635 FXUS61 KBOX 100837 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 437 AM EDT Wed Oct 10 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 well southeast of the 40N/70W benchmark, keeping the threat for heavy rain across far southeast New England with the best chance across Nantucket Friday. High pressure will build in for the weekend, bringing dry but rather cool fall-like weather. Showers may return during Monday or Tuesday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 1015 PM Update... High pressure ridge oriented NW-SE from western NY to NJ and off the coast S of the 40N latitude at 02Z. Dewpoints remain in the mid-upper 60s as temps start to fall back close to the dewpoint values. Low clouds have been developing along the S coast since around 00Z as seen on GOES-East Nighttime Fog product, but have been pushing E-NE and filling in across central and western areas as well as from N CT into N RI and interior SE Mass. This is well depicted on latest short range high res guidance. However, only noting spotty fog with locally reduced visibilities, with the lowest visibility at KPVC at 1/2SM. Some question how much fog will develop. The T/Td spreads are low, but SW winds at 5-10 kt keeping things stirred up. Could see area of dense fog develop off the S coast which could move into portions of Cape Cod and the islands, with only patchy fog further inland mainly after 08Z or so, especially across the higher inland terrain. Have updated near term forecast to bring conditions current and incorporated high res model trends through the remainder of the overnight. Previous Discussion... The high dewpoint airmass in place along with the cooling boundary layer will allow low clouds to develop across much of the region overnight. Some fog is expected as well, but it probably will not be widespread/dense in most locations given enough southwest flow in the boundary layer. The exception to that might be near the south coast, Cape and Islands where some dense fog is possible with moist southwest flow off the ocean. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM THURSDAY/... 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 NIGHT THROUGH TUESDAY/... Big Picture... Atlantic high pressure retreats south. Northern jet stream digs south with falling heights over the northern USA. Upper low associated with Hurricane Michael or its remnants moves north from the Gulf of Mexico, then gets caught in the sinking mid-latitude flow and its movement turns toward the northeast and passes well south of New England. One shortwave from the Pacific Northwest sweeps east and over our area Saturday. A second shortwave from Northern Canada digs south to the Great Lakes and Northeast USA early next week. Upper contour heights are similar through Sunday morning but then show differences. In particular the second of the shortwaves crosses our area at different times depending on the model. The GFS is about 12 hours faster than the ECMWF but similar to the GGEM. Confidence is high Friday, then diminishes to low by Monday and Tuesday. Contour heights are above normal through Friday, then fall to below normal over the weekend and rise a little early next week. We agree with the trend to colder temps behind the Thursday night front and lingering through the weekend. Guidance temps show a warming trend early next week, but not as much as the height increase suggests. We will go with the slower warming. Concerns... Thursday night-Friday... Cold front moves through the region from 00Z to 06Z. Michael or its remnants moves northeast from coastal North Carolina following the upper flow as noted above. This takes the center across 38.5N/70W, well south of Nantucket during Friday. The plume of high PW air remains over Srn New England through midnight and along the South Coast through the night. This suggests showers and embedded local downpours for part of the night inland and all night near the South Coast. A SW 40-50 knot low level jet is along the south coastal areas Thursday night, then shifts south and offshore overnight as the cold front moves through. This may bring wind gusts of 30-40 mph in southern areas until the cold front approaches, then winds will diminish. We will continue the potential for showers/thunder/local downpours, diminishing most areas overnight. Winds then shift from the northwest for Friday. Temperatures will cool overnight as northwest winds bring in lower dew points inland. Less certain of that along the coast where precipitation will continue to moisten the air. Min temps from around 50 northwest to the lower 60s over the Islands. The plume of moisture will linger along the South Coast Friday morning, but should shift offshore during the afternoon. Expect dry weather in most of the area, but morning showers southeast of a Plymouth-Providence line. Finally, the passage of Michael will bring a period of high surf to the exposed South Coast, especially the Islands and east- facing coast of Outer Cape Cod. Some of this will linger into Saturday. Weekend... Weak shortwave and cold pool move across our area Saturday. Cross sections show medium depth of moisture with this shortwave, so a period of clouds is likely. But the air below 850 mb is expected to be dry so no precip is forecast. Temps aloft are forecast at an 850 mb equivalent of 0C to -2C. Max temps are based on this as well as the expected morning temperatures, adding 5-7F to the latter. High pressure overhead and fair skies should allow radiational cooling, and with dew points in the mid 30s to low 40s we expect min temps in the mid 30s to mid 40s. This means potential for frost in some areas, including northern Mass and the low spots/bogs in the coastal plain. Fair weather Sunday with mixed layer temps supporting max temps in the mid 50s to low 60s. Monday-Tuesday... As noted above, timing of the next shortwave and its cold front are in question. But expect clouds and showers to arrive either later Sunday night through Monday, or later Monday through early Tuesday. We will go with a compromise on timing, with chance pops for showers each day. && .AVIATION /09Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...Moderate confidence. Overnight...Mainly VFR through 06Z, except MVFR-IFR CIGS across N CT/RI/SE Mass, with localized LIFR on portions of the immediate S coast. May see improving conditions along the S coast, with IFR CIGS moving further inland around or after 08Z, as well as across the higher inland terrain. Local LIFR VSBYS in patchy dense fog possible along the S coast, Cape/Islands and perhaps the high terrain. Wednesday...Gradual improvement. 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... MVFR to brief IFR CIGS through the morning push with SW winds 10 kt or less. CIGS should improve to VFR by around 14Z Wed. SW winds increase with gusts up around 20 kts. KBDL Terminal... MVFR CIGS may fall close to IFR at times through the overnight hours, into the morning push. CIGS should improve to VFR by around 12Z. Lower confidence continues with reduced VSBYS, but have kept VFR for now. Can not rule out a period of IFR from about 08Z-13Z. Outlook /Thursday Night through Sunday/...Moderate Confidence. Thursday night... MVFR/IFR cigs/vsbys in rain with local downpours. Areas of BR. Scattered thunder. Low level wind shear early with southwest winds at 2000 feet 30 to 40 knots, diminishing by midnight. Friday...VFR most areas. Patchy IFR in morning fog. Areas of lingering MVFR/IFR cigs/vsbys in showers along the South coast during the morning. Northwest gusts 20 knots, except to 30 knots on the Cape and Islands. Winds diminish Friday night. Saturday and Sunday... VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. 10 pm update... SW winds continue at 10-15 kt with gusts up to 20 kt, highest on the outer waters, but diminish somewhat overnight. Local visibility restrictions possible overnight especially across the southern waters, which should improve during Wed morning as SW winds increase with gusts to around 20 kt. Winds diminish again Wed night with areas of fog redeveloping, mainly from around 03Z or so especially on the southern waters. Outlook /Thursday Night through Sunday/...Moderate Confidence. Thursday night... Moderate risk for Small Craft Advisory with gusts to 30 knots. Low risk for Gales with gusts to 40 knots, mainly south of the Islands. Southwest winds turn from the Northwest overnight as a cold front moves through. Seas up to 6 ft, mostly south of Cape Cod and Islands. Rain showers, scattered thunder, areas of fog. Friday... Moderate risk for Small Craft Advisory with gusts to 30 knots. Low risk for Gales with gusts to 40 knots, mainly south of the Islands. Seas up to 7 feet, mostly south of Cape Cod and Islands. Diminishing showers and thunder by afternoon. Remnants of Michael will pass well south of Nantucket. Saturday and Sunday... Winds less than 25 kt. Seas 5 to 6 feet on the outer waters Saturday, then less than 5 feet Saturday night and Sunday. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 2 PM to 6 PM EDT Thursday for ANZ231>234-236-255. Small Craft Advisory from 1 PM to 6 PM EDT Thursday for ANZ235- 237. Small Craft Advisory from 4 PM to 6 PM EDT Thursday for ANZ250- 254. Small Craft Advisory from noon to 6 PM EDT Thursday for ANZ256. && $$ SYNOPSIS...WTB/Sipprell NEAR TERM...EVT SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell/EVT MARINE...WTB/Sipprell/EVT