Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 222036 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 436 PM EDT Mon Oct 22 2018 .SYNOPSIS... High pressure will bring dry and unseasonably chilly weather tonight. Low pressure will bring a few showers to the region on Tuesday, particularly across northern Massachusetts where a period of steadier showers are possible. Some showers linger into Wednesday as low pressure pushes across the Canadian Maritimes. High pressure moves across through late this week, bringing the coldest air of the season late this week lingering into early next weekend. Strong low pressure may work up the eastern seaboard, with rain possible this weekend and a mix of snow possible at the onset inland. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Southwest winds have brought in more clouds this afternoon with some mid level moisture into SNE, but plenty of sun to be had for most under partly sunny skies. Temperatures have climbed into the upper 40s in the interior; more like low 50s along the coast with generally light winds. This comes courtesy of the area of high pressure currently centered to our south and the mid level ridge centered over New England. This all moves off to the east overnight ahead of a low pressure system and shortwave trough dropping down from the Great Lakes region on Tuesday. Ahead of that system tonight winds will be light with partly cloudy skies, allowing good cooling into the mid 30s (quite a bit warmer than we saw last night given the WAA). Early Tuesday morning we`ll see our first shower chances over the Cape and islands ahead of the incoming shortwave, but the majority of the region won`t see rain until after sunrise. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Low pressure and the mid level shortwave approach from the northwest bringing increased moisture, instability, and a tightening pressure gradient. Precipitable waters will be on the increase, as will cloud cover early Tuesday morning and throughout the day; PWATs increasing to 0.5 to 0.75". A weak warm front moves north during the day, though some hi-res guidance has it stalling out along the MA/CT border, so that will be something to watch with the temperature forecast. For now going with temps warmer than today, in the upper 40s/low 50s to the north and the upper 50s to near 60 along the south coast. Given the front, moisture, and mid level instability from our 500 mb cold pool aloft showers are expected to expand through the morning after the initial round for the southeast coast. No huge rain totals expected, generally a few tenths of an inch. Pressure gradients will tighten as the low passes over the region, and a 850 mb LLJ of ~35 kts passes just to our south. Given potential for mixing we could see gusts of 30-35 mph over our south and east waters with 20-30 mph gusts over the Cape and islands. Tuesday night the low moves off the coast and we`ll be under northwest winds with decreases rain chances. Winds will calm a bit as well the jet moves off shore. Temperatures will be several degrees warmer than the previous night, in the 40s (upper 30s for higher elevations). && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Overview... Noting a highly amplified mid level steering pattern across North America and beyond through most of the week into next weekend. Large high pressure builds E across the northeast U.S. late this week. This will bring a rather cold airmass across, with H925 temps dropping to -3C to -6C by early Friday morning, running up to around 3 SD (10-15 degrees) below seasonal normals, though not quite to record levels for late October. 12Z medium range model suite continues to signal the potential for a strong nor`easter remains in the forecast for the upcoming weekend. However, this is still high uncertainty of how this system will evolve, its ultimate track, and its impacts on the region. Details... Wednesday... Cutoff H5 low across northern New England moves slowly E, taking its associated surface low with it. NW wind flow in place, which will start to bring colder air across the region. May see leftover isolated showers linger across central and eastern areas, with the best chance across Cape Cod and the islands through early afternoon. Thursday through Friday... Dry conditions as large high pressure crosses the region. Very cold airmass builds SE out of the Arctic Circle across Hudson Bay into the region on the backside of exiting cutoff H5 low pressure. Expect highs both Thu and Fri in the 40s, with some readings possibly holding in the upper 30s across the E slopes of the Berkshires on Thu. Will also see NW winds on Thu, which will produce wind chill values in the upper 20s to upper 30s. Friday night through Sunday... Strong low develops across the southern stream and, with another digging H5 long wave trough across the Ohio Valley into the interior SE U.S., models continue to push a strong surface low off the SC/GA coast late Friday. Still a high amount of uncertainty in how this low will track up the eastern seaboard, how fast it will push up the coast, and how strong this low will be. Models have quite a spread amongst their solutions at this time, will continue to monitor closely. Could see the precip onset in the form of snow across the interior late Fri night or Sat morning, which could cause problems especially with leaves on the trees. Another concern may be with how strong the onshore winds may be, which may bring a storm surge with it. Boston`s high tides will be around 11 feet this weekend, which could bring some storm surge if the easterly winds are strong enough. Will monitor this potential closely. Monday... Again, quite high uncertainty with the exit of this low, and whether another surface system tries to wrap around digging cutoff H5 low and trough out of the western Great Lakes. Have kept low chance POPs for now, though temps will moderate some so, at this point, less chance for mixed precip/snow. Still quite a bit of time to watch this system. && .AVIATION /21Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/... Through this afternoon...High confidence. VFR conditions continue with W-SW winds 5 to 10 kts, some locations gusting to 20 mph. Tonight...High confidence. Generally remaining VFR, with potential for MVFR cigs under SHRA over KHYA, KFMH, and KACK after 05Z. Spotty SHRA possible over the rest of the region toward 12Z. Tuesday...Low to moderate confidence. Uncertainty remains with regards to northward extend of a warm front. Best chance for MVFR/IFR under scattered showers continues to be over northern MA. Shower chances decrease after around 21Z. KBOS Terminal...High confidence in TAF through tonight. KBDL Terminal...High confidence in TAF through tonight. Outlook /Wednesday through Saturday/... Wednesday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. Wednesday Night: VFR. Windy with local gusts to 30 kt. Thursday through Friday: VFR. Breezy. Friday Night: Mainly VFR, with local IFR possible. Breezy. Slight chance SN. Saturday: Mainly MVFR, with local IFR possible. Windy with gusts to 35 kt. Chance RA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/... Tonight...High confidence. A weak pressure gradient will keep winds/seas below small craft advisory thresholds. Tuesday...Moderate to high confidence. Expecting a period of southwest wind gusts of 25 knots and 3 to 6 foot seas developing across our southern and eastern waters during the afternoon. Small Craft Advisories have been issued. Wednesday: Gusty winds up to 30 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Outlook /Wednesday through Saturday/... Wednesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Wednesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Thursday: Moderate risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Local rough seas. Thursday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Friday Night: Winds less than 25 kt. Slight chance of rain. Saturday: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Chance of rain. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 5 PM Tuesday to 5 AM EDT Wednesday for ANZ232-235-237. Small Craft Advisory from 11 PM Tuesday to 2 AM EDT Wednesday for ANZ233. Small Craft Advisory from 2 AM to 8 AM EDT Wednesday for ANZ250. Small Craft Advisory from 2 PM Tuesday to 8 AM EDT Wednesday for ANZ254>256. && $$ SYNOPSIS...EVT/BW NEAR TERM...BW SHORT TERM...BW LONG TERM...EVT AVIATION...EVT/BW MARINE...EVT/BW

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