Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201548 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1148 AM 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/... 935 AM Update... Winds shifting to E-NE as high pressure ridge builds S out of Quebec at 13Z. Light pressure gradient in place, so expect winds at 5-10 kt, though a bit higher along the immediate E coast through midday with some sea breeze component working in as temps rise to the upper 50s to around 60 through midday. Visible satellite trends showing northern edge of cloud deck shifting steadily S, so expect the clouds to push off the S coast by early afternoon. Have updated near term conditions to bring current and incorporated into afternoon trends. Previous Discussion... H925 temperatures much cooler ranging from 8-12C E to W. Sea breezes will develop along the E MA coast where coolest temperatures today will be in the lower 60s. Otherwise, highs should reach upper 60s to lower 70s in the lower CT Valley, roughly 20-30 degrees cooler than Friday. && .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 /16Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. 15Z update... Through this afternoon... VFR. N-NE winds interior with E-NE winds along the coast. Gusts up to around 25 kts from KBOS south to Cape Cod and the islands through 18Z-19Z, then diminishing. Tonight and Sunday... VFR. Light winds tonight, then sea breezes developing around midday Sunday. KBOS TAF...High confidence. KBDL TAF...High confidence. Outlook /Sunday Night through Wednesday/... Sunday night...Moderate confidence. S winds into Sunday as CIGs lower and thicken. A mix of MVFR-IFR into Monday morning W to E with onset of -RA/RA. 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. 1135 AM update... E-NE winds have kicked up again across Nantucket Sound as well as along the near shore waters of Cape Cod due to brief increase of low level jet as it crosses waters. Have resent Small Craft for Nantucket Sound through 4 PM, and also extended Small Craft for ANZ254-ANZ255 for same timeframe. Through this afternoon... E-NE winds gusting to 25 kt over the southern outer waters then will diminish by mid to late afternoon. Otherwise winds below small craft. 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 is the U.S. government's official web portal to all federal, state and local government web resources and services.