Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 152148 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 448 PM EST Wed Feb 15 2017 .SYNOPSIS...
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Rapidly intensifying low pressure will be lifting into the Canadian Maritimes Thursday morning. This will result in rain changing to accumulating snow across eastern Massachusetts overnight into early Thursday morning. Otherwise, dry but blustery and cold weather continues for the remainder of the work week. Dry weather continues this weekend, but with a significant moderation in temperatures especially by Sunday when much of the region may see highs exceeding 50.
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&& .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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***Accumulating snow likely late tonight into early Thursday morning across eastern MA especially on the north shore*** Rain was currently overspreading eastern MA late this afternoon from the south. This was a result of developing low pressure south of the region. In addition...there was a narrow band of scattered rain and high terrain snow showers moving across western MA and northern CT. This activity was associated with a vigorous upper level shortwave/cold pool aloft approaching from the west. The main focus for precipitation tonight will become focused across eastern MA and particularly northeast MA. This a response to vigorous shortwave energy to our west coupled with a rapidly intensifying ocean storm that will be lifting into the Canadian Maritimes. This will setup the inverted trough which are notoriously tough to forecast. Ptype/Uncertainty... As mentioned above the scattered rain and high terrain snow showers across the interior should come to an end this evening. Most of tonight will be dry in these locations with the focus shifting to eastern MA. The guidance is in general agreement in showing a period of good low level convergence with the inverted trough overnight...particularly in northeast MA. This coupled with a period of strong omega in the snowgrowth region/deep moisture and total totals between 55 and 60 will result in an ideal setup for moderate to heavy snow. However, there are a couple of limiting factors that need to be take into consideration. The first is that the boundary layer will likely be too warm for snow into the first half of the evening. Based on model soundings...rain will probably change to snow between 9 pm and midnight across most of eastern MA. The second is that the inverted trough looks to be progressive...not stationary over our area for an extended period of time. Therefore...the duration of the moderate to heavy snow will probably be limited to a few hours. The last and most important factor is that the models often perform quite poorly in depicting the axis of heavy snow associated with inverted troughs. This can result in a significant snowfall bust on the high or low side for specific locations. In this case...the biggest uncertainty is does the axis of heavy snow drop for enough south to affect our region. Snow Accumulations/Headlines/Timing... Based on the above reasoning have opted to go with a Winter Weather Advisory for 3 to 6 inches of snow across Essex county. Have also issued a special weather statement for the possibility of an inch or two of snow further south across the rest of eastern MA. While the heaviest snow will probably be over by mid morning on Thursday... roads may be snow covered and slippery for the morning rush hour. As mentioned will have to closely monitor trends this evening given the uncertainty. Winds... Winds will shift to the northwest and increase overnight behind the rapidly intensifying low pressure system. Should see northwest wind gusts of 25 to 40 mph develop across the coastal plain. Will be close to wind advisory criteria across the Cape/Nantucket but kept things just below for now.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
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Thursday... While the steadiest snow should be over by mid morning on Thursday across eastern MA...enough low level moisture/steep lapse rates coupled with diurnal heating may allow hit and miss scattered snow showers to develop into mid afternoon. In fact...may see the activity develop further back across the interior as a result of the diurnal heating depending on how much moisture is left. It will also be windy with high temps in the 30s.
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&& .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
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Highlights... * Generally dry conditions through most of this period * Blustery and cold Friday * Moderating temperatures Saturday, and unseasonably mild on Sunday * Above normal temperatures continue into early next week Details... Thursday night... Leftover NW winds at the surface and aloft will continue. Skies will become mostly clear, though some clouds may linger across the E slopes of the Berkshires and outer Cape Cod, along with a few leftover snow showers Thu evening. Winds will gust up to 25-30 kt mainly along the coast Thu evening, then should diminish as the pressure gradient from storm system over the Maritimes decreases. With the NW flow in place, will see H85 temps will fall to -10C to -13C, lowest across the interior higher terrain. Expect lows in the mid to upper teens across N central and W Mass, ranging to lower-mid 20s along the immediate coast. Friday and Saturday... High pressure ridge builds across the eastern seaboard Friday, then will steadily shift E Fri night into Sat. As the H5 vortex across the Maritimes finally lifts toward Greenland Sat, W-NW winds will relax with the ridge`s arrival. Mid level winds back to W as well. Another H5 short wave will slide SE out of central Canada by later Sat, so some mid and high level clouds will increase from the NW during the afternoon across western areas. Temps will return close to seasonal levels, with highs in the mid 30s to lower 40s. Saturday night-Sunday... As a cutoff low moves eastward across the mid Mississippi valley, the other system over southern Canada remains N of the region. This system will push a warm front into northern New England, with only some clouds working across the region Sat night into early Sunday. The main moisture from both systems will remain outside the region, so expect little if any precip. There may be enough moisture for some patchy fog to develop inland late Sat night into Sun morning. W-SW early Sunday will bring milder air to the region, so expect temps to reach the mid 40s to lower 50s, up to 10 degrees above seasonal normals under mostly sunny skies. Winds will shift back to NW late Sunday into Sunday night as both systems push off the coast. Monday through Wednesday... Quite a bit of solution spread amongst the model suite during this timeframe, mainly due to continued somewhat amplified but progressive split mid level flow across the lower 48. Looks like a fast W-NW flow continues across the northern stream, while a cutoff low moves along the southern stream down to the Gulf of Mexico. So, have rather low confidence with this portion of the forecast. At this point, may see another warm front approach around late Mon night or Tue as low pressure moves across Quebec. May see a chance for rain and/or snow showers, but timing is in question. May see another weak system cross during Wed, but again timing and track are in question. Have carried slight chance POPs for this system.
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&& .AVIATION /22Z WEDNESDAY THROUGH MONDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Range /through Thursday/... This afternoon...Moderate confidence. Mainly MVFR conditions expected for the remainder of the afternoon. Band of rain showers will affect portions of Eastern MA/RI through early evening...while scattered rain/snow showers will be possible across western MA/northern CT. Tonight...Moderate confidence. MVFR conditions again probably dominate with rain changing to snow across eastern MA between 01z and 05z. This should result in periods of IFR to LIFR conditions in snow. Focus for snow will be across northeast MA but may affect the rest of eastern MA for a time late tonight into early Thu am. Thursday...Moderate confidence. MVFR CIGS look to hang around much of the day. There will also be scattered snow showers...especially across eastern MA during morning hours. This should result in localized lower conditions. Northwest wind gusts of 20 to 30 knots expected. KBOS TAF...Moderate confidence in TAF. Best chance for a period of accumulating snow will be between 04z and 12z. KBDL TAF...Moderate confidence in TAF. Outlook /Thursday Night through Monday/... Thursday night...High confidence. A few leftover -SHSN possible across the E slopes of the Berkshires Thu evening. May also see leftover MVFR CIGS across the E slopes of the Berkshires and along the outer Cape. Should improve by around 05Z. Otherwise, VFR conditions. Friday and Saturday...High confidence. Mainly VFR conditions. Saturday night-Sunday...Moderate to high confidence. May see patchy MVFR-IFR CIGS/VSBYS develop Sat night across the CT valley and E slopes of the Berkshires ahead of warm front. MVFR-IFR CIGS may linger Sunday and early Sunday night along the E slopes of the Berkshires. Elsewhere...mainly VFR. Monday...High confidence. Mainly VFR conditions.
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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 Range /through tonight/... Tonight and Thursday...Moderate to high confidence. Southeast winds shift to the northwest and increase to between 30 and 40 knots later this evening and overnight. Winds will diminish a tad on Thursday...but strong cold advection will result in gusts between 25 and 35 knots. Gale warnings posted for all waters except small craft headlines for Boston Harbor/Narr Bay. Outlook /Thursday Night through Monday/... Thursday night...High confidence. Expect NW winds gusting to 30-35 kt Thu evening, then will diminish. Gale warnings will likely be lowered to small crafts over the open waters. Light freezing spray is likely along the near shore waters. Seas up to 6-9 feet on the outer waters. Friday-Friday night...High confidence. W-NW winds will continue to gust up to 25-30 kt so small crafts will likely continue. Light freezing spray will likely continue through Fri morning, then should end as winds diminish. Seas 6-9 ft early, then will slowly subside but remain at or above 5 ft through Fri night on the outer waters. Saturday through Sunday...High confidence. W-SW winds gusting up to 20 kt. Seas below 5 ft. Monday...Moderate confidence. NW winds may approach small craft criteria across the open waters.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Winter Weather Advisory from 9 PM this evening to 10 AM EST Thursday for MAZ006-007. RI...None. MARINE...Gale Warning from 10 PM this evening to 10 AM EST Thursday for ANZ231>235-237-251. Small Craft Advisory from 10 PM this evening to 7 PM EST Thursday for ANZ230-236. Gale Warning from 10 PM this evening to 7 PM EST Thursday for ANZ250-254>256.
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&& $$ SYNOPSIS...Frank/EVT NEAR TERM...Frank SHORT TERM...Frank LONG TERM...EVT AVIATION...Frank/EVT MARINE...Frank/EVT

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