Area Forecast Discussion
Issued by NWS Boston, MA

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874 FXUS61 KBOX 181805 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 105 PM EST Thu Jan 18 2018 .SYNOPSIS... Dry and seasonably cold conditions today and Friday, then a warming trend this weekend with continued dry weather. A storm system will affect the region sometime Monday night into Tuesday with mostly rain and coastal wind, but there is a risk of some mix/ice across the interior. Blustery and colder weather follows next Wednesday. && .NEAR TERM /THROUGH TONIGHT/... 1 pm update... Storm digging off the Mid-Atlantic coast, trowaling back along 290-300K isentropic surfaces but running up against subsidence, drier air from the NW as discerned from latest satellite, the cloud shield eroding. But watching low level stratus off the lakes and over the Gulf of Maine. Mesoscale processes occurring as boundary layer mixes out, becoming trapped beneath the dry subsidence inversion as discerned from 12z soundings from Buffalo NY to Portland ME. So the biggest forecast nuisance as of present: stratus decks over the Berkshires, downsloping and eroding S/E with the NW flow, and marine stratus over the Gulf of Maine and Nantucket clearly seen via satellite. Expecting the marine stratus to push SSE with time while eroding, clipping the Outer Cape and Nantucket with low cloud decks. Otherwise high pressure in control well S/W over the SE CONUS as energy continues to stream to our N over S Canada. Subsequent westerly gradient lending to winds around 10 mph with gusts up to 20 mph especially over the high terrain. Temperatures are over-achieving where snow pack is less, warmer with highs in the low to mid 30s. Colder to the N/W with deeper snow pack as well as some scattered to broken cloud decks, highs upper 20s to low 30s. Tonight... Deamplifying H5 impulse sweeping across the region overnight. Accompanying ascent with differential vorticity advection and parent jet streak. Over-running response but atmosphere fairly void of moisture given precipitable waters up around 0.2 inches. Green`s and White`s likely to see some snow potentially extending as far S as the Berkshires as flurries. Otherwise scattered to broken cloud decks while monitoring low-level stratus lingering along the high terrain and along the Outer Cape. With clouds anticipated, leaning away from coldest of guidance, that being MET/MAV. && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... Friday... Departing H5 vortmax with accompanying enhancement to the low- level wind profile. Mixing out to around H9 through the day, there`s the possibility of some breezy W winds during the morning, otherwise relaxing through the remainder of the day beneath a follow-up brief, weak ridge of high pressure. Boundary layer lapse rates remaining well mixed, beneath a dry inversion, and anticipating temperatures to warm into the mid and upper 30s, with snow melt, can`t rule out some few to scattered pancake cumulus. Friday night... Continued energy and moisture accelerating through the pseudo- zonal flow, sagging S into the Great Lakes region. Up against high pressure over the SE CONUS, an amplified gradient wind response with accompanying speed max at H925. Warmer, dry air pushing out of the SW, warming within H9-7 apparent within model forecast soundings, limitations on mixing down to the surface. Thus expecting a gradient wind response with strongest winds potentially in excess of 30 mph along the S-coast, across the Cape and Islands. Keep it dry but also mild with the winds. Lows down in the 20s for most locations. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Highlights... * Dry and mild this weekend * Another storm will bring mostly rain and gusty coastal winds Mon night/Tue with a risk of some interior mixed precip/ice Mild and dry conditions expected this weekend under zonal flow and rising heights. Temps should reach well into the 40s both days with the chance of a few 50 degree readings Sat. It will be a bit breezy Sat with potential for 25-35 mph SW gusts near the coast which will temper the milder temps but less wind Sunday. Some light precip in the form of rain/snow showers possible Mon in the developing warm advection pattern, but models have trended a bit slower with timing of next storm, with greatest impact likely sometime late Mon night into Tue. Deep mid level trough/low approaching from the west is expected to lift NE across the Gt Lakes and into SE Canada. A robust pre-frontal low level jet will likely bring a decent slug of rain to SNE with potential for heavy rainfall and strong coastal wind. PWAT and low level wind anomalies are 2+ SD which is pretty good for a day 5-6 forecast. Exact timing of the rain and wind is uncertain and there also remains enough uncertainty with thermal profile that some interior mix/ice possible at the onset. ECMWF is more robust with the colder air but it will take awhile for these finer details to be resolved. As storm exits, blustery and colder weather will likely follow next Wednesday. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. Rest of today into tonight... VFR. Monitoring MVFR marine stratus over the Gulf of ME and across ACK. Anticipating low CIGs to remain immediately off- shore. W winds continue around 10 kts. SCT-BKN high clouds towards evening, eroding into Friday morning. Friday... VFR. Brief period of breezy W winds as SCT-BKN high clouds erode. FEW-SCT 035 possible during the day. Friday night... VFR. W winds on the increase towards Saturday morning. Mainly a gradient wind with the potential of sustained values up to 25 kts, gusts up to 30 kts. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Saturday through Monday/... Saturday: VFR. Windy with gusts to 30 kt. Saturday Night: VFR. Breezy. Sunday through Sunday Night: VFR. Monday: MVFR/IFR conditions possible. Chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. SMALL CRAFT ADVISORIES trimmed as W winds remain around 10 kts which have allowed seas to diminish. Expect lower sea trends and continued W winds through Friday. Greater concern is on the Friday night into Saturday morning period with increasing sustained winds potentially up to 25 kts, gusts up to 30 kts. Headlines may need to be renewed and there is a low risk of gales. Outlook /Saturday through Monday/... Saturday: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 11 ft. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday Night: Winds less than 25 kt. Monday: Winds less than 25 kt. Seas locally approaching 5 ft. && .HYDROLOGY... A Flood Warning remains in effect for the Connecticut River at Middle Haddam, where ice jams are causing river fluctuations. Cold conditions will persist through Thursday which will limit additional runoff. Continued ice jams will remain on some of the rivers. There will be some increase of snow pack across interior southern New England as well. For details on specific area rivers, including observed and forecast river stages, please visit the Advanced Hydrologic Prediction Service (AHPS) graphs on our website. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 6 AM EST Friday for ANZ254>256. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Sipprell SHORT TERM...Sipprell LONG TERM...KJC AVIATION...KJC/Sipprell MARINE...KJC/Sipprell HYDROLOGY...WFO BOX Staff

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