley, this threat will drop quickly. Tomorrow night... Arctic fropa is mainly dry outside of a few isolated flurries or light snow continuing in the E slopes of the Berkshires as mentioned above. Sfc dwpt depressions continue to increase and H92 temps drop as low as -6C by around 12Z on Fri. Therefore, advective cold (rather than radiative given the increase pres gradient following the frontal passage) should allow more widespread 20s across the region, even with continued SCT-BKN cold advection cloudiness.
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&& .LONG TERM /FRIDAY THROUGH TUESDAY/...
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Highlights... * Unseasonably cold but mainly dry Fri into Sat * Period of light snow possible Sun * Uncertain late Sun night/Mon whether a secondary low brings more significant rain/ice/snow or misses us to the south * Shot of arctic air likely mid to late next week Details... Friday... Colder air will continue to work into the region on Friday accompanied by northwest wind gusts of 25 to 35 mph. Given low sun angle and 850T dropping to around -14c by late afternoon, expect high temps to only reach the 30s. Model cross sections indicate a fair amount of strato-cumulus clouds. While dry weather will dominate the vast majority of the day, a brief flurry/snow shower or two is certainly possible particularly along the east slopes of the Berkshires. Friday night and Saturday... Still can not rule out a spot flurry/snow shower or two Friday evening. Otherwise, expect low temps to bottom out in the teens to around 20 in many locations. Winds will not decouple which will allow wind chill values to drop into the single digits. Despite partial sunshine, high temps will only recover into the upper 20s and lower 30s. Saturday night... Large high pressure settles across southern New England. This will result in mainly clear skies/light winds and an ideal night of radiational cooling. Low temps should drop well down into the teens for most locations, with some single digits likely in the normally coldest outlying locations. Sunday into Sunday evening... High pressure moves off the coast as a weak wave of low pressure moves up towards the Great Lakes. Enough of a pressure gradient may allow the development of a southwest low level jet of 30 to 40 knots. This will over ride the cold dome at the surface and may allow light snow to break out from west to east Sunday into Sunday evening. If this occurs, antecedent airmass is quite cold so mainly light snow would be expected near and northwest of the Boston to Providence corridor. Southeast of that area, ptype probably should start as light snow but warming boundary layer should allow for a change to light rain. One interesting thing to note, 850T are still below -10C at along the south coast at 18z Sunday with southerly winds developing. Ocean SST between 50 and 55 may even allow scattered ocean effect snow showers to develop along the south coast, before any change over to light rain. Overall, there really is not any organized system Sunday into Sunday evening. We are mainly just looking at southwest low level jet, over riding the cold dome at the surface. So while snow accumulations are certainly possible, amounts look to be on the light side at this time. Late Sunday night and Monday... Forecast confidence is quite low at this time. A stronger wave or two of low pressure may develop and threaten the area with more significant precipitation. Model guidance and ensembles are all over the place on the location/track and potential strength of any low pressure system. There are 3 main scenarios on the table right now. The first solution would be a stronger inside runner which would bring mainly rain to the coastal plain and a mixture of rain/ice/snow to the interior. The second would be a weaker wave passing to our southeast, bringing mainly accumulating snow from the I-95 corridor points northwest. The third scenario would have the wave sliding so far southeast that we end up mainly dry late Sunday night and Monday. All options remain on the table at this point, but certainly is the potential for some wintry weather Sunday into Monday. Tuesday and Wednesday... Specific timing remains uncertain, but cross polar flow will result in a shot of very cold air by mid week. In fact, a period of arctic air appears likely some time in the mid to late next week time frame. Specific timing through is uncertain and some models hint at a wave of low pressure, slowing its delivery.
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&& .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. Short term...through Thursday night. Remainder of today...High confidence in trends, lower confidence in timing. Mix of IFR/MVFR conditions will give way to lower end VFR conditions through 20Z - 00Z this evening. This will come as a result of fog lifting and CIGS increasing in height before some breaks are observed. Timing in TAF may be off by a bit. Otherwise, winds gradually shift around to the W, but remain relatively weak. Overnight...High confidence. Mainly VFR once the last of the low clouds lift out. Winds out of the W, but may increase during the early morning hours to around 10 kt after being light most of the overnight. Tomorrow and tomorrow night...High confidence. Mainly VFR. Some MVFR possible in higher terrain due to lower CIGS and even a low risk for light snowfall late in the day. W winds increase, with gusts 15-20 kt at times by late afternoon, continuing into the overnight. Some gusts +20kt along the immediate coast. KBOS TAF...High confidence in overall trends. Lower confidence in exact timing. Expect IFR CIGS/MVFR vsbys to linger through 20Z then slowly attempt to lift out to low end VFR later this afternoon and this evening as NW flow becomes predominantly W. KBDL TAF...Moderate confidence in TAF, mainly due to timing issues. Outlook /Friday through Monday/... Friday...High confidence. Mainly VFR, but brief marginal MVFR CIGS possible at times mainly across the interior. Northwest wind gusts of 20 to 30 knots during the daylight hours. Friday night and Saturday...High confidence in VFR conditions. Sunday and Monday...Low to moderate confidence. A period or extended period of MVFR-IFR conditions are likely later Sunday and possibly into Monday in some rain/snow and/or ice.
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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 Night/... Through tomorrow morning...High confidence. Mainly quiet boating weather under light W flow. Only caveat is the outer waters E of Long Island, where lingering swell from low pres passage hovers near 5 ft through the overnight hours. Small craft advisory will be issued for this risk. Tomorrow afternoon and overnight...High confidence. With cold frontal passage late tomorrow, expect an increase in W-NW winds late in the day and continuing into the overnight. 25-30 kt is likely at times. Seas gradually build as well reaching about 6 ft by early Fri AM on the outer waters. Small craft advisories will be issued for this threat. Outlook /Friday through Sunday/... Thursday night and Friday...High confidence. Small craft advisories are likely with winds gusting over 25 knots and seas on the outer waters 5 to 6 feet. There is a low probability of NW gales. Saturday...Moderate confidence. Winds and seas slowly diminish, but generally remain above SCA criteria until late Saturday night. Sunday...Moderate confidence. Seas below 5 feet will slowly increase once again from the south as low pressure approaches the waters. Winds increase as well, with small craft advisories likely. There is a low probability of S-SW gales.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 7 PM Thursday to 9 AM EST Friday for ANZ230>234-236-251. Small Craft Advisory from 3 PM Thursday to 9 AM EST Friday for ANZ235-237. Gale Watch from Friday morning through late Friday night for ANZ250-251-254. Small Craft Advisory from 1 PM Thursday to 9 AM EST Friday for ANZ250-254-255. Small Craft Advisory until 9 AM EST Friday for ANZ256.
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&& $$ SYNOPSIS...Frank/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...Frank AVIATION...Frank/Doody MARINE...Frank/Doody

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