Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201505 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1005 AM EST Mon Nov 20 2017 .SYNOPSIS... Low pressure in eastern Quebec provides blustery and chilly conditions across southern New England today. The high moves south of the region Tue as winds shift to the southwest along with milder temperatures. A cold front approaches the region Tuesday night, and will combine with moisture working up the eastern seaboard to bring showers across the region through Wednesday. Dry and cold conditions expected Wednesday night through the end of the week. A cold front may bring showers Friday night into Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7am update... Forecast generally remains on track this morning although temps/dwpts are running a bit higher than previous forecast suggested. So will update these to bring them up to current trends. Also noted that daytime mixing is reasonably efficient given the shortwave rotating through, which suggests highs a bit warmer as well. Otherwise, breezy conditions persist until this wave is fully exiting the region. Previous Discussion... Fairly high amplitude mid level mean trough moving through the region this morning with 500 mb temps down to about -36C over Lake Ontario per early morning SPC mesoanalysis. This instability combined with cyclonic flow over the northeast yielding periods of cloudiness across southern New England. Embedded with this large scale flow are a few lake effect snow streamers from NY state traversing CT into RI with another band over northwest MA. 00z NAM and other hi res guidance capturing these narrow lake effect bands very well, so leaned on this guid for the morning hours. Not expecting much snow but a few of these bands may briefly lower vsby and leave a quick dusting or coating behind. However most locations will remain dry. By afternoon model low level streamlines indicates trajectory will shift into northwest MA. Thus any snow shower/flurries this afternoon will be focused over this region. Otherwise a mix of sun and clouds today, chilly with blustery W-NW winds up to 35 mph. Colder than normal with highs only 40-45 except only mid to upper 30s across the high terrain. It will feel even colder give the gusty W-NW winds. Normal highs for 11/20 should be in the upper 40s to lower 50s.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... 330 AM update... Tonight... Mid level mean trough kicks eastward with height rises overspreading the region as mean ridge pushes toward New England. Anticyclonic flow will provide dry weather. Some mid/high clouds may clip western MA as warm front lifts northward across NY state into VT. This transition/airmass change will result in temps not as cold as this morning along with less wind, with winds shifting from WNW to SSW. So coolest temps likely this evening and then temps leveling off or rising toward morning especially along the coast. Tuesday... Short wave ridging across New England as next northern stream trough dives into the Great lakes. Thus dry weather expected. Tightening SSW pgrad as high pres moves offshore as cold front enters eastern Lakes. A robust low level SSW jet of 40-45 kt develops over the area Tue. Given WAA pattern blyr not fully mixed but model sndgs indicate up to 35 kt/40 mph possible. So a windy day but this low level WAA pattern will provide mild conditions as 925 mb temps warm to about +5C, supporting highs in the mid to upper 50s, except low 50s across the high terrain. About 5-8 degs warmer than normal. Should be a mostly sunny day as well. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Leftover SW wind gusts Tuesday evening ahead of approaching cold front * Atlantic moisture will stream across the region as the cold front pushes across late Tuesday night and Wednesday * Dry but chilly for Thanksgiving Day and Friday * A cold front may bring some showers Fri night into early next weekend Overview... Continued progressive mid level steering pattern noting during most of this forecast period. However, will see amplified but progressive H5 trough dig across the eastern U.S. Tuesday night into Wednesday. Most models still signaling low pressure developing off the SE coast and will combine with H5 short wave approaching New England to bring tropical moisture up the coast late Tue night and Wednesday. As the front pushes offshore Wed night, will see mainly dry weather but colder than average temperatures for Thanksgiving and Friday as high pressure ridge builds in. Models showing some timing and track differences of another low that looks to pass S and E of Cape Cod Thu night into Fri as another H5 trough digs across the eastern seaboard. Strong low pressure across southern Canada will bring another cold front moving across late Fri night or Sat that may linger further into next weekend. Expect temperatures to run close to or below normals through this period. Details... Tuesday night and Wednesday...High confidence. High pres at the surface and aloft moves off the eastern seaboard Tue night. Rather strong pressure gradient on western periphery of the high. Low level jet at 45-50 kt from H95 to H925 Tue evening across S coastal MA and S RI along with the leftover low level lapse rates (on order of 7-8C/km) through around midnight may be enough for some SW wind gusts up to 30-35 kt or a bit higher. During the early morning hours, will see pres gradient between the high and approaching front relax and the low level jet move offshore. Noting inverted troughing off the Carolina coast with S-SE winds bringing surge of low level moisture up the coast on the backside of the high, then gets caught up in the S-SW flow ahead of approaching cold front. Models continue to signal weak tropical or subtropical low form off the FL coast which will ride NE in the low level flow. The moisture plume will enhance the precip across eastern areas as the low passes E of the 40N/70W benchmark. Showers will move into S coastal areas late Tue night with the best QPF amounts moving in Wednesday, with the highest amounts forecast along E coastal areas on the order of 0.4 to 0.7 inches. The cold front pushes through late Wed/Wed evening, with rain possibly changing over to snow showers across the E slopes of the Berkshires before ending. Not expecting much in the way of accumulations there. The colder air works in Wed night as skies become mostly clear, though some clouds linger along S coastal areas. Thanksgiving and Friday...High confidence. Behind the front on Wednesday, CAA will move back into the region for Thursday. High pressure ridging builds across the region with a dry but cool day expected for Thanksgiving. Another mid level short wave moves across Thu night and Friday, but the moisture remains N of the region so expect to see continued dry and cool conditions. Friday night through Sunday...Low to moderate confidence. Expect dry conditions Fri night as high pressure moves east. May see a dry cold front move across, then stall S of the region. Low pressure moves across central Canada, with its associated front approaching. This may bring a chance of light mixed precip to the E slopes of the Berkshires after midnight then will change over to rain Sat morning as the precip progresses eastward. Not a lot of confidence on the timing of this front, and whether precip may linger into Sat night. Looks like another shot of cold air works in late next weekend but mainly dry conditions seen for now. && .AVIATION /15Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence. 1140Z update... Through Today... VFR conditions with SCT-BKN clouds at 5-6Kft, though may see brief MVFR conditions in isolated snow showers which should taper off by mid morning. W-NW winds with gusts up to 30-35 kt, highest across central and western areas. Higher terrain may see gusts up to 40 kt. Tonight...VFR. Diminishing wind and becoming SSW. Tuesday...VFR but increasing SSW wind gusts 25-35 kt. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Scattered light snow showers til 07z or so with marginal MVFR-VFR cigs and vsby possibly briefly lowering to 4SM in -SHSN. Otherwise VFR and dry. Outlook /Tuesday Night through Friday/... Tuesday Night: Mainly VFR, with local MVFR possible. Windy with gusts to 35 kt. Chance SHRA. Wednesday: Mainly VFR, with areas MVFR possible. Breezy. SHRA likely. Wednesday Night: VFR. Breezy. Thanksgiving Day through Friday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...high forecast confidence. 640 AM update... Today... Marginal W-NW gales with gusts up to 35 kt. A few brief rain/snow showers through mid morning, otherwise mainly dry weather prevails. Tonight... WNW winds slacken and shift from WNW to SSW as high pres slides south of New England. Dry weather and good vsby prevail. Tuesday... SSW winds may approach gale force especially near shore. Otherwise dry weather and good vsby prevail. SSW winds will provide a long fetch of wind along the south coast and will yield building seas. Outlook /Tuesday Night through Friday/... Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of rain showers. Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Rain showers. Wednesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of rain showers. Thanksgiving Day through Friday: Winds less than 25 kt. Seas up to 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Warning until 6 PM EST this evening for ANZ231>235-237- 250-251-254>256. Small Craft Advisory until 6 PM EST this evening for ANZ230- 236. && $$ SYNOPSIS...Nocera/EVT NEAR TERM...Nocera/Doody SHORT TERM...Nocera LONG TERM...EVT AVIATION...Nocera/EVT MARINE...Nocera/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.