403 FXUS61 KBOX 092020 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 420 PM EDT Tue Oct 9 2018 .SYNOPSIS... Warm and humid conditions continue into Wednesday. An approaching cold front will bring showers to the region Thursday into Thursday evening with locally heavy rainfall. Michael will likely track southeast of the Benchmark...keeping the threat for heavy rain across far southeast New England with the best chance across Nantucket Friday into Friday evening. High pressure will build in for the weekend, bringing dry but rather cool fall-like weather. A period of wet weather may return sometime early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 2 pm update... Scattered to broken low clouds continue. Under dominance of a mid to upper level high SE, a dry inversion in place along with prevailing subsidence, mixing is limited. Derived from the 12z Albany sounding, it is going to take a lot of heating to mix out the boundary layer and erode the low cloud decks pooled beneath the inversion. Additional challenge of increasing humidity in place mixing up through the boundary layer, dewpoints across the region ranging from the mid to upper 60s, even if we were able to erode, the presence of such humidity mixing out will only aid in additional cloud development at the LCL. So in other words, and in short order, partly to mostly cloudy conditions prevail. Wherever there is clearing, warming up nicely and mixing out. An uptick in SW winds gusting up around 20 to 25 mph as has been observed. Holding highs around the low to mid 70s. Tonight... Widespread low clouds and the potential for fog. Light S winds prevailing and not much mixing during the day, dewpoints should stay relatively in the mid to upper 60s. Boundary layer cooling, an abundance of moisture, would point towards lower conditions similar to that we saw last night. Less emphasis on lift, the ridge of high pressure in place with the warm front to our N. Some indication per BUFKIT soundings of stratus more favored. HREF confirming, highest probabilities of 1/2 visibility along only the S coast whereas widespread ceilings below 1000 feet for S New England. Humidity in place, temperatures remaining stable, lows down in the low to mid 60s. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday... Persistence forecasting as low clouds gradually clear similar to that which was observed earlier this morning (Tuesday). Limited mixing to H925 where temperatures reside around +20C and SW winds are around 25 mph. As the low cloud deck lifts becoming broken to scattered, should see temperatures warm into the upper 70s to low 80s as SW winds gust up around 20 mph. A slight drop in dew- points, but humidity remaining high with dewpoints around the mid 60s resulting in it feeling a bit warmer with the early October sun. Wednesday night... Increasing rain / PoP chances. Frontal boundaries converging on the region along with H925-85 S flow impinging. This ahead of stronger synoptic forcing from the W, falling heights, and lead energy. However the mid to upper level ridge still in place, a measure of subsidence, the focus mainly upon low-level lift. Have more likely PoPs out W whereas lower threats S/E given the prevailing mid-upper level ridge. Cloud decks thickening and lowering, the return of dense fog along the S/SE coast. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Highlights... * Showers with locally heavy rainfall possible Thu/Thu evening with a low risk of flash flooding * Michael likely spares most of our region heavy rain...but will have to keep an eye on far southeast New England esp. Nantucket * Dry and cool fall-like weather this weekend * A period of wet weather may return early next week Details... Thursday and Thursday Evening... Quite an interesting setup across southern New England Thursday into Thursday evening. While Hurricane Michael will still be across the southeast states...there will be a backdoor cold front bisecting our region. There also will be the main cold front approaching from the northwest. This will pull deep tropical moisture northward into southern New England. In fact...Pwat anomalies on the GEFS ensembles are indicated to be between 4 and 5 standard deviations above normal. There is also a 40 to 50 knot southwest low level jet that will work on the approaching cold front. All this put together means there will be periods of showers with locally heavy rain and a low risk for isolated flash flooding. Any time a tropical system gets moisture pulled northward along an approaching front this is a concern. The problem is these areas can be localized and it is always very uncertain as to where one might setup. The models are not too robust on QPF in our region...which may allow us to escape this threat. However...there is concern given the very anomalously high Pwats coupled with a strong LLJ. The threat for heavy rain and isolated flash flooding would increase if a boundary gets hung up across our region providing focus for the strong LLJ/high Pwat plume. Friday... Attention will then turn to Michael...which is expected to be a post tropical system at this time. As Michael emerges somewhere off the mid Atlantic coast...it will be picked up by the westerlies aloft and probably pass southeast of the Benchmark. This seems reasonable, as it appears that there is not enough trough amplification to pull the system northward. This should spare at least most of our region the threat of heavy rainfall directly from Michael. The exception might be extreme southeast New England and especially Nantucket. If Michael moves a bit further north this area could become prone for heavy rain...but this remains uncertain at this time. Also, the risk for 40 to 50 mph wind gusts across the Cape/ACK if Michael tracks close enough. Saturday and Sunday... High pressure builds into the region from the west this weekend. This brings dry weather and also rather cool fall-like weather. High temperatures this weekend will only be in the 50s to around 60. In fact...some outlying locations may see overnight lows drop well down into the 30s requiring frost headlines. Monday and Tuesday... Low confidence forecast in this time range. Appears that another trough will be approaching from the west early next week...which may bring a period of rain to the region but timing is highly uncertain. && .AVIATION /20Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...Moderate confidence. 18z update... Rest of today... SCT-BKN MVFR to low-end VFR CIGs prevailing, SW winds gusts up around 20 kts at times, especially along the coast and coastal plain. Tonight... Lowering down IFR-VLIFR after sunset, as late as 6z. Higher chance 1/2SM FG along the S/SE coast and over the high terrain but most other terminals remaining with low OVC CIGs around 004-008. Light S winds. Wednesday... IFR-LIFR lifting CIGs, gradually. S/SE coastal 1/2SM FG slow to lift out. Leaning similar trends to that observed earlier today with lifting 13-14z to MVFR to low-end VFR, becoming BKN to SCT 14-16z. Increasing SW winds with BKN-SCT decks, gusts up around 20-25 kts, strongest gusts again across the coast and coastal plain. Wednesday night... Lowering back down with the increasing threat of -RA towards morning, IFR-LIFR. S winds continuing. Dense fog potentially returning to the S/SE coast. KBOS Terminal... SCT-BKN MVFR CIGs for the rest of today. Gusty SW winds at times around 20 kts. Will see CIGs lower back down IFR-LIFR for the AM push with light S winds. Lifting around 13-14z Wednesday, MVFR to low-end VFR, as SW winds increase with gusts up around 20-25 kts. KBDL Terminal... SCT-BKN MVFR to low-end VFR CIGs presently will lower into the evening hours and overnight to IFR-LIFR. Lower confidence concerning VSBY impacts, thinking they'll hold MVFR-IFR at worst. Slow improvement into Wednesday. Outlook /Thursday through Sunday/...Moderate Confidence. Thursday: MVFR/IFR conditions possible. Breezy. SHRA, patchy BR. Thursday Night: MVFR/IFR conditions possible. Local gusts to 30 kt across the Cape/Islands. Chance SHRA, patchy BR. Friday: Mainly VFR, with local MVFR possible. Local gusts to 30 kt across the Cape/Islands. Chance SHRA. Friday Night through Sunday: VFR. Breezy. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High confidence. 2 pm update... Seas remaining below 5 feet as gusts along the coast are up around 20 kts for the remainder of today. Winds may diminish slightly overnight but there is the greater threat of locally dense fog along the S/SE coast developing tonight, slow to move off into Wednesday as SW winds once again increase with gusts around 20 kts. Potentially eroding during the day with the strong SW flow, it is forecast to return Wednesday night with lighter winds into Thursday morning. Outlook /Thursday through Sunday/...Moderate Confidence. Thursday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Rain showers, patchy fog. Local visibility 1 to 3 nm. Thursday Night: Low risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Seas up to 5 ft. Rain showers, patchy fog. Areas of visibility 1 to 3 nm. Friday: Moderate to high risk for Small Craft Advisory winds with gusts up to 30 kt. Low risk for brief gale force wind gusts around 35 knots...mainly south of the Cape and Islands. Areas of rough seas. Chance of rain showers. Friday Night: Moderate to high risk for Small Craft Advisory winds with gusts up to 30 kt. Low risk for brief gale force wind gusts around 35 knots...mainly south of the Cape and Islands. Areas of rough seas. Slight chance of rain showers. Saturday: Winds less than 25 kt. Local rough seas. Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Sunday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Sipprell/Frank NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...Frank AVIATION...Sipprell/Frank MARINE...Sipprell/Frank