Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 190753 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 353 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/... Early AM convection now predominantly focused in the convergence of a 40 kt SSW LLJ apparent at H92 along immediate SE MA. Given the slow eastward shift and remnant CAPE values 500-1000j/kg per latest LAPS/RAP data, we continue to receive renewed development and training across the Cape/Islands which have led to the issuance of various flood products. PWATs in this location are still running 2.00-2.20 inches, but as the LLJ pulls E, noting that individual cells are progressively waning thanks to a lack of forcing within the modest instability/moisture. This trend should continue, with the bulk of the action offshore around sunrise. 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 /08Z 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/... 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 except for Nantucket, which may see IFR/MVFR conditions linger in a mix of low CIGS and fog, but then mainly just low CIGS late this afternoon and evening. Some improvement possible there, so leaned somewhat optimistic in TAF for ACK. 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 especially after 12Z. Occasional LIFR until winds shift mainly to the W, through 10Z this morning. Sea breeze possible but with a late start. KBDL Terminal...Moderate confidence. MVFR-IFR through 10-12Z but improvement to VFR thereafter. Timing may be off a bit 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. S-SW winds will remain gusting to 30 kt, and slightly higher in remnant thunderstorms, across the SE waters through 12Z. The winds should then gradually recede thereafter, allowing seas to recede as well. 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 9 AM EDT this morning for ANZ231>234. 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...Doody SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.