Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 132101 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 401 PM EST Mon Feb 13 2017 .SYNOPSIS...
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Intense low pressure was located 170 miles E of Cape Cod early this morning. It will continue to deepen today before moving out to sea tonight. A few more bands of snow will impact E coastal MA through this afternoon. But the main impact will be the threat of power outages due to damaging winds across much of the region today, especially when combined with snowfall already on trees and wires. High pressure and cold conditions prevail Tuesday. Another storm system is forecast to strengthen too far offshore for significant impact to Southern New England Wednesday and Wednesday night. A renewed period of cold weather is forecast Friday into the weekend, however there is a moderating warmer trend into next week.
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&& .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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135 PM Update... * Strong to damaging winds expected to cause numerous power outages today * Periods of heavy snow this afternoon on mid-outer Cape Cod, quick 1-3 inches of snowfall. Issued a Winter Weather Advisory for Cape Cod from 1 to 4 pm today. Wrap-around snow band became ocean-enhanced, with radar and surface obs indicating periods of heavy snow on mid-outer Cape Cod. Provincetown and Chatham ASOS showing visibility down to 1/4 mile at times. Could see a quick 1 to 3 inches of snowfall on the mid-outer Cape. Elsewhere, will see some light snow lingering across E coastal Mass into mid-late afternoon, then diminishing. Also noting some snow showers across the E slopes of the Berkshires, which will linger through around mid afternoon. The storm center was located just south of Nova Scotia at 18Z. In southern New England, strong NW winds will persist into early this evening. High Wind Warning continues until 00Z for RI and eastern MA, with a Wind Advisory for central/western MA and north central- northeast CT zones. Will continue to see gusts up to 35-45 kt, possibly up to 50-55 kt along E coastal Mass through this afternoon. The gusts will gradually diminish across W Mass/N central CT by around 21Z-22Z.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Clouds will clear out for the most part tonight, allowing temperatures to drop to the teens, except 20s on Cape Cod and the Islands. Strong winds in the evening will slowly diminish to 10-20 mph toward daybreak. Wind chill values will drop to the single digits across much of the western and central parts of the region. && .LONG TERM /WEDNESDAY THROUGH MONDAY/...
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*/ Highlights... - Another storm system for Wednesday into Wednesday night - Seasonably cold conditions for the end of the week - Warming trend into next week Overview and model preferences... As mentioned by the previous forecaster, the long-term is defined by a gradual shift in the pattern predicated on the AO/NAO remaining at least weakly positive as currently progged by ensembles. This more progressive pattern allows the mean trof across Quebec/Labrador to shift further E than much of the last week. The pseudo-rex block that has been a feature of the plains through the Continental Divide, yielding decent warm anomalies will also begin to shift E, reinforced secondarily by large-scale warm advection ahead of a SE Pacific wave. What this all means is a slight tendency toward warmer conditions for the weekend and early next week. However, there remains some uncertainty in the finalized pattern due to the complex interactions of several waves in a highly amplified pattern, but have no qualms about continuing the trends suggested by previous forecast. The one caveat? One more quick-moving arctic-influenced shortwave for Wednesday, which models generally agree the core of which will remain to the N, different than the much stronger systems of the past week. This northerly passage of the mean jet and its poleward exit region suggests the deepening of the low pres occurs well into the Bay of Fundy and beyond the dome of major impact to S New England. Still some complexity to work out regarding thermal profiles, but at this point, it definitely looks like the weakest of the last three systems. Details... Wednesday... The parent low pres of this system is almost clipper-like in its moisture and thermal profiles initially until the transfer of energy begins by late Wed afternoon-evening. Moisture is generally relegated to the lowest sfc-700mb with PWATs capped at 0.5 inches. Thermal profiles supportive of all SN at the onset early Wed morning, but H92 temps warm to as high as +2C approaching the Mass Pike. This warming is coincident with a dry slot building in from the SW as the transfer begins late in the day. Timing is somewhat uncertain with this dry slot as GFS/NAM are much faster while ECMWF/CMC are much slower, with ensembles favoring either speed. This will impact how quickly snowfall, or a mix of rain/snow/sleet is able end by Wed evening with low pres developing offshore. Snow totals are likely to be on the lighter side, and may be sub-advisory across much of the region. Noting peak omega and moisture is below the dendritic growth zone, suggesting lower SLRs. QPF averages 0.05-0.2 widespread so a generally coating to 2 cannot be ruled out. The highest of which will be N of the Pike and especially along and N of Route 2 in MA. Timing suggests it begins lightly during the AM commute but the bulk is after. One interesting note however is the highlighting the SN Squall parameter on the tail end of this run of the BTV WRF. Not surprising as LLJ 20-30 kt and the sfc-H7 lapse rates are nearly dry adiabatic, suggesting it may not be the snowfall totals but the brief snowfall rates that impact the early half of the day. Mitigating factors are of course the lacking upward motion in the dendrite regime, and SW flow aloft is not a favorable trajectory for squalls. However, would not be surprised if a quick burst of heavier snow occurs along with some gustier winds. This will be refined over the next 24-36 hours. H92 temps starting around -2C and warming to +2C or across the S suggests highs reach low-upper 30s, a bit colder given the likely wet-bulb effect. Overnight mins drop back into the 20s as the core of the cold air associated with the shortwave will be advecting over the region. Thu and Fri... Last vestiges of cold, arctic-influenced air will be sliding across the region through late week. H92 temps average -8C to -10C through the period with H85 temps roughly -10C to -14C. This suggests mainly seasonable to slightly below normal temps, exacerbated by remaining snow pack. Highs mainly upper 20s to low-mid 30s while lows drop back into the upper teens and 20s, colder for sure if some decoupling is allowed. Regarding this, moderate pres gradient remains in place combined with H92 winds (should be able to mix at least to this lvl by day) nearly 30 kt. Therefore breezy conditions expected, which may lead to some very cold wind chills especially overnight, possibly in the single digits. This weekend... Ridging suggests broad scale subsidence (mainly dry wx) but also introduces some warming through the weekend. There is some variability in low lvl temps but noting H85 temps across ensembles generally rising above 0C and H92 above 6C by Sun. ECMWF ensemble probabilities are already showing nearly 80 percent probabilities of highs exceeding 50 5 days out. So will continue to hint at warmer than normal temps supported by CPC and previous forecast, but this will also be dependent on what is left of our snowpack by this time. Early next week... Ensemble means suggest the warming trend continues but given that shortwave energy will be attempting to slide out of the Arctic thanks to broad trof associated with Aleutian vortex flattening the PNA. However, far too early to tell how this will ultimately break down for mesoscale or smaller sensible wx details. Will continue with ensemble trends.
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&& .AVIATION /21Z MONDAY THROUGH SATURDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 15Z Update... Areas of MVFR CIGS across E Mass into RI from about KBED-KOWD- KEWB at 15Z, and mainly VFR across from western Worcester county and NE CT westward. Also lingering MVFR CIGS/VSBYS in -SHSN across E slopes of the Berkshires. Elsewhere, mainly VFR conditions. Will see periods of -SN across E coastal Mass, where MVFR-IFR CIGS/VSBYS are likely. Expect slowly improving conditions across E Mass around or after 20Z. NW winds will gust to 40-50 kt throughout the region this afternoon. A few gusts to 55 kt along the coast from Boston to Cape Cod and along the S coast from Nantucket to S RI. KBOS TAF...Moderate confidence. MVFR conditions in -SN/SN with periods of IFR possible in heavier snow bands. SN ending roughly around 18Z-19Z. Improvement to MVFR at that point then VFR tonight. Wind gusts to 45-50 kt this afternoon. KBDL TAF...Moderate confidence. Mainly VFR. NW winds gusting to 35-40 kt through 20Z-21Z then diminishing. Outlook /Wednesday through Saturday/... Wednesday...Moderate confidence. Areas of IFR or low end MVFR in SN mixing with light RA at times mainly terminals S of the MA PIKE and SE MA. Conditions improve Wed night back to VFR. Some near shore wind gusts 20-30 kt possible as winds shift southeasterly to westerly Wed night. Thursday and Friday...High confidence. Mainly VFR. W-NW winds with gusts 20-30 kt during daylight hours. Sat...High confidence. Mainly VFR with lighter winds.
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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. *** Dangerous Coastal Storm impacts the waters *** Storm center about 100 miles SW of Yarmouth Nova Scotia will continue to deepen as it spins there through most of the afternoon before shifting E. Expect NW winds gusting to 50-55 kt, highest across the eastern outer waters into early tonight. Seas up to 20-25 ft on the eastern outer waters. Visibility restrictions in bands of moderate to locally heavy snow across the eastern waters into Cape Cod and Mass Bays. Expect light freezing spray. The NW winds will slowly diminish tonight, but gusts up to 45-50 kt may linger through midnight on the eastern outer waters. Storm warnings may linger there. Seas will slowly subside, but will remain very rough. Light freezing spray expected. Outlook /Wednesday through Saturday/... Wednesday and Thursday...Moderate Confidence. Weak low pressure inland will lead to a stronger low pressure developing in the Gulf of Maine late Wednesday into Thursday. Initially, southeasterly winds will gust to around 25 kt. However as winds shift to the west at night into Thursday some gusts may reach 35 kt. Therefore, at the very least, small craft advisories will be needed, but there is moderate confidence Gale Warnings may be needed. Seas peak Wed night, 8-10 ft closest to the Gulf of Main. Otherwise, light rain expected over the waters may lead to a period of ocean effect rain/snow showers Wed night into Thu. Fri...High confidence. Small craft advisories likely to linger out of the W-NW with gusts around 25 kt. Seas 5-6 ft. Sat...High confidence. High pres should bring a period of quiet boating wx by the weekend.
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&& .TIDES/COASTAL FLOODING...
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Have issued a Coastal Flood Advisory for a fairly large portion of the Massachusetts east coast for the upcoming late night high tide. Although the late night astronomical high tide is lower than the Monday early afternoon high tide, a subtle shift of low level winds to a little more NNW and large swells generated from the intense storm over the Gulf of Maine reaching the Massachusetts shoreline will likely increase the surge again for a time tonight and cause some overwash over those portions of northeast and north facing shorelines vulnerable to impacts from wave run-up. The large swells with wave periods between 10 and 12 seconds may also produce significant erosion for some portions of the shoreline in the Plum Island area, Hull to Plymouth, Sandwich to Eastham, ocean side of Cape Cod,and the east side of Nantucket. Impacts from the Monday early afternoon high tide appear to have been mostly minor and less than predicted. This was due to the fact that the powerful storm formed further northeast over the Gulf of Maine than expected. This resulted in lower wind speeds, more of a westerly component to the wind, and less of a fetch of wave energy onto the Massachusetts coastline.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Wind Advisory until 7 PM EST this evening for CTZ002>004. MA...High Wind Warning until 7 PM EST this evening for MAZ005>007- 013>024. Coastal Flood Advisory from midnight tonight to 4 AM EST Tuesday for MAZ007-019-022-024. Wind Advisory until 7 PM EST this evening for MAZ002>004- 008>012-026. Winter Weather Advisory until 4 PM EST this afternoon for MAZ022. RI...High Wind Warning until 7 PM EST this evening for RIZ001>008. MARINE...Storm Warning until 11 PM EST this evening for ANZ231-232-250- 251-254>256. Storm Warning until 7 PM EST this evening for ANZ230-233>235- 237. Gale Warning until 1 AM EST Tuesday for ANZ236.
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&& $$ SYNOPSIS...Doody/EVT NEAR TERM...EVT SHORT TERM...EVT LONG TERM...Doody AVIATION...Doody/EVT MARINE...Doody/EVT TIDES/COASTAL FLOODING...Thompson

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