254 FXUS61 KBOX 040741 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 341 AM EDT Sun Sep 4 2022 .SYNOPSIS... A cold front moves slowly south from northern New England on Sunday, with increasing heat and humidity, along with scattered afternoon showers and thunderstorms developing. The front stalls across southern New England Sunday night into Monday, then pushes south of the coast Monday night into Tuesday. This front will bring multiple rounds of showers, with areas of heavy rainfall likely. After a period of below normal temperatures Monday and Tuesday, mainly dry weather with a warming trend follows for midweek into next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 330 AM update... * Warmer and more humid Sunday Sunday... Weak frontal boundary drops south into the region today. Low level warm air advection ahead of this feature, with 925 mb temps rising to about 21C. Partial sunshine combined with these temps aloft will support highs in the mid to upper 80s, almost 10 degs warmer than normal! Not as warm along the coastline, as seabreezes develop given weak pressure gradient. This warmer airmass will be accompanied by more humid conditions, with dew pts rising into the mid and upper 60s. Definitely summer-like airmass. Very warm and humid airmass will yield MU Capes on the order of 1000- 1500 j/kg. This combined with frontal convergence, should be sufficient to trigger scattered showers/thunderstorms this afternoon and evening, mainly along and west of I95. Hence, mainly dry weather today for southeast MA, Cape Cod and Islands. Weak winds aloft result in a low shear environment. PWATs surge to 1.5 - 2.0 inches this afternoon. The combination of copious deep layer moisture, modest instability and weak winds aloft supports a risk of heavy rain with potential training storms. Not expecting widespread/ organized convection, given weak shear and probability of severe storms is low given absence of strong winds aloft. Although, can't rule out low risk of an isolated wet microburst given modest CAPE and anomalous moisture. However, the more likely scenario is isolated heavy rain with localized risk of urban/poor drainage flooding, given slow moving, training storms possible. HREF suggest strongest updraft potential is across western MA/CT. Light and variable winds, except becoming onshore along the coast as seabreezes develop. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... 340 AM update... Sunday night & Monday... * Moderate to heavy rain at times Frontal boundary draped across SNE will become an active front, as northern stream short wave energy interacts with southern stream moisture, with PWATs of 1.75 to 2 inches. While instability is not impressive, deep warm cloud layer depths of around 11 kft late Sunday night into Monday, may enhance rainfall via warm rain processes. Exact location of heavy rain is somewhat uncertain, given slow movement/location of frontal boundary. Timeline for rain appears to be, scattered showers and thunderstorms Sunday evening, then dissipating later evening and giving way to a drier period of weather, followed by numerous showers with heavy rain at times beginning Monday morning and continuing into the afternoon. Not as cool Sunday night given dew pts in the 60s, then cooler Monday in response to cloud cover and precip. Highs in the 70s across CT/RI and southeast MA, but highs only in the upper 60s across northern MA, given position of frontal boundary. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... 340 AM update... Highlights... * Showers and localized downpours will continue Monday night and Tuesday * Trending dry and warmer mid week through next weekend Details... Monday night and Tuesday... The upper trough axis drops through southern New England during this period while the stalled front slowly slips south placing all of the region on the cool side of the boundary. As the front is displaced so too is the axis of heaviest rainfall, shifting further south toward the coast. The column will remain saturated but drier air begins to invade from northern MA moving south with PWATs dropping below 1.5 inches by Tuesday morning and below 1 inch by Tuesday evening. Not expecting embedded thunderstorms given the cooler airmass and lack of instability, but some localized heavy downpours are possible and urban and poor drainage flooding is possible. By the end of this prolonged event some locations could reach rainfall totals of 2 to 3 inches. One adjustment to the forecast was to lower temperatures on Tuesday by a few degrees, into the 60s and low 70s thanks to abundance of cloudcover, rain, and cool northeast flow. Rain comes to an end Tuesday evening into the overnight hours as mid level ridging approaches. Wednesday through Saturday... By Wednesday the upper trough moves out and is replaced by a ridge which will help to suppress a disturbance to our south and keep things dry with sunny and cooler than average temperatures thanks to northeast flow around the surface high pressure. High temperatures on Wednesday were lowered a bit on this account, into the mid 70s (low 70s along the east coast where onshore flow will keep things cooler). There remains some disagreement as to the longevity of the subtropical ridge, but the majority of guidance would have pleasant weather through next weekend beneath a building ridge of high pressure. This would kick temperatures back up into the mid-upper 80s by next weekend. && .AVIATION /08Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. 06z update... Sunday...high confidence. VFR and light S-SW winds this morning, becoming S-SE this afternoon. Dry this morning, then scattered showers/thunderstorms this afternoon along and west of I95. Hence, terminals in southeast MA, Cape Cod and Islands likely remains dry. MVFR conditions possible with any given shower/thunderstorm. Sunday night...high confidence. Scattered showers/thunderstorms in the evening dissipate late evening, but then showers may redevelop after midnight from west to east. VFR in the evening, trending MVFR/IFR after midnight. Light south-southeast winds, possibly becoming N-NE across northern MA as frontal boundary drifts southward. Monday...high confidence on trends, lower in timing and details. VFR/MVFR southeast MA, MVFR/IFR elsewhere with LIFR possible inland across the hilly terrain. Showers likely widespread along with areas of fog. S-SE winds across RI and southeast MA, N-NE winds elsewhere. KBOS Terminal..High confidence in TAF thru Sunday evening, with VFR, but MVFR at times in scattered showers/thunderstorms Sunday afternoon & evening. Then some uncertainty on exact timing and details late Sunday night into Monday. KBDL Terminal...High confidence in TAF. VFR this morning, then MVFR at times this afternoon and evening in scattered showers/thunderstorms. Outlook /Monday Night through Thursday/... Monday Night: Mainly VFR, with areas IFR possible. SHRA likely. Tuesday: VFR/MVFR conditions possible. Chance SHRA. Tuesday Night: Mainly VFR, with local IFR possible. Breezy. Wednesday: VFR. Breezy. Wednesday Night through Thursday: VFR. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. 340 AM update... Sunday... High pressure just offshore, yielding light S-SE winds across the MA/RI waters. Mainly dry weather, but risk of scattered showers/thunderstorms near shore late Sunday. Some storms may contain gusty winds and hail. Vsby reduced at times in heavy showers/thunderstorms. Sunday night... Any early evening showers/thunderstorms will dissipate by late evening. S-SE winds, except N-NE winds late across northern MA waters. Patchy fog may reduce vsby especially across the eastern MA waters. Monday... Frontal boundary draped across MA/RI waters, with N-NE winds north of the front, S-SW winds south of this boundary. Showers with heavy rain will increase in coverage during the afternoon. Outlook /Monday Night through Thursday/... Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers. Tuesday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. Wednesday: Winds less than 25 kt. Seas up to 5 ft. Wednesday Night through Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/BW NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...BW AVIATION...Nocera/BW MARINE...Nocera/BW