490 FXUS61 KBOX 091815 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 215 PM EDT Tue Oct 9 2018 .SYNOPSIS... Warm and humid conditions continue through this afternoon and especially on Wednesday. Showers with local downpours will move through on Thursday ahead of a cold front and may linger into Friday. High pressure brings cooler and drier air over the weekend. Another cold front approaches with showers early in the week. && .NEAR TERM /THROUGH TONIGHT/... 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 /WEDNESDAY 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/... Big Picture... Longwave scale maintains ridges off each coast and a trough over the Rockies and Central Plains. Shortwave scale ejects a trough out of the Plains and across the Northeast USA Thursday and Friday. A second shortwave moves east into our area early next week. Mass fields among the models are similar through Saturday, bringing confidence to that part of the forecast. The GFS and ECMWF have timing differences with the system early next week...the GFS passes it through our area on Monday while the ECMWF is about 18 hours slower. Thus there is confidence in an early week system, but low confidence in exactly when it will move through. Concerns... Thursday-Friday... Shortwave driven by a 130 kt jet moves up the St Lawrence Valley. Favorable upper jet dynamics are forecast to remain west of us Thursday per the GFS but the ECWMF does bring favorable upper divergence late in the day. Both models then show couplets of low level convergence/upper divergence Thursday night and Friday. Precipitable Water values climb to 2.0-2.25 inches, which is well above normal during summer, let alone October. Cross sections show deep moisture and deep lift Thursday and Thursday night with lift as strong as 50 mb/hour. This suggests a cold front crossing our area late Thursday and Thursday night...with showers and a potential for downpours. Stability parameters are marginal for thunder, but close enough to allow for a few rumbles. We will need to monitor for potential urban/poor drainage flooding. Hrcn Michael moves northeast from the Gulf Coast, reaching the Carolina coast Thursday night. With the East Coast upper ridge shifting east and the upper flow diminishing in amplitude, model consensus takes Michael or its remnants well offshore and passing south of 40N/70W. High precipitable water values mostly shift offshore, but remain close enough to the South Coast to suggest continuing showers/downpours in that area Friday. Clouds will hold temperatures from reaching their full potential Thursday, with max temps in the low to mid 70s. Cooler air then starts moving in behind the cold front for Friday and Saturday with high temps in the upper 50s and lower 60s, low temps in the 30s north and 40s South. This suggests a potential for frost in the interior over the weekend. Early next week... The next shortwave dives into the plains over the weekend, then ejects through the Great Lakes early in the week. As noted above, timing is in question...but there remains enough potential to indicate some chance of showers on Monday. && .AVIATION /18Z 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: Mainly IFR, with local MVFR possible. Breezy. SHRA likely, chance TSRA, patchy FG. Thursday Night: MVFR/IFR conditions possible. Windy with local gusts to 30 kt. SHRA likely, slight chance TSRA, patchy BR. Friday: VFR. Breezy. Slight chance SHRA. Friday Night through Saturday Night: VFR. Breezy. Sunday: VFR. && .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 30 kt. Seas locally approaching 5 ft. Chance of rain showers, slight chance of thunderstorms. Thursday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Rain showers, chance of thunderstorms. Friday: Winds less than 25 kt. Local rough seas. Chance of rain showers. Friday Night: Winds less than 25 kt. Areas of rough seas. 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...WTB/Sipprell NEAR TERM...WTB/Sipprell/EVT SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell MARINE...WTB/Sipprell