Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 232004 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 404 PM EDT Sun Oct 23 2016 .SYNOPSIS... Winds diminishing ahead of a clipper-like system overnight tonight into Monday morning bringing with it a widespread area of rain with an embedded line of heavier outcomes. With its departure, winds become blustery out of the W/NW beneath a cool, dry airmass which continues though midweek. A period of rain is likely Thursday into Friday as low pressure crosses the region. Seasonable temperatures are expected for next weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... 4 pm update... */ Highlights... - Winds diminishing out of the W as we go into evening - Rain overnight, possibly moderate, maybe a clap of thunder */ Discussion... Into this evening... Daytime heating and diurnal mixing concluding as the pressure gradient and isallobaric component of the winds relax across the region. Will still remain breezy out of the W, but the threat of damaging winds diminishes allowing the Wind Advisory to conclude. Cold air advection proceeding, will begin to see temperatures drop with clouds on the increase over W/SW New England late. Gusts around 30 to 35 mph with strongest gusts over the high terrain, as well as out across the warmer ocean waters which will result in impacts along the immediate shorelines, especially more so for the Cape and Islands. Tonight... Perhaps an under-estimated overnight disturbance? Clipper-like low sweeping S of New England with attendant vortmax through the overall broadly cyclonic open wave trough. Along the parent warm front, tight thermal packing especially along the 290-300K isentropic surfaces in tandem with the nose of a h925-85 low level jet. Subsequent strong low to mid level isentropic and frontogenetical lift. Accompanying continental polar airmass while latching into a higher tongue of theta-E offshore, there`s a decent shot of moisture with precipitable waters up to 1 inch. Strong forcing and dynamics apparent beneath decent venting aloft with a measure of elevated instability within the mid-levels, consensus of guidance forecasting strong omega values. Expecting a W to E band of moderate rainfall with amounts around 2 to 3 tenths. Can not rule out some isolated thunder and will insert wording into the forecast mainly over the waters S of New England. Difficulty is nailing down the N/S gradient of expected outcomes of which there could be multiple depending on the W-E level of frontogenetical/isentropic forcing from which it emerges. Mesoscale outcomes to which forecast guidance is struggling with specificity. A couple of camps with the RAP/HRRR a bit N than all other guidance. Latest EC has nudged slightly N, GFS has remained persistent. This would be more worrisome if it was colder and this was snow. Systems like these have a big bust potential. Consensus of forecast guidance places high chance to likely PoPs S along and S of the Mass Pike to the S coast and adjacent waters. Can not rule out a N-S wobble in the forecasts as we get closer to the actual event. Winds taper and turn more S in advance of the system. Looking at milder conditions overnight with lows ranging around the upper 30s to low 40s. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Monday into Monday night... Cooler, dry and breezy weather. Strong cyclonic flow aloft pushing an impressive H5 polar airmass S with temperatures of -30C. H5-7 mid- level lapse rates becoming unstable. This within a atmospheric column relatively dry. But can not rule out some isolated shower activity especially off the Great Lakes. Attendant mid-level forcing with a vortmax rounding the cyclonic flow. But with downsloping along the 280-290K isentropic surfaces within the low levels, feel any shower activity is mainly confined N/W of our region and any impacts, if any, will be across the high terrain. Scattered cloud decks during the day with deep layer mixing up to H85 where temperatures around 0 to -2C with cold air advection ongoing. Will see the mix down of faster momentum and drier air so expect very low dewpoints in the 20s with NW wind gusts of 20 to 30 mph. Highs around the mid to upper 50s with abundant sunshine but feeling cooler given the breezy winds. Winds taper slightly during the overnight hours. Lows around the mid 30s with cold air advection still dominating. Given the winds, will not go with any frost in the forecast. More concerned about freezing temperatures. But given how marginal overnight lows will be, at or above freezing, in the areas of Southern New England that the growing season is still considered, will pass on headlines at this time and mention in the hazardous weather outlook. && .LONG TERM /TUESDAY THROUGH SUNDAY/... */ Highlights... * Unseasonably chilly but mainly dry Tue/Wed other than a few ocean effect rain showers across Plymouth County/Cape/Islands * Lows should drop well down into the 20s Wed night/Thu am * Period of rain likely sometime Thu into Fri */ Details... Tuesday and Wednesday... Strong 1035+ MB High pressure north of Lake Michigan builds eastward into Quebec. At the same time, low pressure over the Canadian Maritimes will slowly shift east. The result will be a north to northwest flow of unseasonably chilly but mainly dry weather into the region Tue/Wed. 850T are expected to drop to between -6C and -8C. In addition, the delivery of this airmass will be from more of a north northwest direction allowing for less modification / downslope than the one this weekend. Therefore, expect highs Tue/Wed to be generally in the 40s but it will feel colder on Tue with gusty winds. Low temps Tue night and Wed night will be below freezing in most locations, other than downtown Boston and portions of southeast New England coast where ocean effect clouds are expected. Wed night should be the coldest night when many locales drop well down into the 20s as winds diminish. While mainly dry weather is expected, the setup is favorable for a few ocean effect rain showers from Plymouth county and onto the Cape/Islands Tue night/Wed. Surface winds will turn more northerly as strong Canadian High pressure builds into eastern Canada. Given ocean water temps still near 60F, the 850mb/sea surface differential should exceed 20C. Although 850 mb temps will be around -5C, the boundary layer will be way too warm to support anything more than a few ocean effect rain showers. Thursday and Friday... Upper trough quickly lifts east of the region as low pressure from the midwest lifts northeast towards the Great Lakes. This will increase the forcing for ascent and allow for a period of rain sometime Thursday into Friday. Specific timing remains uncertain this far out, but latest guidance suggests highest risk will be late Thu into early Fri. The antecedent airmass ahead of this system is quite cold/dry for late October standards. Therefore, it is not out of the question for a very brief period of snow and/or sleet at the onset across the high terrain along the east slopes of the Berks and perhaps even the northern Worcester Hills on Thu. Even if this were to occur for a short time, it would have little impact with a quick change to rain. Lastly, there are some signs that a secondary low will try to develop near the south coast. If this occurs, there might be some enhancement to the rain across portions of our region but it is just too early to say. Saturday and Sunday... Low confidence in this time range. A cold front may bring a few brief showers, but guidance suggests that much of the time may end up dry with temperatures not too far from normal readings. && .AVIATION /19Z SUNDAY THROUGH FRIDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... 20z update... Into this evening...High confidence. VFR. W gusts 30-35 kt with up to 40 kts over high terrain. Tonight...Moderate confidence. Winds tapering, turning S, gusts dropping off expect for coast- line terminals where gusts of 15 to 20 kts are possible. Initially VFR, conditions drop to low-end VFR to MVFR mix beginning around midnight. -RA/RA sweeping shortly after into Monday morning, TEMPO MVFR-IFR VSBY restrictions possible. S winds becoming W by morning. Monday into Monday Night...High confidence. Low-end VFR / MVFR quickly clearing S/E. Increasing W/NW winds with gusts 20 to 25 kts, up to 30 kts along the coast and perhaps across the high terrain. FEW to SCT cigs 4-5 kft agl. KBOS TAF...Lower confidence with respect to -RA/RA impacts. Will hold the bulk of MVFR impacts S of the terminal. KBDL TAF...Confident in -RA/RA impacts to the terminal overnight with a mix of low-end VFR to MVFR conditions. Outlook /Tuesday through Friday/... Tuesday...High confidence in VFR conditions. Northwest winds may gust to around 30 knots. Tuesday night and Wednesday...High confidence in mainly VFR conditions. A period of MVFR CIGS are possible in ocean effect clouds and a few rain showers across Plymouth county and onto the Cape/Islands. Thursday and Friday...Moderate confidence. A period of MVFR-IFR conditions expected in rain along with some lower clouds and fog patches. Timing uncertain but greatest risk right now appears to be late Thu into early Fri. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...High confidence. 4 pm update... Tonight... W gales persist into the evening hours, concluding shortly before midnight as winds turn out of the S ahead of an area of rain and possible visibility restrictions, maybe even a clap of thunder over the S waters, from shortly after midnight into early monday morning. Gales will be converted to small crafts as winds diminish. Waves slowly subside as well. Monday into Monday night... Behind the disturbed weather clearing out Monday morning, W/NW winds increase to near gale force over the waters out through Tuesday morning. Waves remaining 5 to 7 feet. Outlook /Tuesday through Friday/... Tuesday...High confidence. Northwest winds should gust to 30 knots over most waters in the strong cold advection pattern given the relatively mild ocean. There is even a low risk for brief winds up to Gale Force. Seas build to between 5 and 6 feet across our outer- waters. Wednesday...Moderate to high confidence. Pressure gradient weakens some, but still expect northerly wind gusts up to 25 knots given good mixing over the relatively mild ocean. Thursday and Friday...Low to moderate confidence. Winds turn to the southeast Thu ahead of an advancing low pressure system and probably more southerly by Thu night. Behind the low pressure system, should see more of a westerly component to the winds develop on Friday. Probably will see a period of small craft wind gusts and seas late Thu into Fri, but still a lot of uncertainty this far out especially with timing. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Wind Advisory until 6 PM EDT this evening for CTZ002>004. MA...Wind Advisory until 6 PM EDT this evening for MAZ002>024-026. RI...Wind Advisory until 6 PM EDT this evening for RIZ001>008. MARINE...Gale Warning until 2 AM EDT Monday for ANZ231>235-237-250-251- 254>256. && $$ SYNOPSIS...Frank/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...Frank AVIATION...Frank/Sipprell MARINE...Frank/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.