113 FXUS61 KBOX 232302 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 702 PM EDT Thu Jul 23 2020 .SYNOPSIS... Scattered thunderstorms will continue into the evening ahead of an approaching cold front. Some of the storms may produce localized damaging wind gusts along with torrential rainfall/localized flooding. Behind the front, not quite as hot or humid Friday along with mainly dry weather except for a few showers possible near the south coast. Drying-out for the weekend. Saturday appears to be a rather comfortable day and likely the pick of the weekend. Oppressive heat and humidity does return Sunday through early next week. A frontal boundary brings showers and storms to southern New England by mid next week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... 7 pm update... Bulk of the region is finished with the severe weather threat this evening. Mesoanalysis indicates the rain cooled atmosphere has been worked over enough in northern MA that the environment isn't as favorable to sustain the few isolated storms moving in from NH. With this evening update I've brought down temperatures several degrees in areas that saw the bulk of the rain today (generally north of the MA pike). Exception to the nil severe threat would be for southwestern MA/northwestern where a few cells continue to take advantage of several thousand J/kg of CAPE and could produce some damaging winds as they advance east over the next hour or so. No chance to the previously mentioned thinking that scattered showers and thunderstorms may persist overnight near the south coast as the LLJ surges over top the stalled boundary. Previous discussion... * Severe Thunderstorm Watch into this evening for the entire region except for the Cape/Islands and the south coast of RI * Localized flood threat exists too Scattered to numerous thunderstorms have developed across western/central and northern MA and now were working their way into northern CT and I-95 corridor late this afternoon. Despite poor mid level lapse rates...70 to 75 degree dewpoints have resulted in surface capes between 2500 and 3000 J/KG. This combined with bulk shear of 30 to 35 knots was enough for some of the storms to become strong to severe, producing localized damaging wind gusts. In addition, high Pwats on the order of 2+ inches was resulting in torrential rainfall with the strongest storms. This has allowed for localized flooding, especially across northeast MA where training thunderstorms have resulted in significant urban flooding. The shortwave/pre-frontal trough will continue to push southeast into the first half of the evening. While the forcing/shear is not quite as strong across RI/southeast MA plenty of instability remains. Therefore, a few strong to severe thunderstorms will remain possible into the evening. In addition, modest Cape values persist across parts of NH/VT which has allowed additional scattered convection to develop to our north. Therefore, a few more strong to severe t-storms will be possible across our interior zones and northern MA into the evening with the main threat being locally damaging straight line wind gusts. Therefore, the Severe Thunderstorm Watch continues through 8 PM for all but the south coast of RI as well as Cape Cod/Islands. Torrential rainfall with the strongest storms will continue to pose a localized urban flood threat. The bulk of the showers and thunderstorms should wind down by late evening as the frontal boundary sags towards the south coast. However, the boundary does stall out near the south coast coupled with a modest southwest LLJ. This may allowed scattered showers and a few thunderstorms to persist overnight near the south coast, Cape and Islands. Meanwhile, across the rest of the region partial clearing may occur in some locations allowing for the formation of patchy dense ground fog in the typically prone locations. Overnight low temps should bottom out in the middle 60s to the lower 70s. && .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/... * Mainly dry and a bit less humid Friday, but a few showers will remain possible mainly towards the south coast Friday... The frontal boundary will remain stalled out near the south coast on Friday. This combined with high pressure across the Canadian Maritimes will result in drier northeast flow of air across much of the region. Expect partial sunshine with high temps mainly in the middle to upper 80s, although it will be a few degrees cooler along the immediate coast with sea breezes. Dewpoints will remain in the 60s, but it will not be quite as oppressive as today. Dry weather should dominate, but a few low topped showers will remain possible mainly towards the south coast near the frontal boundary. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Highlights... * Sunny and dry weather through the weekend and into early next week * Seasonably comfortable to start the weekend, then becoming hot and humid with a possible heatwave Sunday/Monday/Tuesday Friday night through Saturday... Weak surface trough will move off shore during the predawn hours of Saturday, behind it, 1020 mb high. The area of high pressure will move south out of Ontario and into the Great Lakes Region Saturday. Clouds will clear from northwest to southeast overnight. A few areas could see their morning lows dip down into the upper-50s across northwest Massachusetts, great sleeping weather! The remainder of southern New England is still comfortable, but a bit warmer, in the low and middle-60s. Saturday afternoon and evening still appears to be the pick of the weekend with dry and sunny weather. Additionally, the dewpoint values are in the 50s and lower-60s. 850 mb temperatures are forecast between +15C and +18C and we should mix down. Mixing to 850 mb would yield temperatures between the mid-80s to the lower-90s. Those nearing the low-90s would be the Merrimack and Connecticut Valley. Sunday though Tuesday... Whether you like it or not, the heat and humidity returns. At 850 mb the temperature, each day, ranges between +17C and +20C. Mixing those temperatures down should give us daily high temperatures in the upper-80s to the middle-90s, plus daily dewpoints in the upper- 60s to middle-70s. As previously mentioned, models agree that Monday will likely be the hottest day with the probability of 90+ temperatures between 90% and 100%. Headlines for a 'heat advisory' will likely be needed once again as we enter our next heatwave. Tuesday late afternoon could feature an isolated pop-up thunderstorm, otherwise, dry conditions will persist. Wednesday through Thursday... A frontal boundary will move across southern New England during the middle part of next week. Models are still not in the best of agreement on which day. The Euro has this front moving through Wednesday afternoon and the GFS on Tuesday afternoon. We are still several days away and will continue to monitor future developments. && .AVIATION /00Z FRIDAY THROUGH TUESDAY/... 00Z Update... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...High confidence in trends, but lower confidence in timing. The scattered thunderstorm threat will end across much of the region by 02z-03z, but persist a bit longer in parts of RI/SE MA. Otherwise, VFR conditions dominate but IFR to even LIFR conditions may develop across the Cape/Islands in lower clouds and fog. More patchy fog may develop overnight in the typically prone locations. Winds becoming light overnight. Friday...High confidence. VFR with any remaining low clouds/fog burning off during the morning across the Cape/Islands. Isolated showers possible Fri afternoon mainly towards the south coast, but the areal coverage will be limited. NE winds 5 to 10 knots. KBOS Terminal...High confidence in TAF. Scattered thunderstorms expected between 19z and 23z. KBDL Terminal...High confidence in TAF. Scattered thunderstorms expected between 19z and 23z. Outlook /Friday Night through Tuesday/... Friday Night through Saturday Night: VFR. Sunday through Sunday Night: VFR. Breezy. Monday: VFR. Breezy. Slight chance SHRA, slight chance TSRA. Monday Night: VFR. Breezy. Slight chance SHRA. Tuesday: VFR. Breezy. Slight chance SHRA, slight chance TSRA. && .MARINE... Tonight and Friday...High confidence. Main concern will be a few strong thunderstorms tonight. Initially they will be along the waters adjacent to the eastern MA coast, but push further south towards the south coastal waters later this evening and overnight. There may also be some fog that develops overnight across our southern waters. Vsbys should improve though on Friday with winds/seas remaining below SCA thresholds outside of a few strong t-storms tonight. Outlook /Friday Night through Tuesday/... Friday Night through Saturday Night: Winds less than 25 kt. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday Night through Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/Gaucher NEAR TERM...Frank/BW SHORT TERM...Frank LONG TERM...Gaucher AVIATION...BW/Gaucher MARINE...Frank/Gaucher