Area Forecast Discussion
Issued by NWS Boston, MA

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182 FXUS61 KBOX 231409 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1009 AM EDT Fri Aug 23 2019 .SYNOPSIS... A wave of low pressure tracking along a cold front south of the Islands will bring a period of light rain to the south coast of Massachusetts and Rhode Island the remainder of the morning. Then clouds will thin and give way to some breaks of afternoon sunshine as this wave of low pressure moves offshore. A fall- like airmass will then invade southern New England tonight into the weekend as large high pressure moves from the Great Lakes today into the Maritimes by Sunday. Mild afternoons and cool nights will be featured. Dry weather Saturday with possibly a few showers Sunday especially eastern MA into RI, but a washout is not expected. Cool Maritime high pressure remains in control early next week along with breezy conditions across Cape Cod and the Islands. Meanwhile we`ll be watching a potential tropical system tracking from the Bahamas to well south of New England. && .NEAR TERM /UNTIL 8 PM THIS EVENING/... 10 AM Update... Back edge of rain shield now confined to the south coast of MA/RI and continuing to move southward as post frontal airmass surges southward. Thus trend toward drier and less humid weather this afternoon. Back edge of cloud shield over northwest CT into central MA along with cloud shield thinning. So continue to expect clouds thinning and giving way to afternoon sunshine from northwest to southeast this afternoon. Previous forecast captures this trend well so no major changes with this update. Previous Discussion... Once the flow shifts, expect clearing skies, earliest in northern MA and latest along the South Coast. Cross sections show drying aloft and at the surface, but a lingering moist layer around 850 mb. The mixed layer is expected to reach a little past 850 mb, so there should be some diurnal cu formation over the interior. The dry air should limit the extent. Temperatures at 850 mb will be 10-12C, so we expect max sfc temps of 76F to 82F. Best of all, dew points will be in the upper 40s and 50s everywhere this afternoon/evening. && .SHORT TERM /8 PM THIS EVENING THROUGH 6 PM SATURDAY/... A fair weather period, quite welcome to most after the recent spell of humidity. High pressure over Northern Ontario builds east, reaching the Maritimes late Saturday. This brings a light north flow to New England tonight, turning to a northeast flow by Saturday afternoon. With dew points upper 40s and 50s, expect min temps tonight in the 50s most places...with upper 40s in the cold spots and around 60 in large urban areas. The developing northeast flow will play havoc with mixing, but daytime heating and 8C at 850 mb suggest sfc temperatures into the 70s. If anyone stands a chance at 80, it will be the CT Valley of Hartford-Springfield. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * Expect much cooler than normal temperatures Sunday and Monday especially along the east coast * Onshore winds will bring spotty light rain or showers at times into mid week * Temperatures return to near normal levels around mid week * Another chance for showers Wednesday into Thursday Details... Saturday night through Monday... With a large high pressure area extending across Quebec and northern New England, a return northeasterly wind flow will bring cool temperatures across the region especially along east coastal areas into early next week. With the onshore wind flow, the long ocean fetch will bring increasing low level moisture westward. So, can not rule out periods of spotty light rain and/or showers at times mainly over the weekend. By Monday, it appears the southern extend of the large ridge will start to shift into N Mass, which will bring drier conditions across the entire region with good subsidence. So, any spotty precip early should end, though clouds may linger across eastern and S coastal areas through the day. Expect daytime highs mainly in the lower to mid 70s, but may not break 70 along the immediate E coast as well as outer Cape Cod and Nantucket. With the long fetch across the western Atlc, will see E to NE winds gusting up to 25-30 mph along the E coast both Sunday and Monday. Monday night through Thursday... The large high center across the Maritimes will begin to shift E during this timeframe as a cold front slowly shifts out of the Great Lakes. Winds will diminish somewhat as the pressure gradient relaxes, then will veer to SE-S by later Wed into Thu. Increasing uncertainty as to how quickly the ridge will shift further offshore along with the timing of the approaching cold front. At this point, the ridge will keep dry conditions in place on Tuesday, except for spotty light showers mainly near the S coast. Deepening layer moisture as well as slowly but steadily increasing dewpoints will approach starting later Tuesday and continue through Thursday. Expect highs on Wed to be close to seasonal levels, then upward to around 5 degrees above normal on Thu. The approaching cold front will bring a more organized shot for showers starting late Tuesday night across the E slopes of the Berkshires, then slowly shifting E Wed and Thu. Timing of the actual front still in question due to the exit of the Maritime high pressure. && .AVIATION /14Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...Moderate confidence. 15Z update ... Rain shield confined to the south coast of MA/RI at 15z and continuing to move offshore. Thus conditions all terminals by this afternoon along with VFR and NNW winds. Previous discussion below. Tonight... VFR with mostly clear skies and light north winds. Saturday... VFR. North wind turns from the northeast during the afternoon. KBOS Terminal...High confidence in TAF KBDL Terminal...High confidence in TAF. Outlook /Saturday Night through Tuesday/...Moderate to high confidence. Saturday Night: VFR. Breezy. Sunday through Monday: Mainly VFR, with local MVFR possible. Breezy. Monday Night: Mainly VFR, with local IFR possible. Breezy. Patchy BR. Tuesday: VFR. Breezy. Slight chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. Today...Scattered showers this morning with brief poor vsbys. Conditions should improve from north to south through around midday, with drying in the afternoon. Winds will be less than 25 kts through the day. Seas 4 feet or less. Tonight...Winds less than 20 kt and seas 4 feet or less. Saturday...Winds less than 20 kt. Seas 4 feet or less. Outlook /Saturday Night through Tuesday/...Moderate confidence. Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain. Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain. Monday: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Slight chance of rain. Monday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain. Tuesday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/Nocera/EVT SHORT TERM...WTB LONG TERM...EVT AVIATION...WTB/Nocera/EVT MARINE...WTB/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.