Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 250319 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1119 PM EDT Mon Oct 24 2016 .SYNOPSIS... Blustery west to northwest winds beneath a cool, dry airmass will continue through midweek. Some of the coldest air of the season impacts the region Wednesday into Thursday. Quite cold Thursday morning, this prior to a strong storm system into Friday which may bring a period of wintry weather prior to moderate to heavy rain along with gusty winds. Another shot of wet weather late Saturday into Saturday night, then possibly again early the following week. Overall an active, cooler than average weather pattern is advertised. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 1050 PM Update... large low pressure across the Canadian Maritimes continues to spin cold air across the region at 02Z, as well as bands of low and mid level clouds across northern and central Mass. Dewpts have lowered to the upper 20s to mid 30s across the region, a bit lower than previously forecast. Downsloping winds have caused the clouds to dissipate for the most part across central and southern areas this evening. Will still see some bands of mid clouds cross the region in the W-NW flow at the surface and aloft, otherwise expect partly cloudy skies on average overnight. With continued W-NW winds, even as temps fall back through the 30s across most areas, except 40-45 across outer Cape Cod and the islands. Frost will be limited as winds will continue to stir though not quite as strong as this afternoon. Will see gusts up to around 20 kt mainly across the higher terrain and along the immediate coast, though should diminish somewhat overnight. Have updated near term forecast to bring conditions current. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Tuesday... Expecting another day of gusty west to northwest winds to develop by mid morning. Some diurnal clouds develop, too. More clouds should be along the coast, and over the ocean, where the humidity is more favorable. High temperatures should be about 10 degrees below normal. Tuesday Night... Anticipating the start of a rather cool, if not cold, period. Northwest winds should be still be blowing, so not thinking frost will be the real concern. Instead, looking more at freezing to subfreezing temperatures across portions of southern New England. Will hoist a freeze watch where there is moderate confidence for a potential freeze, and the growing season has not already ended. Farther inland, we would expected those same conditions, even though no watch will be issued. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... */ Highlights... - Coldest temperatures of the season Thursday morning possible - Winter-weather threats possible with onset of storm Thursday - Transitioning to flooding threats with heavy rain into Friday - Another shot of wet weather into the weekend, maybe early week - Overall pattern of cooler-than-average temperatures */ Overview... Looking at the bigger picture. Last week saw impressive warmth as a fair number of teleconnections were in a near-neutral state. The H5 pattern relatively flat with some slight ridging gave us a period of above-average temperatures and record warmth. That has all changed. A cooler than average, active weather pattern lies ahead. Roughly 3 features worth noting: H5 lows over the Bering Straight, NE Pacific, and the N Canadian Maritimes. Round these more stable features over the N Hemisphere, energy is promoting both an amplified and somewhat buckled pattern. In particular with respect to the NE CONUS cyclonic flow prevails round the base of which individual impulses undergoing amplification are promoting deepening areas of surface low pressure over the forecast area. With the Madden-Julian Oscillation forecast into phase 8 as the AO/NAO/EPO phases have shifted negative, there are several signals indicating that the Arctic door is open for business allowing colder air to spill S and overall temperatures into early November to be below-average. As the previous forecaster mentioned, the Labrador trough downstream of which there is a combination of favorable ridging and weak blocking. */ Discussion... Bringing it home. Initially a shot of some of the coldest air of the season. H85 temperatures around -8C by Wednesday morning. Very steep lapse rates, looking at temperatures well-below average with breezy NW flow. Luckily abundant sunshine but still it is going to feel cold. Clouds on the increase into Thursday morning, it becomes a question as to whether radiational cooling proceeds which could knock out a fair number of forecast areas still observing a growing season. A majority of the overnight period Wednesday night into Thursday morning with light winds and clear conditions, will go with lows down into the 20s for many locations, but again, on the cusp of that all changing is if mid to high clouds move in faster. Thereafter, looking at 3 shots of disturbed weather: Thursday into Friday, late Saturday into Saturday night, and possibly again early next week. Focusing on the first two: Thursday into Friday... Storm characteristics similar to a Miller-B undergoing rapid cyclo- genesis beneath a deepening H5 closed low. Strong frontogenetical / isentropic lift ahead of the low with accompanying strong mid to upper level forcing and favorable venting, along with a measure of instability. Looking at a good forward-thump of rain exacerbated by strong low to mid level moisture convergence in response to the deep- layer forcing. Thunder possible given instability with higher thetaE slipping in aloft within a moist-adiabatic / conditionally unstable atmospheric profile. With onset, cold air damming, even drier air, may not only delay the storm but also invite some initial mixed wintry precipitation. The details will be ironed out with time but agree with the previous forecaster: One can get burned by the isallobaric component of the wind when out of the N. Promoting less veering of winds within the interior valleys results in colder air holding in longer rather than scouring out. Warmer air proceeds above a shallow inversion of cold air trapped in a valley, concerns grow as freezing rain is possible. It will be very cold Wednesday and dewpoints are low. We`ll have a better idea as more high-res forecast guidance becomes available to capture such small-scale processes. Timing difficult given fly in the ointment of drier air. Would expect with the fast movement of the low center that shallow cold air should scour out, especially as the low sweeps across the interior valleys, as presently forecast. Expect threats to transition towards moderate to heavy rain with time and given leaf-clogged drains and flooding that just occurred over central portions of New England, there are underlying flooding concerns. Potential for faster momentum to mix down to the surface with heavy rain, gusty S winds. As the storm lifts out and occludes, trowaling rearward and wet-bulb-zero values falling to at or below freezing, likely contending with wet-snow again for the high terrain mainly N/W and on Friday. Multiple headlines may be needed but too early to say exactly what and when just yet. Will go with a mention in the hazardous weather outlook for the potential of urban, poor drainage flooding. A quick shot of breezy winds and cooler air immediately behind the system dampening with mid-level ridge building across the region Friday night into early Saturday. Late Saturday into Saturday night... A lull prior to a quick follow-up clipper-like disturbance. Moderate confidence given recent swings in forecast guidance. Another shot of wet weather is possible along with the possibility of some wet snow for the higher terrain given wet-bulb-zero heights. Front-end thump with a majority of the isentropic / frontogenetic forcing collocated ahead of the accompanying sweeping cold front to the surface low. But so far out in the forecast, nothing yet sampled, wanting to hold off on any further details. Behind this system it would appear given the broader pattern that another shot of blustery winds will usher colder and drier air S. Early next week... Still a lot of uncertainty. Either a continental polar airmass with an accompanying cold front drops S (per EC) or a warm front lifts N across the region (per GFS). Continued low confidence but err to a cooler than average, active pattern given teleconnections. && .AVIATION /03Z TUESDAY 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/... 11 PM update... VFR. Mainly FEW clouds at 5-7kft, except SCT-BKN at times across NE Mass and outer Cape Cod. W-NW winds gusting up to 25 kts across higher terrain and coastal terminals, gradually diminishing overnight through Wednesday morning. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Wednesday through Saturday/...Moderate confidence. Wednesday... VFR. Breezy NW winds diminishing and becoming N into evening. Gusts up to 25 kts during the early half of Wednesday. Thursday into Friday... Lowering and thickening cigs down to IFR-LIFR centered around Thursday night. Initial SN/IP possible with potential accompanying LLWS threats transitioning to -RA/RA with embedded +RA, possible TSRA mostly S/SE. Blustery S/SE winds. Gusts up to 30 kts along the immediate shoreline terminals. Improving late Friday with winds turning out of the W. Saturday... Brief period of VFR before conditions return low-end VFR to MVFR with another chance of -RA/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/...High confidence. 11 PM update... Noted S-SE swells up to 6-8 ft across the outer waters, highest from E of Cape Cod to the southern waters. Gusts up to around 30 kt at buoys 44017 and 44018 at 02Z. NW winds will continue to gust up to 25-30 kt through Tuesday, then gradually diminish Tuesday night. Seas will remain up to 5-7 ft, occasionally up to 8 ft on the outer waters. Small craft advisories remain in effect. Outlook /Wednesday through Saturday/...Moderate confidence. Wednesday... Seas remaining rough with persistent NW winds becoming N with time and dissipating. Wave heights gradually diminishing below 5 feet towards the later half of the day. Thursday into Friday... Rain on the increase through Thursday, moderate to heavy at times with the possibility of thunderstorms. S/SE winds strengthening ahead of a deepening area of low pressure across the region. Seas building 8 to 10 feet the height of which will be on Friday, slowly dropping off thereafter as winds turn out of the W behind a cold front but remain breezy. Saturday... Breezy W winds continue as there is a brief lull in the weather. Seas remain above 5 feet on a good chunk of the waters as winds increase once again along with wet-weather late Saturday into Saturday night. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Freeze Watch from late Tuesday night through Wednesday morning for CTZ003. MA...Freeze Watch from late Tuesday night through Wednesday morning for MAZ006-007-013-014-018. RI...Freeze Watch from late Tuesday night through Wednesday morning for RIZ001-003-006. MARINE...Small Craft Advisory until 2 AM EDT Wednesday for ANZ230>232- 235-237-251. Small Craft Advisory until 8 PM EDT Tuesday for ANZ233-234-236. Small Craft Advisory until 6 AM EDT Wednesday for ANZ250- 254>256. && $$ SYNOPSIS...Belk/Sipprell NEAR TERM...EVT SHORT TERM...Belk LONG TERM...Sipprell AVIATION...Belk/Sipprell/EVT MARINE...Belk/Sipprell/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.