Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 080257 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 957 PM EST Wed Dec 7 2016 .SYNOPSIS... Dry weather expected tonight but residual ground moisture may result in patchy black ice developing. A strong 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. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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Lower clouds breaking up a bit over land, but plenty of higher clouds to take their place. Low clouds remain prevalent across the coastal waters. Overall, still expecting mostly cloudy skies to persist overnight. Light winds are already established across southern New England. Dew point depressions are not large though. Even if clouds depart from an area for an extended time, am not thinking temperatures will drop too much. At most, looking at another 3-4 degree drop from the 10 PM readings. The other issue will be areas of fog. Most areas had visibility greater than 2 miles in fog. However, the Connecticut River valley region would be more likely to have visibility one half mile or less in fog. Will continue the Special Weather Statement which addresses both these areas of fog, and the possibility of black ice. Previous Discussion... An abundance of low clouds lingered across southern New England early this evening, but there were a few breaks developing. Winds will be quite light tonight, so while low clouds will gradually lift this has been a slower process than guidance has indicated. In addition, model cross sections indicate an abundance of mid level cloudiness that will work in from the west. So expect considerable cloudiness persisting overnight, but with some breaks at times. The other issue is that residual ground moisture/light winds and temps dropping below freezing across much of the region, may lead to both patchy fog/black ice. We have gone ahead and issued a special weather statement to highlight these concerns. Low temps will bottom out mainly in the upper 20s to the lower 30s.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... 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. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/...
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Highlights... * Unseasonably cold but mainly dry Fri into Sat * Period of light snow possible Sun into Sun Eve * 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 line, 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 the middle of next week. In fact, a period of arctic air appears likely some time in the mid to late next week time frame. Specific timing though is uncertain and some models hint at a wave of low pressure, slowing its delivery until late next week.
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&& .AVIATION /03Z 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 Thursday night. Tonight...Moderate confidence. Mainly VFR cigs across the region. However, light winds/wet ground will lead to patchy ground fog and localized MVFR, to perhaps even IFR conditions, in the typically prone locations. 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 TAF. Leftover MVFR CIGS should improve to VFR by 02 or 03z. KBDL TAF...Moderate confidence in TAF, but risk for some patchy ground fog developing tonight with light winds/residual ground moisture. 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... 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 Monday/... Friday and Friday night...High confidence. Strong cold advection will result in widespread northwest wind gusts of 25 to 35 knots. Opted to issue a Gale Watch across our eastern outer-waters, where the winds look to be the strongest. At the very least strong small craft winds are expected elsewhere, with even the risk of brief marginal gale force gusts. Saturday...High confidence. Despite weakening pressure gradient enough cold advection over the relatively mild ocean will allow northwest 20 to 30 knot wind gusts to continue. Small craft headlines will be needed. Sunday...High confidence. High pressure builds overhead and finally allows winds and seas to drop below small craft advisory thresholds. Sunday night and Monday...Low confidence. Winds and seas are quite uncertain and will depend on location/track and strength of a potential wave of low pressure. && .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...Belk/Frank SHORT TERM...Doody LONG TERM...Frank AVIATION...Belk/Frank/Doody MARINE...Frank/Doody

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