869 FXUS61 KBOX 251429 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1029 AM EDT Tue Aug 25 2020 .SYNOPSIS... Another round of scattered showers and thunderstorms is expected this afternoon into early evening as a strong cold front sweeps across the region. Much drier and cooler weather will follow tonight into Wednesday with a touch of fall. Unsettled weather may return Thursday with the risk of showers and thunderstorms as a frontal boundary tracks over or near the region. Dry weather may briefly return to the area Friday followed by a cold frontal passage and scattered showers/T-storms Saturday/Sat night. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1030 AM update... The cold front which will lead to our chance for storms this afternoon is moving into western NY this morning. Clearing skies have allowed for increasing instability (up to >1000 J/kg) and low level lapse rates, especially over CT/RI (to 7 C/km). These should continue to increase into early afternoon but the fly in the ointment remains...obs show westerly flow over our area (limiting convergence) and drier air at the mid levels moving into western zones this afternoon. Overall not expecting a very widespread severe event (moreso to our southwest) but it is fairly likely we'll see a few strong to severe storms. Previous Discussion... Low clouds and fog across the CT River Valley and also across RI and southeast MA will advect offshore by 9 AM as boundary layer winds are from the WSW including elevated platforms such as KORH 27008 KT and BUZM3. Upstream convection over the Catskills of eastern NY continues to weaken as cloud tops warm. Should just be some decaying showers upon entering western CT/MA in an hour or two. Otherwise previous forecast remains on track. Earlier discussion below. ======================================================================= A few strong to severe t-storms possible this afternoon... Line of convection moving through central NY ahead of mid level trough expected to weaken as it approaches western New Eng with hi-res CAMs indicating just a few weakening showers possible moving into western MA this morning. A robust mid level trough will be moving SE into New Eng with modest mid level jet punching into the region this afternoon/early evening. The attendant cold front moves quickly south across SNE mid/late afternoon reaching the south coast by early evening. Moderate instability develops ahead of the front with CAPES 1000-2000 J/kg with effective bulk shear increasing to 30-40 kt in the afternoon. Instability/shear parameter space supports organized updrafts and potential for strong to severe t-storms, however there remains some uncertainty with overall areal coverage of storms. This mainly due to westerly flow ahead of the front which may limit low level convergence. Hi-res CAMs not bullish on widespread storms and indicating more scattered activity. Current thinking is there may be a focus for storms this afternoon south of the Pike and especially across eastern CT, RI and SE MA ahead of the front where instability will be greatest given higher dewpoints and more heating, as well as localized convergence as winds turn SW near the south coast. A second area of storms with the actual front may impact northern MA. Potential for storms to organize into line segments with damaging wind gusts the primary threat given steep low level lapse rates approaching dry adiabatic. Hail is a secondary threat with increasing deep layer shear and modest mid level lapse rates. Confidence is above average for a few strong to severe t-storms, however the areal coverage of storms is less certain. It will be another hot and humid day, especially across RI and eastern MA where temps expected to reach the lower 90s. Highs 85-90 elsewhere, except cooler higher terrain. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight... Any lingering convection, mainly near south coast and Cape/Islands will dissipate early in the evening as the front moves offshore. Then clearing with much cooler and drier weather. Impressive cold advection develops behind the front with 925 mb temps dropping from 20C at 00z to 7C at 12z. Temps expected to fall into the 50s, but lower 60s outer Cape/Islands. The cold advection will be accompanied by gusty NW winds, especially near the coast where gusts 25-30 mph possible. There will be a touch of fall in the air Wednesday morning. Wednesday... High pres builds to the west with very dry column resulting in lots of sunshine. Core of low level cool air will be brief as 925 mb temps moderate to 15-16C by late in the day. Highs will reach the mid 70s with some upper 70s possible in the coastal plain, but close to 70 over higher terrain. Refreshing airmass with dewpoints in the 40s. Elevated fire potential Wed afternoon as RH drops to near 30 percent with NW gusts around 20 mph. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights * Unsettled weather Thu with frontal boundary nearby * Risk of Storms Sat/Sat evening, drier Sunday * Another fall-like airmass possible Sun ngt/Mon Precipitation... Tricky setup over southern New England Thu with frontal boundary over or near the region. WNW flow aloft should preclude warm sector from advancing northward thru the region but it will be close with boundary possibly draped across the region. This may set the stage for strong to severe storms given boundary and associated strong low level shear. SPC has placed southern New England in a slight risk in its day 3 outlook. Friday could turn out to be a mainly dry day, as short wave energy enters the Great Lakes and results in downstream ridging over southern New England. This short wave trough over the Great Lakes will eventually result in showers and thunderstorms (possibly widespread!) across southern New England sometime Sat/Sat night. Meanwhile, remnants of Laura will be tracking across the Mid Atlc states. Models not phasing Great Lakes trough with Laura but looks like northern stream trough ingest/captures tropical moisture plume from Laura northward into southern New England. Thus risk for heavy rain Sat/Sat night. Ensembles and deterministic guidance suggest dry post frontal airmass overspreads southern New England Sun and continuing into Monday. Temperatures... Tricky temp forecast Thu with frontal boundary draped across or near southern New England, with a sharp poleward thermal gradient across the region. Some of the guidance has 90 at HFD while northeast MA remains in the upper 60s! Given the WNW flow aloft warm sector likely remains south of New England or possibly bleeding into CT but it will be a close call. Trending warmer Friday given dry weather likely. Saturday some uncertainty given timing of warm front and possible arrival of warm sector. In addition with showers/T-storms potentially widespread, this may keep highs in the 70s to near 80. Post frontal airmass likely sometime Sunday, thus not as warm and considerably less humid with 850 mb temps falling from +12C 12z Sun to about +9C by 00z Mon. The core of the cool fall-like airmass per EC ensembles settles over the region 12z Mon with 850 mb temps lowering to about +7C = refreshing! Highs Sunday and Monday 75-80, lows in the 50s, 40s northwest MA. && .AVIATION /14Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 11z update...IFR/LIFR conditions over southeast MA, parts of RI and the CT River Valley, will improve to VFR by 13z. Thunderstorms over the Catskills of eastern NY at 11z will continue to dissipate as cloud tops warm. Earlier discussion below. ================================================================ Today...Moderate confidence. Mainly VFR after stratus and fog burns off. Scattered t-storms are expected to develop in the afternoon with brief lower conditions, and a few strong storms may produce strong wind gusts. Tonight...High confidence. A few showers/t-storms possible over the Cape/Islands early evening, otherwise VFR with clearing skies. NW gusts to 25-30 kt developing after midnight over Cape/Islands and portions of east coastal MA. Wednesday...High confidence. VFR. NW winds 10-20 kt. KBOS Terminal...Moderate confidence in TAF. Main concern is the threat for a few thunderstorms in the vicinity of the terminal 18-22z. KBDL Terminal...Moderate confidence in TAF. Main concern is the threat for a few thunderstorms in the vicinity of the terminal 17-22z. Outlook /Wednesday Night through Saturday/... Wednesday Night: VFR. Thursday: Mainly MVFR, with areas VFR possible. Breezy. Chance SHRA, slight chance TSRA. Thursday Night: Mainly VFR, with local MVFR possible. Chance SHRA, slight chance TSRA. Friday: Mainly VFR, with local MVFR possible. Slight chance SHRA, slight chance TSRA. Friday Night: Mainly MVFR, with areas VFR possible. Chance SHRA, slight chance TSRA. Saturday: Mainly MVFR, with areas IFR possible. Breezy. SHRA likely, slight chance TSRA. && .MARINE... Today...Pre-frontal SW gusts around 20 kt develop this afternoon with a few higher gusts over south coastal waters where seas may build to 5 ft late today. Chance of t-storms this afternoon. Tonight and Wednesday...Winds shift to NW this evening, with post- frontal gusts to 30 kt developing later tonight with building seas, especially eastern waters. Building seas. SCA all waters except Narragansett Bay. NW winds and seas gradually diminish Wed but gusts to 20 kt. Outlook /Wednesday Night through Saturday/... Wednesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, slight chance of thunderstorms. Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, slight chance of thunderstorms. Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms. Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, slight chance of thunderstorms. Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain showers likely, slight chance of thunderstorms. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 11 PM this evening to 10 AM EDT Wednesday for ANZ232>235-237. Small Craft Advisory from 8 PM this evening to 8 AM EDT Wednesday for ANZ230. Small Craft Advisory from 8 PM this evening to 10 AM EDT Wednesday for ANZ231-251. Small Craft Advisory from 8 PM this evening to 2 PM EDT Wednesday for ANZ250-254. Small Craft Advisory from 4 PM this afternoon to 10 AM EDT Wednesday for ANZ255-256. && $$ SYNOPSIS...KJC/Nocera NEAR TERM...KJC/Nocera/BW SHORT TERM...KJC LONG TERM...Nocera AVIATION...KJC/Nocera MARINE...KJC/Nocera