Area Forecast Discussion
Issued by NWS Boston, MA

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398 FXUS61 KBOX 200212 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1012 PM EDT Tue Mar 19 2019 .SYNOPSIS... High pressure will bring dry weather with moderating temperatures Wednesday afternoon. A coastal storm impacts the region Thursday into Friday morning with a period of heavy rain and strong winds possible. Becoming very windy behind the departing storm Friday along with a shot of cold air into Saturday, then more seasonable conditions return Sunday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 10 pm update ... 1030 MB high south of New England this evening providing light winds across the region. These light winds combined with mostly clear skies, and a dry airmass with dew pts in the teens and 20s will provide another chilly night. Low temps before sunrise will be in the 20s regionwide, except around 30 in the city of Boston. At 10 pm already down to into the upper 20s across parts of eastern MA including the upper Cape. Previous forecast captures this well so no major changes with this update. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Rather chilly start to the day by mid March standards. Lots of sunshine should permit temperatures to rebound nicely. High pressure further offshore should result in a S to SW wind. Local seabreezes are possible during the afternoon. This high pressure moves even farther offshore Wednesday night. Expecting increasing clouds, especially after midnight. Thinking dry weather prevails, with only a slight risk for some light precipitation late Wednesday night west of the CT River. There is the possibility for some light snow should the precipitation arrive before daybreak. Little to no accumulation before a change to all rain. && .LONG TERM /THURSDAY THROUGH TUESDAY/... */ Highlights ... - Widespread rain, embedded heavy showers, thunder Thursday night - Wet snows, becoming blustery Friday night into Saturday - Mild, dry conditions Sunday - Wintry weather potential early next week, turning colder - Late week high pressure, return mild */ Overview ... See-saw pattern. Seemingly +EPO with NE Pacific low pressure that aids in maintaining a +PNA over western N America, Pacific / Arctic energy downsheared W of a Baffin Bay tropospheric polar vortex into preferred H5 troughing S into the E CONUS. An occasional cyclonic wave break over the W CONUS adding to forecast challenges, looking at a late-week potent coastal storm system tapping into sub-tropical energy and moisture. Maturing, later occluding quickly E, a brief window of wet snows and colder air before milder air returns. But early next week, the H5 trof reloading S as a Central CONUS storm system ejects E, could see one last shot of wintry weather before March concludes. Meteograms continue to tell the story. Forecast details below. Echoing the prior long-term forecaster, continual spread in model forecast solutions in storm morphology, spread in surface pressure ranging between 995-970 mb as the Thursday night coastal storm sweeps over S New England. Consensus forecast approach preferred. */ Details ... Thursday into Friday ... Coastal storm. Initial Pacific-waves downsheared phasing as they dig cyclonically through a preferred E CONUS H5 trof regime tapping into maritime-tropical air drawing N into an initial towards maturing low center with increasing surface pressure falls / lowering heights. Associated dynamics / ascent upon a high thetaE / instability tongue with precipitable waters close to an inch, expecting widespread rain with embedded heavier showers around Thursday night. Concurrent with low maturation can`t rule out some rumbles of thunder. Likely PoPs, potential for areas to see over an inch of rainfall, can`t rule out overnight nuisance flooding issues, ponding of water on roadways. Flush probably the last of the river ice, rivers which are flowing near-average. Fairly wet soils, expect majority rainfall as runoff. Given consensus model forecast of a more robust system, fairly warm, leaning towards warmest 2m temperature / dewpoint guidance feeling preceding mild air daytime Thursday plus onshore flow. Bigger concern is the strength of the low center and respective wind fields concurrent with expected high astronomical tides. Strong S/SE H925 jet also centered around Thursday night, yet a shallow boundary- layer inversion in place. Limits on potential mix-down of fast winds but for sake of argument, will go with a short-duration of onshore winds with gusts up around 25 mph, minor coastal flood impacts for the E MA coastline around the near-midnight high tide are possible. Unsure as to S-coastal high tide 8p Thursday and 8a Friday. Mainly in low tide throughout the event. Perhaps a potential for emerging impacts with the 8a Friday high tide. Friday night into Saturday ... SE Canada storm occlusion. Follow-up downsheared Pacific + N stream energy pivots, later phasing and closing off with initial energy in association with the coastal storm. Through the E N America H5 trof, cyclogenesis / surface pressure falls increase over SE Canada with the low center occluding down towards 980 mb. Expect a trowaling comma-head storm appearance reaching back into the NE CONUS. Cold air advection with blustery NW easily mixing down to the surface, there are two concerns: 1.) Potential for high-terrain wet snows given deformation / orographic lift along W-slopes, and 2.) WIND ADVISORIES with mix layer top close to H8 where winds will be in excess of 50 mph. Chance to likely PoPs, highest along Berkshires and Worcester Hills coinciding with ECens probabilities for 1" of snow in 24 hours. Majority focus Friday night into Saturday morn with pivoting secondary mid-level disturbance that later trowals energy back W prior to ejecting E into the wide open N Atlantic. Push wind gusts around 30-40 mph, conservative for now. Broken to overcast cloud decks, lean towards the cooler side of forecast guidance with low wind chills in addition to winds. Coating to 1 inch of snow, again highest confidence high terrain. Sunday onward ... Quick rebound with W/SW warm air advection, end up mild Sunday with winds becoming light, sun and cloud mix. Watching a cut-off Central CONUS storm system as energy continually dips S through continual preferred H5 troughing over E N America. Early next week looks some- what interesting if cold air can build S as over-running continental tropical air edges N. Perhaps a shot at some wintry weather to close out March. High pressure seemingly lingers for the rest of the week as temperatures become more seasonable, mild. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High confidence. 03z update ... No major changes. Overnight... VFR, light winds mainly out of the W-NW and dry weather. Winds becoming light SW late. Wednesday... VFR, dry weather with light winds becoming SSW by late morning. Local seabreezes possible during the afternoon. Wednesday night... VFR. KBOS Terminal...High confidence in TAF. Lower confidence in a seabreeze developing late Wed morning into the afternoon. KBDL Terminal...High confidence in TAF. Outlook /Thursday through Sunday/... Thursday: Mainly VFR, with local MVFR possible. Breezy. Chance RA. Thursday Night: Mainly VFR, with areas IFR possible. Breezy. RA. Friday: Mainly VFR, with local IFR possible. Windy with areas of gusts up to 30 kt. Chance RA. Friday Night: Mainly VFR, with local MVFR possible. Windy with gusts up to 35 kt. Chance RA, chance SN. Saturday: VFR. Windy with gusts up to 35 kt. Slight chance SHSN. Saturday Night: VFR. Windy with gusts up to 35 kt. Sunday: VFR. Breezy. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High confidence. 7 pm update ... No major changes. Previous discussion ... Rather tranquil boating conditions across the coastal waters as a high pressure moves E to the south of New England. Might see a few gusts approaching 25 kt late tomorrow into Wednesday night. Seas generally less than 4 feet through this period. Good visibility. Outlook /Thursday through Sunday/... Thursday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain. Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain, slight chance of thunderstorms. Friday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 11 ft. Chance of rain. Friday Night: Moderate risk for gale force winds with gusts up to 35 kt. Rough seas up to 11 ft. Chance of snow, slight chance of rain. Saturday: Moderate risk for gale force winds with gusts up to 35 kt. Rough seas up to 11 ft. Slight chance of snow showers. Saturday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. && .CLIMATE... Average first occurrence of 70 degree high temperatures (since records began as early as 1872) Boston.......April 8th Hartford.....April 1 Providence...April 8th Worcester....April 11th Average first occurrence of 70 degree daily average temperatures Daily average temperatures = (high + low) / 2 (since records began as early as 1872) Boston.......May 27th Hartford.....May 10th Providence...May 20th Worcester....May 29th && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Belk/Nocera/Sipprell SHORT TERM...Belk LONG TERM...Sipprell AVIATION...Belk/Nocera/Sipprell MARINE...Belk/Sipprell CLIMATE...

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