Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201953 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 353 PM EDT Thu Jul 20 2017 .SYNOPSIS... After some wet weather tonight, one more day of hot and humid weather for Friday. Cooler and drier air moves in for Friday night. Warm and dry Saturday. Cooler, and more humid but with unsettled weather follows for Sunday through Tuesday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... 330 pm update... */ Highlights... - Late afternoon scattered showers and thunderstorms expected mainly for N/E portions of MA through evening. There`s a low risk of storms becoming strong to severe. - Into this evening, complex of showers and thunderstorms out of W NY pushes S/E. Expecting rain to clip S/W portions of S New England with the possibility of some embedded thunder. */ Discussion... Scattered showers and thunderstorms forecast in association with a moist, unstable boundary layer being acted upon mid to upper level ascent. However a limiting culprit across the region of mid- level dry air. With favorable ascent across N New England associated with the mid-level vortmax and LFQ of an upper level jet through the cyclonic flow acting upon a destabilized surface-H8 boundary layer, already seeing shower and thunderstorm activity initiating along the high terrain with better convergent focus. Some of that mid to upper level ascent should lobe S far enough to spark some activity as suggested by the HRRR, especially over N/E areas of MA where chance PoPs are presently forecast this evening. But after more thorough analysis, the strong to severe chances are limited by the presence of mid level dry air. Even the NCAR ensembles show low probability of lightning in concert with a present area of low K-indices. Meanwhile S/W, monitoring the upstream parallel-leading stratiform mesoscale convective system (MCS). S-flank is anchored to the low level SW inflow of better moisture. However as the complex drops S, it will depart from areas of more favorable venting and shear. The modification of the environment looks to weaken lessening the maintenance of low level inflow as W winds become SW during the evening period over Upstate NY / PA. Thinking that S/W areas of New England will see widespread rain with the leading stratiform from the MCS as it weakens. Perhaps some embedded thunder. But two issues, 1.) cloud shield associated with the MCS, and 2.) mid- level dry air. HRRR is suggesting some heavier precipitation with the parent outflow boundary acting on the destabilized airmass over S/W CT, but not totally sold on it. In all, will limit the thunder threat across the region, opt out of any strong to severe wording, and mainly focus on chance PoPs for scattered to widespread rain over N/E MA and S/W CT. In-between may see little if anything. Heat diminishing, lows dropping down to around the upper 60s to low 70s. The W/SW flow prevailing allowing the maintenance of higher dewpoints along the S/SE coast, likely to see the return of low clouds and fog. If anything dense, mainly over Nantucket. Will issue another SPECIAL WEATHER STATEMENT highlighting the threat along with the potential of a dense fog advisory. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Friday... One more day of heat and humidity. Little if any lifting and forcing mechanisms, looking like a quiet day beneath a fairly dry airmass. The boundary layer mixing out to around H85 where temperatures of +16-17C still reside, should see scattered cumulus develop as both drier air and faster westerlies mix down to the surface. Highs upper 80s to low 90s with mid to upper 60s at first quickly mixing out to upper 50s to low 60s. This will limit heat indices from climbing into the mid 90s. No HEAT ADVISORY headlines are being considered with this forecast. Friday night... Turning cooler and drier. Low to mid level cool-dry front sweeps S/E. Winds shifting NW ushering in drier air with a slightly cool airmass aloft. Humidity dropping beneath some scattered to broken cloud decks. Lows dropping into the low to mid 60s with a few spots N/W in the upper 50s. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Highlights... * Sat: Dry and very warm afternoons but with lower humidity * Sun into Tue: Cooler than normal + unsettled with showers at times * Drying trend likely by the middle of next week Overview and model preferences... Noting that the time-mean jet remains firmly entrenched across New England through the long term period. This generally has two consequences, one being the inability to get true subtropical warmth into New England and the second being the continuation of a relatively active pattern. As the previous forecaster noted, this by no means suggests a washout. The first Canadian vortex shifts from the Ungava Bay region offshore of Newfoundland-Labrador by the end of the weekend. This will initially yield more zonal flow, but it is short lived as a second wave phases and re-deepens the longwave trof in place across the NE CONUS. This trof remains the key synoptic player in sensible wx details across the region through much of next week, and its trof center`s location with respect to S New England will determine whether the active, wet pattern continues or drier air is allowed to move in. Details... Sat... Current indicators suggest that with the vortex to the N shifting E, this allows a ridge of high pres to move into New England, keeping the slow moving frontal boundary well to the S and limiting destabilization somewhat thanks to subsidence and a dry air intrusion which lower overall lapse rates and moisture (lowered K- indices). Therefore, will maintain a mostly dry forecast for the day on Sat. The ridging and afternoon mixing will provide a bit of relief in afternoon dewpoints, possibly mixing them down into the upper 50s. H85 temps, should be reached, and given they run around +16C, afternoon highs could be as warm as near 90 where downsloping is maximized, mid-upper 80s elsewhere. Sat night into Sun... Will need to watch another MCC develop in the Great Lakes region and gradually shift along and S of the stalled frontal boundary through the OH Valley to the mid Atlantic states. Given the better thermodynamic parameters remain S of this stalled front, and weak lapse rates remain in place, am not too keen on nocturnal convection, as this is a typical case where the best instability/moisture will pull the MCC to the SW of New England. However, one factor to consider as we approach will be the remnant cold pool and whether a weak cold pool low pres could allow the remnants of the system to slide E, or re-form convection in situ. Will keep POPs mainly S through the overnight hours, but will still need to monitor this as we approach given the close proximity. Otherwise, noting an attempt to lift the front N on Sun, and with the additional moisture load could see a round of SHRA develop but given weak lapse rates remain and better CAPE remains to the S, only looking at a spot TS if at all. Still relatively unsettled, and should the front shift further N than currently progged, it has the possibility of being more active. Generally cooler (mainly 70s to around 80 for highs) with more clouds to contend with, but dewpoints will generally be higher thanks to the increase in column moisture. Mon into Tue... With deepening trof move in aloft, am noting guidance in favor of developing weak slow moving low pres which will gradually pivot across the region during this 48-hour period. Given some cooling aloft with the trof, am noting a slight increase in lapse rates along with CAPE values. Shear does increase too thanks to cyclonic curvature yielding stronger directional shear. Overall, a window of potential continuing unsettled wx with a mix of showers and occasional T-storms. Not a washout, as dry air will rotate into the region at times as the low pres moves. However, some heavy rain possible given PWATS hover near 1.50 inches. Not a great setup for severe wx, but given the shear/slightly higher lapse rates this still may need to be watched. Another, mitigating factor may be clouds, which are likely to be dominant (with generally a few breaks of sun). Highs likely remain at or below normal thanks to the cloud cover while overnight mins should rest in the 60s thanks to sustained dwpts. More to come given the uncertainty in development of the weak low pres. Wed into late next week... Base of trof finally shifts E of the region allowing for ridging and attendant NW flow. This drying and influence of high pres associated with the ridge should allow for a trend of drying/less unsettled wx for the late week period. Will trend dwpts down suggesting more comfortable conditions and mainly dry/NIL POPS. && .AVIATION /19Z THURSDAY 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/...High confidence. Through tonight... VFR. W/SW winds with gusts up to 20 kts diminishing. -RA possible mainly N/E MA and S/W CT with a lower risk of TSRA. IFR-LIFR CIGs and VSBYs advecting back in along the S-coast and along the Cape and Islands with ACK seeing 1/2SM FG again. Friday... S-coast IFR CIGs and VSBYs eroding, especially over ACK. W/SW winds continuing with gusts up to 20 kts. VFR with SCT CIGs around 4-5 kft. Friday night... Continued VFR. Winds veering out of the W/NW with time early on. KBOS Terminal... Low risk of SHRA/TSRA activity into evening. Otherwise VFR. KBDL Terminal... RA possible this evening. Will continue with VCSH. Lower risk of TSRA. Outlook /Saturday through Tuesday/... Sat...High confidence. Mainly VFR. A spot shower or thunderstorm possible late, mainly overnight. Sun through Tue...Low confidence. VFR will likely dominate especially inland, however a complex mix of clouds, occasional SHRA and isolated TSRA could lead to occasional dips to MVFR/IFR. Otherwise, some clouds and fog may lead to low CIGS/VSBYS (especially along the coast) to yield periodic IFR/MVFR conditions as well. Timing uncertain. && .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... Through tonight... Breezy W/SW flow continues. Scattered showers and thunderstorms possible off the waters off NE MA and S CT. Waves below 4 feet. Renewed fog forecast along the near-shore S-coast, especially around Nantucket. Friday through Friday night... W/SW winds through the day dissipating fog. Will shift W/NW overnight ushering in drier air. Gusts below 20 kts. Seas below 4 feet. Outlook /Saturday through Tuesday/... Sat and Sun...High confidence. Mainly quiet boating weather expected. Winds although shifting from mainly W-SW to the E on Sun, should remain below 20 kt. Seas too, remain low, mainly 3 ft or less on the ocean waters. Only caveat could be a localized shower or thunderstorm, which may exhibit stronger winds/higher seas locally. Mon and Tue...Moderate confidence. With a risk for low pres slowly approaching then moving over the waters seas may build in response to nearly 5 ft on the ocean waters. Winds are likely to generally remain below small craft advisory thresholds, but given the risk for higher seas, headlines may be needed for a time during this period. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Heat Advisory until 6 PM EDT this evening for CTZ002>004. MA...Heat Advisory until 6 PM EDT this evening for MAZ017>021. RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>008. Heat Advisory until 6 PM EDT this evening for RIZ001>004-006. MARINE...None. && $$ SYNOPSIS...Doody/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...Doody AVIATION...Doody/Sipprell MARINE...Doody/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.