834 FXUS61 KBOX 300723 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 323 AM EDT Mon Aug 30 2021 .SYNOPSIS... Warmer more humid air moves in today, generating instability for scattered thunderstorms this afternoon and evening. Hazards from thunderstorms will include strong/damaging wind gusts, brief local downpours and possible flash flooding, and cloud to ground lightning. A cold front moves through during tonight, bringing somewhat less humid air. High pressure then builds for Tuesday, bringing fair weather. Tropical moisture associated with the remnants of Hurricane Ida arrives Wednesday and Thursday. There is a risk of heavy rain and flooding, but the axis of heaviest rainfall remains uncertain. Drier and cooler conditions arrive for Labor Day weekend bringing a touch of fall. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Light variable winds turning from the south as high pressure moves farther offshore. Note a small area of showers popping over SE Pennsylvania, ahead of a warm front. The showers are moving east at a rate that would take them south of Block Island around 9-10 AM and farther east through midday. If the showers should inch northeast the next few hours, it would give a chance of a shower or thunderstorm along the South Coast during the mid-morning to midday. Expected POPs are around 20 percent, or slight chance. But this will be monitored. Dew points upstream beyond the warm front are 70-73. These should move into Srn New England this morning, resulting in an uncomfortable airmass by midday and afternoon. This moisture is also consistent with forecast PW values aoa 2 inches as forecast. Clouds will lift and thin somewhat toward midday and afternoon. This should generate mixing to about 900-mb. Temps at that level are at 20C, equiv to 15C at 850 mb. Tapping the temps at that level would support max sfc temps in the mid 80s. Stability parameters are in the 1500-2500 J/Kg range, and LI continues at minus 5 to minus 7. The instability is most noted in Western/Central MA and Northern CT, but with convection moving east with time we would expect RI and Eastern MA to be affected later in the afternoon. Upper jet is mostly to our north and northwest, but it does move into range to assist convection around 21-23Z. Winds at 850-mb and 500-mb are lighter than ideal, so expect instability and low- level convergence to drive any thunderstorms. Main concerns from storms will be damaging wind gusts and local flash flooding. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Outlook... Cold front slides south and southeast of Srn New England. High pressure builds in on Tuesday even as vertical low pressure sits over Northern Quebec. Tonight... Showers and thunderstorms in the evening will dissipate early with loss of heating due to sunset. As noted, cold front moves through early tonight bringing a wind shift and moving somewhat drier air into the area overnight. Dew points should drop back into the 60s. This will support min temps in the low-mid 60s north and 65 to 70 South Coast. Tuesday... High pressure from central Canada builds southeast toward New England. This will bring dry weather with dew points in the 60s. Mixing to near 850 mb would tap 14-15C, supporting another day of temps in the 80s. If mixing expands to 800-mb, this would be equiv to 15-17C and focus on the mid to upper 80s. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights * Increasing likelihood for heavy soaking rain associated with deep tropical moisture from remnants of Ida later Wednesday into Thursday. Greater concern for areal flooding as opposed to flash flooding due to cool-season type stratiform precipitation. * We dry out with temperatures trending below to near normal heading into Labor Day weekend. Will certainly feel like early Sep in Southern New England. Details... Tuesday night into Thursday... Hurricane Ida made landfall near New Orleans, Louisiana on Sunday. While Southern New England was far removed from the direct impacts, model guidance does direct tropical moisture towards the Northeast US around the later Wed/Thu timeframe. While there remains plenty of moving parts to resolve, it appears increasingly likely that at least portions of our region will see heavy rain and associated flooding risks in that time frame. And the setup favors a cool- season type of stratiform soaking rainfall, so unfortunately for those with outdoor plans, a good part of late Wed into Thu time frame appears to be a washout. The silver lining is that while the threat for areal flooding is elevated, the risk for widespread flash flooding is actually not as high because we are not looking at intense rainfall rates associated with convective precipitation. The consensus of most deterministic and ensemble guidance now shows an axis of heavy rain setting up across our area late Wednesday into Thursday. The Mon 0z NAM gives a good idea of the dynamics in play, even though it only goes through Thu 12z. S New England looks to be situated in the right entrance region of a 90-100+ kt jet streak, which is more typical of mid-Autumn. Looking at 850mb, there is clearly going to be an area of speed and directional convergence that looks to set up from roughly southern CT to southeastern MA. Just north and west of this convergence zone is where we can expect the band of heavy precipitation to set up. Kind of similar to winter! A look at the positions of the low pressure center from the EC and GFS ensemble guidance shows a modest clustering near the 70W/40N benchmark, which would suggest the band of heavy rain setting up near the BOS-PVD corridor given the anomalously high PWAT environment. Of course, it wouldn't take much for the band to shift 50 miles to the west of to the east. Regardless, the threat for heavy rainfall is there and with northeast wind, later Wed into Thu will feel rather raw. Then the other question is, how much rainfall can we expect? Again, with the possible event being 60-72 hours out, we lean more on ensemble rather than deterministic guidance. The GFS ensemble shows 30-40 percent probability of 24-hour QPF exceeding 2 inches south of the Pike and 20-30 percent elsewhere. Whereas the EC ensemble is even more bullish, with up to 50-60 percent probability of 24-hour QPF over 2 inches across the southern zones. Forecast wise, used exclusively WPC guidance (we had a collaboration call early this morning for initial coordination) which is reasonable and for consistency purposes with neighboring offices which yields 2-4" across much of the region with isolated 4-6". So overall, while we have increasing confidence that heavy rainfall will occur, we have low confidence on where the axis of heavy rain will set up. On the other hand, there remains a low likelihood that the trough on Tuesday comes in stronger than expected and suppress the tropical moisture from Ida to our south. If that's the case, then dry and tranquil weather will occur but that is looking unlikely at this time. In addition to the heavy rain threat, as the remnants of Ida moves into the Northeast US as an open wave, strong low to mid-level flow associated with the system could result in isolated tornado threat if sufficient buoyancy is in place. Once again, predictability is very low at this time. Friday into Sunday... Finally, it looks like after possible tropical moisture from Ida exits the area late Thursday into early Friday. However, there are signs of a secondary low developing south of us that may lead to rain showers lingering into a good part of Friday. But eventually high pressure builds in and we head into Labor Day weekend with seasonably cool conditions. There are increasing signs that we get into a mean H5 trough as we begin meteorological fall. The latest 6- 10 day CPC outlook has 33-40 percent probability (slightly above average confidence of seeing below normal temperatures heading into Labor Day weekend. For reference, normal daytime highs for early September in Southern New England are in the mid 70s to right around 80 with normal overnight lows in the upper 50s to low 60s. Perhaps the worst of this summer's oppressive heat and humidity is behind us with things finally feeling more Autumn-like. Could potentially see some precipitation moving in on Sunday/Monday as a shortwave lifts in from Northern Plains/Great Lakes. Still a lot of uncertainty this far out, so have kept with the NBM guidance. && .AVIATION /08Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...High confidence in trends, moderate in timing. Variable sky conditions to start, with VFR in RI and SE Mass, IFR in Central Mass and much of CT, and MVFR between. Expect conditions to improve to VFR all areas by late morning. A passing warm front may generate a few showers/thunder off the RI Coast this morning, but little to support that in the observations. Daytime heating and unstable airmass will favor development of showers/thunderstorms this afternoon/evening. These afternoon storms will have potential for damaging wind and local downpours. Best chance will be in Western/Central Mass and parts of Northern CT. TAFs will include mention of VCTS for most of the time from 18Z through 01Z. Tonight...High confidence. Cold front sweeps through, bringing a wind shift from the west. Anything less than VFR will quickly improve with this wind shift. Tuesday...High confidence. VFR with clearing along the South Coast. Diurnal clouds with bases at 4500 feet from mid morning through sunset. Dry weather KBOS Terminal...High confidence in TAF. Chance of TSRA 19Z-01Z this afternoon/early evening. Strong winds and heavy rain are the main threats from thunderstorms. KBDL Terminal...High confidence in TAF. Chance of TSRA 17Z-00Z this afternoon/early evening. Strong winds and heavy rain are the main threats. Outlook /Tuesday Night through Friday/... Tuesday Night: VFR. Wednesday: MVFR/IFR conditions possible. Chance SHRA. Wednesday Night: Mainly MVFR, with areas IFR possible. Breezy. SHRA likely. Thursday: Mainly MVFR, with areas VFR possible. Breezy. Chance SHRA. Thursday Night: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA. Friday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...Patchy fog this morning along with scattered thunderstorms south of the Islands. Vsby then improves this afternoon with a wind shift from the South-southwest. Late afternoon/evening thunderstorms are possible. Some storms may contain strong winds and frequent lightning. Winds less than 25 kt except gusts to 50 kt possible in thunderstorms. Brief vsbys below 3 nm in downpours. Seas less than 5 feet. Tonight...Scattered early night thunderstorms will dissipate early tonight. Winds shift from the west-northwest as a cold front moves through after midnight. Winds less than 25 kt and seas less than 5 feet. Tuesday...Drier weather as high pressure from central Canada builds across the waters. West northwest winds will be less than 25 kt. Seas less than 5 feet. /Outlook /Tuesday Night through Friday/... Tuesday Night: Winds less than 25 kt. Wednesday: Winds less than 25 kt. Chance of rain showers. Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Rain showers likely, isolated thunderstorms. Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain showers. Thursday Night: Winds less than 25 kt. Rough seas up to 11 ft. Chance of rain showers. Friday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Chai NEAR TERM...WTB SHORT TERM...WTB LONG TERM...Chai AVIATION...WTB/Chai MARINE...WTB/Chai