Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191403 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1003 AM EDT Sat Aug 19 2017 .SYNOPSIS... A cold front will slowly cross the region today, shifting offshore by evening. High pressure brings fair seasonable weather Monday and Tuesday. Another cold front sweeps through New England Wednesday, followed by more dry weather Thursday and Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10 AM update... Not much change from previous forecast. Post frontal airmass continues to slowly overspread the region with NNW winds across CT-RI and western-central MA. SPC mesoanalysis shows winds at 925 mb westerly all the way to the coastline. So as solar heating increases westerly winds just above the surface will help mix down drier air aloft to the surface. However the process will be slower across eastern MA as very moist low level airmass with dew pts into the lower 70s across southeast MA will remain trapped given drier air aloft. Although eventually westerly winds and drier air aloft will win out with low clouds eroding late morning into early afternoon with at least partial sunshine developing across all of eastern MA. Cape Cod and Islands will likely be the slowest regarding this trend. Given southwest flow aloft cold front either gets hung up or washes out across eastern MA this afternoon. This will result in an area of surface convergence enhanced by afternoon seabreeze along the eastern MA coastline. Thus low risk for a few low top (given dry air aloft) showers later this afternoon across eastern MA. All hi res guid including the NCAR ensembles, ARW, NMM and HRRR in agreement on this. Otherwise a warm summer afternoon with highs well into the 80s. Less humid across western MA/CT but remaining somewhat humid across eastern MA with seabreeze precluding drier/less humid air from advecting seaward. Previous discussion... Otherwise, cold front still very slowly moving into S New England just within the last hour or so, and has likely only just recently reached the BOX CWA per latest MSAS/OBS. WV imagery suggests a slight shift to the mid-upper flow such that its likely now more parallel to this steering flow. Therefore expect a very slow passage of the sfc front through the day today, and it may wash out in-situ, as drier air is already filtering in aloft per GOES-16 low-upper lvl WV products. This is supported by LAPS/RAP data as well, with K-indices already in the 20s across W-MA and CT. While the dry air aloft continues to build, it will trap the lower lvl moisture, maintaining the high dwpts through the day today. While this will maintain the humid conditions across S New England, the capping implied by the dry air should limit further redevelopment of convection today in spite of CAPE holding thanks to rising mid lvl lapse rates under cyclonic curvature. There is one short window this afternoon, when the remnants of the front reach E MA/RI where modest moisture pooling could allow for a few SHRA to develop, hinted at in latest mesoscale guidance. Given the dry air however, capped POPs at slight chance. Warm and humid today, even with early cloud debris, gradual clearing and afternoon mixing should still allow temps to reach well into the 80s, with dwpts even at peak mixing remaining in the mid-upper 60s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... Cold front continues to move offshore and wash out, but drier air at the sfc will still lag well behind due to the inability to mix it out during the daylight hours. Meanwhile, low pres continues to wrap up in Quebec as fairly robust upper lvl shortwave rotates toward New England during the late evening hours, shifting E of the area by early AM Sun. This is accompanied by a secondary spike in K-indicies and PWATs through the column as it shunts the initial dry slot to the E, similar to comma-head style wrap-around. While forcing attendant to the wave is relatively strong, the remnant dry air should mitigate shower development, but the risk is non-zero. Some QPF on recent model runs, but given the initial drying, and the moisture spike mentioned above is quite modest, will maintain a generally dry forecast with this update and only suggest slightly more clouds than previous forecast. Day crew can reassess if POPs might be needed. With little change in low-lvl moisture, maintaining dwpts in the 60s, and increased cloud cover expected, overnight mins should once again only fall into the mid 60s at the coolest spots. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Big Picture... Subtropical high maintains sultry summer weather over the Southern USA. Zonal flow over the Northern USA and Canada early week amplifies during the mid and late week. Contour heights are normal to a little above normal early week, but fall below normal by late week. This supports warm summery weather early week, followed by cooler, drier air moving into New England late week. Model upper contour fields are similar through much of next week. Thermal fields and surface pressure fields show a similar distribution. This maintains confidence in the forecast data for our area. Details... Sunday through Tuesday... High pressure builds over the region during this period. Upper trough is directly overhead Sunday morning with lingering moisture below 800 mb but dry air above. Mid level air is at -10C or warmer, while the destabilizing part of the cold pool is near the Canadian border. Expect dry weather Sunday with some diurnal clouds but otherwise a fine day. Temps aloft at 13-14C should support max sfc temps in the low to mid 80s. Expect dry weather Sunday night to Tuesday, although increasing mid and high clouds during Tuesday. Light wind flow will allow sea breezes on Monday afternoon, but gradient on Tuesday suggests a general southwest flow. Mixing should reach between 800 mb and 850 mb each day. Temps of 14-16C Monday and 16-18C on Tuesday suggest max sfc temps in the 80s Monday and 85-90 Tuesday...cooler along the coast. Slow increase in humidity with dew points in the 60s. Winds in the mixed layer Tuesday suggest southwest gusts 15-20 knots. Tuesday night-Wednesday... 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. Cold fropa Wednesday brings an end to the showers, but sufficient mixing to bring northwest gusts to 20 knots behind the cold front. Thursday-Friday... Surface high pressure builds over the region while upper trough moves overhead. Cold pool in the upper trough moves in with 500-mb temps forecast to reach -16C to -18C by Friday. Temps at 850 mb will be 8-10C. Moisture profiles show lots of dry air during that time but with a moist layer at 850 mb. Expect dry weather with potential for some diurnal clouds. Expect max temps in the 70s, possibly near 80 in spots. && .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 Tonight/... 10 AM/14Z update... Only change to the forecast is for a slightly slower erosion to the low clouds across eastern MA. Earlier discussion below. ==================================================================== Through 15Z...Moderate confidence. Mix of IFR/MVFR with the continuation of low CIGS and areas of fog. After sunrise, conditions will gradually improve especially across W MA/CT, but this may take until mid morning to fully break out to more widespread VFR. SHRA/TSRA mainly Cape/Islands continue through 10Z this morning, then shift offshore. IFR remains through that area through 15Z. Winds gradually shift to W-SW and recede. After 15Z through tonight...Moderate confidence. Gradual improvement to VFR all terminals Winds mainly W-SW, with sea breezes possible E coast. KBOS Terminal...Moderate confidence. Mix of IFR this morning will gradually give way to improvement to VFR. Sea breeze possible but with a late start. KBDL Terminal...High confidence in TAF. Outlook /Sunday through Wednesday/... Sunday through Tuesday...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. Tuesday night-Wednesday...Moderate confidence. VFR with areas of MVFR cigs/vsbys in showers and scattered thunderstorms. IFR cigs possible in early morning fog/low clouds especially in areas that have had some rain. && .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. Winds and seas will diminish this morning. Timing of current small craft advisories seems reasonable, although conditions may drop off more quickly than current advisories suggest. After advisories are dropped boating conditions should remain relatively quiet into the overnight hours, with the only issue being some localized marine fog which may restrict visibilities, especially on the S waters. Outlook /Sunday through Wednesday/... Sunday through Tuesday...High confidence. High pressure builds over the waters. Winds remain 15 knots or less, and seas 3 feet or less. Winds pick up a little on Tuesday, but remain 20 knots or less with seas 4 feet or less. Tuesday night-Wednesday...Moderate confidence. A cold front approaches from the Great Lakes, crossing the waters late Wednesday or Wednesday night. Southwest winds increase with gusts 20 to 25 knots during Wednesday. Winds shift from the northwest Wednesday night. Seas build Tuesday night and Wednesday reaching 5 to 6 foot heights on the outer waters. The southwest flow may also nudge those higher seas into RI Sound as well. A Small Craft Advisory may be needed on some of the waters during this time. && .CLIMATE... Highest recorded dew points on August 18 in any year PVD 76 in 2002 BDL 82 in 2002 BOS 77 in 1994 ORH 75 in 1987 Highest dew points yesterday PVD 76 from about 1130 to midnight EDT BDL 76 from about 630 PM to midnight EDT BOS 74 from about 11 PM to 1 AM EDT ORH 72 from about 740 PM to 1220 AM EDT So the PVD value ties highest for August 18. Highest value at PVD for August is 80. Highest value at PVD for anytime is 81. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 11 AM EDT this morning for ANZ235- 237. Small Craft Advisory until 5 PM EDT this afternoon for ANZ250- 254. Small Craft Advisory until 8 PM EDT this evening for ANZ255- 256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Nocera/Doody SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Nocera/Doody MARINE...WTB/Doody CLIMATE... is the U.S. government's official web portal to all federal, state and local government web resources and services.