099 FXUS61 KBOX 051736 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 136 PM EDT Wed Aug 5 2020 .SYNOPSIS... A weak cold front moves through Southern New England late in the day, and may generate isolated showers. High pressure then builds over the region tonight and Thursday with very warm and drier air. While generally dry and seasonable Thursday night into Saturday, cloudy and unsettled conditions for the South Coast of MA and RI and Cape Cod. Warm and dry weather for the rest of weekend. Our weather then turns very warm and increasingly muggy into next week, with daily chances for showers and thunderstorms. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1:30 PM Update: Forecast is still on track, sct'd cumulus clouds have bubbled up with an isolated shower/t'storm moving across eastern CT and moving towards western RI. The threat of a VERY isolated shower or garden variety thunderstorm will continue this afternoon and diminish moving past sunset. 10 AM Update: Only a few minor tweeks to the near term forecast this morning. We've updated sky conditions to reflect the few clouds that remain along the coast of RI and the south coast of MA; including the Cape and Island. Otherwise it's a mainly dry and sunny day. As previously discussed a few pop-up showers are possible across eastern MA/RI - coverage will be isolated. Otherwise it's a warm and muggy day with temperatures in the upper 80s and low 90s. 7 AM Update: No significant changes needed to the going forecast with this update. Upper shortwave disturbance aloft passing to our northwest with its main effect being a modest increase in coverage of cumulus. Possibility that a few could pop a shower or two but it's a fairly remote possibility and many will stay dry today. Turning quite warm today and given that folks may head to the beach, a high surf advisory is in effect for south- facing beaches due to sea/swell from Isaias leading to a higher risk of rip current and hazardous surf conditions. Highs look on track in the 80s to lower 90s with generally decreasing humidity levels. Previous discussion... Upper low over the Great Lakes will swing a shortwave across Srn New England today. The associated upper jet will place its right entrance region overhead, providing some lift that could generate a few showers. PW values are around 2 inches along the South Coast, but diminishing farther north and west. Favorable CAPE is limited to the South Coast. With limited moisture and instability, confidence in precip is limited. Will continue to forecast widely scattered showers with no mention of thunder. Mixing is forecast to reach 800-mb, where temperatures will be 11-13C. These temperatures will support surface max temps of 85 to 90. Dew points in the 60s, so the humidity will be noticeable but much more comfortable than yesterday. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Tonight... The weak cold front moves offshore, and surface winds turn out of the northwest. Dew points will continue to fall to more comfortable levels, reaching the mid 50s to mid 60s. Expect min sfc temps in the upper 50s and 60s, possibly mid 50s in extreme northwest MA. Thursday... High pressure over the region will bring dry weather and continued comfortable humidity. Mixing to 850-mb will tap temperatures of 14-15C, supporting max sfc temps in the mid 80s except a little cooler on the South Coast. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Highlights... * Unsettled showers South Coast, Cape and Islands Thurs nite thru Sat, generally dry for remainder of SNE. Seasonable temps. * Warm and dry weather for the rest of the weekend. * Increasing heat and humidity early next week with rising heat indices. Daily thunder chances, better t-storm coverage Tues-Wed. Details... Thursday Night into Saturday: SW flow aloft regime prevails in mid-levels through this period, established by an upstream mid-level trough which moves from the Great Lakes into the Northeast states late in the week into Sat. While a broad surface ridge associated with dry weather and low humidity levels is expected to extend across a part of interior New England, the primary surface feature is a SW-NE oriented stationary boundary from the mid-Atlantic states into our far southern offshore waters. Weak large-scale ascent acting on the stalled boundary appears to be enough across most of the 00z NWP suite to permit a weak surface wave to develop on the boundary near the DE/NJ coast early Fri. As this feature ripples NE along the boundary, it shifts its effective position slightly northward as a warm front. Main source of uncertainty in this period is timing when and how far north will the stationary feature return as a warm front, spreading clouds and related showers into a part of Southern New England. In general, the ECMWF/Canadian GEM keep the feature suppressed to the south enough to allow for little if any measurable precip. The GFS and the SREF on the other hand keep much of this period wet/unsettled, bringing showers to the South Coast/Cape by Thurs nite and spreading the precip shield northward across much of Southern New England thru Fri/Sat. Ensemble means also offer no preferred outcome and generally lend support to their respective deterministic model solution. Can't dismiss a wetter solution yet, but am thinking given the weaker surface low shown across most models that it probably won't be enough to really produce as much northward advance to the boundary/precip as depicted in the wetter guidance. Thus following along the lines of the ECMWF/GEM. Will thus keep most of MA, northern CT into northern RI generally dry, and confine more clouds with Chance-ish PoPs to the MA/RI South Coast, Cape and adjacent waters. That said, doesn't look to be a washout by any stretch but enough to potentially ruin any beach days. With more clouds/showers south than north, temperatures should have more of a diurnal range to them (e.g. warmer highs/cooler lows) as one moves north from the South Coast/Cape. More clouds and rains to the south will allow for narrower diurnal ranges in SE MA and coastal RI. Overall temps should be fairly seasonable, though highs may run cooler than average towards the South Coast/Cape. Saturday Night into Sunday Night: Much nicer stretch of weather for the rest of the weekend with high pressure building in from the Great Lakes Saturday night, then moving offshore Southern New England late Sunday into Sunday night. Look for dry conditions with plenty of sun, and should be great for any plans outdoors. 850 mb temps initially around the mid teens (+13 to +15C) with warm advection on Sunday rise to the upper teens Sunday (+16 to +18C). Should see highs in the mid to upper 80s, though a few sites in the CT Valley and eastern MA could reach 90 given WSW downsloping. Will also start to see a rise in humidity levels come Sunday but nothing too humid or oppressive. Monday through Wednesday: Pattern aloft turns zonal across CONUS, allowing for significant heat (upper teens to low-20s Celsius 850 mb air) to advect eastward into Southern New England. Humidity levels will rise to very humid to oppressive levels especially by Tuesday, so the main story in this period will be increasing heat and humidity. With high temps in the upper 80s to lower 90s and dewpoints looking to rise into the upper 60s to mid 70s, heat indices approaching Advisory levels appear possible in parts of this period. Given the heat and humidity, daily thunder chances should then resume. Difficult at this time range to pinpoint how strong they may be and/or on what days. Do note the GFS advecting in some steeper mid-level lapse rates that could really boost instability but appears flow aloft is very weak for most of the period. GFS/ECMWF differ on timing a cold front through which would offer greater convective coverage, but that may be sometime either Wednesday (ECMWF) or Thursday (GFS). Will follow blended PoP guidance this period. && .AVIATION /18Z WEDNESDAY THROUGH MONDAY/... 1:30 PM Update: Fairly quiet weather leading to VFR conditions for many this afternoon. Isolated patchy fog could develop tonight into Thursday morning, reducing some locations along the coast to IVFR. Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 12z TAF Update: Today: High confidence. VFR all terminals with bases around 050. Outside shot at showers but too remote a possibility to mention in the TAFs. Daytime mixing will also draw southwest wind gusts of 20 kt down from aloft. Tonight...High confidence. VFR. Possible IFR/LIFR cigs/vsbys in fog and low clouds moving off the ocean across ACK and possibly Cape Cod. Thursday...High confidence. VFR. Light northwest wind. KBOS Terminal...High confidence in the TAF. KBDL Terminal...High confidence in the TAF. Outlook /Thursday Night through Sunday/... Thursday Night through Friday: VFR. Slight chance SHRA. Friday Night: Mainly VFR, with local MVFR possible. Slight chance SHRA. Saturday through Sunday: VFR. && .MARINE... 10 AM Update: We've allowed the Small Craft Advisory to expire for waters east of Boston. Otherwise, moderate to high risk for rip currents on southern facing waters. Today... Southwest winds with gusts to 20 kt. Lingering rough seas on the outer waters and RI/Block Island Sounds, with 10-12 feet. Expect these seas to diminish this afternoon. Tonight... Southwest winds shift from the northwest by morning, as a weak cold front moves across the waters. Seas will continue to diminish, dropping below 5 feet overnight. Thursday... High pressure builds over the waters. Light northwest winds become variable. Seas remain less than 5 feet. Outlook /Thursday Night through Sunday/... Thursday Night through Friday Night: Winds less than 25 kt. Chance of rain showers. Saturday: Winds less than 25 kt. Slight chance of rain showers. Saturday Night through Sunday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Surf Advisory until 6 PM EDT this evening for MAZ020-023- 024. RI...High Surf Advisory until 6 PM EDT this evening for RIZ006>008. MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ235- 237. Small Craft Advisory until 6 AM EDT Thursday for ANZ254>256. && $$ SYNOPSIS...WTB/Loconto NEAR TERM...WTB/Loconto/Gaucher SHORT TERM...WTB LONG TERM...Loconto AVIATION...WTB/Loconto/Gaucher MARINE...WTB/Loconto/Gaucher