Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 150618 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 118 AM EST Wed Nov 15 2017 .SYNOPSIS...
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High pressure brings drier air and clearing conditions today. Low pressure from the N Plains sweeps E, eventually redeveloping along the New England coast Thursday and bringing rain to the region. High pressure Friday, followed by rain over the weekend. Drier into early next week.
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&& .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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1 AM update... Mean mid level trough axis now over eastern MA and continuing to advect eastward. Ridge axis over western PA/NY advancing eastward toward New England, resulting in anticyclonic mid level flow and associated subsidence is eroding cloud cover from west to east across CT/RI per latest GOES-16 satellite imagery. This trend will also result in decreasing cloudiness across MA during the predawn hours. Otherwise previous forecast remains on track. No major changes with this update. ==================================================================== Previous discussion... With clearing conditions and light wind, expect temperatures in W MA and CT to radiate for a few hours and reach near the dew point. Farther east where clouds persist longer, temps will remain a few degrees above dew point. Overall range should be mid 20s northwest to near 40 Nantucket.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM THURSDAY/... Wednesday... High pressure overhead will bring mainly clear skies and light wind. The actual center of the high will pass to our north, allowing the gradient wind to turn from the east through the day. Mixing to 925 mb will tap temps equiv to 850 mb temps of -7C to -10C. This supports max sfc temps in the low to mid 40s. Some question remains as to lingering clouds over the Cape and Islands. RH fields at 925 mb show 90-95 pct values there during Wednesday while 850 mb RH values are high mainly to the east of the Outer Cape. We will show 50 to 55 pct sky cover over the Cape and Islands with less cover for the remainder of our region. Wednesday night... High pressure maintains dry weather much of the night as it moves off to the east. The next shortwave comes from the Canadian Prairies and reaches the Great Lakes Wednesday night. The surface weather system moves from the Great Lakes to Quebec Wednesday night. The nose of the upper jet approaches the New England border around 06z with favorable jet dynamics over our area during the late night. Low level moisture increases during this same period. Precipitable water values only climb a little above normal by morning, and the low level jet forcing is concentrated in Northern NY overnight, so expect slow eastern progress into Southern New England. All of the operational models keep measurable QPF to our west through 06Z, then move the measurable values into at least western and central sections by 12Z. We move chance pops into Western sections by 3 AM and to Eastern Mass by 12Z. Likely pops to Western Mass and CT by 10-11Z. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... */ Highlights... - Progressive pattern however storm maturity favored over SE Canada - Quick shots of wet weather, milder temperatures in advance - Followed by cooler air, breezy conditions as disturbances depart - Wet weather forecast Thursday and again over the weekend */ Overview... Presently contending with a mainly zonal flow, closely monitoring the evolution of a series of impulses out of the Pacific sweeping E into the N Atlantic, whether a H5 block develops around the Davis Straight between SE Canada / Greenland. Focusing on ensemble means, teleconnections are largely -AO/-NAO around the weekend into early next week, the PNA subdued. Favor a less amplified and faster flow over the deterministic 14.12z GFS a bit more amplified with respect to teleconnection trends, stronger with the block over the Davis Straight. Leaning a slow down of energy towards roughly the weekend into early next week. This will allow opportunistic storm maturation around SE Canada behind which will be closely monitoring dips in the polar jet N, and draw of energy S, tightening the thermal wind and aiding in greater baroclinicity. Depending on specifically where ultimately determines the outcomes as to whether sub-tropical warm moist air pumping N ahead and/or the magnitude of arctic air being dragged S behind will impact our region. Will focus on these points in the discussion below. */ Discussion... Thursday... An approaching, progressive, open-wave system. Maturation occurring downstream as the system obtains cyclonic curvature drawing S Arctic energy rearward subsequently deepening with the tightening thermal wind / baroclinic zone. Initially the surface low an inside runner, high pressure slides E allowing the warm front to approach ahead of which isentropic over- running proceeds. Need to watch closely as to whether some of the initial light precipitation is of a wintry mix before changing to rain during the day with S draw of sub-tropical air. Low risk given the overnight airmass lacking an Arctic connection. Flow reverting S quickly, highs and surface dewpoints becoming mild. The system sliding and immediately departing, it begins to close off and stack. Could see a departing slug of precipitation and breezy W winds as the secondary low deepens NE into SE Canada, trowaling of the warm conveyor belt emerging, deformation focusing in the NE-quad of the H85-7 low. Magnitude and positioning crucial with respect to cooling towards the wet-bulb, potentially drawing S air below H925 right around freezing given the absence of an Arctic connection. The 14.12z EC seemingly the outlier with respect to position / strength of the surface low, however the 14.12z NAM not far off. Will keep with a cold rain this forecast thinking the flow would be more E and progressive even with the secondary low circulation and mid-upper level features, less opportunity for maturation and surface low backing / retrograding beneath lower heights. Drier air quickly wrapping, precipitation quickly comes to an end overnight and cold air advection proceeds, steepening low level lapse rates, keeping those breezy winds into Friday morning. Will go with highest sustained up to 25 mph and gusts up to 40 mph across the Cape and Islands. Not ignoring signals of H925 winds in excess of 45 mph. Lapse rates easily mixed beyond that (top of the layer around H9). Over the high terrain and across E/SE MA there is the potential for wind headlines. Height of the winds close to Friday morning. Friday... High pressure building in. NW winds continuing, strongest early on, relaxing towards evening. Going to be breezy all day with potential gusts out across the Outer Cape up to 35 mph. Scattered to broken cloud decks, more than likely NE, gradually clear out towards evening. The winds become light overnight, and anticipated clear conditions, looking at an opportunity for radiational cooling. Keeping highs near to below average for mid-November, upper 30s to mid 40s, lows overnight back down into the 20s, teens in spots. Weekend and beyond... Considering a block over the Davis Straight, trapping a storm system over SE Canada during the weekend into early next week, we`ll get a quick shot of wet weather and mild temperatures given an inside- runner and quickly progressive, followed by the return of colder air out of the N/W. Wobble of the SE Canada storm system crucial, a measure of forecast uncertainty, will keep the remainder of the forecast period dry. However, with additional Pacific impulses through the continued progressive flow, expect warmer air ahead and shots of wet-weather followed by colder air behind. Likely impacts to the wind profile with periods of gusty winds with the passage of storm systems. The colder air following behind over anomalously warm ocean waters, expect marine stratus offshore. && .AVIATION /06Z 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/... 06z update... Thru 12z...high confidence especially in trends. VFR cigs (CIGS OVC040-060) will give way to SKC toward sunrise. Exception will be over Cape Cod and Islands where VFR cigs may linger. Light to modest NNE winds. After 12z...high confidence. VFR conditions. Low risk of MVFR cigs over Cape Cod and the islands. Light NNE winds becoming E/SE during the afternoon. Tonight...High confidence. VFR to start the evening except marginal MVFR-VFR along south coast including Cape Cod and islands. Then trending to MVFR all terminals as the night progresses with risk of showers toward morning. Thursday...Moderate confidence, higher on trends than details. Strong cold front approaches from the west with rapidly intensifying low over southeast MA Thu afternoon then into Gulf of Maine Thu evening. MVFR with showers overspreading the area in the morning. Brief possible wintry mix across northern MA in the morning. Otherwise showers become heavy during the afternoon with isolated T-storm possible. Gusty SE winds possible over Cape Cod and the islands. KBOS Terminal...high confidence in TAF. Tranquil weather until Thu. NNE wind this morning becoming SE late today and this evening. KBDL Terminal...high confidence in TAF. Tranquil weather until late tonight into Thu. Outlook /Thursday Night through Sunday/...High confidence. Thursday Night through Friday: VFR. Windy with gusts to 35 kt. Friday Night: VFR. Windy with local gusts to 30 kt. Slight chance FZRA. Saturday: Mainly VFR, with local MVFR possible. Windy with local gusts to 30 kt. SHRA likely, chance FZRA. Saturday Night: VFR. Windy with gusts to 30 kt. SHRA likely. Sunday: VFR. Breezy. Slight 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 Thursday/... 940 pm update... Modest NNE winds 10-20 kt but long fetch yielding seas 4-7 ft across the eastern MA ocean waters. This will continue overnight as high pressure remains over PA/NY state. Otherwise dry weather and good vsby prevail. Earlier discussion below. ================================================================== High pressure builds over the waters tonight and Wednesday, then moves off to the east Wednesday night. Expect winds to remain below 25 knots through the period, and below 20 knots tonight and Wednesday. Seas have lingered at 5 feet on the outer waters east of Massachusetts and look to remain that way through Wednesday night. We have issues a Small Craft Advisory for Seas for this area. No headlines on the remaining waters. Rain will move in from the west late Wednesday night. Outlook /Thursday Night through Sunday/... Thursday: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Seas up to 5 ft. Rain showers likely. Thursday Night: Moderate risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Slight chance of rain showers. Friday: Moderate risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas locally approaching 5 ft. Slight chance of rain showers, slight chance of freezing rain. Saturday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Rain showers likely, chance of freezing rain. Saturday Night: Low risk for gale force winds with gusts up to 40 kt. Areas of rough seas. Rain showers likely. Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 8 PM EST this evening for ANZ250-254-255.
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&& $$ SYNOPSIS...WTB/Sipprell NEAR TERM...WTB/Nocera/Sipprell SHORT TERM...WTB LONG TERM...Sipprell AVIATION...WTB/Nocera/Sipprell MARINE...WTB/Sipprell

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