Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201820 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 220 PM EDT Sat May 20 2017 .SYNOPSIS... High pressure will build across the region this weekend with much cooler conditions. Turning wet Monday ahead of a sweeping cold front then turning dry and seasonable for Tuesday. Shower chances and near seasonable conditions for the remainder of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 155 PM Update... A band of mid and high clouds remains along the S coast of Mass and RI as seen on latest visible satellite imagery. High pressure ridge building S across the region at 17Z. Light pressure gradient along the E coast has allowed sea breezes to develop mainly from Plymouth northward, while NE winds remain in place S of there. Wind gusts up to 20-25 kt continue across the islands and coastal waters E and S of Cape Cod, Nantucket and the Vineyard. Temps along the E coast remain in the mid-upper 50s, but have recovered to the 60s W of there. Readings may rise a few more degrees, possibly touching 70 across portions of the CT valley. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Tonight... High pressure in control across New England will result in good radiational cooling with clear conditions and light winds. It will be a chilly night with mins upper 30s to lower 40s, but some mid 30s possible in colder spots of interior E MA and upper CT Valley. Sunday... High pressure remains in control with lots of sunshine and light winds leading to seabreezes developing along the coast. Expect temperatures similar to today with highs upper 60s to lower 70s, except lower 60s near the coast. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... */ Highlights... - Widespread rain, embedded heavier showers Monday - Dry and seasonable on Tuesday - Shower chances through the mid to late week period with near-seasonable temperatures */ Overview... An upstream pseudo-static wave pattern of trough-ridge-trough that extends from the NW Pacific into the Central and E CONUS. Building persistent ridge over the W CONUS puts the brakes on the upstream progressive pattern, yet amplifies the downstream trough shearing energy equatorward capturing sub-tropical air and drawing it N. So looking at an active weather pattern with a series of disturbances through the broader trough delivering shots of light to moderate, perhaps heavy rain at times, drier periods in-between. Seasonable overall given the exchange of airmasses, the series of sweeping cold fronts keeping any airmass similar to that which deliver the recent heat wave well S. */ Details... Monday... Robust sub-tropical plume of deep-layer moisture with precipitable waters exceeding 1.5 inches. Freezing levels up around 11-12 kft with column saturation below signaling an efficient warm-rain process. Instability is rather weak, but given the +1-2 standard deviation of low-level inflow and precipitable water, think we`ll easily see widespread moderate rain with embedded heavier showers. Can`t rule out a rumble of thunder. Could be some issues for the AM commute. Nothing flooding-wise, simply nuisance, however will have to watch for a possible meso-low focusing the low-level inflow. The near-term guidance is suggesting a localized area of heavy rain in and around the S-coast of New England. Tides are only just starting their march towards high astronomical levels peaking May 27th, so less exacerbation seemingly of poor-drainage issues. Timing still in question, however likely PoPs for Monday into Monday evening. Fog, visibility issues, all in the mix with moderate to potentially heavy rain events like these, though with the quick- movement of the system 60+ degree dewpoints may remain S. Tuesday... Consistently dry the last several forecast model runs. Brief period of high pressure, light winds, looking at sea-breeze potential along the coast. Seasonable conditions with highs upper 60s to low 70s. Wednesday onward... Forecast guidance struggling with individual waves and potential out- comes for the mid to late week period, possibly extending on into the weekend. However it does appear that the broad troughing period is slightly deeper S/E and given the downstream progressive pattern there is less opportunity for slugs of sub-tropical moisture to be drawn into S New England. Chance PoPs. Temperatures hovering around near- to below-seasonable. An active weather pattern at least and there should be periods of dry weather in-between, however nothing is clear, nor is there any confidence, with respect to outcomes and timing of particular features. The spread is far too great. But with an eye towards targets of opportunity and/or threats, nothing stands out. && .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/... Through 00Z...High confidence. VFR. Light/variable winds away from the E coast, with winds ranging from NE to SE mainly in sea breezes. Across Cape Cod, just enough pressure gradient and mixing to bring 20-25 kt gusts. Expect winds to diminish after 21Z. Tonight...High confidence. VFR. Light/variable winds. Clear skies through midnight, then high clouds move into western areas. Sunday...High confidence. VFR. Mid level clouds move in from W-E during the afternoon. Light N-NE winds early, shifting to E-SE up to around 10 kt along the S coast. Sunday night...Moderate confidence. VFR to start, lowering to MVFR from W-E in developing -SHRA, with IFR CIGS/VSBYS possible after 08Z-09Z. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Monday through Wednesday/... Monday into Monday night...Moderate confidence. MVFR-LIFR. Lowest conditions likely for the S-coast with potential fog impacts. Increasing S winds ahead of a line of widespread -RA with embedded RA/+RA. Gusts up to 25 kts. Low risk TSRA. Possible LLWS impacts with S/SW winds 2 kft AGL 40-50 kts. Tuesday into Tuesday night...Moderate confidence. VFR. Clearing. Possible SKC if not SCT-BKN low-end VFR CIGs and possible mid-level CIGs late. NW winds initially turning S. May be light enough to allow onshore sea-breezes. Wednesday into Wednesday night...Low confidence. MVFR-IFR mix possible with -SHRA. Onshore E wind. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. Through 00Z... NE winds gusting to 20-25 kt across the southern waters, strongest E and S of Nantucket and Cape Cod as well as Nantucket Sound. Small crafts continue. Winds should diminish later this afternoon. Seas up to 4 ft. Tonight and Sunday... Quiet boating weather with light winds becoming SE Sunday afternoon. Outlook /Sunday Night through Wednesday/... Going into Monday, S winds on the increase with gusts up to 25 kts ahead of a cold front along with a widespread line of rain with embedded heavier showers beginning Monday morning and dissipating late afternoon into evening. A low risk of thunderstorms. Clearing out after midnight into Tuesday morning though waves build 5 to 6 feet prior and are slow to diminish under NW winds. Winds are light through Tuesday turning S so this should allow seas to subside. Then turning E into Wednesday with seas remaining below 5 feet. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 4 PM EDT this afternoon for ANZ232- 254>256. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Sipprell/EVT SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell/EVT MARINE...KJC/Sipprell/EVT

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