Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 181712 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 112 PM EDT Fri Aug 18 2017 .SYNOPSIS... Out ahead of a deep low pressure center lifting into S Canada, rain, possibly heavy at times, along with the threat of localized flooding, is expected for today and tonight. A low risk of strong to severe thunderstorms mainly over S/W portions of MA and CT. A cold front moves through New England Saturday with a few leftover showers and thunderstorms, then moves off to the east Saturday night. High pressure returns with fair seasonable weather Sunday through Tuesday. Another cold front sweeps through New England Wednesday, followed by more dry weather Thursday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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*/ Highlights (1p update)... - Simply put, widespread rain with embedded heavier showers and thunderstorms. - Higher confidence with respect to localized flooding impacts given forecast ascent upon the tropical environment. With heaviest storms earlier today, rainfall rates of around 1 to 2 inches have been observed, storm total rainfall amounts up to 3 inches. - Lower confidence with respect to strong to severe storms. Greater focus over S/W portions of MA and CT. Potential of gusty winds. Lower risk of a brief tornado given the environment. - No headlines will be issued with this update. */ Overview (1p update)... Scattered showers and thunderstorms evolving within a tropical environment drawn N by a closed low pressure system over the N Great Lakes region. The attendant surface warm front lifting NE across N/E areas of New England as discerned by SPC H925-85 meso- analysis bucks up against the mid-upper level ridge S/W. Warm- moist tropical surging N with surface dewpoints in the 70s, over- running and convergence, especially along the nose of H925-85 winds, heavy rain has been observed and should continue. Further S/W, greater instability lending to the threat of shower and thunderstorm development, some strong to severe with heavy rain, out ahead of a surface cold front and mid-level dry punch poised to sweep through the region overnight into Saturday. */ Discussion (1p update)... Straight-forward, continued forecast of widespread rain with embedded heavier downpours and thunderstorms. Two things of concern: 1.) localized flooding, and 2.) strong to severe storms. Through afternoon, warm front presently over the immediate E waters stretched N into ME per SPC H925-85 dewpoint mesoanalysis is the continued focus for heavy rain given convergent focus of H925-85 winds advecting tropical moisture N. Better chances of embedded thunder S/W within greater instability. This area continues to progress slowly N/E, hung up against the S/W mid-upper level ridge and anticyclonic flow. Expecting the focus of heavy rain to continue right up till the cold front sweeps through the area Saturday. Towards evening and overnight, monitoring upstream trends. Per GOES-16, clearing emerging across SE PA into the Delmarva Peninsula and per SPC mesoanalysis it is this area which has highest surface dewpoints and greatest instability within a region of modest shear. Expectation is that within this area strong to severe storms will develop and they may rob the environment N up across Albany into S/W portions of MA and CT. Moving N/E with the mean wind, will have to watch the evolution and morphology closely. As the boundary layer decouples with warm-moist air continuing to surge aloft, expect storm activity and enhanced H925-85 low- level convergence to meet and potentially surge over S/E New England during the overnight hours. It is this timeframe where we may see the greatest threat of localized flooding if activity does not emerge immediately S/W ahead of the pre-frontal trough presently observed via radar SW to NE across PA, Upstate NY, into New England. Agree with SPC. We`re still within a strongly sheared environment within a measure of helicity as signaled via SPC mesoanalysis and NCAR ensembles. Marginal instability that could be compensated by the shear, can`t rule out strong to severe weather across S/W areas of MA and CT. However, greater confidence of outcomes is further S/W of our region around SW PA, NJ, Delmarva with the more potent focus of thermal-instability axes. Better low-level lapse rates which are mediocre over S New England. In greater agreement with WPC. Anomalous moist environment up to 3 standard deviations above normal. Tropical. This morning has been an indication of how with any ascent rainfall rates of 1 to 2 inches per hour are possible. Precipitable waters well over 2 inches with K indices close to 40. The difficult part is nailing it down, however with the latest guidance, the concern is over S/E areas of New England during the overnight period as convection emerges around the Delmarva, fueled by SW warm-moist flow, enhanced by boundary layer decoupling, pushing NE with the mean wind and maintained until the dry punch and cold front begin to sweep into the region towards Saturday morning. No FLASH FLOOD / FLOOD WATCH at this time. Difficult to nail down as it is possible heavy rain may stay offshore going into this evening. However, can`t rule out urban / poor-drainage issues even ahead of the pre-frontal trough approaching from the W.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/... Tonight... Thermal axis will peak over the region tonight as upper level trough begins to approach. LLJ will be on the increase early this evening. This combined with the warm, moist airmass and approaching upper level trough expect the first half of the night to be busy with ongoing precipitation. Conditions will slowly improve from west to east through the early morning hours, however the approaching cold front appears to stall either over or west of the region. Thus expect a very mild night as dewpoints will remain high, in the 70s. Could see a round of fog overnight as temp/dewpoint spread will be low. In fact, cannot rule out the dense fog potential. Overall, anticipate ongoing precip and fog through the night with conditions slowly improving from west to east by the morning hours. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Big Picture... Subtropical high remains in control across the Southern USA while a general zonal flow continues across Canada and the Northern USA. The flow becomes less zonal next midweek as Alaska closed upper low digs south over the Gulf, subtropical upper ridge builds north over the Western USA, and upper low over Hudsons Bay digs south into the Northeast USA. One shortwave moves across New England Saturday night, with brief cooling aloft that destabilizes the airmass Sunday. A second shortwave crosses New England Wednesday as the Hudsons Bay low approaches and moves a supporting jet max across the region. Contour heights are normal to a little higher than normal through early next week. As the Quebec closed low deepens, heights over New England dip below normal midweek. Model mass and thermal fields from the 00Z suite are in general agreement through Wednesday, building forecast confidence through that time. Less agreement in the fields Thursday. Details... Saturday... Moderate confidence. Models hold the the cold front over Southern New England while moisture fields race east and drier air moves in during the late morning and afternoon. PW fields show 1.5 inch values across much of the region in the morning, but moving east of Cape Cod by evening. CAPE fields show 1000-1500 J/Kg over Easter CT/RI/Eastern Mass midday. Winds aloft are marginally significant, especially at 500 mb with speeds of 40-45 knots. Stability parameters are mixed, with Totals in the mid to upper 40s in the early night, and LI values subzero over RI and Eastern Mass. Expect the main area of showers/heavy downpours to move east of Massachusetts during the morning. Drier air moves in at low levels, with lingering moisture around 850 mb. This suggests at least some clouds during the day, but with potential for sunny breaks. This heating should push temps into the 80s, with enough instability to generate scattered showers/thunder along and ahead of the cold front. Saturday night... Cold front moves offshore, and surface winds become northwest. Skies will either clear or partially clear. Dew points will linger in the 60s, with min temps close to that level. Sunday... Upper shortwave and the coldest of the cold mid-level temps will be over New England. The more significant cold temps as well as cloud- level moisture will be over NH-VT. Cross-sections show Southern New England will have a moist layer based at 850 mb, but with drier air above and below that layer. Expect diurnal clouds, probably a bit more than guidance is suggesting, but otherwise a fair day with less noticeable humidity. Temps aloft 12-14C support max sfc temps in the upper 70s to mid 80s. Surface high pressure builds in with light variable wind Sunday night. With dew points in the upper 50s and around 60, expect min temps around this same range. Guidance temps were nudged a couple of degrees colder. Monday-Tuesday... High pressure builds over the region Monday and shifts offshore south and east by Tuesday. Cross sections show dry air in place at most levels through Tuesday evening, so expect dry weather both days. Increasing mid and high clouds Tuesday ahead of the next weather system. Temps aloft 14-15C Monday and 15-17C Tuesday, supporting low-mid 80s Monday and 85 to 90 on Tuesday. Tuesday night-Wednesday-Thursday... Closed low digs over Quebec and the Northeast USA with shortwave driving a cold front into/through New England. Expect showers/scattered tstms with the front Tuesday night/Wednesday. The airmass dries out Thursday, but lingering patches of moisture in the mixed layer should generate some diurnal sky cover. && .AVIATION /18Z FRIDAY 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 Tonight/... Today and tonight...Moderate confidence. Trending MVFR to IFR spreading N/E if not already. Can`t rule out partial improvement over S/W New England late at times. Overall, expect -RA/RA this morning, becoming RA/+RA/TSRA later today and continuing overnight, SW to NE, as conditions lower IFR over most locations. With RA/+RA, expect TEMPO MVFR-IFR VSBY impacts, locally dense IFR fog over the S coast overnight. Strong S winds around 10-15 kts with gusts up to 25 kts, strongest over S/SE New England. Potential LLWS overnight with 40-45 kt SW jet across S/SE New England. All conditions improving as winds turn W into early morning Saturday. KBOS Terminal...Moderate confidence. VFR with a gradual transition through the morning to MVFR/IFR along with a band of -RA and occasional +RA/RA. Low risk for occasional TSRA through the day. KBDL Terminal...Moderate confidence. Hovering around MVFR. Some improvement is possible into the later-half of the day prior to possible RA/+RA/TSRA encroaching from the S/W. Low confidence concerning potential, as well as timing and impacts. Outlook /Saturday through Tuesday/... Saturday... Moderate confidence. MVFR/IFR cigs and vsbys in showers and fog Saturday morning, then lingering showers/thunderstorms in Eastern Mass and RI in the PM. Trend all places will be to VFR. A cold front over the region will move offshore to the east by Saturday evening with winds shifting from the west. Areas of IFR cigs/vsbys in fog and low clouds later Saturday night. Sunday through Tuesday...Moderate-High confidence. VFR. Areas of IFR in early morning fog/low clouds, then improving to VFR by each mid morning and continuing the remainder of each day.
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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 Tonight/...Moderate confidence. S winds will be on the increase, gusting up to 25 kts across the S/SE waters. SMALL CRAFT ADVISORY remain as winds will result in heightened seas up around 5 feet. Heavy rain with embedded thunderstorms sweeping W to E across the waters today and tonight. Will likely be some visibility restrictions. Outlook /Saturday through Tuesday/... Saturday...Moderate confidence. Lingering potential for scattered showers/tstms through the day, diminishing west to east late Saturday. Diminishing wind through the day. Some gusts 20-25 knots in the morning. Seas near 5 feet on the outer waters and on RI Sound, but trending lower through the day. Sunday through Tuesday...Moderate-High confidence. High pressure builds over the waters. Winds remain 20 knots or less, and seas 4 feet or less. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 5 PM this afternoon to 9 AM EDT Saturday for ANZ231>234. Small Craft Advisory from 5 PM this afternoon to 2 AM EDT Saturday for ANZ236. Small Craft Advisory until 11 AM EDT Saturday for ANZ235-237. Small Craft Advisory from 8 PM this evening to 2 PM EDT Saturday for ANZ250-254. Small Craft Advisory until 8 PM EDT Saturday for ANZ255-256.
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&& $$ SYNOPSIS...WTB/Dunten NEAR TERM...Dunten/Sipprell SHORT TERM...Dunten LONG TERM...WTB AVIATION...WTB/Dunten/Sipprell MARINE...WTB/Dunten/Sipprell

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