Area Forecast Discussion
Issued by NWS Boston, MA

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533 FXUS61 KBOX 200831 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 331 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 shifting winds to the southwest along with milder temperatures, followed by a cold front sweeping across the region that will bring showers late Tuesday night and Wednesday morning, especially across Rhode Island and eastern Massachusetts. Dry and colder conditions expected later Wednesday through the end of the week. Colder than normal temperatures overspread the region next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 330 AM Update... 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 latest 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 WNW 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 WNW winds. Normal highs for 11/20 should be in the upper 40s to lower 50s. && .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... * Warm but windy for Tuesday * Cold frontal passage on Wednesday will drop temps through the day with showers. * Dry but chilly for Thanksgiving day Pattern Overview/Confidence... Building high pressure over Baja California will push the ridge into the Western CONUS for next week. This in combination of the blocking high over northeastern Canada and Greenland will favor East Coast troughing through the period. 12z Models and their ensembles are in good agreement on the large scale pattern. However, some differences will remain on timing and amplitude of each wave. Overall trend in the forecast for southern New England will be wet weather for Wednesday with drying trend into the end of the week. Will have to continue to watch the weekend as with offshore low to the east and approaching system from the West and how those two systems will impact each other. Otherwise temperatures will trend below average for the week. Details... Tuesday night into Wednesday...High confidence. Mid-level ridge will build over the region on Tuesday as winds switch to the SW and WAA returns. Temperatures will warm within this southwesterly wind as 50`s return back to the region. A few sites may be near 60F depending on how much mixing occurs. Approaching cold front from the west will result in a developing LLJ with 925mb winds reaching 40-45 kts. Once again another windy day on Tuesday. Cold front from the Great Lakes will pass through the region on Wednesday. A coastal low off the coast of the Carolinas appears to ride along or out ahead of the front. Latest guidance continues to trend this low closer to the region, thus allowing for more moisture in the mid-levels. The GFS appears to be on the slow end of the developing low, thus not capturing enough moisture when the front moves through. Therefore trended with the EC/UKMET/NAM for this portion of the forecast. Precip will begin to overspread the region overnight and result in widespread rainfall Wed morning. Highest confidence is south and east of I-95, closer to the mid-level moisture. Regardless appears that most of the region will see some rainfall, which highest amounts closer to SE MA. One thing we will have to watch on Wednesday is the available moisture and timing of the CAA. Some guidance indicates that a few flakes can fly on the backside of the front. Uncertain on if there will be moisture. Regardless, trend is only trace to less than if it does accumulate on the roads. Thanksgiving into Friday...High confidence. Behind the front on Wednesday, CAA will move back into the region for Thursday. High pressure will move south of the region resulting in a chilly but dry day for Thanksgiving. Passing shortwave to the north Friday morning may bring in some clouds, but another dry but more seasonable day. The weekend...Low confidence. High pressure and mid-level ridge Friday night will lead to a warming trend on Saturday. Winds will back to the southwest as another front from the west approaches. Behind this system anomolous cold air will spill into the region putting temps well below average. While a few waves will move through this period, overall precip looks light. && .AVIATION /09Z 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. 06z update... Thru 12z... Mainly VFR but marginal MVFR-VFR in isolated snow showers. Vsby briefly lowering to about 4 miles. Perhaps a quick dusting or coating possible, otherwise no accumulations. Light WSW winds across RI and eastern MA then becoming WNW 07z-09z with gusts up to 30 kt. Farther west across CT/western-central MA WNW 10-20G30KT. After 12z... VFR with SCT-BKN050-060. WNW winds gusts 25-35 kt. Monday night...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 areas 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. 330 AM update... Today... Marginal WNW gales with gusts up to 35 kt. Other than a brief rain/snow shower 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: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Tuesday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Rain showers likely. Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Rain showers likely. Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Thanksgiving Day: Winds less than 25 kt. Areas of rough seas. Thursday Night 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 2 PM EST this afternoon for ANZ232>235-255- 256. Small Craft Advisory until 6 PM EST this evening for ANZ230- 236-237. Gale Warning until 6 PM EST this evening for ANZ231-250-251- 254. && $$ SYNOPSIS...Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...Dunten AVIATION...Nocera/Dunten MARINE...Nocera/Dunten

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