Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211346 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 946 AM EDT Sat Jul 21 2018 .SYNOPSIS... High pressure over the Gulf of Maine provides another dry day across New England along with comfortable humidity. Low pressure with tropical moisture moves northward along the New Jersey coast tonight with its attending warm front approaching New England. This setup will bringing showers and thunderstorms with torrential downpours and gusty winds tonight into Sunday. Bermuda high pressure then builds west toward southern New England next week bringing tropical humidity and warm temperatures. Scattered showers and thunderstorms are possible Monday and again Thursday with drier conditions likely Tue and Wed. && .NEAR TERM /UNTIL 7 PM THIS EVENING/... 945 am update... Plenty of sunshine late this morning...except for a bit of strato CU across northwest MA and some low clouds across the Cape/Islands. Given the strong July sunshine...we should see this erode some by afternoon before it returns towards evening. Otherwise, as previous forecaster stated one more pleasant day with comfortable humidity levels with high pressure located across the Gulf of Maine. Highs should be well up into the 70s to the lower 80s in most locations...but probably only top out in the lower 70s along the immediate coast with the onshore flow. && .SHORT TERM /7 PM THIS EVENING THROUGH 6 PM SUNDAY/... Tonight into Sunday... */ Highlights... - Highly anomalous synoptic setup for mid July - Tropical downpours, excessive rainfall, potential flooding - Embedded thunderstorms, possible severe weather elements - High shear, low instability, potential for brief spin-ups - However a measure of uncertainty, set-up bears watching */ Overview... Sub-tropical low rotating round a negatively tilted cut-off closed low across the Mid-Atlantic coast NW into Upstate NY. An attendant anomalous S to N low-level wind / precipitable water axis, low-level convergent beneath broad diffluence, along a lifting warm front to the sub-tropical low. Deep moist profile with high shear, especially 0-1 km, low instability. Forecasting a slug of tropical downpour / thunderstorm activity during the early morning hours, potentially evolving further later Sunday within the warm sector as sunshine breaks out over S New England. Potential severe weather elements, flooding possible, a highly anomalous situation for mid July. However, sub-tropical low focus rotating into, as mentioned prior, the NYC tri-state region into Upstate NY. A low-level focus is still possible into S New England, however the system is transitional as a mid to upper level ridge builds into the region, heights rising. A measure of uncertainty. Split between mesoscale and global runs for the 36-48 hour period. Difference in axis of heavy rainfall with global guidance leaning E over the W leaning mesoscale guidance. It is perhaps an outcome of mesoscale guidance beyond 36 hours which tends to do poorly. Not much clarity from ensemble means / probabilistics, as well as CIPS Analogs. Threats are there, but nailing them down remains difficult. Break down the potential threats below. */ Discussion... Flooding... Potential for tropical downpours, pockets of +1"/hr rainfall rates, yielding 1-2" total rainfall amounts, locally higher. Leaning with an initial burst of rainfall along a convergent low-level jet, then S to N training of tropical elements. Deep, warm column moisture, precipitable waters 2.0-2.5", high freezing level heights, efficient rainfall processes. Sultry environment with mid 70s dewpoints, H85 dewpoints +15C. Instability present, conditionally unstable profile, invoking updrafts, heavy rain potential. Localized flooding is possible. However low confidence. Spread continues within mesoscale and global forecast guidance. No clear signals within ensemble probabilistic guidance and means. Greater heavy rain focus may occur towards the Delaware River Valley. Meanwhile H5 ridge / rising heights building into the region during the forecast period. Can`t rule out a S-N training of tropical elements, potential flooding over New England, but not enough clarity / consistency to warrant any flood headlines. Still a challenge to nail things down. Winds... Combined with potential heavy rain and/or severe weather elements, forecast S gusts around 35 mph. Roughly around 2 kft agl sustained winds 40-50 mph per forecast model consensus ahead of the sub-trop low. GEFS / SREF +5 standard deviation low-level S flow, highly anomalous for mid July. Waters not as cold, low-level inversions not as stout. Can`t rule out locally higher gusts up as high as 45 mph especially with potential heavy rain / thunderstorms. Impacts especially along S-coastal New England around Sunday morning possibly continuing through the day with any breakout sunshine within the warm sector. Tornado / Waterspout... Potential for a brief spin-up. A high shear, low instability setup. Echoes of recent Concord and Revere early morning tornadoes which had a similar environment. Allegory to a landfalling tropical rain- band associated with a cyclone, the interaction with terrain and subsequent frictional component could potentially aid in a brief spin-up for the early morning period with any onshore thunderstorm. Then focus on later in the day within the warm sector. Continued low- level jet / shear within weak, subtle synoptic scale forcing as heights rise with the enhancing Atlantic ridge. Can`t rule out additional break-out midday into the afternoon activity. Low LCLs throughout with dewpoints getting into the mid 70s. SREF probs highlighting a high probability of exceeding 0-1 km helicity over 150 m2/s2, and some indication of sigtor`s over 1. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Highlights... * Tropical humidity through at least Thursday * Greatest risk for showers/T-storms Sun ngt-Mon & again Thu * Drier weather possible Tue and Wed Sunday night... Tropical moisture plume aligned along the eastern seaboard into New England via deep southerly flow courtesy of anomalous 597 dam ridge south of the Maritimes and a closed low over the eastern Great Lakes/OH Valley. Within this tropical moist conveyor belt scattered showers and embedded thunder will continue across southern New England. Global guidance and the NAM indicating torrential downpours possible. This combined with ongoing convection during the day Sunday will yield a low risk of localized flash flood threat. WPC recognizes this with a slight risk in their excessive rainfall graphic. However given the time range here still enough uncertainty to hold off on a Flash Flood Watch but later shifts may have to consider pending model trends. Otherwise oppressive tropical humidity within this airmass along with warm overnight temps given dew pts of 70-75. Breezy as well in response to lingering southerly low level jet. Monday... Anomalous subtropical ridge retrogrades westward into New England with 594 dam heights into the region! These rising heights shifts tropical moisture plume westward into NY state as the day progresses. Thus drier weather advecting into the area from east to west as seen in model PWAT and K indices fields decreasing. Yes, not your typical advection pattern. So risk for showers in the morning and then a drying trend from off the ocean into eastern MA trending westward in the afternoon. Given rising heights/warming temps aloft will not include thunder in the forecast. Very tropical feel to the day with highs 80-85 and dew pts in the mid 70s! Tuesday and Wednesday... Good agreement among ensembles and deterministic data sets for subtropical ridge to continue building westward into New England with 500 mb heights of 591-594 dam across southern New England. This will shunt tropical moisture plume north and west of our region. Thus Tue and Wed could turn out to be mainly dry given subsidence and deep layer dry air aloft. However can/t rule out a few tropical isolated low top showers. Tropical airmass will remain over the region with highs 80-85 and dew pts 70-75. Other than some patchy morning fog possible, could be looking at two good beach days especially across Cape Cod and Nantucket, closest to the subtropical ridge...a taste of Bermuda weather. . Thursday... Ensembles and deterministic guidance both suggest northern stream trough enters the Great lakes. This provides cyclonic flow into New England along with cold front approaching New England. Thus probability of showers and T-storms on the increase. Warm and humid conditions continue with highs 80-85 and dew pts 70-75. Friday... Ensembles suggest FROPA sometime Fri-Sat time period so expecting the tropical humidity to finally break. However still warm with ensembles offering +16C or so at 850 mb over New England. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...Moderate to high confidence. Today...Mainly VFR conditions continue through the afternoon. The exception is mainly across the Cape/Islands where some IFR to even LIFR on Nantucket were impacting the region late this morning. The strong July sun angle may allow this to scatter out for a time this afternoon before returning toward evening. Always tricky with these low clouds...so will continue to monitor trends closely. Tonight... Lowering IFR-LIFR, widespread. RA with embedded +RA/TSRA towards early morning Sunday, roughly after 6z. Gusty winds possible 25-35 kts with TSRA. Both CIG and VSBY impacts. Sunday... IFR-LIFR slowly lifting through the day towards MVFR to low-end VFR. Some locations may remain IFR throughout. Morning widespread RA with embedded +RA/TSRA becoming more localized along a S-N band. Continued strong to severe weather potential with gusty winds. KBOS Terminal...VFR conditions probably hold into late afternoon/early this evening before conditions deteriorate. Increasing E onshore flow. Onset wet- weather towards early morning Sunday with gusty winds, potential +RA/TSRA. KBDL Terminal...VFR conditions probably hold into late afternoon/early evening before conditions deteriorate tonight. Lowering conditions overnight Saturday into Sunday morning with onset of widespread RA, embedded +RA/TSRA elements with TSRA potentially being strong to severe. Outlook /Sunday Night through Wednesday/... Sunday Night: Mainly MVFR, with local IFR possible. Breezy. Chance SHRA, slight chance TSRA, patchy BR. Monday: Mainly MVFR, with areas VFR possible. Breezy. Chance SHRA, patchy BR. Monday Night: Mainly MVFR, with local IFR possible. Chance SHRA. Tuesday through Tuesday Night: Mainly VFR, with areas MVFR possible. Chance SHRA. Wednesday: Mainly MVFR, with areas VFR possible. Chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. */ Highlights... - A Gale Watch continues for Narragansett Bay and the waters south of Rhode Island for tonight and Sunday. - A Small Craft Advisory has been issued for all other waters for tonight and Sunday. */ Discussion... Today... Increasing onshore E flow ahead of a lifting warm front. Will see marine stratus / fog envelop the waters, advect towards the shore during the day. Low visibility possible. Tonight through Sunday... A storm system will be moving N along the mid-Atlantic coast and passing well W of the waters by Sunday. A strong warm front will be moving across the waters tonight into Sunday morning. Showers and thunderstorms with torrential rain and gusty winds are expected late Saturday night into Sunday. Gale force wind gusts are possible, hence the Gale Watch in the waters along the RI coast. Winds are forecast to be upwards of 30 knots over all other waters late tonight and Sunday, but it is possible that portions of these areas may need to be upgraded to Gales in subsequent forecasts. In addition, there is the threat of a brief spin of a waterspout. Continued marine stratus / fog, the threat of low visibility. Strong wind and wave action, there is the threat of high surf especially along S-coastal New England, wave action building up to 10 feet. This will also pose the threat of dangerous rip currents. Outlook /Sunday Night through Wednesday/... Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 9 ft. Chance of rain showers, slight chance of thunderstorms, patchy fog. Visibility 1 to 3 nm. Monday: Winds less than 25 kt. Areas of rough seas. Slight chance of rain showers, patchy fog. Local visibility 1 to 3 nm. Monday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. Tuesday through Tuesday Night: Winds less than 25 kt. Seas up to 5 ft. Wednesday: 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...Small Craft Advisory from 2 AM to 6 PM EDT Sunday for ANZ230>234-250-251-254-255. Gale Watch from late tonight through Sunday afternoon for ANZ235>237-256. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Frank SHORT TERM...Sipprell LONG TERM...Nocera/Sipprell AVIATION...Frank/Nocera/Sipprell MARINE...Nocera/Sipprell

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