Area Forecast Discussion
Issued by NWS Boston, MA
Issued by NWS Boston, MA
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497 FXUS61 KBOX 080208 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1008 PM EDT Sun Jul 7 2024 .SYNOPSIS... Oppressive heat and humidity to start the week, which may linger into Wed. Remains humid throughout the rest of the week, but temperatures return to more seasonable levels. These cooler temperatures bring increased shower and thunderstorm chances with locally heavy downpours possible. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... 1000 PM Update... Stratus and fog has begun its northward creep from the southern coastline, with the leading edge of the stratus layer aligned along a line roughly from Mystic CT to Westerly RI to Bristol RI on to Plymouth MA. Continued northward expansion is expected of both; it still is unclear on its northward extent but it seems likely it would not reach any further north than a Windsor Locks to Norwood line. With regard to fog, lowest visbys seem more likely on the immediate southern coast. North from there, clear skies are expected. Could have patches of radiation fog develop into northern MA but should be less pervasive than along the southern coast. Overall forecast has this handled well with no major adjustments needed. Previous discussion... Mid level cap still evident with the radar largely quiet. Scattered cumulus on the visible satellite imagery, but not much else. Stratus persisted across portions of the Cape and islands this afternoon, and will only expand after sunset in the very humid air. Thinking this stratus will stay generally south and east of a line from Marshfield to Smithfield to Willimantic. Areas of fog also should expand, and will issue an Special Weather Statement to account for some patchy dense fog. Not much change in low temperatures from past nights given the high humidity continuing. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Yet another day of heat and humidity for Monday. Mid level dry air gets event deeper, meaning even less risk for showers. based on the 2-day threshold of heat indices 95 or greater, will go forward with a Heat Advisory start Monday where confidence is highest. To keep things simpler, will continue the headline through Monday night even though heat index values are expected to fall back into the 70s. It is likely this Heat Advisory will be expanded with later forecasts. Above normal temperatures and humidity continue. Stratus around the Cape and islands could linger through Monday night, although do expect some improvement during the day. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Highlights * Hot and humid Tue and Wed. Heat Advisory may need to be extended in Wed. Remains humid through the rest of the week, but temps return to more seasonable levels. * Unsettled with a stalled front nearby/overhead through much of the week. The result is daily opportunities for showers/storms. More widespread rain possible late in the week associated with the remnants of Beryl. Tuesday and Wednesday... Stuck under cyclonic flow through this timeframe. A shortwave trough will slide through portions of northern New England on Tue. Should see ridging setting up for Wed across the Mid Atlantic/eastern Great Lakes, but a shortwave rides on top of the ridge. A quasi-stationary front will be positioned over portions of the interior on Tue. It may become more oriented east to west just to our north or over us for Wed. Main story for this timeframe will be the heat and humidity. Have already hoisted a Heat Adv that extends into Tue as we meet the 2 day criteria. Though this will likely need to be extended into Wed and perhaps expanded a bit more in future updates. Have held off for now due to uncertainty on heating given increased cloud cover. Both days featuring 925 hPa temps roughly of 22-26 degrees Celsius with prolonged SWly flow. The result are highs in the upper 80s to low 90s. The only exception being along the immediate south coast where readings generally will be in the 80s. Expect dew points in the low to mid 70s. The result will be heat indices of roughly 90-100 degrees across much of the region, other than the immediate south coast where it will be cooler. The other risk for this timeframe is showers and thunderstorms given we will have a stalled out frontal boundary overhead/nearby. Risk on Tue appears to be highest across western/central MA and CT. This is where have chance PoPs. A bit trickier on Wed as the stalled out front may reorient itself to more of an east-west configuration over our area or just to the north. For now have gone with consensus of guidance, which focuses activity more over NH/VT/MA border. Both days will have the risk of heavy downpours, though a bit higher on Wed vs Tue. Should see PWATs around 2 inches on Tue, which is 1.5-2 STD above model climo per the NAEFS. This climbs firmly to 2-2.5 inches on Wed with NAEFS showing us 2-2.5 STD above model climo. Warm cloud layer depths range from 3.5-5 km on both days. Instability somewhat similar with a few hundred to roughly 2000 J/kg of MLCAPE. The big difference between the two days is the upper forcing is a bit more removed further to the NW on Tue as opposed to Wed it is closer by. Should note that deep layer shear in 0-6 km layer is 30-40 kts. Mid level lapse rates are pretty meh as well as low level lapse rates. The risk is higher for flash flooding given this environment per the marginal ERO from WPC on both days. Will note that ML guidance per CSU does highlight a severe risk on Wed across our region, so will be something we need to monitor. Not highlighted yet at this point per SPC Outlook and will need a better handle on where the frontal boundary set up. Thursday through Saturday... Still unsettled through this timeframe with cyclonic flow in place. The remnants of Beryl and moisture associated with it remain either nearby or over our area for a portion of this timeframe. The result is chances for more widespread showers/storms. Though at this point will need to really hone in on where frontal boundaries are setup as these will be key for where there will be an increased risk for flash flooding. Generally have stuck with the NBM at this point in time as it looks reasonable given the uncertainties in the forecast. Temperatures returning to more seasonable levels, but the humidity remains given the prolonged southerly flow. Still quite moisture laden with PWATS of 1.5 to 2.5 STD above model climo per the NAEFS and warm cloud layer depths of roughly 4-5 km. Think anywhere where we`ve got a frontal boundary will be the focus for heavier rainfall and perhaps flash flooding. This is highlighted per Day 5 ERO per WPC and CSU ML Excessive Rain probs guidance. Will be something to monitor as the week progresses. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight through Monday...High confidence in trends, moderate in exact timing. IFR to LIFR fog/stratus across the Cape/Islands. Think this does slip into PVD and perhaps OWD, but think will remain SE of BOS/ORH as well as BDL. If it does move into BOS it would probably be heading into the AM push. Should see the stratus/fog lift fairly quickly for PVD/FMH roughly 13-17Z. May take awhile for HYA, while ACK may still have IFR/LIFR ceilings linger with improved vsby Mon afternoon. Light winds tonight and Monday, with seabreezes at BOS and PVD. Monday Night...Moderate confidence. VFR conditions across interior MA and BOS. Roughly to the S/SE of I-90 will have IFR to LIFR stratus and fog lifting in. Risk greatest along the immediate south coast. S/SSW winds 5-10 kts. KBOS TAF...High confidence in TAF. VFR with seabreezes likely to develop around 14-16Z. IFR/LIFR stratus possible after 04Z into the AM push, but confidence higher to the SE. Have held off from including in TAF for now. KBDL TAF...High confidence in TAF. VFR through the forecast. MVFR to IFR stratus possible Mon Night into early Tue. Outlook /Tuesday through Friday/... Tuesday: VFR. Breezy. Chance SHRA, isolated TSRA. Tuesday Night: VFR. Breezy. Slight chance SHRA, isolated TSRA. Wednesday: VFR. Chance SHRA, isolated TSRA. Wednesday Night: Mainly VFR, with local MVFR possible. Chance SHRA, isolated TSRA. Thursday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, isolated TSRA. Thursday Night through Friday: Mainly VFR, with local IFR possible. Breezy. Chance SHRA, isolated TSRA. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Through Monday Night...High confidence. Relatively light winds and seas across the waters through Monday night. The main concern will be poor visibility in fog tonight. Visibility should improve Monday, then not get quite as low Monday Night. Outlook /Tuesday through Friday/... Tuesday: Winds less than 25 kt. Tuesday Night through Wednesday: Winds less than 25 kt. Slight chance of rain showers, isolated thunderstorms. Wednesday Night: Winds less than 25 kt. Slight chance of rain showers. Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers. Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. Friday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers, isolated thunderstorms. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Heat Advisory from noon Monday to 8 PM EDT Tuesday for CTZ002>004. MA...Heat Advisory from noon Monday to 8 PM EDT Tuesday for MAZ005- 012-013-017-018. RI...Heat Advisory from noon Monday to 8 PM EDT Tuesday for RIZ001- 002. MARINE...None. && $$ SYNOPSIS...Belk/BL NEAR TERM...Belk/Loconto/BL SHORT TERM...Belk LONG TERM...BL AVIATION...Belk/Loconto/BL MARINE...Belk/Loconto/BL