039 FXUS61 KBOX 220603 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 203 AM EDT Wed Jul 22 2020 .SYNOPSIS... Oppressive humidity returns Wednesday into Thursday along with scattered thunderstorms possible each day. Best chance for storms Wednesday appears across CT, RI and south of the Massachusetts Turnpike, then across much of Southern New England Thursday ahead of a cold front. Not as hot and much lower humidity Friday and Saturday behind this cold front along with drier weather. Heat and humidity may return later Sunday and into Monday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 140 AM Update... Forecast remains on track so no changes made. An area of showers with embedded thunderstorms is lifting northeast from NYC. There is little to no instability to work with, however in our area so only expecting scattered showers and maybe even a rumble of thunder for the rest of tonight. Higher dew points are being advected northwards so could be some patchy fog, especially along the south coast and the Cape and Islands. PREVIOUS DISCUSSION... 1030 PM Update... Forecast is largely on track so only brought near-term forecast more in line with obs. Did notice that mid to upper 60 dew points have made it as far north of Taunton and Providence. A stationary front current just south of DC and southern Delaware will lift north as a warm front. The speed at which the warm front lifts north will be key to the coverage of severe weather in our area. More to follow in subsequent discussions. 7 PM Update... Evening skies were mostly clear. An area of thicker clouds over the Catskills of NY were moving east into Western MA/NW CT. Expect these clouds to move across Srn New England tonight, but mostly after midnight. Regional radar mosaics show showers/thunder over Central NY/PA with a few additional showers over Ohio. The forecast continues to expect a warm front to our southwest to move toward Srn New England overnight. Showers/thunder along/ahead of the front may reach Wrn MA/Nrn CT late tonight. This timing continues from the previous forecast. Small changes in timing of the sky cover tonight, but otherwise no change to the forecast. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM THURSDAY/... * Strong to locally severe storms possible with a warm front lifting north. Higher humidity also returns. Threat of severe weather will depend greatly upon the location of a warm front. Should this warm front move north faster across our region, we would be better positioned to access higher surface dew points, and be much more likely to break up morning clouds. Even so, model soundings still indicated most-unstable CAPE values of 1-2 kJ/kg. HREF probabilities on about 20-30% of surface-base CAPE values in excess of 1 kJ/kg. So, still a mixed message for strength of thunderstorms Wednesday. This will need to be monitored over the next 24 hours. Shower chances will continue into Wednesday night, with the risk for thunderstorms diminishing as the night progresses. Near normal high temperatures towards the east coast due to an onshore wind, trending to above normal in the CT River valley. Above normal temperatures Wednesday night. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Big Picture... Zonal flow late week, but building ridge over the Plains and Great Lakes brings increasing 500-mb heights over the weekend. Several shortwaves move through this flow through next Tuesday. Heights at 500-mb will be in the mid and upper 580s through the weekend, so warmer-than-normal temperatures are expected. Models differ after that, with the GFS cooling while the GGEM and ECMWF maintain above-normal heights through early next week. With model agreement through the weekend but a zonal flow, forecast confidence is moderate-high. Model differences early next week bring a lower level of confidence...call it low-moderate confidence. Details... Thursday...Southwest flow and a warm sector in place over Srn New England. Cold front over the Canadian border moves southeast through the day, crossing Srn New England late in the day or early night. Could be a few morning showers as the warm humid air moves in, but then a rain-free period possible late morning and early afternoon. The favorable right-entrance region of the upper jet moves over Srn New England from mid afternoon through early night. This dynamic lift will join with PW values around 2 inches and CAPEs of 1000-2000 J/Kg to generate a period of showers and thunderstorms. Upper winds are marginal...25-30 kt at both 850 mb and 500 mb...so strong wind gusts possible in thunderstorms. PW values suggest local downpours will again be possible. Mixing will reach between 850-900 mb, with temps 16-17C. So surface max temps of 85-90F expected. Dew points 70- 75 during the day, lowering to 65-70 Thursday night. Humid day, improving night. Friday through Sunday... High pressure builds over New England bringing dew points in the 60s. Winds under the high will be light Fri and Sat, so expect sea breezes to develop each day, keeping max temps along the coast a little cooler than the interior. There is some uncertainty of showers for Friday, with afternoon CAPE around 1000 J/Kg and afternoon LI sub-zero. Will mention low-end chance pops Friday afternoon/evening, and rain-free Saturday and Sunday. Mixing reaches to 850 mb or a little above, tapping temperatures 14-15C Friday- Saturday and 17-20C Sunday. That suggests max sfc temps in the 80s Fri-Sat and upper 80s to mid 90s Sunday. Monday-Tuesday... Building ridge to our west brings a Northwest upper flow to New England. Possible shortwaves move through both Monday and Tuesday. Will mention chance pops for showers/tstms each day. GFS shows 850 mb temps 18C Monday and 14C Tuesday, while ECMWF shows temps at 850 mb 20-22C each day. Mid 80s or mid 90s? For now the forecast will go with a compromise, with max temps upper 80s and lower 90s inland and 80s along the coast. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...High confidence. VFR for most locations. There may be localized IFR to MVFR due to development of patchy fog for the coastal terminals and scattered showers across the western terminals after 08z. Winds turning N and then NE. Wednesday: Moderate confidence. Ceilings lower to MVFR and low VFR for most locations. Winds turning S to SE at 5 to 10 kts as a warm front moves north across our region. Greatest risk for showers and isolated thunderstorms over western MA and CT. Wednesday Night...Moderate confidence. Mainly VFR, lowering to MVFR for most. Areas IFR towards the south coast of New England in marine stratus. Scattered showers and thunderstorms, mainly across the western half of southern New England during the evening hours. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Thursday Night through Sunday/... Thursday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, slight chance TSRA. Thursday Night: Mainly VFR, with areas IFR possible. Chance SHRA, slight chance TSRA, patchy BR. Friday: Mainly VFR, with local MVFR possible. Slight chance SHRA, slight chance TSRA. Friday Night: Mainly VFR, with areas IFR possible. Patchy BR. Saturday: VFR. Saturday Night: Mainly VFR, with local IFR possible. Sunday: VFR. Breezy. && .MARINE... Winds and seas remain below Small Craft Advisory criteria through Wednesday night. High pressure passes by to the north of the waters tonight, with a warm front moving north across the waters Wednesday and Wednesday night. Increasing humidity will lead to areas of fog on the southern coastal waters with visibility likely reduced to 1 NM or less, especially Wednesday night. Outlook /Thursday Night through Sunday/... Thursday: Winds less than 25 kt. 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. Seas locally approaching 5 ft. Friday Night: Winds less than 25 kt. Patchy fog. Visibility 1 to 3 nm. Saturday through Saturday Night: Winds less than 25 kt. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Belk NEAR TERM...WTB/Belk/Chai SHORT TERM...Belk LONG TERM...WTB AVIATION...WTB/Belk MARINE...WTB/Belk