Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 232336 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 636 PM EST Thu Nov 23 2017 .SYNOPSIS... Quiet weather through Friday with dry and cool conditions. Milder air returns Saturday ahead of a cold front. That cold front swings through Southern New England late Saturday and early Saturday night. Blustery and cooler weather follows Sunday. Conditions trend toward dry and milder again Tuesday and Wednesday, and then cool again Thursday behind another cold front. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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635 pm update... Satellite images show high clouds moves across New England, and an area of mid-level clouds diminishing as it approaches from our west. No precip upstream. So expect a fair weather night with patchy clouds overnight. We made a few minor adjustments based on observed temperatures and dew points, but otherwise no significant changes to the forecast. Previous discussion... Scattered to broken mid to high cloud decks filtering across the region could put a blanket on Thanksgiving festivities keeping temperatures slightly warmer. Positively- tilted open wave and attendant weak vortmax sweeps through the region early across the N-stream, a flattening feature, as energy sweeps up from the SE. Mid to high clouds sweeping through however into a region of drier air and subsidence, can already see some erosion in the latest GOES-16 satellite. Perhaps some blanket / insolation impacts so did not go with coldest of MOS guidance. Towards the lower end with forecast light winds, looking at lows down into the 20s, warmer along the shore.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Nice day on tap. With daytime mixing under mostly clear conditions, the mid to high clouds eroding out, winds turn S/SW. Warm air advection proceeding aloft, mixing is limited to around H95. Looking at highs in the upper 40s to low 50s, edging towards the warmer end of guidance. With both N and S stream energy still neighboring our region, dry subsidence inversion prevails. As we go towards evening, the cap well in place, mixing up to just beneath, and the boundary layer cools, likely to see the moisture pooling condense and subsequently could see the development of low-level stratus evening and overnight especially over SE New England. Lows down into the 30s most places, but warmer SE around the low 40s. With daytime mixing, dewpoints should be low enough that 5 to 10 degree dewpoint depressions prevail. So despite the overnight N/W looking favorable for radiational cooling, do not expect fog to be an issue. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Big Picture... Longwave scale shows a broad trough along the USA East Coast and West Atlantic, a second trough over the Eastern Pacific, and a ridge over the the Plains. The Eastern trough lingers much of the week while the overall flow trends zonal. Model consensus shows northern stream shortwaves crossing New England Saturday night/Sunday and again Wednesday. The models also show a southern stream low crossing the Southeast USA Thursday trailing the Wednesday northern shortwave. The mass and thermal fields are similar through Mon night/Tuesday, but then diverge by showing different timing of the northern and southern stream features. The feature that becomes the northern and southern shortwaves next week is currently over the Aleutians and diving south over the Pacific Ocean...and it reaches the USA West Coast by Monday. Expect changeable solutions regarding this feature until that time. Contour heights and thermal fields are a little above normal Saturday, trend colder than normal Sunday-Monday, then trend above normal midweek next. Details... Saturday... Low pressure passes well offshore. General agreement that the southern shortwave remains unphased with the northern trough. Meanwhile, the northern trough doesn`t dig much until the offshore low is moving past us. So expect the precipitation to remain over the ocean. Cold front approaches during the afternoon and passes through Southern New England during the evening/early night. Moisture is limited below 700 mb much of the time, but runs deep briefly during the evening. Precipitable water values climb to about 0.85 inches during the evening, which is above normal for late November. We will show low-end chance pops during the late afternoon/evening. Temps in the mixed layer are forecast at 850-mb equiv of -1C to -4C, so expect max sfc temps in the 50s. Behind the front, the airmass cools and dries slowly, so min temps at night should only fall to the 30s and low 40s. Sunday-Monday... Upper trough digs over New England Sunday. Cold pool will be in place with 500-mb temps at -28C working to destabilize the airmass. Moisture lingers below 800 mb during this time. Expect quite a few clouds to pop during the day, but moisture is too limited for showers. Mixed layer temps will support max sfc temps in the upper 30s and 40s. The trough moves off on Monday and high pressure builds surface and aloft. Expect a fair day with max sfc temps in the 40s. Tuesday through Thursday... Another cold front crosses us midweek, driven by the northern stream shortwave. As noted above, there are timing uncertainties among the models, so we used a blend. This favors a Wednesday passage at this time, but the uncertainties noted above may change that exact timing. This will need to be monitored. The blend of model data supports slight chance pops Wednesday with the cold fropa and mainly dry Thursday. && .AVIATION /23Z THURSDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. VFR. W/NW winds diminish overnight, turning S/SW into Friday. BKN-OVC MVFR CIGs possible for SE New England into Friday night. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Saturday through Tuesday/... Saturday through Saturday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. Sunday: VFR. Windy with local gusts to 30 kt. Sunday Night through Tuesday: VFR. Breezy. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... SMALL CRAFT ADVISORIES adjusted to account for seas. Should conclude into Friday morning. High pressure mainly in control. Good boating weather as swells diminish. Outlook /Saturday through Tuesday/... Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers. Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Sunday Night: Winds less than 25 kt. Local rough seas. Monday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 7 PM EST this evening for ANZ250. Small Craft Advisory for hazardous seas until 7 AM EST Friday for ANZ254>256. && $$ SYNOPSIS...WTB/Sipprell NEAR TERM...WTB/Belk/Sipprell SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell MARINE...WTB/Sipprell

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