Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 181421 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1021 AM 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/... */ Highlights... - Simply for today into tonight, widespread rain with embedded heavy rain and thunderstorms - Higher confidence with respect to downpours contributing to localized flooding per tropical environment and already observed 1-2 inch per hour rainfall rates with morning storms. Watching closely areas around NJ / Delmarva and how they could evolve N/E - Lower confidence with respect to strong to severe storms over S/W portions of S New England, especially the CT River Valley, though can not rule out with any the potential of gusty winds, perhaps even, though a low risk, of a brief tornado given the environment - No headlines will be issued with this update */ Overview... Closed low pressure system over the N Great Lakes will be the focus for wet weather today. Mid-level ridge over S New England this morning will move offshore and build over the N Atlantic. Surface warm front will move from SW to NE over the region today. This will keep the region in the warm sector for much of the day before surface trough approaches from the west. Once thing to watch is how far north the front advects. If is slows down today, then it could be the focus for heavy precip and eventually flooding this afternoon. */ Discussion... In simplicity, widespread rain with embedded heavy downpours and thunder. Warm frontal boundary lifting N/E across the region behind which SW low-level flow is surging with higher surface dewpoint air into the low 70s. The combination of confluence and over-running of a very moist, tropical environment contributing to heavier downpours with thunderstorms more prominent over NJ and Long Islands in areas of greater low-level confluence and higher amounts of elevated instability along with potential enhanced ascent with lead short- wave energy. Reports gathered of rainfall amounts of 1-2 inches in an hour. Widespread rain N/W beneath better dynamics, diffluence, and lead shortwave energy out ahead of the closed low over S Canada, away from the mid-upper level ridge S/E For this morning, the warm frontal boundary will continue to lift N/E behind which tropical moisture will surge. Higher cloud bases to the E will gradually lower as the influence of anticyclonic flow beneath the mid-upper level ridge pushes slowly E. Expect activity to wash out E, lesser rainfall amounts for the Cape and Islands and lower PoPs, but remain prominent N/NE along the nose of the H925-85 jet and beneath more favorable dynamics, cyclonic flow out ahead of the deep low center over S Canada. Into afternoon and evening, all eyes S/W on what develops, evolves and transitions. Still a strongly sheared environment across the region along with a measure of helicity. Modest instability however the more potent thermal-instability axes are across NJ and the Delmarva Peninsula within marginal shear. Enough to support storm development, there`s concern that activity may rob our environment N/E in that areas of S/W New England may see little if anything. Higher confidence that activity does blossom out of NJ and the Delmarva and then pushes N/E with the H925-85 jet enhancing out over the waters with the boundary layer decoupling as warmer-moist air pushes in aloft. An area of heavy rain with the tropical-feeding flow, could see heavy rain into the overnight hours impact S/E portions of New England along with some embedded thunder. Sharpening focus out ahead of a H5-7 dry punch and surface cold front. However, don`t want to rule out anything just yet, lots of mesoscale influences, a messy forecast. In agreement with the Storm Prediction Center. Still can`t rule out the threat of strong storms over S/W portions of S New England despite the gut feeling that NJ / Delmarva is where stronger to severe storm activity will emerge. If any storms, gusty winds are possible given the shear / mean-wind profile, and with helicity, can not object to the possibility of a brief tornado, however feel this risk is low at this time. Lapse rates are mediocre as well, 5-6 C/km. Preference to trends with respect to lightning out of the NCAR ensembles. Echoing the previous forecaster, NCAR ensembles do highlight low LCLs, rotation / helicity, and backing surface winds over S/W areas of S New England. BUFKIT hodographs are also quite impressive, so again, will need to watch areas around the CT River Valley closely this afternoon. Stronger agreement with Weather Prediction Center on an excessive rain threat. Precipitable waters well over 2 inches associated with the tropical environment, K indices close to 40, and having already observed the rainfall intensity with morning storms, can not rule out flash flooding. Though it remains difficult to nail down exactly where and when this will take place, and whether it will coincide with any high tide periods. This ultimately lowers confidence with respect to issuing a FLASH FLOOD / FLOOD WATCH. Can`t rule out urban / poor-drainage issues, but given the setup for today, unable to nail anything down however can`t rule out localized impacts. Very anomalous event as we all well know up to 3 standard deviations above average. Warm rain processes expected given high freezing level heights and H85 dewpoints exceeding +12-15C, up around +17C. && .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 /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 Tonight/... Today and tonight...Moderate confidence. Trending MVFR to IFR spreading N/E this morning. 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. Morning -RA, 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. && .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... 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 from 2 PM this afternoon to 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 from 2 PM this afternoon to 8 PM EDT Saturday for ANZ255-256. && $$ SYNOPSIS...WTB/Dunten NEAR TERM...WTB/Dunten/Sipprell SHORT TERM...Dunten LONG TERM...WTB AVIATION...WTB/Dunten/Sipprell MARINE...WTB/Dunten/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.