Area Forecast Discussion
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000 FXUS61 KBOX 180825 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 425 AM EDT Sat Mar 18 2017 .SYNOPSIS...
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High pressure over Maine will provide dry but cooler weather today. A clipper low then tracks S of New England, bringing the potential for accumulating snow Saturday night into Sunday. The highest risk is across RI and Southeast MA, especially Cape Cod and the Islands. Dry and seasonable weather returns Monday, as high pressure builds into our region. Milder weather Tuesday ahead of an arctic front. Then a frigid airmass with near record cold overspreads the area during the middle of next week. Dry and not as cold the end of next week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Thinking the day will be mostly quiet and dry, just the chance of rain / snow encroaching from the S/W late. Increasing clouds lowering and thickening through the day with highs getting into the upper 30s to low 40s with strengthening easterlies.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
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*/ Highlights... - Continued spread in latest forecast guidance, limited confidence - Winter Storm Watch continues for the Cape and Marthas Vineyard - Potential for a 4-6 inch heavy, wet snow event - High Wind Watch continues for Cape and Islands - Potential for northeasterly 40 mph sustained winds, 60 mph gusts - Strong to damaging winds in addition to possible heavy, wet snow - Possibility for downed trees, tree limbs, power outages */ Discussion... Models: Two camps, the 18.0z GFS / Canadian huggers versus the 18.0z NAM / UKmet / EC buggers, as in bugging out to sea. Both exhibit the surface low stacking beneath mid-level H85-7 features. Mesoscale and synoptic forcing leading to deformation within the N/W quadrant of the low with focus within H6-8 with rearward isentropic 290-300K trowaling with both lift and available moisture in the snow growth region in an environment of conditionally unstable lapse rates that yield SW-NE snowbanding signatures. However the location of such signatures with respect to SE New England is either hit or miss. Some indication perhaps correlated to variance of upstream ridging and/or shearing equatorward of mid-level vort lobe energy and its associated magnitude. In the end the spread is noteworthy and simply leads to a lack of confidence. Even the 18.0z WRF solutions vary between NMM and ARW cores. Time to evaluate ensembles. Ensembles: Per NCAR, continued spread in amplitude of H85-5 height features. Unclear is approximate magnitude and troughing of mid- level features, perhaps consequential of model handling of upstream ridge enhancement yielding stronger equatorward winds shearing and potentially cutting-off vort-max energy S. However, despite spread in H925-85 thermal fields, a high degree of confidence the column aloft will be below freezing. Would then expect a rain/snow event that hinges on 2m temperatures and precipitation intensity. Segway, SREF dprog/dt trends, the 17.21z being the latest, lessening impacts over N/W portions of S New England, steady around the I-95 corridor while increasing over SE MA, indications of tight NW-SE gradient of outcomes in agreement with model huggers, 18.0z GFS / Canadian. Forecast thinking: Splitting hairs over a forecast which there is little confidence gleaned from either model or ensemble guidance. Simply comes down to a consensus weighting of all raw forecast data. This results in the likelihood of outcomes across SE MA from roughly 1 am to 4 pm Sunday. However given a heightened consensus of model guidance of offshore outcomes, with wobbling of the storm center noted over the last several forecast runs, will blend the previous forecast with a consensus blend of the latest guidance trending towards a tighter gradient of outcomes along the I-95 corridor with the majority of impacts focused over SE MA especially the Cape and Islands. This agrees pretty well with the latest WPC forecast. Now to the forecast details. Winds: Continued potential of 60 mph northeasterly gusts over the Cape and Islands with sustained winds up to 40 mph around Sunday, the height of the winds during the midday into afternoon hours. Yet given uncertainty and a portion of forecast guidance wobbling the storm center further out to sea, will keep with the HIGH WIND WATCH and forego the issuance of wind advisories with this update. This simply because there is not enough confidence to go with additional headlines, however there remains enough possibility to continue with the current watch. Snowfall: Amounts decreased. Weighting the previous forecast with the latest thinking, there is the potential for accumulating snow along the I-95 corridor and towards the SE with a coating to an inch N/W, more than likely a coating if that for N/NW areas of MA. As to the SE, considerations include the intensity of snowfall, especially during the daylight hours given it`s mid-March and there`s a strong onshore easterly flow off ocean waters in the upper 30s to low 40s. Likely going to be mild prior to the storm with highs Saturday up into the low to mid 40s over SE portions of New England. With light snowfall, more than likely we`ll only see accumulation on non-paved surfaces, whereas more intense snowfall, with rates closer to 1 inch per hour, then roads will more likely become and remain snow covered thinking that with more intense snow surface temperatures will drop. That is if snow isn`t mixing with or changed over to rain. Dealing with milder air with an E onshore flow. Have gone ahead and included a wet-bulb consideration into the 2m temperature forecast centered during the height of the snowfall forecast around Sunday morning. This looks to be one of those events where areas around the Cape Cod Canal including the Upper Cape see the highest amounts while perhaps the Outer Cape sees very little. Again, as noted above, comes down to low level thermal profiles and snowfall intensity. In addition it could be one of those events where accumulations are that of heavy, wet snow weighting on trees, tree limbs, bringing down power lines and leading to power outages. Highest amounts forecast of around 4 to 6 inches, and given potential impacts, will keep with the current WINTER STORM WATCH. In closing: This remains a very tricky forecast and we`re working hard to keep you informed on the latest forecast guidance. Nothing means more to us than our mission of protecting lives and property. With that being said, interests along and S/E of the I-95 corridor including SE MA, Cape and Islands, should continue to maintain a level of awareness for potential impacts being the area of greatest risk. The forecast can still change even within the next 12 hours, the storm center can still wobble, even possibly hugging closer to the coast. There remains a wide spread in the forecast guidance of possible outcomes and thus we will retain the WATCH headlines with this forecast update.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/...
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Highlights... * Moderating temps early next week with 50s possible Tue! * Arctic blast with near record cold returns middle of next week Monday... Dry weather returns as subsidence on backside of departing ocean storm provides sunshine across most of the region. The exception will be over Cape Cod and the Islands where low clouds will linger as ocean storm slowly exits seaward. Clouds may not break here until afternoon or late in the day. Also gusty north winds persist across the Cape and Islands in the morning with gusts up to 40 mph possible. Not quite as windy during the afternoon. However these northerly winds off the chilly ocean waters will provide cool conditions. Father inland model cross sections suggest a fair amount of sunshine, possibly fading behind increasing high clouds late in the day as WAA pattern approaches from the west. Nonetheless fairly pleasant with highs in the 40s, abundant strong March sunshine and a modest NNW wind. Tuesday... Arctic front approaching from the northwest but not expecting a fropa until after sunset. Thus potential for a mild day. Prefrontal environment is relatively mild along with good mixing courtesy of westerly pressure gradient. Bufkit soundings indicate blyr mixing at least up to 850 mb during the afternoon with temps around -7C at the top of the mixed layer. This combined with at least partial sunshine should yield highs at least near 50. However downslope winds and a dry airmass should tack on a few more degs to push highs into the 50s. A gusty west wind up to 30 mph will make it feel a tad cooler but nonetheless temps finally above normal! EC mos is the warmest of the guidance will use it to derive max temps Tue afternoon. Wednesday and Thursday... Arctic front moves across the area Tue night with a frigid airmass (by late March Standards) overspread the region Wed and Thu. Ensembles (both GEFS & EPS) and deterministic guid both bring -18C to -20C 850 mb temps across the region during this time. These values are about -2 standard deviations colder than climo! This will result in near record cold temps for both Wed and Thu along with bitterly cold wind chills. Friday... Not as cold with core of cold air exit New England. Dry weather likely prevails.
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&& .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
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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 Sunday Night/...Moderate Confidence. Today... Possible SCT-BKN MVFR cigs developing late. Increasing E winds with gusts up around 20 kts for S coastal terminals. -RA/-SN late over S/W CT and RI terminals. Tonight... BKN-OVC MVFR cigs overspreading all terminals with increasing -SN/SN coverage over S New England. IFR impacts with visibility of 1-2 miles in SN. Increasing easterlies becoming NE. Gusts up to 40 kts by Sunday morning across the S-coast of RI and MA including the Islands. Roughly 20 to 30 kts along the E coast of MA. Sustained winds around 15 to 25 kts. Sunday... -SN possibly mixing with or changing over to -RA over SE New England, gradually shifting E out to sea. Mix of MVFR/IFR impacts with mainly MVFR cigs 1-2 kft agl. Continued strong E/NE winds the height of which is around 21z with general 30 to 40 kt gusts along the E/SE coast, but to 50 kt over the Cape and Islands with sustained winds up to 35 kts. Sunday Night... -RA/-SN and wind impacts diminishing E. Strongest winds at the beginning of the period with potential 35 kt E/NE sustained winds over the Outer Cape and Nantucket, gusts to 50 kts. Lesser IFR impacts. MVFR cigs lifting out to the E. KBOS TAF...Accumulating snows on area runways are possible for the Sunday AM push and continuing through the PM push. However only moderate confidence at this time. Higher confidence with respect to strong E/NE becoming N with time around Sunday. KBDL TAF...Lesser threat of snowfall. Thinking amounts across the terminal on the order of a coating to an inch. Breezy N winds. Moderate confidence forecast. Outlook /Monday through Wednesday/... Monday... MVFR cigs likely over southeast MA including Cape Cod and the islands. Scattered -SHSN/-SHRA possible here too. North winds may gusts up to 35 kt early in the day. Otherwise VFR and dry conditions elsewhere along with a diminishing north wind. Tuesday and Wednesday... VFR likely with a modest WNW wind Tue but increasing NW winds Tue ngt into Wed with passage of an arctic front.
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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 Sunday Night/...Moderate Confidence. Still uncertainty on a low center tracking S of New England and deepening offshore. It`s proximity to the coast is the biggest question as is its intensity. Subsequent impacts with respect to gale and storm force winds with waves building up around 10 to 14 feet on the outer waters. Given the uncertainty in the forecast will continue with GALE and STORM WATCHES. However, mariners should be aware of the likelihood no matter what for very breezy E/NE flow that will heighten wave activity, mainly centered around Sunday afternoon into evening. Outlook /Monday through Wednesday/... Monday... Northerly gales likely across the eastern MA waters, then slowly lowering during the afternoon. Elsewhere, north winds 20-30 kt. Rain/Snow showers may linger Mon morning across the eastern MA waters. Otherwise dry weather and good vsby. Tuesday... Modest WNW winds during the day along with dry weather and good vsby. However arctic front likely barrels thru during the evening hours with a risk of WNW gales behind the front. Wednesday... WNW gales likely behind departing arctic front. Frigid airmass and WNW gales will increase the risk for moderate freezing spray especially Wed night.
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&& .TIDES/COASTAL FLOODING...
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The possibility of northeasterly winds gusting up to 60 mph across the waters resulting in forecast wave heights of 10 to 14 feet during periods of high tide around Sunday morning and Sunday evening presents the threat of splashover or minor coastal flooding along the E coast of MA, especially in Cape Cod Bay. Surge values of 1.5 to 2.5 especially around areas of Barnstable MA along with increased swell and wave action presents the coastal flood threat, including minor beach erosion. Given the uncertainty of both the proximity, residency, and strength of the storm center anticipated offshore, will hold off on headlines with this update pending further evaluation of later forecast guidance. However, those with interests along the E MA coastline should remain aware of the latest forecast and take actions as necessary.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...High Wind Watch from late tonight through Sunday evening for MAZ022>024. Winter Storm Watch from this evening through Sunday evening for MAZ022-023. RI...None. MARINE...Storm Watch from late tonight through Sunday evening for ANZ231>234-254-255. Gale Watch from late tonight through Sunday evening for ANZ230- 236. Gale Watch from late tonight through late Sunday night for ANZ235-237-256. Gale Watch from late tonight through Monday morning for ANZ250- 251.
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&& $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell TIDES/COASTAL FLOODING...Sipprell

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