000 FXUS61 KBOX 211129 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 729 AM EDT Tue Sep 21 2021 .SYNOPSIS... High pressure nearby will maintain another dry day across southern New England. This high pressure 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 by widespread showers at times Thursday into at least Friday. Improving conditions expected Saturday, but uncertainty increases for the weekend. No big warm ups or cool downs are expected. && .NEAR TERM /THROUGH TONIGHT/... 730 am update... CT River Valley fog is hanging on in patches this morning as indicated by GOES Night Fog channel and surface obs. Dewpoint depressions are zero in these locations, but the fog should continue to mix out as temps are on the rise in the next hour or so. Forecast is on track. Previous Discussion... Strong high pressure centered south of the Maritimes will maintain a SE to S flow across southern New England. This high pressure will be close enough to keep it dry today, with some scattered diurnal fair-weather clouds at times. The exception could be some low clouds and fog in the CT River Valley early this morning. Any stratus and fog should dissipate rather quickly after daybreak. The slight decrease in sunshine will mean slightly lower temperatures this afternoon compared to Monday. && .SHORT TERM /WEDNESDAY/... This high pressure moves farther offshore tonight into Wednesday. As it does, the light SE flow tonight will increase Wednesday. With the core of the subsidence departing to the east, and the increased humidity, expecting more clouds and a chance for showers tonight into Wednesday. The greatest risk for showers will be across the western half of southern New England. Limited instability, so not thinking thunderstorms will be an issue. Slightly above normal temperatures expected due to increased cloud cover and southerly flow. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Increasing temperatures and humidity mid-week * Slow moving low-pressure system brings periods of unsettled weather Wednesday, potentially through the weekend Details... Wednesday night through Friday... Late Wednesday through much of the latter half of the week a slow moving and negatively tilted upper level low will linger over the Ohio Valley with a strengthening surface low at the surface. This places southern New England in moist SSE flow between high pressure to the east and low pressure to the west. The warm front lifts north through the region on Thursday bringing surface dewpoints back up into the upper 60s. This, together with a 30-40 kt low level jet and placement beneath the right entrance region of a 100 kt upper level jet will bring widespread scattered shower showers Wednesday night into Thursday. A few thunderstorms are possible during the day on Thursday as soundings indicate several hundred J/kg of CAPE coincident with modest bulk shear. The cold front slowly approaches around Friday which will provide more focused lift in the lower levels for heavier rainfall. However, a great deal of uncertainty remains as to how the front evolves and when it may cross on Friday, or potentially stall out in our vicinity. Temperatures remain on the high side of normal, in the upper 70s on Thursday, cooling off a few degrees on Friday if the front makes it through. This weekend... For now we've lowered POPs from an initial blend of guidance, leaning away from the slower ECMWF guidance which wants to hang up the front through at least Saturday and keep rain in the forecast for the first half of the weekend. For now, though broad troughing continues over much of the east coast, keeping the forecast largely dry but with widely scattered showers possible beneath the upper level cold pool. We're still waiting on that first sub-70 F high temperatures in Boston; temperatures remain largely in the 70s through the forecast period. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 12Z update... High confidence. VFR at all TAF sites today, though BAF and BDL will be dealing with some patchy fog for the next hour or so. Also will continue to see a brief period of MVFR ceilings across portions of RI and southeast MA this morning. VFR everywhere by this afternoon. Modest southeasterly winds. Conditions lowering to MVFR/IFR across the western half of southern New England tonight into Wednesday, mainly from low clouds. Scattered showers possible, mainly after 22/04Z. KBOS Terminal...High confidence. VFR. KBDL Terminal...High confidence. VFR. Outlook /Wednesday Night through Saturday/... Wednesday Night: Mainly VFR, with local IFR possible. Chance SHRA. Thursday: Mainly VFR, with local IFR possible. Breezy. Chance SHRA. Thursday Night through Friday: VFR. Breezy. Chance SHRA. Friday Night: VFR. Breezy. Slight chance SHRA. Saturday: VFR. Slight chance SHRA. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Through Wednesday...High Confidence Rather tranquil winds and seas expected through Wednesday with high pressure over the waters. Mainly dry weather across the waters will good visibility. Outlook /Wednesday Night through Saturday/... Wednesday Night through Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. Friday through Friday Night: Winds less than 25 kt. Areas of rough seas. Slight chance of rain showers. Saturday: Winds less than 25 kt. Seas up to 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...Belk/BW NEAR TERM...Belk/BW SHORT TERM...Belk LONG TERM...RM AVIATION...Belk/BW MARINE...Belk/BW CLIMATE...