Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 220203 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1003 PM EDT THU JUL 21 2016 .SYNOPSIS... Heat and humidity return Friday along with the risk for scattered showers and thunderstorms during the afternoon and evening, some of which may be severe. There may be a few leftover showers Saturday, otherwise the weather will be rain-free. Hot weather is expected to continue this weekend. An approaching cold front will bring another round of scattered showers and thunderstorms Monday night and Tuesday. More warm weather follows midweek. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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10 pm update... Only minimal changes with the late evening update. Dropped mins by a degree or two thanks to plenty of dwpts still in the 50s at this time. Most locations will remain in the 60s, but some will come very close to 60 itself in NW MA before the night is over. Otherwise, forecast is on track.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Friday into Friday Night... */ Highlights... * SPC Outlook of SLIGHT RISK for scattered severe storms * Scattered severe thunderstorms late Friday into Friday evening * Threats: damaging winds, large hail, an isolated tornado or two * Heavy rain leading to localized minor street flooding possible Scattered strong to severe thunderstorms are expected late Friday into Friday evening across Southern New England. A SLIGHT RISK is currently in effect from the Storm Prediction Center highlighting the potential threats with these storms of damaging winds, large hail, and an isolated tornado or two (low risk threat). Tough forecast so sticking close to 21.0z NCAR ensembles and the latest SREF guidance, though not completely ignoring deterministic solutions but making sure they`re initializing well in capturing the present MCS over the Great Lakes Region. RPM and NAM models are somewhat more favored over other guidance, though WRF solutions are handling well the anticipated forecast MCS overnight. */ Morning into Midday... With regards to the morning, feel we`ll see a pre-trough boundary push S across New England from remnant overnight convection over the St. Lawrence River Valley into the Great Lakes region with the possibility of a MCS W emerging out of the N Great Lakes Region curving S towards the OH River Valley and Mid-Atlantic with the mean-wind / corfidi vector flow. Anticipating isolated to scattered shower activity making its way into our forecast area with scattered to broken cloud decks given mid to upper level anticyclonic flow. Expect activity along the pre-trough boundary to become diffuse and washing out as it progresses S over our region and out to sea. This while anticipating that we`ll see some measure of clearing across N New England rearward of the pre- trough frontal boundary. But there is some question as to the extent and thickness of expected cloud cover across the NE CONUS, how long it will linger and whether it perhaps limits boundary layer destabilization, especially for S New England. This has implications on the outcomes discussed below. */ Afternoon into Evening... Into the afternoon and on into the evening, forecasting scattered thunderstorms to develop across N / Central New England during the afternoon hours, moving S/E with the mean wind towards evening and continuing late. Knowing exactly when and where storms will initiate is the biggest question but a general initiation area over the N to central NE CONUS is favored, evolving S/E into S New England. As to the environment, here`s what a consensus of forecast guidance shows: H5 trough axis and attendant mid-level vortex energy digging across the NE CONUS Friday Night. It`s the environment ahead of this that there is plenty of moisture given low-level RH and theta-E axis with precipitable waters up to 1.75 inches. Plenty of instability as well so long as the boundary layer can destabilize mixing out into conditionally unstable mid to upper levels yielding a deep-layer profile of CAPE. Still some indications of an elevated mixed layer from the W that would only exacerbate the instability threat. In all a general 1000 to 2000 j/kg of CAPE is forecast that, if extended throughout the atmospheric profile should it fully destabilize, presents the threat of large hail as there would be a fair amount of CAPE above the -20C layer. With regards to forcing and lifting mechanisms, increasing cyclonic NW flow aloft with indications of venting via diffluence as lead mid- level vort-max energy is expected out ahead of the main mid-level trough forecast overnight. Focus of shear is within the 0-3 km layer where there is significant turning, more so if you get a S over a SW wind. Roughly 30 to 35 kt 0-3 km shear vectors and helicity values of 150 m2/s2 with energy helicity index values of 1.0 to 2.0 that could potentially be strengthen more so with a S wind. Shear vectors oriented quasi-perpendicular to the aforementioned lead mid-level vort-max energy and the possibility of another surface pre-frontal trough boundary. In all, environment parameters that give way to concerns of organized storms with rotating updrafts, presenting threats of damaging winds and the possibility of a tornado. This is amplified further by consensus forecasts that with boundary layer mixing that lifting condensation levels mix out to around 4000 feet. As the previous forecaster mentioned, high LCLs, surely makes for a low risk tornado threat thus the isolated one to two wording. Will keep with enhanced thunderstorm wording in consistency with SPC`s SLIGHT RISK of scattered severe convection outlook. Damaging winds, heavy rain, frequent lightning, and small hail. Roughly, timing looks to be around 4 pm to 10 pm (20z to 2z) with strongest storms across the interior, perhaps more N/W with the greater opportunity of destabilization beneath and increasing cyclonic flow aloft. In closing, nothing is set in stone. This type of weather comes down to subtle mesoscale details that as of late near-term high- res guidance has been handling well. But in looking at the synoptics of forcing and lift along with moisture and instability, there is the potential for strong to severe storms somewhere across the NE CONUS so long as we get some clearing allowing for destabilization. Exact location as well as specific timing though still remain somewhat indeterminate. Outcomes in some measure reliant upon what happens this evening (Thursday) and overnight. */ Overnight... Expect with the conclusion of diurnal heating that any thunderstorm activity should weaken as it continues to slide S/E. The crux of the mid-level vort-max along the H5 trough axis and associated dynamics sweep across N New England. Still there will be some measure of lift and forcing of available moisture mixed above the decoupled boundary layer. It is within the elevated conditionally unstable layer that there is still a decent measure of instability. Perhaps some low-level h925-85 convergence above the boundary layer, can not rule out the continuation of scattered thunderstorm activity S/E and out to sea ahead of a sweeping cold front. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Big Picture... High pressure covers the Southern and Central USA while troughs dig over the Pacific Northwest and the Maritimes. The high pressure weakens a little and shifts south, opening up a zonal flow across the northern tier of the USA. The Maritime trough directs a shortwave through New England Saturday which then kicks the trough off to the east. The Pacific NW trough ejects into the zonal flow and reaches New England Monday night-Tuesday. During mid-week, the zonal flow remains open to additional shortwaves, but timing consensus among the models is poor. We used a blend of the long range guidance. Forecast contour height fields remain higher/warmer than normal levels, so expect temperatures to run warmer than normal through the period. Details... Saturday-Sunday... Shortwave overhead Saturday moves east of us by Sunday. The supporting 120 knot jet overlays its left exit region over VT-NH and possibly into parts of Southern New England. Moisture fields show some post-frontal drying across our area, but a pool of moisture under the shortwave advects into at least Northern MA during Saturday. This suggests a period of midday-afternoon clouds, especially over Northern MA, and either scattered or widely scattered showers. Total-totals will be near 50 and LI sub-zero, so thunder also possible. Otherwise rain-free with high pressure building over the region especially on Sunday. With cooler temps aloft, mixing may reach deeper Saturday... but should climb to at least 850 mb each day. Temps in this layer suggest max sfc temps around 90, especially on Saturday. Monday-Tuesday... Pac NW shortwave races east across Canada with surface low swinging a cold front across New England Tuesday. GFS shows a two-part shortwave with the main area in Northern Quebec and a detached less- defined shortwave moving across New England. The ECMWF shows a single well-defined shortwave crossing New England Tuesday. Lean toward the ECMWF scenario. This means dry weather much of the day Monday. Scattered showers/thunder possible in the CT Valley and Berkshire slopes late Monday, and all areas Monday night into Tuesday until fropa. Temps aloft around 18C and possibly 20C, so max sfc temps should be around 90 or in the lower 90s. Wednesday-Thursday... High pressure builds over the region. Looks like a quiet weather period. Temps at 850 mb remain 16-18C so surface max temps should be 85-90F. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... 18z update... Today...High Confidence. VFR. Light W/SW flow. FEW to SCT mid-high cigs. Likelihood of sea-breezes along the immediate E-coast. Tonight...High Confidence. VFR. Increasing W/SW flow. Gusts up around 15 kts for S-coast terminals. SCT to BKN mid-high cigs dropping down from the N. Friday into Friday Night...Moderate Confidence. VCSH during the morning period with SCT-BKN cigs. Clearing later half of the day ahead of SHRA/TSRA for the late afternoon into evening period. With any TSRA, TEMPO MVFR-IFR conditions. Some TSRA may be severe with strong to damaging W/NW winds, hail, even an isolated tornado possible. Focus 22.20z to 23.04z. S Winds with the likelihood of gusts up to 25 kts especially for S-coastal terminals. KBOS TAF...Sea-breeze may hang just off the door-step of the terminal around 19-20z. W/SW winds prevailing. KBDL TAF...VFR prevailing. Outlook /Saturday through Tuesday/... Saturday...Moderate confidence. Mainly VFR conditions expected. Brief periods of MVFR/IFR conditions possible in SCT SHRA/TSRA over SE New England. Patchy fog possible during the early morning Saturday. Sunday...High confidence. VFR. Monday...Moderate confidence. Mainly VFR conditions expected. Brief periods of MVFR conditions in SCT SHRA/TSRA possible. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. Tonight... W/SW winds continue with gusts up to 20 kts possibly as high as 25 kts. Seas building towards 5 feet towards morning by which point small craft advisories will be in effect for a majority of the waters. Seas building to 5 feet. Friday into Friday Night... Continued S/SW winds up to 30 kts ahead of a sweeping cold front late. It is also ahead of this front that shower and thunderstorm activity is anticipated. Main concern is along the near shores where there is the possibility for some of those storms could be strong to severe. Low confidence at this time. Small craft advisories continue. Seas 5 to 7 feet. Outlook /Saturday through Tuesday/... Increasing southwest winds ahead of a cold front will continue to generate rough seas, particularly across the southern coastal waters into Saturday. Winds and seas briefly diminish Sunday behind a cold front. Showers and thunderstorms are likely possible over the waters Friday evening and night, and again on Monday afternoon. Storms Friday night may become strong, producing dangerous boating conditions at times. In addition, areas of fog may develop on Friday night, limiting visibilities. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Air Quality Alert from 11 AM to 11 PM EDT Friday for CTZ002>004. MA...Air Quality Alert from 11 AM to 11 PM EDT Friday for MAZ005>007-012>021. RI...Air Quality Alert from 11 AM to 11 PM EDT Friday for RIZ001>008. MARINE...Small Craft Advisory from 8 AM Friday to 8 AM EDT Saturday for ANZ231>237-250-254>256. && $$ SYNOPSIS...WTB/Sipprell NEAR TERM...Doody SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell MARINE...WTB/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.