NWS Forecaster Discussion
BOX
Area Forecast Discussion National Weather Service Boston/Norton MA.
000 FXUS61 KBOX 241356 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 956 AM EDT Sat Jul 24 2021 .SYNOPSIS... High pressure builds over New England today, bringing sunny skies and seasonably mild weather. A warm front brings an increasing risk for showers later tonight, along with increasing humidity levels. Unsettled pattern returns Sunday with chances for showers and thunderstorms Sunday and then each afternoon Tuesday into late week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 950 AM Update... Increased high temperatures across southern New England toward the 90th percentile of guidance. Latest observations show that the warming is occurring a bit faster than the previous forecast indicated, so have tried to bring things in line with that. Otherwise the forecast remains on track with some diurnal CU expected to develop across the interior. 730 am update... Beautiful start to what will be a very nice weather day. Temperatures are starting out in the 60s at 730 am with clear skies and lots of sun. No significant changes with this update. Previous Discussion... Upper trough and cold pool that brought thunderstorms to the region yesterday will move off to the east today. High pressure surface and aloft will build over the region. This will provide subsidence, which should keep the airmass dry through the day. Moisture cross- sections show a moist layer lingering between 700-mb and 800-mb, which could be a source of diurnal cumulus, but the subsidence should limit the extent of coverage. Expect a mostly sunny day. Light wind under the high pressure will allow sea breezes to form once again, starting 10AM to Noon and continuing until 8PM to 10PM. Mixing today will reach around 800-mb, where temps will be the 850- mb equiv of 12-13C. This will support max sfc temps in the upper 70s to lower 80s where full mixing takes place. The sea breeze will keep temps a few degrees cooler at the coast, roughly mid 70s. Dew points will remain in the 50s with low 60s on the Cape and Islands. Thus temperatures today will continue to be comfortable. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Overview... High pressure shifts offshore. A weather system from the Great Lakes, leading with a warm front late tonight and Sunday morning, then a trailing cold front approaches late Sunday. Details... Tonight... Fair weather the first part of the night as high pressure moves offshore. Winds remain light, but swing out of the south. Clouds increase and thicken after midnight ahead of the approaching warm front. Southwest winds behind the warm front will increase, reaching 35 to 40 kt at 900-mb. These winds will ride up over the top of the south winds, with the lift generating showers. The showers should move over Srn New England beginning after 3 AM. Dew points will rise a little into the 60s, so min sfc temps will be in a similar range overnight. Sunday... Weak upper shortwave moves overhead Sunday morning. This will continue to support lift and precipitation during that time. PW values will rise as the warm humid air moves in, reaching 1.75 inches Sunday morning. While not excessive, it will be above normal and could support some local downpours. Hard to tell if there will be a break between the departing warm front and the approaching cold front. Instability increases by the afternoon, with model forecast CAPE values of 1000-1500 J/Kg moving to at least Western/Central MA and Nrn CT during that time. So there will also be a chance of thunderstorms. Mixing will be limited by the warm air overrunning the surface air. Expect the mixed layer to reach 925-mb, with temperatures around 18C, equiv to 10C at 850-mb and supporting max temps in the 70s. Surface dew points will continue to rise during the day, reaching the mid to upper 60s. The humidity will once again be noticeable. The mixing will also try to draw some of the stronger winds aloft down to the surface, but will likely not bring all of that wind down. Expect southwest winds gusting 20-25 kt/25-30 mph, strongest along the coast and on hilltops. Wind could also gust a little stronger in thunderstorms. So to sum up...a cloudy muggy day with showers and scattered thunderstorms, some with local downpours and gusty winds. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Highlights... * Drier conditions return during the day on Monday * Warmer temperatures return on Monday into Tuesday, then cooler mid- late week Details... Monday through Wednesday... An overall drier dry stretch for the first half of the week, but still featuring a shot at showers and thunderstorms each day. Broad cyclonic flow lingers aloft with a mid level trough planted over the east coast through the entire week. Monday showers and thunderstorms will exit offshore by noon as the cold front slides south ushering in a marginally drier airmass. Flow is less amplified the first half of the week but at least one shortwave rounds the base of the trough bringing the return of rain and thunderstorm chances late Tuesday into early Wednesday. Expect a mix of clouds and sun each day with highs in the upper 80s Mon-Tue, then a cool down Wednesday into the 70s and low 80s as cooler air dips south with the shortwave. Thursday and Friday... Did someone ask for more of the same? Well we`ve got it. A more potent shortwave and frontal system approaches around late Thursday/Friday which will bring a better chance of widespread showers and thunderstorms during the latter half of the week. && .AVIATION /14Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...High confidence. VFR. Light and variable winds. Sea-breeze develops along the coastal areas around 14-15Z, and dissipates around 00Z. Tonight...High confidence. VFR continues, but with increasing clouds from the west after midnight with showers possible in Wrn/Central MA and Nrn CT. Ceilings may lower to MVFR in some showers late at night. Sunday...Moderate confidence. Ceilings/Vsbys lowering to MVFR with areas of IFR in showers and thunderstorms. Southwest winds gusting 20 to 25 kt, possibly higher in thunderstorms. KBOS TAF...High confidence in TAF. Brief shower possible toward Sunday morning. KBDL TAF...High confidence in TAF. Showers possible after 06Z Sunday. Outlook /Sunday Night through Wednesday/... Sunday: Mainly VFR, with local IFR possible. Breezy. Chance SHRA, slight chance TSRA. Sunday Night: Mainly VFR, with local IFR possible. Breezy. Chance SHRA, isolated TSRA, patchy BR. Monday through Monday Night: VFR. Tuesday through Tuesday Night: VFR. Slight chance SHRA, isolated TSRA. Wednesday: VFR. Chance SHRA, slight chance TSRA. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...High confidence. Winds and seas below Small Craft Advisory levels. Wind gusts 10 kt or less. Seas less than 3 feet. Sea breezes develop on nearshore waters around 10 AM to Noon. Tonight...High confidence. South wind gusts 20 kt or less. Seas 3 feet or less. An approaching warm front will bring increasing clouds after midnight, with a fog and a chance of showers toward morning. Sunday...Moderate confidence. South-southwest wind with gusts to 25 kt in the afternoon. Seas building to 5 feet in the afternoon. Showers with vsbys occasionally 1 to 3 miles. Chance of thunderstorms. A Small Craft Advisory may be needed for both wind and seas. /Outlook /Sunday Night through Wednesday/... Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers, slight chance of thunderstorms. Monday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers, isolated thunderstorms. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday Night: Winds less than 25 kt. Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, slight chance of thunderstorms. && .HYDROLOGY... No active river flood warnings are in effect for rivers and waterways in BOX`s hydrologic service area. However, after having a tremendous amount of rainfall from early to mid July, many rivers and streams in Southern New England are running much higher than normal. This is resulting in swift currents that would not typically be seen in the summer. These swift currents can catch those swimming or tubing/kayaking on area waterways off guard. There have been 3 total reports of drownings in the last week on the Farmington River in CT and Squannacook River in MA given the higher than usual flows and swift currents. As we move through the weekend, those with interests swimming, tubing or kayaking on area rivers and waterways should be aware that swift currents due to high flows are occurring. This can result in dangerous conditions, and it is generally discouraged to swim, inner-tube, or kayak until flows decrease. && .TIDES/COASTAL FLOODING... Tides reached their monthly astronomical peak and will now trend lower each day. Tides will not flood, but could still cause minor splashover with the evening high tide tonight. Minor splashover is possible on typically vulnerable shoreline roads. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/BL/BW NEAR TERM...WTB/BL/BW SHORT TERM...WTB LONG TERM...BW AVIATION...WTB/BW MARINE...WTB/BW HYDROLOGY... TIDES/COASTAL FLOODING...