Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 140154 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 954 PM EDT Mon Aug 13 2018 .SYNOPSIS... Scattered showers along with areas of drizzle and fog overnight as a warm front approaches the region. The front slowly lifts north Tue, turning warmer and more humid along with scattered afternoon and evening showers and thunderstorms. Two weak cool fronts move across the region Wed and Thu providing slightly less humid and drier weather. Showers and thunderstorms may be on the increase Friday and Saturday as another front moves through. High pressure over Quebec brings drier and less humid weather Sunday and Monday. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 945 PM update... As per previous discussion, water vapor imagery shows pronounced dry slot from the NJ coast northward to the Berkshires. The back edge of the bulk of showers extended from Springfield to Plymouth at 945 PM and this will continue to lift north over the next few hours. Areas of drizzle and fog will continue overnight with the moist easterly flow. The dry slot also adds convective instability. Strong thunderstorms were near Philadelphia and we will be monitoring their progress. Additional showers, albeit more isolated, were still developing over Long Island Sound. So, there will still be some scattered showers during the overnight period across southern New England. Will maintain chance PoPs. Low pressure off the northern NJ coast was slowly starting to lift the warm front slightly northward. Winds have shifted from ENE to SE at Block Island, Martha`s Vineyard, and over Nantucket Sound (per ferry report) as of 945 PM. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Tuesday... Low clouds and areas of dense fog in the morning as the warm front slowly lifts north thru the region. By midday model guidance has warm front vicinity of the Mass pike with Hi Res guidance developing convection along this boundary especially the HRW NSSL. Given dry slot will be over the region at midday this should be low top convection and will limit severity of convection given shallow updrafts. However always concerning for rotating T-storms with warm front draped across the region this time of year (warm ocean not a mitigating factor) combined with 1000-2000 j/kg of CAPE. As warm front lifts north entire region warm sectors Tue afternoon, first CT/RI and southeast MA then northern MA later in the day. It will be much warmer than today with highs Tue 80-85 and feel muggy with dew pts in the low 70s. As mentioned above this will yield SB capes of 1000-2000j/kg. Midday/early afternoon convection will be very isolated/widely scattered and probably focused near warm front. Then later in the day as the mid level low approaches, cold pool aloft with -10C at 500 mb combined with increasing cyclonic flow should support widely scattered storms across western MA/CT. Good deep layer shear in the morning and afternoon exits the area by late afternoon/evening, so instability and shear do not overlap. Thus main threat with late day storms will be hail with cold pool along with heavy rainers as wind field weakens and PWATs recover from about 1.25 inches midday to 1.7 inches late in the day over western MA/CT. Tuesday night... Isolated to widely scattered showers as mid level low becomes an open wave moving across the region. However activity should not be widespread, so not a washout expected. Still prefrontal airmass over the area so remaining warm and humid. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Big Picture... The jet stream is in zonal flow across Canada while subtropical high remains in control across the southern 2/3 of the USA. The USA northern tier is in between, with a lighter west-east flow overhead while most of the jet energy stays to our north. This will be our pattern for most of the week. A closed low over the Appalachians reaches northeast to join with the jet flow, crossing New England late Tues/early Wed as it does so. A shortwave in the jet flow reaches down as it approaches New England Friday, then passes overhead during the weekend. Model mass fields are similar through Friday. Differences from the Pacific Northwest work their way across the country during the weekend. Confidence remains high through Friday, then moderate Saturday through next Monday. Concerns... Wednesday-Thursday possible convection... Ejecting upper low and cold pool linger in the vicinity Wednesday morning, but move off to the east by afternoon. A shortwave sweeps across northern New England Thursday with weak curvature extending over Southern New England. Lots of moisture through an extended layer Wednesday, then limited moisture Thursday. Lifted Index is sub- zero much of the time, with PW values 1-1.5 inches. SREF shows a probability of CAPE at 1000+ at 50-70 pct during Wednesday afternoon. This holds a chance of showers/tstms Wednesday afternoon, but likely rain-free Thursday. Friday-Saturday_Sunday convection... Shortwave from the Pacific Northwest approaches through the Great Lakes Friday and crosses New England over the weekend. SBCAPE shows favorable values over Nrn CT and Wrn/Centrl Mass Friday afternoon, shifting east Friday night/Saturday morning. PW values rise back to around 2 inches Thursday night/Friday, indicating return of the higher humidity as well as increased potential for downpours in any convection. Sunday-Monday weather... Weak high pressure then builds across eastern Canada. This should bring fair weather and less humid air to us for Sunday and Monday. Main concern is the position of the high center, with resulting northeast low level flow. The flow off the water may bring increased sky cover and less warm temperatures than forecast. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/... 945 PM update... Overnight... IFR/MVFR in scattered showers. Lowest conditions in the Worcester Hills. Areas of fog/drizzle. Lowest visibilities will be over RI and Eastern MA, the Worcester Hills, and east slopes of the Berkshires. High confidence. Tuesday... Drizzle and fog in LIFR/IFR in the morning lifts to MVFR/VFR in the afternoon. Two rounds of showers/T-storms possible, first round in the early afternoon along the warm front lifting north thru MA. Then 2nd round late in the day and evening ahead of approaching cold front western MA/CT. High confidence on trends but lower on exact details. Tuesday night... Scattered showers and isolated thunder from west to east in a mix of VFR/MVFR. Moderate confidence given some uncertainty on areal coverage of convection. KBOS Terminal...IFR/LIFR tonight into Tue morning ahead of approaching warm front. Showers transition to drizzle. Wind shift from SE to SW Tue aftn with improving conditions. High confidence on trend but lower on exact details. KBDL Terminal...MVFR in showers and isolated thunder transitions to IFR/LIFR tonight in fog and drizzle. Improving to MVFR/VFR Tue afternoon but late day showers/T-storms likely. High confidence on trend but lower on exact details. Outlook /Wednesday through Saturday/... Wednesday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. Wednesday Night: VFR. Thursday through Thursday Night: Mainly VFR, with local MVFR possible in patchy fog. Friday through Friday Night: Mainly VFR, with areas MVFR possible. Chance SHRA, chance TSRA. Saturday: Mainly VFR. Chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/... 945 PM update... Overnight...warm front moves across the southern waters. Scattered showers. Areas of drizzle and patchy dense fog. Tuesday...warm front slowly lifts north thru the region with wind shift from SE to SSW as the day progresses from south to north. Drizzle and patchy dense fog reduce vsbys in the morning, then improving in the afternoon along with risk for a few showers/Tstms. Tuesday night...SSW winds continue along with patchy fog and isolated/widely scattered showers/tstms. Outlook /Wednesday through Saturday/... Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Thursday through Thursday Night: Winds less than 25 kt. Seas less than 5 ft. Friday: Winds less than 25 kt. Seas less than 5 ft. Slight chance of rain showers. Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms. Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of thunderstorms, slight chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Nocera/Field NEAR TERM...Field SHORT TERM...Nocera LONG TERM...WTB AVIATION...WTB/Nocera/Field MARINE...WTB/Nocera/Field is the U.S. government's official web portal to all federal, state and local government web resources and services.