Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 150847 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 347 AM EST Wed Nov 15 2017 .SYNOPSIS...
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Sunshine returns today as high pressure crest over New England. A cold front approaches Thu with low pressure developing on the front over southeast Massachusetts, bringing a period of rain, heavy at times to the region. Behind the departing low Fri, blustery and noticeably colder conditions overspread the area. Another area of low pressure impacts the region late Saturday into Sunday morning with gusty showers and milder conditions. Drier and colder weather overspreads the area later Sunday into Monday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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345 AM update... Mid level ridge over western PA/NY advects eastward into New England today. This anticyclonic flow and associated subsidence will erode cloud cover currently over the area during the predawn hours. Thus sunshine returns today, at least partial sunshine as clouds may linger over Cape Cod and the Islands. Chilly airmass remains over southern New England with 925 mb temps only ranging from about 0C western areas to -2C across eastern MA. 1024 mb high will limit boundary layer mixing due to subsidence inversion. Thus highs will only recover into the low and middle 40s, about 7-8 degs cooler than normal. However partly to mostly sunny conditions combined with light winds will take some of the edge off the cool temps.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY NIGHT/...
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Updated 345 AM... Tonight... Temps likely fall fairly quickly with sunset given surface ridge remains over the area combined with a dry airmass and mostly clear skies especially away from the south coast where clouds may linger. Then temps level off or even rise toward daybreak as low level flow becomes southeast and clouds quickly increase later in the evening and especially after midnight as WAA commences ahead of approaching short wave. Precip may break out between 09z-12z especially across western-central MA into CT. If precip arrives a bit earlier it may begin as a brief period of snow and/or sleet in western/northern MA. Otherwise ptype will be rain. Dry weather may linger until sunrise Thu across RI and eastern MA, with rain possibly arriving after sunrise Thu here. Thursday... Vigorous but progressive negative tilt short wave trough moves across the area Thu afternoon. This induces rapid developing secondary/triple point low along the south coast of RI/MA then into the Gulf of Maine Thu evening, with rapid falling pres of at least 1 mb per hour! However the system remains an open wave as it tracks across our region and doesn`t close off until somewhere over downeast ME. Thus the brunt of this storm will be felt northeast of our region, across NH and ME. However there will be at least some impacts here as comma head develops over southern New England. All model guid suggest a period of moderate to heavy rain in the comma- head across interior CT-MA Thu afternoon. Mid level lapse rates approach 7C/KM as comma-head develops. Thus have inserted chance of elevated convection. This may yield a period of heavy rainfall with a low risk of minor poor drainage/street-highway flooding. Other concern will be developing low level S-SE jet in the prefrontal environment. This may result in a period of strong winds in the warm sector across the south coast including Cape Cod and the Islands, with dew pts climbing into the 50s and easing low level inversion for wind transfer especially in heavy showers. Fairly strong pressure rise-fall couplet accompanying the rapidly developing secondary/triple point low. This will enhance the isallobaric wind component and increase risk for a brief period of strong winds. Will have to watch this system closely and later model trends. If secondary low develops quicker/sooner, greater the impact for southern New England. Conversely, if system develops later less impact for our region.
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&& .LONG TERM /FRIDAY THROUGH TUESDAY/...
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*/ 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... 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.
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&& .AVIATION /09Z 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 Night/... 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 /Friday 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 Night/... 345 AM update... Today... light winds as high pres moves from New England to the coastal waters. Dry weather and good vsby prevail. Tonight... High pres moves offshore so light SE wind develops. Dry weather and good vsby gives way to risk of showers toward daybreak. Thursday... Rapidly intensifying low pressure moves from the RI/MA south coast to Gulf of ME. SE gales possible on the front end Thu then NW gales on the back end Thu night. Isolated thunder possible. Vsby lowers in widespread showers. Outlook /Friday through Sunday/... 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...Gale Watch from late Thursday night through Friday evening for ANZ230>234-250-251-254-255. Gale Watch from Thursday evening through Friday afternoon for ANZ235>237-256. Small Craft Advisory for hazardous seas until 8 PM EST this evening for ANZ250-254-255.
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&& $$ SYNOPSIS...Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell

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