Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 162031 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 331 PM EST Fri Feb 16 2018 .SYNOPSIS...
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Showers will continue to impact south coastal areas this afternoon as a cold front crosses the region. A return to seasonably wintry temperatures is expected tonight into Saturday. Fast moving low pressure will bring a plowable snow Saturday night into very early Sunday morning, with the heaviest amounts near and south of the Massachusetts turnpike. High pressure brings dry and cool weather Monday. A surge of much warmer air returns Monday night and may linger through Wednesday. A cold front drops through either late Wednesday or Thursday. High pressure brings cooler weather Thursday and Friday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1250 PM Update... The first of two steadily moving cold front will cross the region this afternoon. Scattered light rain showers will continue to cross the region with and ahead of the fronts, which should push off the coast by around 00Z but may linger near Nantucket for an hour or so this evening. Noted winds have shifted to W-NW across portions of central and N Mass at midday, with a gust up to 27 kt reported at KORH at 17Z. Can not rule out more brief wind gusts up to 20-25 kt as the fronts pass through the afternoon. Temps at 17Z ranged from 41 degrees at KORE, up to 56 degrees at KPVD. The lower clouds have lifted across most areas, but still linger along the S coast as well as across the N Mass portion of the CT valley. Back edge of the high clouds pushing E-SE out of the Lake Ontario and Erie regions of W NY/NW PA as seen on latest GOES-East visible satellite imagery. However, with the general W steering flow, will likely not see any breaks in the clouds until late this afternoon across western areas. Have updated short term to bring conditions current and incorporated trends into update with high res model data. && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/... Tonight... With front finally clearing the Cape/Islands this evening, H92 temps will continue to drop, starting around -3C at 00Z, dropping to -10C by 12Z. The clearing and cold advection will lead to more seasonably cold temperatures overnight tonight. Even though by the AM hours, skies will be clear, the maintenance of strong pres gradient will allow winds to continue through the overnight hours. Therefore, ambient temps will not dip as low as they otherwise could, but with winds 10-20 mph with gusts 25-30 mph at times will allow wind chills to drop into the low teens even while 2m temps remain in the upper teens to mid 20s. Will need to monitor roadway drying, as the lingering showers and rapid cooling could allow for isolated ice development. Sat... Seasonably cool, with highs mainly in the mid 30s, but with lighter winds than the overnight hours. Although increasing moisture is expected, the bulk of any impacts will be overnight Sat night, see below for that. && .LONG TERM /SUNDAY THROUGH FRIDAY/...
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Big Picture... Upper pattern starts with flat flow Sunday, but this quickly amplifies Monday-Tuesday. Shortwave from the Alaska coast dives south over California. This digs a deep trough over the Western USA while ridges build along the East Coast and the Eastern Pacific Ocean. The eastern ridge flattens late in the week. Per the NA Reanalysis the average values of 500 mb heights over Southern New England for February are 5420 meters to 5460 meters. The average values of 850 mb temperatures are -4C to -7C. Model values start near normal Sunday, then quickly climb well above normal Monday through Wednesday before dropping back closer to seasonable late week. Per the SPC Sounding Climatology, record 850 mb temps are roughly 10-11C. GFS, ECMWF, and GGEM forcast 850 temps at or above this value on Tuesday and Wednesday. This projects record warmth by midweek, and even when cooler air moves in it is forecast at near seasonable temperatures. Model mass and thermal fields are similar through Tuesday. The GFS is faster with the subsequent cold front with passage on Wednesday while the ECMWF and GGEM show a Thursday fropa. All models agree on high pressure and dry seasonable weather during the day Friday. Details... Sunday-Monday... Upper shortwave races through on Sunday, pushing the offshore surface low out to sea and making room for cooler drier air. This will mean clearing skies. Pressure rises behind the storm will average 1 mb per hour, tightening the pressure gradient and bringing gusty northwest winds. Mixing should reach to 900 mb, which would support max sfc temps in the low to mid 40s. High pressure moves overhead Sunday night. Should feature mostly clear skies, light winds, radiational cooling. With dew points in the 20s and low 30s, expect min temps in a similar range. High pressure moves off to the east Monday, with an increasing southwest flow aloft. This will bring warm advection aloft, lowering the mixing depth to around 950 mb...temps at that level support highs mostly in the 40s. The flow will also bring increasing moisture aloft, leading to increasing mid and high clouds mainly in the afternoon. Monday night through Wednesday... Warm front moves through during Monday night. A 45 to 55 knot low level jet works over the region Monday night and should set up a period of lift that generates showers. The clouds and incoming warm air will keep temperatures from falling much. More likely we will have steady or slowly rising temperatures during the night. The warm front lifts to the Canadian border Tuesday and Wednesday, leaving Southern New England deep in the warm sector. Temps at 950 mb will be 12-14C each day, supportive of highs in the 60s except cooler along the South Coast in the southwest flow. One uncertainty is whether the southwest flow producing clouds where the flow comes off the ocean. The moist flow may also bring areas of drizzle, especially along the south coast. Cloudier skies would suggest lower max temps. For now we will stay near guidance which has 60s with 50s along the south-facing coasts. Wednesday night through Friday... Multiple low pressure waves move along the front midweek. The final wave will swing the front south as a cold front. Our other uncertainty with this system comes with the timing of that trailing cold front. The GFS brings the cold front through Southern New England during the day Wednesday while the ECMWF does so Wednesday night and the GGEM toward Thursday morning. Whatever the timing, we expect low-end pops with the passage. We took the middle solution for timing, which lines up with the ECMWF. As the cold front moves off to the south, high pressure builds over nearby Canada. This brings a north flow of cooler air to our region. The flow turns from the northeast with time, and eventually turns from the east off the ocean. Expect temperatures trending near normal. Warm air ahead of the next weather system approaches Friday night. This may bring another chance of showers at the end of the forecast period.
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&& .AVIATION /21Z FRIDAY 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 Saturday Night/... Through 00Z...Moderate confidence. VFR conditions across most of the terminals, except some lingering MVFR CIGS/VSBYS along the S coast and down to areas of LIFR on the islands at 18Z. Should see improving conditions across S areas from 21Z-00Z, the last at KACK. Wind shift to W-NW with frontal passage from N-S this afternoon with brief gusts up to 20-25 kt. Tonight...High confidence. Any leftover MVFR conditions in SHRA and patchy fog across KACK should end by 02Z. Otherwise, VFR. W-NW wind gusts to 25-35 kt at times through 06Z, highest along the immediate S coast, then should diminish. Saturday...High confidence. Expect VFR CIGS through the day, VFR VSBYS through 21Z then may lower to MVFR with the onset of -SN across portions of N CT/RI. Light NW winds, but then shift to S-SE at 10 kt or less after 18Z. Saturday night...Low to moderate confidence. Conditions lower to IFR in -SN/SN, heaviest from 03Z-10Z with local LIFR CIGS/VSBYS in +SN at times. PType may be -RA/RA at times through around midnight along immediate S coast/Cape Cod and the islands. Conditions may quickly improve across western and central areas after 10Z. KBOS Terminal...Moderate to high confidence in TAF. KBDL Terminal...Moderate to high confidence in TAF. Outlook /Sunday through Wednesday/...Moderate Confidence. Sunday: VFR. Breezy. Sunday Night: VFR. Washingtons Birthday: Mainly VFR, with areas MVFR possible late. Breezy. SHRA likely. Monday Night: Mainly MVFR, with local IFR possible. Breezy. SHRA likely. Tuesday through Tuesday Night: VFR with local IFR along the South Coast Breezy. Slight chance SHRA. Wednesday: VFR, with local IFR possible along the South Coast. Breezy. Chance SHRA.
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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 Saturday Night/...High confidence. Through 00Z... Winds shift to the W through this afternoon with some brief gusts up to 25 kt. Building S swell continues, with 5-9 ft seas possible through the morning across the S waters, requiring Small Craft Advisories. Rain/fog lead to reduced vsbys. Tonight... W winds increase as skies clear through midnight. Gusts up to 30-35 kt, highest across the eastern waters. Gale Warnings continue, along with Small Craft Advisories elsewhere. Sat... Winds and seas slack, especially as the winds shift to the S-SE. Small Craft Advisories will end through the morning. Outlook /Sunday through Wednesday/...Moderate to High Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Washingtons Birthday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Monday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Rain showers likely. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain showers. Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Slight chance of rain showers. Wednesday: Winds less than 25 kt. Local rough seas. Chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Storm Watch from Saturday evening through Sunday morning for CTZ002>004. MA...Winter Storm Watch from Saturday evening through Sunday morning for MAZ011>013-015>022. RI...Winter Storm Watch from Saturday evening through Sunday morning for RIZ001>007. MARINE...Small Craft Advisory until 6 AM EST Saturday for ANZ232. Small Craft Advisory until 5 AM EST Saturday for ANZ230-233- 234. Gale Warning until 2 AM EST Saturday for ANZ231-251. Small Craft Advisory until 1 AM EST Saturday for ANZ236. Small Craft Advisory until 7 AM EST Saturday for ANZ235-237. Gale Warning until 3 AM EST Saturday for ANZ250-254. Small Craft Advisory until 10 AM EST Saturday for ANZ255-256.
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&& $$ SYNOPSIS...WTB/Frank/Doody/EVT NEAR TERM...Frank/EVT SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/EVT MARINE...WTB/EVT

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