Area Forecast Discussion
Issued by NWS Boston, MA

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045 FXUS61 KBOX 100531 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1231 AM EST Sun Dec 10 2017 .SYNOPSIS... Low pressure passing east of Nantucket will bring a general snowfall tonight. Mainly dry and cool conditions are expected Sunday afternoon and Monday. Low pressure approaching from the west late Monday night into Tuesday will likely bring a period of snow and/or rain. A shot of arctic air follows Wednesday into Thursday with wind chill values falling below zero. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 10 pm update... Low pres currently resides E of S New England and continues to show deepening per latest MSAS. This continues to draw low-mid lvl f-gen toward the low center and across E MA/RI. Have watched the banding signature become more defuse over the last few hours as a result and expect that this trend will continue as the f-gen shifts E and dry air begins to filter in above H6 per wv imagery. Will see a slow dissipation of snowfall rates through early AM, so will maintain current warnings/advisories and re- evaluate at that time. Focused an area of higher snow/qpf mainly N of Plymouth through the S shore of MA where good coastal front signature resides, right along the current SN/RA line per latest 0.5 CC image. Otherwise, am a bit concerned for freezing drizzle as snowfall rates recede. As mentioned above, good dry air infusion already occurring aloft, and a resulting subsidence inversion is apparent on soundings. This is likely to trap some of the lower lvl moisture through much of the overnight hours until BL mixing can help to scour our the moisture near sfc. Will be adding FZDZ to the forecast overnight, and this may keep conditions a bit slippery even after snows drop off. This may warrant the maintenance of at least some winter wx advisories, but again will reassess this risk later. Previous Discussion... Surface low will pass near the 40N/70W benchmark by around 00Z-01Z, then will shift NE across the eastern Gulf of Maine and S of Nova Scotia overnight. Besides bringing the snowfall to an end, winds will shift northwest and become gusty late. Min temperatures should be slightly be normal, with wind chill values in the teens and 20s. Just a reminder, this is the first snowfall of the season so motorists should use extra caution. Untreated surfaces will be slippery, and visibility can drop quickly in heavier snow bands. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/... Sunday... Strong shortwave energy will swing across the region Sunday. Still thinking this will likely result in a few lingering snow showers Sunday morning, especially across eastern MA where moisture is deeper. Otherwise, a mainly dry and blustery day is expected. High temperatures slightly below normal despite more sunshine. Sunday Night... Increasing clouds after midnight as another mid level trough and shortwave approach from the west. Gusty west winds continue, which could result in some ocean-effect clouds and showers south of the Cape, and especially across the outer coastal waters. Low risk for some showers across the Vineyard and Nantucket, too. Thinking mostly rain, with an outside shot at some light snow, especially late. Near normal low temperatures. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Highlights... * Dry but chilly conditions Monday * Light snow and/or rain likely Mon night-Tue across the interior, with snow changing to rain along and southeast of the BOS-PVD corridor * Batch of arctic air moves in late Wed into Thu with bitter wind chill values at times * May see another quick moving low push across late next week but timing is in question Overview... Highly amplified mid level pattern continues across North America early next week, with deep troughing along the eastern seaboard and ridge nosing into western Alaska and the Northwest Territories early next week. 12Z models showing some weakening of the long wave trough, but keeps lower H5 heights across the northeast U.S. through a good portion of next week. With the western ridge holding in place through a good portion of next week, the base of the trough will hang across the northeast, will likely see a couple of waves move along in this general flow. Timing of each feature is in question, especially around Thu-Fri timeframe, as another cutoff H5 low develops and moves out of central Canada. Could see the coldest air of the season so far around the Wed-Thu timeframe as the cold pool moves across, along with some gusty winds bringing bitter wind chill values. A lot of questions beyond Thursday as the western ridge shows signs of breaking down and moving slowly east, but it appears that long wave troughing will continue through the remainder of the long term period at this point. Details... Monday... Expect the H5 short wave to push offshore early Monday, with weak high pressure quickly moving across. Expect mainly cloudy skies early, then becoming partly cloudy by late morning. Mid level flow briefly flattens around the base of the trough. However, the next H5 short wave quickly moves out of the western Ohio valley late Mon, so clouds will move back in again during the afternoon. Temps will only rise to the upper 20s to lower 30s across the higher inland terrain ranging to 35-40 near the coast. Noting H85 temps dropping during the day, down to -7C to -10C by evening. Monday night and Tuesday... Low pressure moves out of the Great Lakes, bringing precip shield eastward Monday evening. Expect the best chance for precip around or after midnight Mon night through around mid morning Tue, then will push eastward. Thermal profiles signaling mainly light snow except along the immediate coast Mon night then, with marginal temps rising with light S-SE winds along the shore, snow will mix with or change to rain along a good portion of the coastal plain by mid-late Tue morning. This, along with QPF values on order of 0.1 to 0.3 inches for this event, should see only light snowfall amounts. Temps will top off in the lower-mid 30s well inland, ranging to the lower-mid 40s along the shore. Tuesday night through Thursday morning... Winds shift to W Tue night, but may see some spotty snow showers develop as upper level system starts to approach. Leading edge of arctic air will approach during Wed, along with scattered snow showers. Temps will drop from midday onward on Wed, with readings already down to the 20s to lower 30s away from the immediate coast by 00Z Thu. W winds will become gusty, especially by late Wed morning through Wed night. Could see gusts up to 25-35 mph, highest along the coast, along with the colder air working in. Wind chill values will drop below zero from central Mass/NE CT westward, down as low as -5 to -10 by Thu morning across the E slopes of the Berkshires. Expect frigid temps on Thu with highs only in the 20s to lower 30s for most areas, ranging to the mid 30s along the S coast, running 10 to 15 degrees below seasonal normals. Winds will diminish, but will still gust up to 20-30 mph through midday across the higher terrain and along the coast. Clouds will increase during the day as another low approaches in the W-NW flow aloft. Thursday afternoon through Friday... Another short wave moves quickly toward the region out of the Great Lakes. Will see periods of light snow moving into S coastal areas by Thu evening, then pushing N during Thu night. Not a lot of moisture with this system so should only see light snow amounts. Due to fast flow aloft, though, having some timing and track issues amongst the model suite. At this point, the precip should end from W-E Fri morning. Saturday... Low confidence on this portion of the forecast, but for now carrying a dry forecast for most of Saturday, but could see yet another system bring light precip late Sat/Sat night. High temps look to run up to 5 degrees below seasonal normals. && .AVIATION /05Z SUNDAY THROUGH THURSDAY/...
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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 Monday/... Through 12Z...High confidence in trends, lower confidence in exact timing. A mix of IFR/MVFR conditions in light SN most areas (except SE MA/RI where RA will transition briefly to SN before ended) will continue through about 08Z, then gradually give way to VFR and no SN between 08Z and 12Z. However, occasional FZDZ will continue through about 12Z except where current RA, dissipating as CIGs rise. Today into tonight...High confidence. Mainly VFR after early MVFR conditions dissipate early this AM. A few lower VFR CIGS tonight. W winds will gust 20-25 kt at times late today. Tomorrow...High confidence. Mainly VFR. W winds 10-15 kt. KBOS Terminal...High confidence in TAF trends. Remnant MVFR conditions dissipate as SN ends this AM. Some FZDZ lingers through 11Z otherwise improvement may be off a bit in TAF. KBDL Terminal...High confidence in TAF trends. Outlook /Monday Night through Thursday/... Monday: VFR. Breezy. Monday Night: Mainly VFR, with areas IFR possible. SN likely. Tuesday: MVFR/IFR conditions possible. Breezy. SN likely, chance RA. Tuesday Night: Mainly VFR, with local IFR possible. Breezy. Slight chance SN. Wednesday: Mainly VFR, with areas IFR possible. Windy with gusts to 35 kt. Slight chance SHSN. Wednesday Night: VFR. Windy with gusts to 35 kt. Thursday: Mainly VFR, with local IFR possible. Breezy. Slight chance SHSN.
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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 Monday/...High confidence. Today and tonight... Low pres off the NC coast this morning tracks over or near 40N/70W this evening. NE winds increase to 20-25 kt by evening resulting in the building of seas. SCA have been issued. Rain and snow will reduce vsby today and tonight and slowly taper by daybreak. Sunday...Northwest wind gusts of 25 to 30 knots anticipated in the cold air advection pattern. Marine headlines continue. Sunday Night...W winds briefly diminish, but rough seas take longer to subside. Marine headlines should continue, especially across the outer coastal waters. Outlook /Monday Night through Thursday/... Monday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain. Local visibility 1 to 3 nm. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Rain likely. Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of snow. Areas of visibility 1 to 3 nm. Wednesday: Strong winds with gusts up to 35 kt. Areas of rough seas. Chance of snow showers. Visibility 1 to 3 nm. Wednesday Night: Moderate risk for gale force winds with gusts up to 35 kt. Rough seas up to 11 ft. Slight chance of snow showers. Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Slight chance of snow showers. Areas of visibility 1 to 3 nm.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Weather Advisory until 7 AM EST this morning for CTZ002- 003. Winter Storm Warning until 7 AM EST this morning for CTZ004. MA...Winter Weather Advisory until 7 AM EST this morning for MAZ002- 003-008>011-016>021. Winter Storm Warning until 7 AM EST this morning for MAZ004>007-012>015-026. RI...Winter Weather Advisory until 7 AM EST this morning for RIZ002- 004>007. Winter Storm Warning until 7 AM EST this morning for RIZ001- 003. MARINE...Small Craft Advisory until 7 AM EST Monday for ANZ231>235-237- 250-251-254>256.
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&& $$ SYNOPSIS...Belk/EVT NEAR TERM...Doody SHORT TERM...Belk LONG TERM...EVT AVIATION...Doody/EVT MARINE...Belk/EVT

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