842 FXUS61 KBOX 101045 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 645 AM EDT Wed Oct 10 2018 .SYNOPSIS... Warm and humid conditions continue today. Scattered showers may move into the region late tonight into early Thursday as a backdoor cold front slowly pushes south before stalling across central New England. Showers and a few thunderstorms will push across the region ahead of an approaching cold front Thursday through Friday. 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 outer Cape Cod and 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 PM THIS EVENING/... 7 AM Update... Several breaks in the clouds, especially across Northeast Mass. But satellite and obs show a broad area of low clouds across Southern New England with a layer of higher clouds moving over the top. Adjustments were made to temps and dewpts, as well as a few sky cover grids. But otherwise the forecast remains on track. Previous Discussion... Area of low clouds and patchy fog continue across the region as seen on latest GOES-East Nighttime Fog product loop. Lowest cloud bases noted along the S coast, Cape Cod and the islands as well as across portions of NE CT and the mid and upper CT valley at 08Z. With a steady SW wind flow in place across most locations, T/Td spreads remain low so the lower clouds will continue through around mid morning or so when they will tend to dissipate. Also noting some high clouds passing over the lower clouds on the satellite imagery, which will continue through the day as they work NE out of the mid Atlantic region, with the most clouds lingering along S coastal areas. General W-SW wind flow from H95 through H85 will bring some good mixing away from the S coast this afternoon. Could see wind gusts up to 20 kt especially across the higher inland terrain and along the coast. High pressure ridge S of New England will settle offshore, so expect dry conditions during the day. It will be warm, humid day with H85 temps up to +15C to +16C. Remained close to previous forecast, which bumped temps up a few degrees. Expect highs in the upper 70s to lower 80s away from the S coast, where readings will top off from 70 to 75. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Tonight... Mid level ridge/cutoff high across the western Atlantic pushes offshore, allowing H5 heights to lower. Noting pressure surge working S-SW along the Maine coast, with an associated backdoor cold front trying to shift S. May see some showers try to push into N central and NE Mass around or after midnight, but it will be tough for the front to work much further S. In the meantime, strong low pressure will push across the Great Lakes, with its associated cold front trying to push E from the Ohio Valley to the mid Gulf Coast. Lastly, the moisture plume feeding NE ahead of the front, aided by the approaching Hurricane Michael into the FL panhandle, will see surge of tropical moisture northward. PWATs across central and W areas increase to around 2 inches, or around 3-4 SD above normal. Expect increasing chance for showers across the region, with the best chance across W Mass/N central CT after midnight. Will also see patchy fog develop overnight, with visibility lowering to around 1SM across the E slopes of the Berkshires and possibly into the Merrimack Valley. May see quite a range in temps overnight, depending upon how much the cooler air pushes into N central and NE Mass. Current thinking suggests temps bottoming out in the 55-60 degree range from about a KBOS-KORH line N and E, ranging to the upper 60s across the mid and lower CT valley. Thursday... Deep moisture ahead of the approaching front and from Michael down the coast will keep an increasing PWAT plume pushing across the region during the day. PWAT values increase to 2.1 to 2.3 inches, up to 4-5 SD above normal. Showers will become widespread from W-E as the cold front approaches from the W, while the front across central New England retreats back N. With the high moisture plume in place, will see locally heavy downpours that may cause brief poor drainage and urban flooding in some spots. Also noting increasing instability ahead of the front, with K indices up to the mid 30s moving across the region during the day. Have also mentioned a low chance for thunderstorms as well. May see SW winds gusting to 20-25 kt along the S coast during Thu afternoon as good mixing returns along with a good low level jet passing along S coastal areas. Expect temps to range from the mid 60s to near 70 across N central/NE Mass to the mid 70s across the lower CT valley. && .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 /12Z 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. Today...Conditions improve across most areas by around midday except the S coast where MVFR to areas of LIFR CIGS/VSBYS may linger through the day with the onshore winds. SW winds gusting to around 20 kt along the coast. Tonight...Showers develop from W-E overnight with approaching cold front. Expect CIGS to lower to IFR-LIFR by around 04Z-06Z. VSBYS lower to W-E after midnight to MVFR-IFR. Thursday...CIGS may improve to VFR across most areas, expect MVFR conditions lingering across the higher terrain and possibly along the S coast. Mix of MVFR-IFR VSBYS. Showers continue, with chance for thunderstorms. Locally heavy rainfall possible. KBOS Terminal...VFR CIGS may briefly lower to MVFR from time to time through late morning. VFR VSBYS. SW winds gusting up around 20 kt this afternoon. Conditions may lower to IFR around midnight tonight. KBDL Terminal...Mainly VFR conditions, though CIGS may lower to MVFR from time to time through mid morning. VSBYS may lower to IFR after midnight tonight as scattered showers approach. 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. SW winds will gust up to 20 kt from midday through this afternoon, then diminish tonight. Have hoisted Small Craft Advisories as SW winds gust up to around 25-30 kt across most of the water Thu afternoon. Seas also build up to around 5 ft late in the day on the open waters S of RI. Visibility restrictions in lingering fog mainly across the southern waters through mid morning, then again tonight into Thursday. 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/EVT NEAR TERM...WTB/EVT SHORT TERM...EVT LONG TERM...WTB AVIATION...WTB/EVT MARINE...WTB/EVT