Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 200556 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 156 AM EDT Tue Jun 20 2017 .SYNOPSIS... A cold front moves across Southern New England tonight, and then moves offshore Tuesday. Another weak front moves across Southern New England Wednesday. High pressure then builds over the region later Wednesday and Thursday with seasonable temperatures and lower humidity. Warmer temperatures return late in the week as the high moves off to the east. Another cold front crosses from the west over the weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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150 AM update... Showers and isold t-storms assocd with the low level jet lifting north into SNE. Main focus for a brief t-storm with heavy rain through 12z will be across SE New Eng in close proximity to low level jet and best instability. Previous discussion... Convection has weakened considerably as it moves into RI and eastern MA while back edge of the showers moving through western MA. Flash flood watch has been cancelled. Still some marginal instability combined with low level jet and good moisture flux for showers and a few t-storms to persist and move across eastern and SE New Eng into the early morning hours as convection lifts NE from the mid Atlc coast. Adjusted PoPs for latest radar trends and hi-res guidance. Other concern for tonight is areas of fog which may be locally dense along the south coast. Cold front not expected to reach the south coast until Tue morning so higher dewpoints will persist which will allow for areas of fog, locally dense, to persist overnight. Gusty SW winds in the coastal plain will slowly diminish overnight as the low jet shifts eastward, but the gusty winds will continue overnight for the Cape/Islands.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/... Cold front pushes east of southern New England during the day. Much drier air works its way into the region. Low chance for scattered to isolated showers eastern MA/RI, mainly in the morning. However a cap around 10-15 kft accompanied by drier air aloft will limit the vertical extent of any convection. Breezy with W/SW wind gusts 20-25 mph. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Big Picture... A mainly zonal pattern over the Northern USA through the week. A shortwave within this flow moves across New England during midweek and off to the east Thursday. Closed low ejects from the Gulf of Alaska through Canada by midweek and swings a shortwave axis through New England toward Saturday. Another shortwave approaches from the west on Monday. Contour heights are near normal midweek, and then climb above normal late week and the weekend. Expect temperatures near normal midweek and above normal by the weekend. Model mass fields show agreement through Thursday, with the ECMWF and GFS in general agreement through Friday. Increasing differences, especially regarding timing of weather features, and resulting decreasing confidence after Friday. Details... Tuesday night-Wednesday...Moderate-High confidence. RH fields and cross sections show some lingering shallow moisture Tuesday night. We will be in the right entrance region of the upper jet, which will provide upper venting, but with little to work on we will expect patchy clouds and no pops. Upper trough and its cold pool move overhead Wednesday. The surface reflection will provide some low level convergence, while the cold air aloft will combine with daytime heating to destabilize the airmass. Moisture remains limited but concentrated near the top of the mixed layer. Lifted Index shows negative values while Totals show mid to upper 40s and a couple of patches around 50. We will show low-end chance pops for showers/tstms. Any showers may linger into the evening but then diminish. Mixing to 775 mb with temperatures supporting sfc temps in the lower 80s. Thursday...Moderate-High confidence. High pressure builds over the region. Expect dry weather with temps aloft supporting upper 70s and lower 80s. Friday...Moderate confidence. Diverging mass fields depicted by the various models, but all agree on surface low pressure passing across Northern Quebec. This puts our area in the warm sector with dew points back in the 60s. Upper jet is over or north of the St Lawrence Valley and rather far off for dynamic support. But consensus model sfc pressure and thickness fields show a lee trough along the coastal plain with cold front trailing in the Eastern Great Lakes. Lifted Index is negative and Totals are in the upper 40s. With this pattern we expect showers with scattered thunder, especially Friday afternoon/evening. The actual cold frontal passage looks like either later Friday night or early Saturday. Saturday through Monday...Low confidence. Model mass fields continue to diverge, but consensus suggests the cold front moving offshore early Saturday with an end to precipitation and slight lessening of the dew points to near 60 by Saturday evening and into the 50s Sunday and Monday. Weak high pressure surface and aloft should bring dry weather Saturday night and Sunday. Of concern are signs of shortwaves in the flow that might cross New England Sunday night and Monday. These would support a chance of showers as they pass. We will show chance pops for showers much of the area Sunday night which lift out to the northeast Monday. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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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 Wednesday/... Through 12z...Widespread MVFR/IFR cigs with areas of LIFR stratus and fog near the south coast. Sct showers moving north into SNE with an isold t-storm possible S New Eng and particularly the Cape/Islands. Today...Moderate Confidence. Improving to VFR 12-15z but areas of IFR may persist over the Cape and especially ACK until 15-18z. SW wind gusts to 25 kt developing in the afternoon. Tonight...High confidence. Mainly VFR but some fog is possible over ACK. Wednesday...High confidence. VFR with sct-bkn cigs 5-7k ft developing. An isold t-storm possible in the afternoon. W/SW gusts to 20 kt. KBOS Terminal...Moderate confidence. Lower cigs should improve to VFR by 12-14z. KBDL Terminal...Moderate confidence. Lower cigs should improve to VFR around 12z. Outlook /Wednesday Night through Saturday/... Wednesday through Thursday...Moderate-High confidence. VFR. Developing daytime cumulus clouds Wednesday with scattered or widely scattered showers/thunder. One of these showers may briefly lower vsbys to MVFR levels. Southwest wind Tuesday night/Wednesday gusting to 20 knots...turning from the west late Wednesday and becoming variable Thursday. Patches of IFR in fog are possible each night, especially on the South Coast. Friday... Moderate confidence. Cold front to our northwest with warm humid air in place over our area. Scattered showers/tstms possible, especially in the afternoon/evening. Generally VFR, but with brief MVFR/IFR in afternoon/evening showers/tstms. Southwest winds gusting to 20 knots. Saturday... Low confidence. Conditions improving to VFR as the cold front moves offshore.
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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 Wednesday/... Tonight...High confidence. Strong low level jet will result in S/SW wind gusts of 20 to 30 knots into tonight. Persistent S/SW flow will build seas to between 5 and 8 feet across our open waters. Small craft headlines remain posted for all waters. The other big concern for mariners will be areas of fog, which will be locally dense at times across our S waters. Scattered thunderstorms are possible across the waters late this evening, diminishing to showers Tuesday morning. Tuesday...Moderate confidence. Overall SW winds around 10 to 15 kts with gusts 20 to 25 kts, with strongest winds during the day. Seas 4-7 ft linger thru the day on the outer coastal waters. SCA headlines posted for outer coastal waters as well as RI/BI Sounds on Tuesday. Outlook /Wednesday Night through Saturday/... Tuesday night and Wednesday...Moderate-High confidence. Winds from the southwest with gusts reaching 25 knots on the Cape and Islands waters Wednesday. Winds shift from the west late Wednesday as a weak cold front moves through. Possible shower or thunderstorm with the front as it moves through. Seas 5 to 6 feet on the outer waters and on RI and Block Island Sounds. Thursday...Moderate-High confidence. Winds diminish and become shifting as high pressure moves overhead. Seas 4 feet or less. Friday-Saturday...Low-moderate confidence. Cold front in the St Lawrence Valley sweeps southeast, crossing the waters Friday night or Saturday morning. Areas of poor visibility in fog Friday, then locally poor in scattered showers/thunderstorms Friday evening/night as the cold front moves through. Non-thunderstorm winds will gust from the southwest 20-25 knots Friday afternoon and night ahead of the front, then less than 20 knots from the west-northwest Saturday. Seas less than 5 feet both days.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 AM EDT this morning for ANZ231>234-251. Small Craft Advisory until 2 AM EDT early this morning for ANZ230-236. Small Craft Advisory until 5 PM EDT this afternoon for ANZ235- 237-250-254>256.
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&& $$ SYNOPSIS...WTB/NMB NEAR TERM...KJC SHORT TERM...NMB LONG TERM...WTB AVIATION...WTB/KJC MARINE...WTB/NMB

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