Area Forecast Discussion
Issued by NWS Boston, MA

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722 FXUS61 KBOX 181203 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 703 AM EST Wed Jan 18 2017 .SYNOPSIS...
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A mixed bag of wintry weather will continue to impact S New England this morning, spotty showers lingering through the day into the overnight hours. High pressure builds over the region beginning Thursday and through the weekend. A weak cold front slips through the high late Saturday, followed by Canadian high pressure and slightly colder temperatures for Sunday. Developing low pressure over the Southern Plains will bring rain and possibly some initial ice to New England Monday, with the rain lingering into Tuesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 AM update... One area of rain showers moving through RI and Southeast MA... additional showers of rain and snow moving from NY State into Western MA. Back edge of the showers lines up with axis of northern shortwave moving east toward New England. This shortwave crosses our area during the afternoon/evening. Expect pcpn to diminish by that time. No significant changes made to the forecast with this update. Previous discussion... Cold, dreary, overcast day. Breezy N/NE low-level fetch of moisture on the backside of the immediate offshore deepening surface low. Undergoing weak orographic ascent beneath a measure of mid-level forcing brought upon by a sweeping H5 positive-tilted trough axis. Precipitation output trending moderate to light likely continues, especially over E/NE MA and up against the E-facing slopes of high terrain. There is, however, uncertainty on precipitation type, dependent on surface temperatures and whether ice is present in the column. No longer is there an issue of a warm-layer aloft as the column cools within the low levels as colder air is drawn S behind the departing low. It`s either rain, snow, or freezing rain. Water vapor satellite presently shows the wedge of drier air moving into S New England, however mid to upper level cloud decks, likely composed of ice nuclei, are apparent out of the Great Lakes region in association with a positively tilted trough sweeping into the area. Also considering onshore E flow of marine influence and CCN which could aid in the presence of ice and development of snow wherever the column and surface temperatures are sub-freezing, rather than freezing rain / freezing drizzle. Overall, going to keep it simple. Mainly across N/NE MA it`ll be a mix of rain or snow changing over to rain as we warm at the surface and the low-levels cool aloft. Higher confidence in the presence of ice nuclei. Over N/W MA, especially the Berkshires, will go with freezing rain or snow combination. This approach given the overall uncertainty of ice present in the column. Precipitation becomes light throughout the day, so weather-types will alter becoming drizzle, freezing drizzle, or light snow.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Tonight... Continued ascent of low level moist profiles ahead of weak mid-level shortwave energy and associated positively tilted trough axis across the region by morning. Stronger signal of drier mid to upper levels and thus the lack of ice nuclei favorable towards snow development. But still a good onshore flow and maritime influence. Temperatures dropping overnight, there is the expectation over a good chunk of S New England of transitioning back over to wintry precipitation types. Precipitation still rather light given the weak ascent, will keep it simply and go with freezing drizzle OR light snow for those areas where temperatures are at or below-freezing. Confidence that the window of freezing drizzle will be from sundown to roughly around midnight. After midnight, another mid-level shortwave impulse should yield the right ingredients to make the dominant precipitation type as snow for any sub-freezing locales. Will see the light precipitation move offshore towards morning with winds reverting northwesterly. The drying process begins as cloud decks become broken W to E. Thursday... High pressure and drier air working into the region. Will see clouds become scattered W to E through the day allowing for sunshine prior to sundown over a good portion of S New England. Continued NW flow. Will see temperatures warm into the low 40s as heights rise across the region and warmer air moves in from the W. H85 temperatures nudge above freezing towards the end of the day. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Big Picture... Upper ridge remains in control of the flow over the Eastern USA Thursday night through the weekend. A couple of weak shortwaves move through New England on Saturday. A more prominent shortwave currently south of the Aleutian Islands will reach the Pacific coast Friday and re-form over the Southern Plains Saturday night. The associated surface storm moves to the Southeast Atlantic coast Monday, then up the coast Tuesday. Upper height contours remain higher/warmer than normal much of the period, with heights dropping Monday night/Tuesday as the Southeast storm moves up the coast. Model mass fields are similar through Sunday but then diverge. Early next week the Canadian GGEM attempts to develop not one but two lows along the East Coast ahead of the GFS and ECMWF Sunday night, and the GGEM shows a much more intense storm than the other models for off our coast Tuesday. The differences limit forecast confidence, however impressive the storm is depicted. With no clear favorite, we will use a blend of model data. Details... Thursday night-Friday... High pressure over the region will maintain dry weather. Lingering moisture at 850 mb suggests some sky cover. Thursday night dewpoints of 25-35 will make room for min temps in that same range. Mixing reaches to 950 mb, with temps in the layer that will support max sfc temps in the upper 30s and lower 40s. Saturday-Sunday... High pressure remains in control. One southern stream shortwave slips through the ridge Saturday, while a northern stream shortwave slide through the Maritimes Saturday night. These bring an increase in moisture below 750 mb Saturday and Sat night, but little jet dynamics to promote lift. A slightly colder high pressure area moves in for Sunday. Expect partly sunny skies each day. Temps aloft support max sfc temps in the 40s Saturday and mid 30s to low 40s Sunday. Nightime temps continue in the mid 20s to mid 30s. Monday-Tuesday... The main concern for the long-term forecast period is, as noted above, a system south of the Aleutians that may bring precip to our area to start the week. The presence of Sunday`s high pressure parked over the Maritimes suggests cold air lingering in the interior. The thermal fields do show warming at all levels with time, so any initial ice should eventually change to rain. We will show chance pops for precip...based on the model differences and the current distance of the parent system from the North American upper-air network. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
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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/...Moderate Confidence. Today... MVFR-IFR mix with mostly RA/DZ across the terminals. FZRA/SN for terminals N/W of the Mass Pike and I-495 corridor. N flow through the CT River Valley while blustery E/NE winds along the immediate E MA coast. Gusts up to 30 kts, especially across the Cape and Islands. Tonight... Precipitation tapering off as DZ for most terminals, though FZDZ or -SN is forecast for interior terminals, again the focus being N of the Mass Pike and W of the I-495 beltway. Roughly speaking W and Central MA. Winds turning more N/NW and diminishing into the morning period. Will begin to see cigs become BKN W to E, lifting low-end VFR. But most of the overnight period is characterized as a mix of MVFR-IFR. Thursday... Conditions improving as cigs become BKN to SCT, lifting to low-end VFR. NW winds continue. Specific Terminals... KBOS TAF...will hold rain across the terminal throughout the duration of the TAF. With the E/NE onshore flow will keep cigs IFR down around 800 ft agl into Thursday morning. Can not rule out cigs lifting briefly at times up to 1500. KBDL TAF...RA/DZ for the most part. Will be watching the overnight period as to whether sub-freezing temperatures impact the terminal thereby creating a risk of icing with FZDZ. Low confidence at this time as the expectation is that NW winds will proceeding during the overnight period with drier air allowing conditions to begin to improve into morning. Outlook /Thursday Night through Sunday/... Moderate confidence. VFR through the period. Patchy MVFR cigs possible especially along the South Coast.
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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/...Moderate Confidence. Today... Blustery E/NE winds which will push seas up to 8 to 10 feet on the outer waters, especially SE. Gusts up around 30 kts. Can not rule out gales for the SE waters so will continue with gale headlines, though the threat seems lesser for the inner waters, so have reverted inner water areas around Cape Cod back to small craft advisories. Rain overall and visibility reductions at times. Tonight into Thursday... Winds turn out of the NW and taper. Drizzle will linger into Thursday yielding some visibility restrictions out on the waters. Otherwise waves dampen as conditions improve going into Thursday. Will see see small craft advisories persist throughout the period over the outer waters. Outlook /Thursday Night through Sunday/... Moderate confidence. North to northeast winds 15 knots or less. These become variable Friday as high pressure moves overhead, then west Saturday and north- northeast again Saturday night as a weak cold front sweeps through. Seas of 5-6 feet Thursday night will subside as high pressure builds overhead Friday. Seas will be mostly below 5 feet during the weekend, although areas of 5 foot seas may briefly return on the outer waters Saturday and Sunday.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Winter Weather Advisory until 10 AM EST this morning for MAZ002>004-008-009-026. RI...None. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ230>234-236. Small Craft Advisory until 1 AM EST Thursday for ANZ235-237. Small Craft Advisory until 7 PM EST Thursday for ANZ250-251-256. Gale Warning until 4 PM EST this afternoon for ANZ254-255.
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&& $$ SYNOPSIS...WTB/Sipprell NEAR TERM...WTB/Sipprell SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell MARINE...WTB/Sipprell

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