Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 171400 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1000 AM EDT Mon Jul 17 2017 .SYNOPSIS... High pressure offshore will bring a flow of warm and humid air during the first half of the week. A cold front approaching from the Great Lakes will bring a risk of afternoon/evening thunderstorms today and Tuesday before dissipating. Typical summer conditions will continue through the remainder of the week. There will be the chance for Wednesday afternoon and evening showers and thunderstorms. Another cold front approaches with a renewed threat for showers/thunderstorms Thursday, which will slowly pass during Friday. Cooler and somewhat less humid conditions look to push in for next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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10 AM update... Early AM soundings and latest obs continue to indicate modest capping inversion at the base of a moderate EML with temps at 750mb nearly +12C. Given the presence of the EML, lapse rates above this inversion are moderate (hovering around 7.0C/km) this cap may limit destabilization processes throughout the day. However, the lapse rates may still lead to MU CAPE values around 1500j/kg. Am combining this thinking with the fact that there is little shear, and strongest low-mid lvl convergence (lifting mechanism) are mainly across central NY into the St. Lawrence valley. Best upper lvl jet/shortwave also remain closer to these regions as well. Given this, am not feeling as strongly about convective chances (at least until later) across even interior S New England. Latest mesoscale guidance and morning NAM update do support this thinking as well, raising overall forecast confidence. Therefore, will be slowing POPs down mainly until mid-late evening. This does not mean there will be no activity as any E propagating cell/cluster could move in quicker., but am feeling chances are lower with this update and will adjust forecast to reflect this. Mainly used a blend of WRF/NAM/HRRR data as a baseline. Otherwise, sped up timing of morning sea breeze development on E coast. In spite of the gradient winds being mainly S, winds aloft seem weak enough to suggest standard sea breeze development 14Z-15Z (10-11 AM local). Temps and dwpts will also be adjusted, primarily to increase daytime max temps as little airmass change and plenty of sunshine for mixing, and relatively weak S flow should allow some areas to reach mid-upper 80s. Previous Discussion... Cold front over the St Lawrence Valley supported by closed low over the Great Lakes. As the low deepens, it is turning the upper flow parallel to the cold front and slowing/stopping its forward motion. Expect the front to remain to our north and west through the day. Weak high pressure off into Mass Bay and the Gulf of Maine will continue to bring a south flow across the region. This allows fair skies for much of the day, with mixing generating high temps in the low to mid 80s. Main question is the potential for convection to break through the cap, or for convection in Upstate NY to move into our area. CAPE in Western Mass is forecast to reach 1000-2000 J/Kg, with totals in the low 50s and Lifted Index in around -5. Winds at 850 and 500 mb remain weak, thus weak dynamics. So the potential for damaging weather will depend on strength of the instability. Best chance of thunderstorms will be in Western Mass with diminishing likelihood farther east. Precipitable water values will be around 1.5 inches, so some local heavy downpours are possible. We will also mention potential for strong wind gusts with any afternoon thunderstorms, primarily in Western Mass.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... The upper low and trough weaken as they move east, crossing New England on Tuesday. We note little change in the airmass as the shortwave and its surface cold front move through. Any frontal characteristics shift back to the northwest. Instability weakens overnight but remains sufficient for a few showers and thunderstorms overnight and Tuesday. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Threat for scattered showers and thunderstorms at times Wednesday into Wednesday evening. * A slow moving cold front will bring a better chance for showers/thunderstorms Thursday into Friday morning * The front should push off the coast by late Friday, with somewhat cooler, drier conditions for next weekend Overview... Not noting many big differences amongst the 00Z model suite into the early Thursday timeframe, then models showing timing and track differences with the approach of a cold front. Big issue will be how far south and how quickly the front will move across the region late this week. Details... Tuesday night through Wednesday night... Will see typical mid summer conditions across the region, with warm and humid weather. Noting high H5 heights continue through Wed, on order of 586-587 dm. Should see high temps Wed running up to around 5 degrees above seasonal normals away from the coast. A weak surface trough may provide enough of a trigger for scattered showers/isolated thunderstorms. Some models also signaling a weak H5 short wave moving across Wed afternoon/ night which could keep convection going through a portion of Wed night. Thursday through Sunday... A cold front starts to slide SE out of southern Canada late this week. With model solutions diverging on the timing of this system, not a lot of confidence during this timeframe. However, some run-to-run consistency does suggest the front should move across the region Thu night into Fri morning. Should see drier and somewhat less humid conditions forecast for Saturday and Sunday. && .AVIATION /14Z 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/... Today...Moderate to high confidence. Patchy fog, some locally dense across portions of Cape Cod and the islands as well as S coastal RI, should continue to dissipate through around 14Z-15Z. Some fog might linger a bit longer across Nantucket, but should improve. Otherwise VFR through the day. Most showers/tstms should remain to our N and W, but with potential for scattered storms in western/central Mass and possibly Hartford County CT. Brief MVFR-IFR conditions in any such storms, which could have strong wind gusts and would be most likely in the late afternoon. Light S-SW wind with local sea breezes along the coast, including at KBOS. Tonight...Moderate confidence. VFR in general but with MVFR/IFR cigs/vsbys developing overnight in fog, especially along the south coast. A few lingering showers possible early in the night. Tuesday...Moderate confidence. Low ceilings/vsbys in fog early. Otherwise VFR. Showers and a few thunderstorms are possible in the afternoon with brief MVFR cigs/vsbys. KBOS Terminal...High confidence in TAF. South winds tonight becomes SE in a sea breeze late morning. Dry weather prevails. KBDL Terminal...High confidence in TAF. VFR, except a period of MVFR later tonight in patchy fog. Outlook /Tuesday Night through Friday/... Tuesday night through Friday...Moderate confidence. Expect mainly VFR conditions. Will likely see SCT-BKN low end VFR CIGS with a chance for scattered SHRA/isolated TSRA mainly during the afternoon and evening hours. Best chance for SHRA/TSRA should occur Wed night and Thu. Will see local MVFR- IFR CIGS/VSBYS in any shower/thunderstorm activity. && .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence. Light S-SE winds continue thru Tuesday along with tranquil seas. Areas of patchy fog this morning especially near Nantucket and Marthas Vineyard. Risk of more widespread fog tonight with vsbys less than a mile. Outlook /Tuesday Night through Friday/... Moderate confidence through the period. Expect winds and seas to remain below small craft criteria through Friday. SW winds may gust up to 20 kt on the outer waters Wed night and Thu, then shift to W late Thu into Fri. Seas up to 4 ft on the outer waters E and S of Cape Cod and Nantucket as well as the southern outer waters Thu night and Fri. Mainly dry Tue night, but may see patchy fog during the night with visibility restrictions which should improve by midday Wed. Scattered showers, along with isolated thunderstorms, may develop any time Wed through Thu night, with the best shot for thunderstorms occurring Thu afternoon and night. Will also see continue patchy fog developing during the late night/early morning hours each day with locally reduced visibilities. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/Doody SHORT TERM...WTB LONG TERM...EVT AVIATION...WTB/EVT MARINE...WTB/EVT

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