000 FXUS61 KBOX 220212 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1012 PM EDT Tue Sep 21 2021 .SYNOPSIS... High pressure centered south of the Canadian Maritimes moves slightly farther offshore Wednesday, resulting in increasing humidity and a risk for showers. A slow-moving front approaching from the west will be accompanied bring showers into western sections Thursday and spread across the region Thursday night and Friday. Improving conditions expected Saturday, one or two more periods of showers are possible Sunday. No big warm ups or cool downs are expected. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... More minor tweaks to rainfall chances based on observed trends. Latest runs for the HRRR and RAP seem to be verifying reasonably well, as both captured the light showers across the Cape and islands this evening. Since these were the only guidance sources to do so, leaned heavily on their solutions to update the timing and location of precipitation overnight. Temperatures were also brought back line with observed trends and updated expectations from near term guidance. 7 PM Update... High pressure centered south of Nova Scotia. Local winds show a southeast surface flow while winds at 2000 feet or higher, per the KBOX VWP, are out of due south. Increasing low level moisture. Radar shows light showers popping near the Vineyard and moving toward Cape Cod. Another batch of showers near Montauk Pt and along the RI Coast. All showers moving north. Of the CAMS, the NAM Nest is the only one to fully capture these showers. But it and the other CAMS generate a new area of showers moving toward Wrn Long Island near midnight that spreads across Srn New England overnight. So expect some light rain/showers during the night. POPs at each core should range from 25-30 pct, within a broader area of 20 pct. The forecast expands the coverage of the rain but otherwise follows the general theme of the previous forecast. Previous Discussion... The surface high pressure and mid level ridge move a bit farther offshore, with a deepening mid level trough to our west. Across southern New England, S/SE flow will bring increasing low level moisture. This allows for an increase in clouds, and the chance for patchy light rain and/or drizzle mainly over the interior. Higher surface dew points will prompt a more mild night, with overnight lows in the upper 50s to mid 60s, with areas of fog developing. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday...Surface high and mid level ridge remain centered offshore, while deepening mid level trough and associated surface low move slowly eastward through the eastern Great Lakes Region/Ohio River Valley. S/SE flow persists and pressure gradient tightens somewhat. Saturated layer once again remains below 700 mb. Expecting partly to mostly cloudy skies, with a continued chance of light rain or drizzle mainly over the interior. Above normal temperatures, with highs in the low to mid 70s. Wednesday night... Models show mid level trough becoming cut off/blocked and halting eastward progression, with surface low retrograding somewhat. Low level moisture remains in place, with a continued S/SE flow. Thus expecting another night of considerable cloudiness, with patchy light rain and drizzle mainly over the interior. Surface dew points in the mid and upper 60s, very muggy for mid-September, with overnight lows also falling into this range. Areas of fog will likely redevelop. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Big Picture... Closed and vertical low pressure over the Great Lakes lifts north into Canada. A second upper low then drops southeast from the Canadian Prairies across the Great Lakes and Northeast by the weekend, with the axis crossing New England Sunday. High pressure builds across the Northeast Monday and Tuesday. Contour heights, reflecting the deep layer, are above normal Thursday and Friday, then in the normal range Saturday through Tuesday. This may briefly dip below normal Sunday night as the trough axis moves through...but then recover to normal Monday as the trough moves off through Eastern Canada. Mass fields are similar through Friday, then show the same trend but different positioning over the weekend. Greater differences in the fields Monday and Tuesday. Overall confidence is high Thursday/Friday trending to low-moderate early next week. Details... Thursday-Friday High pressure centered over the West Atlantic, while low pressure moves slowly north through Michigan and Srn Ontario. This pressure pattern maintains a southeast flow over Srn New England, bringing low level moisture and marine temperatures. Cross sections show plenty of convergence below 925-mb and just as much divergence between 925-mb and 700-mb. Moisture cross sections show plenty of moisture below 800-mb and pronounced dry air above 700-mb. Low-end CAPE stretches north over the region, with a limited 200-600 Joules/Kg available over the region. Given the shallowness of the moisture, and the dry air aloft, this seems more like a warm frontal pattern with rain/drizzle rather than showers. Mid-level moisture is forecast to move into Wrn New England Thursday afternoon as does the right entrance region of the supporting upper jet. This may be enough to mention chance pops for afternoon showers/thunder in the CT Valley/East Slope regions. The upper support shifts east Thursday night and Friday, allowing the surface cold front to enter Western MA/CT Friday morning. The front crosses to the Central Hills by early afternoon and the East Coast Friday evening. The cold front should provide good low level convergence, with adequate mid level moisture with the front. PW values increase Thursday and peak near 1.75 inches along/ahead of the front Friday, high enough above normal to support local downpours. CAPE builds to 1000-1500 J/Kg Friday, especially in RI and Central/Eastern MA. Convective parameters are favorable for thunder. Will forecast chance pops for showers/thunder. Mixing temperatures Thursday around 10-11C support max temps mid to upper 70s. Similar temps aloft Friday but more rain suggest max temps more broadly in the 70s. Saturday-Sunday... Cold front moves offshore Friday night but lingers over the nearby offshore waters through the weekend. Passage of the second trough on Sunday may generate a wave along the stalled front. Also, the upper trough and its cold pool aloft may generate enough instability for showers on Sunday. Will forecast chance pops for showers over the southeast waters over northern/eastern areas Saturday and northern areas Sunday. Will also forecast chance pops for showers over Nrn MA Saturday night and Sunday. Monday-Tuesday... Upper low moves off to the northeast, with showers diminishing Sunday night. All models generate a wave passing south of New England Monday. ECMWF then conjures up another system coming through the Great Lakes Tuesday, while the GFS shows high pressure. With low confidence in this part of the forecast, will keep the forecast dry at this stage with mostly clear skies but with limited confidence as noted above. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight...Scattered to widely scattered light rain/showers moves north from the South Coast. Greatest coverage should be over CT and western MA around and after midnight. Conditions lowering to MVFR/IFR across most terminals overnight into Wednesday morning, from low clouds and areas of fog. Localized VSBYs as low as 1/2 mile. Wednesday...IFR/MVFR CIGs improving to VFR, except for far interior terminals, where CIGs only improve to MVFR. Patchy fog lingers until 12-14Z. Patchy -RA mainly over the interior. SE wind 5-10 kts with gusts to 20 kts possible south coast/Cape/Islands. Wednesday Night...Conditions once again lower to MVFR/IFR across most terminals, with low clouds and areas of fog. Slight chance -RA. KBOS Terminal...High confidence in trends. Moderate confidence in timing. KBDL Terminal...High confidence in trends. Moderate confidence in timing. Outlook /Thursday through Sunday/... Thursday: Mainly MVFR, with areas VFR possible. Breezy. Slight chance SHRA, isolated TSRA. Thursday Night: Mainly MVFR, with local IFR possible. Breezy. Chance SHRA, slight chance TSRA. Friday: Mainly MVFR, with areas VFR possible. SHRA likely, chance TSRA. Friday Night: Mainly VFR, with areas MVFR possible. Chance SHRA. Saturday: Mainly VFR, with local MVFR possible. Saturday Night through Sunday: Mainly VFR, with areas MVFR possible. Slight chance SHRA. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight through Wednesday Night...High Confidence Tonight and Wednesday...High pressure centered offshore maintains a south-southeast surface wind 10 to 15 kts. A few gusts to 20 kts possible. Patches of light rain/drizzle with patchy fog possible. Wednesday Night....Seas building to 4 to 5 feet on the southern outer coastal waters, thus SCA headlines may be needed there. Otherwise SE winds 10 to 15 kts continue. Patchy fog possible with visibility as low as 1-2 miles. Outlook /Thursday through Sunday/... Thursday: Winds less than 25 kt. Local rough seas. Slight chance of rain showers, isolated thunderstorms. Thursday Night through Friday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, slight chance of thunderstorms. Friday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers. Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. && .CLIMATE... The latest first sub-70 degree high temperature heading into astronomical Autumn for BOS was September 20, 1947. Still waiting on the first sub-70 max temperature after Summer. The latest first sub-70 degree high heading into astronomical fall for PVD is September 26, 2015. Both BDL and ORH have already recorded sub-70 degree highs earlier this month. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/NMB NEAR TERM...WTB/Belk/NMB SHORT TERM...NMB LONG TERM...WTB AVIATION...WTB/Belk/NMB MARINE...WTB/NMB CLIMATE...