Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 171052 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 652 AM EDT Sat Mar 17 2018 .SYNOPSIS... Mainly dry but unseasonably cold weather will continue into early next week. A multi-part storm system moves to the Mid Atlantic states midweek. This system may affect parts of Southern New England during midweek, but it is also possible the storm stays to our south. Currently there is low confidence on the eventual behavior of this storm. High pressure builds with drier weather Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 645 AM Update... Mostly clear skies across Southern New England. But satellite imagery does note a patch of clouds off the Cape and Islands, and a second area of clouds over VT and Northern NY, the latter showing mainly mid-level clouds. No changes in conditions, with a Northwest upper jet diving across New England. This along with 7C/km lapse rates should generate lift. Daytime sunshine will support mixing through the lower layers, and draw strong wind gusts to the surface. The result will be cloud development during the morning, temperatures climbing into the 30s, and gusts to 35 mph. No significant changes to the forecast planned at this time. Previous discussion... WV imagery suggests core of a strong vort-max and attendant H5 cutoff is rotating through central Quebec. Meanwhile slight warm advection to the has allowed heights to build overnight. This has lead to mainly clear conditions, except where a very narrow band of moisture, between H85 and H7 has led to some SC enhanced by higher terrain of W MA. This squeeze play has allowed for some continued cold advection near the surface, enhanced by a stronger height rise-fall couplet and resultant winds. Otherwise, the vort-max moves through N New England through the day, leading to a kink in the MSLP fields mid day. This implies convergence at the sfc. Noting also through the morning that continued low-mid lvl cold advection will lead to enhanced lapse rates from the sfc to the top of the mixed layer (around H75) near 7C/km. The lake enhanced moisture field between H85 and H7 rests around the top of this mixed layer. Based on all of this, cannot rule out a brief snow shower or even squall (thanks to gusty winds 25-35 mph remaining an issue). Otherwise, flurries or sprinkles. The limiting factor will be the moisture as it remains low and only in the layer mentioned above. Therefore, little to no accumulation is expected, but a few spots on the grass are possible. This risk ends by late afternoon/evening as height rises lead to stronger subsidence, both eroding the moisture layer and supressing any weak lift. Highs will be limited to the 30s for the most part, as H85 temps remain at or below -11C all day, and even if mixing reaches higher it would be limited by the continued lower lvl cold advection. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Overnight... With rising heights aloft, subsidence will lead to clearing and finally slacken the sfc pres gradient. This suggests clearing skies and a weak enough pres gradient for some radiational cooling. This will allow min temps to drop into the teens and even single digits at the obs sites normally prone to decoupling. Otherwise a dry ngiht, one of the colder nights in recent past. Sun... A stretched vort-max pivots through the SW flow aloft, on the W periphery of a longwave trof across the Maritimes. Once again, the majority of the column is moisture starved, but with weak lift and NW flow some CAA SC is likely throughout the day, along with a risk for ocean enhancement along the Cape. Mainly dry otherwise. NW winds, but generally weaker than previous days as the core of the sfc pres gradient change is to the E. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Big Picture... Closed low over the Maritimes slowly ejects northeast. This keeps a cyclonic flow over New England early in the week. Southern jet stream remains in place most of the period with one shortwave moving east to the Mid Atlantic coast Tuesday and a second digging in over the Midwest Wednesday. This sets up a complicated flow to our south that generates multiple weather systems. These could easily pass to our south, or one of them may turn north and bring pcpn to Southern New England. Run-to-run trends on the operational models maintain dry weather Monday, so high confidence in that part of the forecast. Trends also push midweek precipitation farther south and mostly offshore. Ensemble runs still show possible scenarios that bring precip into our area midweek. Confidence in this part of the forecast remains low. Details... Sunday night and Monday... High pressure centered over Northern Ontario noses southeast over New England, and should provide sufficient subsidence for mostly clear skies. Mixing depth is forecast at 900 mb, with temps at that level supporting max temps in the 30s. Clear skies, light wind, and dew points in the teens should support Monday night min temps in the mid teens to lower 20s. Tuesday through Thursday... As noted, a complicated pattern is being projected by the models. There is a consistant trend of pushing the storm track farther south, with all opertional models showing QPF mainly south of our region. The GFS does show 6 hours of precip south of a Plymouth- Providence line Tuesday afternoon. Also the ensemble members show greater potential for precip reaching farther north, contrary to the operational models. If there is any confidence in the model data, it is in showing multiple storm segments passing us during this period. Temperature profiles mostly support snow, expect for a rain-snow mix Wednesday. We have lowered pops a little, but maintain at least slight chance pops in general and chance pops across southern RI and parts of southeast Mass. This remains a low-confidence forecast, with all scenarios...a complete miss, a South Coast special, or a broader Southern New England event...continuing in play. Friday... High pressure builds over the region with dry weather and seasonably cool temperatures. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. Through this evening... Mainly VFR. Although occasional light SHSN/SHRA possible mid morning through mid afternoon with very brief MVFR/IFR conditions. Winds W-NW shift to mainly NW late afternoon. Gusts 25-35 kt at times. Tonight and Sun... Mainly VFR with clearing. Winds also diminish this evening but remain out of the NW. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Sunday Night through Wednesday/... Sunday Night through Monday: VFR. Breezy. Monday Night: VFR. Tuesday: VFR. Breezy. Tuesday Night: VFR, with with areas of IFR possible on the South Coast. Windy with gusts to 35 kt. Slight chance SN. Wednesday: VFR, with areas IFR possible on the South Coast. Strong winds with local gusts to 40 kt. Slight chance SN. && .MARINE... Short Term /through Sunday/...High confidence. Today... W-NW Gales continue through the day. mainly on the eastern and ocean waters to the S of New England. These dissipate late this afternoon and evening, with lingering Small Craft Conditions to follow. Seas peak this morning 8-10 ft, and then begin to diminish. Tonight into Sun... Winds seas diminish overnight such that a brief period with conditions below Small Craft Advisory thresholds for a brief period late into early Sun AM. The winds turn NW and pick up 25-30 kt during the day on Sun, sugggesting a need for Small Craft Advisories to continue. Outlook /Sunday Night through Wednesday/... Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Monday through Monday Night: Winds less than 25 kt. Tuesday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas locally approaching 5 ft. Slight chance of snow. Areas of visibility 1 to 3 nm. Tuesday Night: Moderate risk for gale force winds with gusts up to 40 kt. Areas of rough seas. Chance of snow, chance of rain, chance of sleet. Visibility 1 to 3 nm. Wednesday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 11 ft. Chance of rain, chance of snow. Areas of visibility 1 to 3 nm. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until midnight EDT tonight for ANZ232>235- 237. Gale Warning until 6 PM EDT this evening for ANZ230. Gale Warning until 8 PM EDT this evening for ANZ231-250-251- 254>256. Small Craft Advisory until 6 PM EDT this evening for ANZ236. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.