Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 200814 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 414 AM EDT Thu Jul 20 2017 .SYNOPSIS...
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Mainly dry but hot and humid weather persists today. There is growing concern for a round of showers and thunderstorms tonight, a few of which might be strong and produce heavy rainfall. Dry and very warm afternoons expected Friday and Saturday, but with lower humidity. Unsettled weather with showers at times are expected Sunday through Tuesday with below normal temperatures.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... 400 AM Update... Boundary still set up over southern New England as dewpoints are still in the 70s across RI and SE MA. North of the boundary, dewpoints are in the mid to low 60s as winds aloft are in a more westerly direction. These higher dewpoints has allowed for fog and stratus to remain over the CAPE and Islands. Aside from some minor updates the forecast, overall trend remains on track for this morning. Today... A warm start today as temperatures will warm well into the upper 80s to low 90s. In fact, a few sites may be warmer than yesterday as 850mb temps warm to 18C. Some dry air will push into the region, which will help increase mixing potential. A few sites may see westerly winds increase to 15-20 MPH. These winds will allow for mixing especially across the western half of the region. Thus have lowered dewpoints in that region. However across RI and SE MA, 70+ dews will pool during the afternoon. This combined with the warm temperatures will push Heat Indicies values to 95F. Went ahead an issued a heat advisory for regions that have the best shot for 95F heat indicies and that observed them yesterday. Aside from the heat and humidity, falling heights through the day may trigger a few isolated showers and/or thunderstorms. Best region for this to occur will be across the MA east coastline where sea breeze boundary will the area of focus. A strong storm cannot be rule out as CAPE values will reach above 1000 j/kg and increasing shear values. Gusty winds and heavy rain will be the main threat. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/... Tonight... Hi risk, low confidence forecast for tonight as there is still some model spread on the convection potential tonight. Models have trended farther south with a potential cluster of showers and thunderstorms propagating from the Great Lakes region. With an upper level ridge firmly entrenched across the central CONUS, thinking is that convection over the western Great Lakes region will track more southeast than eastward, following the better instability and flow. Biggest question is where this flow will set-up. The latest EC and UKMET continue to bring the cluster through the region, while the Hi- Res guidance and NAM have trended south of the Pike. Will trend the forecast towards the later as the bulk of the instability axis is south as well as the nose of southerly LLJ and the flow of the Corfidi vectors. 850 mb theta-e gradient also has trended a bit further south than 24 hours ago. Best timing for the cluster of thunderstorms to move through the region will be after 7PM and exit around 2 or 3AM. Steep lapse rates above 6 C/km combined with 40-50 kts of 0-6 km Bulk shear values and falling heights ahead of the complex will lead to a few strong to severe storms. Greatest risk will be gusty winds and heavy rainfall as PWAT are well above 1.5 inches. Agree with SPC on the marginal risk as the storms could be strong to perhaps severe. Again greatest potential will be south of the Pike. Will need to continue to monitor trends for the complex, especially this afternoon once the storms eventually develop. This will help determine SNE actual risk for this evening and early tonight. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Fri & Sat: Dry and very warm afternoons but with lower humidity * Sun into Tue: Cooler than normal + unsettled with showers at times * Drying trend likely by the middle of next week Details... Friday and Saturday... Anomalous closed upper level low across eastern Canada will result in northwest flow aloft into southern New England Fri & Sat. The result will be dry weather into at least Saturday afternoon. Plenty of sunshine should push high temps well into the 80s on both days, and a few locations may reach 90 on Friday. However, drier air will mix down resulting in lower humidity than what we have experienced the last few days. Sunday through Tuesday... The anomalous closed low across the Canadian Maritimes will help to carve out an upper level trough across the northeast. The result will be temps averaging below normal. Specific daily high temps are uncertain and will depend on timing, cloud cover, and location of any surface boundaries. Highs will mainly be in the 70s over this time frame, to perhaps a few lower 80s. It is also possible that a some locations have a day where it struggles to break 70 as a result of a surface boundary to the south coupled with northeast winds/low clouds. Plenty of shortwave energy rotating around the upper trough will result in periods of showers and perhaps isolated thunderstorms at times. It does look rather unsettled, but the entire period will not be a washout it is just impossible to try to pin down timing this far out. Wednesday... Forecast highly uncertain in this time range, but guidance shows upper level trough finally lifting out which would result in dry and pleasant weather. && .AVIATION /08Z THURSDAY 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 Term /through Tonight/...High confidence. Before 12z...High confidence. Mainly VFR with patchy fog in typical low lying terminals. Today...High confidence. VFR for most terminals. Shallow sea breezes may the focus for a isolated showers/-TSRA during the afternoon. Tonight...Moderate confidence. Chance SHRA/TSRA for the entire region. TSRA may contain strong to damaging winds along with +RA. TEMPO MVFR-IFR impacts. Majority of impacts evening and towards midnight, clearing out during the early morning hours with SW winds becoming W. Great risk is terminals south of the Pike. KBOS Terminal...VFR. Could see a sea-breeze around 15z that holds into the afternoon hours before mixing out and returning W. A spot SHRA/-TSRA is possible this afternoon. KBDL Terminal...SHRA/TSRA impacts possible Thursday evening. Outlook /Friday through Monday/... Friday and Saturday...High confidence in VFR conditions. Sunday and Monday...Moderate confidence. MVFR to IFR conditions likely at times in periods of showers, low clouds and fog patches. VFR conditions will also likely occur, but timing of all this is uncertain.
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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 Tonight/... Today and Tonight...High confidence. SW flow becoming W towards the morning before returning SW. During daytime periods and along the S/SE near shore, gusts up to 20 kts are expected. Possibility of showers and thunderstorms sweeping the waters tonight. Southern waters have the greatest chance for this to occur. Seas remaining below 4 feet. Outlook /Friday through Monday/... Friday and Saturday...High confidence. Pleasant boating weather as weak pressure gradient will keep winds/seas below small craft thresholds. Sunday and Monday...Moderate to high confidence. East to northeast winds may gust between 15 and 20 knots at times, but probably remain below small craft thresholds for the most part. Areas of fog and showers may reduce visibilities for mariners at times.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Heat Advisory from 1 PM this afternoon to 6 PM EDT this evening for MAZ017>021. RI...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for RIZ001>008. Heat Advisory from 1 PM this afternoon to 6 PM EDT this evening for RIZ001>004-006. MARINE...None. && $$ SYNOPSIS...Frank/Dunten NEAR TERM...Dunten SHORT TERM...Dunten LONG TERM...Frank AVIATION...Frank/Dunten MARINE...Frank/Dunten

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