Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 200757 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 357 AM EDT FRI MAY 20 2016 .SYNOPSIS...
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High pressure over the Northeast USA will bring dry mild weather today and much of Saturday. Coastal low pressure then passes south of New England Saturday night and Sunday. It then stalls in the Gulf Of Maine Sunday with rain transitioning to showers. The unsettled conditions continue Monday and Tuesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Clear skies, light wind, and moist ground from showers have created patches of locally dense fog across the region. This fog may affect parts of the morning rush hour especially in the CT River Valley of MA. Fog will burn off shortly after sunrise. Upper shortwave and cold pool are moving off to the east, but still linger close enough this morning for potential destabilization as the sun heats the surface. This will generate another round of diurnal clouds. There is less moisture available aloft for clouds so sky cover should be less than on Thursday. Light north flow, but the pressure gradient under the high will be light enough to allow sea breezes to develop late morning and afternoon. Temps at the top of the mixed layer will support max sfc temps in the mid 70s. If the mixed layer goes a little higher/deeper, then a few upper 70s may be possible. Sfc temps will be cooler at the coast where sea breezes develop.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
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Tonight... High pressure remains overhead through the night and provide fair dry weather. Increasing high level moisture will bring some increase in high level sky cover. Radiational cooling for at least a part of the night. Dew points of 45 to 50 will allow temps to fall to near that level, with low 50s in some of the urban centers. Saturday... High pressure moves off to the east but remains close enough to maintain dry air over New England below 10 thousand feet. Meanwhile moisture increases above 10 thousand feet bringing increasing/thickening sky cover. Overall a fair day with temps aloft supporting max sfc temps in the 70s, except cooler along the south coast. The main question for Saturday is the eventual timing of rain/showers from coastal low pressure. Shortwave from the Southern Plains generates surface low that moves through KY/TN later today and tonight, then reforms along the Mid Atlantic coast. There is a fair consensus among the models, so we will use a blend of the group. Even so, there is some contradiction in the forecast data. Measureable QPF amounts reach a Windsor Locks- Woonsocket-Plymouth line by 8 PM Saturday, but system-generated lift is forecast only to the south coast. Cross sections of RH also show plenty of dry air below 700 mb through at least 8 pm. This would evaporate any rain that attempted to fall. With this in mind, we have trimmed POPs for Saturday late afternoon by about 15 percent with likely pops only to the South Coast and chance pops across CT-RI-SE Mass. We also maintained chance pops in the CT Valley.
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&& .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
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Highlights... * Rain, especially along the S coast Sat Night into Sun. * Unstable but milder Monday and Tuesday. * Warmer still and drier Wednesday. Overview and Model Preferences... 20.00Z model guidance suite still shows reasonably good agreement through the middle of next week in the synoptic-scale, impressive given the highly amplified flow pattern across the CONUS. Building ridge upstream of New England across the plains and Great Lakes, leads to gradual deepening/digging of the trof developing across New England. This feature then cuts off and meanders near to south of the region for much of the late weekend and early half of this coming week. Ensembles show H5 anomalies almost 10dm below normal. The attendant cold pool reaches -20C at it`s coldest, which suggests unstable conditions linger into the early portions of the week until it fills and warms aloft. Since there is good agreement with all of these over-arching issues, a consensus blend will work as a starting point. Only caveat will be the initial coastal low pres developing Sat night into Sun, will lean more on ensembles due to differences in the precip shield spatially. Details... Sat night... Low pres develops near the Delmarva and then slides E and deepens as it approaches the 40/70 benchmark. Primary issue is how far the wrap-around precip shield reaches into Srn New England. ECMWF is currently the driest with only the immediate S coast seeing measurable precipitation. NAM/CMC are much further N...covering much of Srn New England. The ensemble probabilistic approach brings nearly 70 percent along the S coast, 40 percent along the Mass Pike, then near 20 percent at the MA/NH border. With the uncertainty, this is actually a good representation for POPs during the overnight hours, and will lean quite close to these numbers. Otherwise, the rain comes to an end as drier air gets wrapped into the system after sunrise. However this dry slot will also lead to a push in the mid- lvl baroclinic zone shifting the shield to the N, so expect rising pops into the daylight hours until ending mid morning. Temps mild with rising dwpts, expect mins only in the 50s. Sun... Low pres will be shifting into the Gulf Of Maine yielding breezy NE flow, clouds and continued low-mid lvl moisture spilling into the region. Therefore...expect a period of gray, cool and damp conditions for much of Sun. This also will limit destabilization in spite of cooling at H5. Latest guidance has shifted the instability trof further W where more sunshine is expected. Will follow this trends given the flow off of the Gulf. Highs mainly in the upper 50s and low 60s. May need some wind headlines /mainly advisories/ for portions of E MA and RI as pres gradient deepens further with the low pres shifting E. Mon and Tue... Cutoff low pres in place will allow several shortwaves to rotate across Srn New England. The most robust looks to occur late Mon night into Tue, so will feature an increase in POPs during this time. Diurnal cloud cover is likely with cold H5 temps in place, and could allow for the development of some T-storms occasionally with the wave passages. Too uncertain for exact timing, but expect on-off showers and potential storms through the period. Temps near to slightly above normal thanks to H85 temps still holding near +8C in spite of the cold temps above. Wed... Brief ridge of high pres regains control before sfc warm front approaches from the W. Will continue with mainly dry conditions and mild temps, as highs shift solidly in the 70s across much of the region. Thu... With the potential for a warm front to shift N of the region, looking at temperatures reaching above normal. However, weak cutoff energy to the E could allow for diurnally driven shra/tstms each day thanks to the warm and somewhat destabilized airmass. No certain solution here, but both ensembles and operational ECMWF and GFS are supporting this broad thinking.
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&& .AVIATION /07Z FRIDAY THROUGH TUESDAY/...
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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 Saturday/... Through 12Z...High confidence. Brief MVFR vsbys possible Cape Cod and Islands in showers. IFR/LIFR cigs/vsbys in the CT Valley region of Western Mass in fog. Patchy IFR in fog in regular fog spots farther east. Otherwise VFR with a light north wind. Friday...High confidence. VFR and dry weather. Diurnal clouds develop but with bases at or above 4000 feet. Sea breezes develop along the coast late this morning. Inland areas will have a light NW wind shifting from the SW during the afternoon. Friday night...High confidence. VFR in dry weather and light winds. Saturday...Moderate to high confidence. VFR conditions likely through the day with a light South Southwest wind. Increasing sky cover through the day. There may be scattered showers in Western MA/CT during the afternoon. There is a low potential for MVFR cigs/vsbys developing in the late afternoon/evening, earlier than forecast, in light rain. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook...Saturday night into Tuesday Sat night into Sun...Moderate Confidence. Mix of MVFR/IFR likely as rain, fog and low clouds build from S-N over the region and NE flow develops on Sun. Wind gusts 20-30 kt possible at times Sun morning. Mon and Tue...Moderate Confidence. Although VFR is likely to dominate, some periods of MVFR/IFR are possible in shra and fog development especially during the overnight hours.
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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. Today...High forecast Confidence. High pressure provides light NW winds in the morning then becoming light and variable. Seas remain below 5 feet. Dry weather and good vsby. Tonight...High forecast Confidence. More of the same...high pressure overhead provides light winds/dry weather and good vsby. Seas remain below 5 feet. Saturday...High forecast Confidence. Low pressure develops along the Mid Atlantic coast. Winds will increase from the south but remain below 25 knots. Seas will remain below 5 feet. Outlook...Saturday night through Tuesday... Sat night into Sun...Moderate Confidence. Winds will be shifting to the N-NE with time and increasing, especially over the SE waters. Some gusts to gale force possible, but small craft thresholds can be expected. Seas increase to as high as 8-10 ft on the ocean waters. Mon and Tue...HIgh Confidence. Low pres churning in the gulf of maine will keep a swell going for much of Monday even if seas drop below SCA thresholds. Therefore, expect small craft advisories to continue Mon, but possibly drop by Tue as the low pres moves east.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/MD NEAR TERM...WTB/MD SHORT TERM...WTB LONG TERM...MD AVIATION...WTB/MD MARINE...WTB/MD

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