Area Forecast Discussion
Issued by NWS Boston, MA

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777 FXUS61 KBOX 211833 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 233 PM EDT SAT MAY 21 2016 .SYNOPSIS... * Updates to near- and short-term forecast portions only... Mid-Atlantic low pressure moves up the coast overnight passing southeast of New England in the process into the Gulf of Maine for Sunday. A return of cooler conditions with wet weather mainly confined to E/SE portions of New England. Tapering off Monday into Tuesday, a warming trend begins Wednesday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Into this evening... Apparent through an observational analysis is the struggle of the warm moist conveyor belt ahead of the troughing pattern W and surface cyclogenesis off the Mid-Atlantic up against high pressure and drier air over N/E New England into SE Canada. The initial isentropic push has already swept N/E across S New England as indicative of the increasing and thickening clouds along with the very light returns per WSR-88D at around 9 kft agl. Top-down moistening proceeding will only increase the effectiveness to which rain can reach the surface. So as we go into this evening low pressure strengthens off the Mid- Atlantic coast and the Delmarva Peninsula. Will see the emphasis of precipitable waters over the NW Atlantic focus to the N/NE quadrant of the low. And with the low pushing N/E towards the 40N/70W benchmark, will see the chance of rain along the immediate S-coast of New England just prior to sunset. So expect the rain-shield off the New Jersey coast to take a greater role in making a wet-weather impact to our area over the next several hours. Tonight... Surface low pressure undergoing maturation S/E of the 40N/70W benchmark will likely result in wet-weather outcomes for SE New England with chance elsewhere. Could actually remain dry over N/W interior New England. Aside, lead mid-level vortex energy ahead of an open-wave trough over the OH River Valley invokes forcing upon low-mid level isentropic ascent and convergent nose of a low level jet in the N/NE quadrant of the aforementioned surface low. But too far S/E and up against developing dry and cold conveyor belt motions, confident in rain over SE New England with a developing dry corridor N/W per isentropic downward motions / sinking air especially towards Sunday morning. PoPs adjusted accordingly. Turning chilly and perhaps dismal and damp especially over E/SE coastal New England with developing onshore flow with NE winds. Lows around the 50 degree mark. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Sunday into Sunday night... Continued low maturation into the Gulf of ME as a cold front pushes S into the NE CONUS. Two separate airmasses between which sinking air and dry weather prevails. While forcing parent to the Gulf of ME low is far removed from New England, cold and dry conveyor belt motions prevail. It is anticipated that across interior New England it will remain quiet with dry conveyor belt motions, while over E New England it remain socked in with the threat of drizzle / light showers. Along with NE flow, looks to be cool and dreary. To the W, cold pool associated with the broad troughing pattern yields an unstable profile ahead of the diffuse weakening cold front. With any sunshine a diurnal storm pattern emerges. But emphasize some wiggle room considering the difficulty already as to which the forecast models are having with the present synoptic situation. This warrants a look at ensemble probabalistic forecasts. Such wiggle room can alter the balance in airmasses in New England with a more unstable environment to the W and a dreary environment to the E. In-between prevailing dry with sinking air. Can not rule out such a shift in activity either E or W. So a cautious forecast that is not concrete. A model blend preferred with equal weighting to a MOS consensus. Does appear a SW-NE line across interior New England has the best chance of remaining fairly dry in comparison to other areas along the immediate W and E. Quiet over most of S New England overnight with only issues across E/SE with continued anticipated cold conveyor belt motions with the initial low dissipating and another redeveloping off the Mid- Atlantic parent with a mid-upper level cutoff low invoked by N- stream energy diving S. Looking at continued drizzle / light showers for E/SE New England and overall cool conditions. && .LONG TERM /MONDAY THROUGH SATURDAY/... Highlights... * Unstable/wet but milder Monday and Tuesday. * Warmer still and drier Wednesday * Warmer still late week, but potentially unstable. Overview and Model Preferences... In spite of what remains a relatively amplified flow pattern across the CONUS for much of next week, even operational guidance is in reasonably good agreement at least until the weekend. Cutoff remains the dominant player through Tuesday, followed by responsive ridging associated with srn stream warming. Therefore, will maintain the status quo for the most part, a seasonable, but unsettled and occasionally wet period for Mon-Tue followed by a warming trend late in the week, which could be accompanied by destabilization featuring shra/tstms. Feel fairly confident in using a GFS/ECMWF blend along with persistence as a starting point for this forecast update. Details... Sun night into Mon... Any remaining shra from either the instability to the W or the low pres exiting the Gulf Of Maine will whither Sun night giving way to mainly drizzle/damp conditions under continued NNE flow. Clouds/damp airmass yields a mild night with temps holding mainly in the 50s. By Tue expect some mixing and drying of the lower lvls as a secondary low pres moves up from the mid Atlantic states. Clouds/fog early still likely as Ely component to the flow continues. Even though unsettled conditions continue, a period with little in the way of measurable precip is likely through the day. The enhanced mixing should also lead to warmer highs than Sun, realizing some of the H85 temps near +8C, so Wrn areas /away from onshore flow/ should make it to near 70 while E areas remain cooler and in the 60s. Mon night into Tue... Secondary low pres associated with -2-sigma H5 cutoff moves up in a similar fashion to the Sat night-Sun feature, although it may actually make a closer pass or even just inside of the 40/70 benchmark. Pwats with this feature approach 2 std deviations above normal, so look for another round of widespread shra to develop early Tue morning with even the low risk of some isolated tstms thanks to H5 temps new -20C and TT approaching 50. Min/Max temps similar to Mon, even though some warming of the mid- lvls is expected, the increased clouds wet-bulb effect will maintain temps similar to those experienced the day prior. Wed... Brief ridge of high pres continues to be advertised as upstream mid and upper lvl ridging builds in response to warming srn stream flow. After early day showers/NE flow wains, expect some improvement through the latter portion of the day. Warming of H85 temps to near +14C and at least some observed sunshine should allow highs to rebound back into the mid 70s to even low 80s. Thu and Fri... Warm front shifts into Canada and Nrn New England with return flow and ridging. Looks like a good warm sector setup with potential destabilization possible. Therefore, diurnally driven shra/tstms are possible and will feature pops as such. Severity will be dependent on timing and mesoscale features such as LLJ. +16C Thu and +18C Fri are the average H85 temps, so with sunshine, highs in the 80s are quite likely. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/... 18z update... Rest of today...High Confidence. Lowering cigs but remaining VFR. Will see sea-breeze along the E coast erode with dominant S/SW winds. Tonight...Moderate Confidence. Keeping optimistic. Approaching offshore storm system. Will see the onset of -RA across S/SE terminals by sunset and deteriorating conditions down to MVFR. S winds backing NE and increasing with gusts up to 20 kts by morning. Sunday...Moderate Confidence. Continued optimistic forecast. MVFR over E/SE coastal New England with -DZ/-SHRA. VFR across the interior. Low risk of TSRA across W CT River Valley. E/NE flow, with gusts up to 30 kts especially over SE New England, Cape Cod, and Islands. Sunday Night...Moderate Confidence. MVFR over E/SE Coastal New England but E/NE winds diminishing. VFR elsewhere. KBOS TAF...Sea-breeze eroding out over the next hour or two, then winds turn variable for a time out of the S prior to backing out of the NE overnight and increasing towards morning. Will keep with an optimistic forecast keeping both -RA and MVFR mainly out of the terminal until Sunday morning. KBDL TAF...Optimistic forecast with keeping -RA and MVFR out of the terminal. Confident this will be the case through the TAF forecast. Outlook...Monday into Wednesday. Monday into Tuesday...Moderate Confidence. Although not a complete washout, periods of showers will continue on and off across the area with northeasterly flow. This also suggests low clouds. Therefore, periods of MVFR/IFR are likely, but it/s possible occasional VFR, especially in the west will occur. Wednesday...High Confidence. Mainly VFR with high pressure. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Through Sunday Night...High Confidence. Low off the Mid-Atlantic / Delmarva lifts NE past the 40N/70W benchmark tonight into the Gulf of ME Sunday and Sunday Night. Will see an increase in areal coverage of rain and reductions in visibility down to roughly 3 to 5 miles starting this evening and continuing through Sunday Night. In addition to the low passage will see NE winds increase with gusts up to 30 kts and waves building to 6 to 8 feet. Low risk of gales so will continue with Small Craft Advisories. Outlook /Monday through Thursday/... Sun night into Mon...High confidence. Winds will diminish Sun night and remain below the 25 kt threshold for small crafts into Monday. However, with low pres just to the east a lingering swell reaching 5-8 ft will last well into Mon, maintaining the rough conditions and need for small crafts. Tue...Moderate Confidence. Winds still remain below 25 kt but a second approaching low pres system will allow for another building swell from the south with 5-6 ft seas possible. Small crafts may need to continue. Wed...High Confidence. High pres briefly builds over the waters, mainly quiet boating weather expected. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 5 AM to 11 PM EDT Sunday for ANZ232>234. Small Craft Advisory from 5 AM Sunday to 8 AM EDT Monday for ANZ231-250-251-254. Small Craft Advisory from 2 AM Sunday to 5 AM EDT Monday for ANZ235-237. Small Craft Advisory from 2 AM Sunday to 8 AM EDT Monday for ANZ255-256. && $$ SYNOPSIS...Doody/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...Doody AVIATION...Doody/Sipprell MARINE...Doody/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.