Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 192325 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 725 PM EDT Fri May 19 2017 .SYNOPSIS... A cold front moving over Southern New England will move offshore this evening. High pressure from Canada will build south over our region tonight and remain in place for much of the weekend. More seasonable weather is expected for the weekend into early next week. Dry initially then turning wet Monday ahead of a sweeping cold front. A brief reprieve Tuesday prior to more rain expected mid to late week along with continued near-seasonable conditions. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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7pm update... If you are sitting near the Cape Cod Canal, or at the very least just northwest of it, you are seeing an impressive little convergence zone between a very weak sea breeze off of the east coast, and the fighting fronts (cold front trying to slide offshore, and ongoing southerly sea breeze) just inland of the south coast. Although waining, the limited remaining instability (ML CAPE values still right around 1000j/kg) has been just enough such that a continual string of updrafts develop at the convergence zone, then as the slide E quickly become cutoff by their own rain cooled downdrafts given they reach a limiting height where drier air continues to filter in aloft. Have seen several showers with a few rumbles of thunder start near the Plymouth/Carver/Wareham lines then travel across the N shore of Cape Cod, cross the Cape then move just offshore of the outer arm. This trend is likely to continue at least a little bit longer until the diurnal temperature trend is enough to allow the cold front to slip quietly offshore and limit any further updraft development. Also, noted a fast moving shallow almost backdoor cold front racing out of the colder waters of the Gulf of Maine thanks to this low lvl warm low development in the convergence zone mentioned above. A lot of very interesting features. Meanwhile, still a mild start to the evening. Have actually slowed diurnal trend a bit given the fact that the front has not been able to out-maneuver the sea breeze. As such, also raised overnight mins just a few degrees (still much cooler in relation to the past couple of nights). This is also supported by some leftover anvil CI breaking through the clearing as it moves NE from developed convection across MD/VA. Previous discussion follows... Tonight... High pressure builds south from Canada tonight. clearing skies and drier air will overspread all of Southern New England. Dew points will fall back into the 40s and eventually the 30s. This will make room for temperatures to fall back into the 40s inland and low 50s near the shore.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Saturday... Fair weather day with high pressure build in. Some high clouds may linger to the south and southwest, but otherwise a sunny day. Mixing should again reach at least 850 mb, where temperatures will be 4-6C. That would support max temps in the mid and upper 60s. If we mix higher, which is very possible in the interior, temps would reach the lower 70s. Winds will turn northeast in eastern Mass and RI in response to the position of the high center. There could also be a sea breeze component along the immediate coast. Either way, temps in eastern Mass should be cooler than farther west. We will go with a range of 60s east to lower 70s west. Saturday night... Fair skies and light wind. Dew points in the mid 30s to low 40s. Expect min temps in the upper 30s to mid 40s. && .LONG TERM /SUNDAY THROUGH FRIDAY/... */ Highlights... - Seasonable and dry for Sunday - Rain with embedded heavier showers for Monday - Dry on Tuesday - Wet for the mid to late week period, remaining near-seasonable Overview and model preferences... The ridging Rex-like block allowing for the nice weekend will be shifting to the E by Mon as the upstream cutoff moving out of the Plains to the Great Lakes interacts with deep trof near Hudson Bay. This forces the cutoff further E and digs/deepens it toward the SE through the end of the week. This provides a conduit for several shortwaves within the cyclonic flow on the E side of the cutoff. Therefore, as previous forecaster noted, an active pattern sets up with periods of showers/storms possible, but not a washout as each individual wave will be followed by weak ridging yielding breaks. Noting fair agreement here between models such that a blend of guidance can be used as a baseline for this long-term forecast update. Details... Sun... The pleasant weekend continues. Plenty of sunshine under high pres which remains in control. May be a few late day clouds building over from the W, but this should not limit the pleasant conditions. H85 temps warm through the period starting around +4C at 12Z but reaching +8C by late afternoon. This should allow temps to inch back into the low-mid 70s away from coastal areas which will be cooled by sea breezes. Sun night into Mon night... Occluding low pres sliding into Ontario and Quebec will be pushing its occluding fronts through the region by daytime Mon. In fact, noting the possibility of triple point low pres formation, slowing the system down by Mon evening. This should be enough lift to interact with modest moisture (PWATs reach about 1.5-1.8 inches) and allow for a period of wet wx with embedded heavier downpours through the period, starting from W-E Mon morning. The risk for downpours amplifies by the potential triple point low. Modest convective risk also adds to the risk for heavier downpours as SLIs are noted to dip to near 0 with some conditionally unstable lapse rates. Therefore, could see widespread rainfall totals of 1.00-1.50 inches with the possibility of a few higher amounts where heavier rains are observed. Flooding risk is very low given the progressive nature leading up to the triple point development. Typical nuisance and poor drainage flooding are the potential impacts here. Did undercut guidance somewhat for highs especially Mon, with the warm front likely never making it through New England (especially given the triple point risk) leaned more heavily on raw model 2m temps which were much cooler. Tue into Wed... Modest ridging is noted in the mass fields this period as the upstream cutoff continues to slowly dig/deepen across the lower Great Lakes. This should allow for a reprieve from precipitation for the most part across the region as the amplifying pattern will slow shortwave progression. Will generally highlight another two day stretch of generally nil POPs given this ridging and associated high pres. H85 temps remain near +10C through the period suggesting highs in the low 70s for the most part while lows (assuming enough clearing could dip in the upper 40s and low 50s). In essence, near seasonal normals. Thu into Fri... Strong shortwave looks to rotate through the longwave trof/cutoff forcing the whole pattern to shift E. Tapping into Gulf of Mexico moisture and yielding good lift, we are looking at low pres development E of the Appalachians then sliding NNE to once again yield a prolonged period of wet wx with pockets of embedded heavier rain. Timing somewhat uncertain given the pattern, in fact at this point, its likely too fast pushing most of the rain through on Thu. This is a typical long-range bias with highly amplified flow. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
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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 Night/... Tonight...High Confidence. VFR. Especially after shower risk ends and haze dissipates on Cape/Nantucket by around 02Z. Then a few mid clouds late tonight. Winds will continue to shift around to the N over the next couple of hours with a few gusts to 20-25 kt just as the shift occurs. Saturday...High confidence. VFR. Onshore wind expected along east coastal MA. North winds continue in the interior. Saturday night...High confidence. VFR. Mostly clear skies, light wind. KBOS TAF...High confidence. VFR expected through the period. Note wind shifts to be from the east on Saturday afternoon. KBDL TAF...High confidence. VFR expected through the period. Outlook /Sunday through Wednesday/... Sunday...High confidence. VFR. S winds at or below 10 kt most of the day. Sea breezes expected all coastlines. Sun night into Mon night...Moderate confidence. MVFR-LIFR. Lowest conditions likely for the S-coast with potential fog/rain impacts. Increasing S winds with gusts up to 25 kts. RA/+RA at times. Low risk TSRA. Possible LLWS impacts with S/SW winds 2 kft AGL 40-50 kts. Tuesday into Wednesday...Moderate confidence. Improvement expected Tuesday with mainly VFR conditions W winds shift to S-SW. Wednesday uncertain, but mix of VFR is possible although if rain occurs, predominant MVFR/IFR expected.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...High confidence. Lingering 5 foot seas on the southern and southeast outer waters this evening, but this should diminish this evening. A cold front moves across the waters early tonight. Winds then turn from the northwest and eventually from the north. Winds will be less than 25 knots and seas less than 5 feet through the night. High pressure builds over the waters Saturday and Saturday night. Winds turn from the northeast Saturday and then become light variable Saturday night. Seas will remain less than 5 feet. Outlook /Sunday through Wednesday/... Sun and Sun night...High confidence. Quiet boating weather expected winds will shift to the S on Sun. Mon into Mon night...High confidence. Period of Small Craft Advisory conditions as low pres passes along the S waters. Winds will shift out of the S to the SE late Mon, then to the W-NW Mon night. Wind gusts 25-35 kt expected, with a risk for low end Gales. Seas will build to 5-7 ft on the ocean waters. Tue and Wed...Moderate confidence. W winds shift to the E on Wed. At this time, it appears winds/seas should remain below Small Craft Thresholds, however there is some uncertainty in the pattern by mid week that may change this thinking. && .CLIMATE... High temperatures through 4 pm... BOS 90 90 (1986) PVD 91 New record, old record 89 (1906) BDL 92 94 (1962) ORH 84 92 (1962) Lost in the talk of high temperatures... all four climate sites broke their high minimum temperatures on Thursday. BOS 71 Old record 63 (1896) PVD 70 Old record 62 (1943) BDL 67 Old record 64 (1906) ORH 69 Old record 63 (1943) && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ254>256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...WTB LONG TERM...Doody AVIATION...WTB/Doody MARINE...WTB/Doody CLIMATE... is the U.S. government's official web portal to all federal, state and local government web resources and services.