Area Forecast Discussion
Issued by NWS Boston, MA

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585 FXUS61 KBOX 241930 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 330 PM EDT Fri Mar 24 2017 .SYNOPSIS... High pressure moves farther offshore tonight bringing a southwest flow of milder air to Southern New England. A cold front will push south across the region Saturday. This front will stall Saturday night, then move northward on Sunday. The resulting battle of airmasses could bring a period of light rain Saturday, then more wintry precipitation Sunday night. Heavy precipitation is possible Monday as low pressure system moves over southern New England. Rain Tuesday ahead of a cold front behind which colder air lags Wednesday into Thursday with breezy NW winds. Another chance for disturbed weather Friday into the weekend given the continued active weather pattern. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Warm front and its precipitation are moving off to the northeast. Southwest winds about 2000-3000 feet aloft reached 50 knots during the afternoon and will slowly diminish tonight. Even so, that will bring potential for gusts to 30 knots through the evening. Upper jet passes north of New England overnight, carrying a wave of low pressure across Quebec. This will in turn move the front south as a cold front after midnight. Low level convergence may generate light showers that reach Northern Mass by morning, but otherwise expect little/no precipitation in Southern New England tonight due to southwest flow and no low level forcing. The mild flow of air will keep temperatures from falling much. There may be a slight cooling this evening, but overall trend through the night should be steady or slowly rising. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Saturday... Cold front sweeps across the region first thing in the morning. High pressure building in behind the front will turn winds from the north Saturday morning and from the northeast in the afternoon. The upper jet referenced earlier will be to our northeast Saturday with our area under the right entrance region. This may maintain upper venting over/near the front while precipitable water values along/south of the front remain at 0.75 to 1.0 inches, which is well above normal for mid-late March...the 90 pct exceed value is about 0.86. This should be enough to support a chance of light rain, along and a little behind the front. This rain shifts south of New England toward evening. With clouds and potential light rain, expect temps to climb just a little, with max values in the 40s. Saturday night... High pressure from Canada builds south into New England and brings colder air. Expect dry weather most of the night. Dew points will not fall as much as with most highs because our low level flow will be out of the east-northeast, more of a marine flow. Expect these dew points to be in the mid 20s to around 30. As such, we expect min temps to be in the upper 20s to mid 30s. With the ridge building in as it shifts toward the Maritimes, the models show a developing cold air damming signature Saturday night. This suggests the low level cold air will become establish in advance of the next weather system. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Highlights... * A period of wintry precip is possible Sunday night * Unsettled weather pattern will continue into Wednesday * Improving conditions during the later half of next week Pattern Overview... 00z models and ensembles are in agreement with the general synoptic pattern for the extended. Still some timing and thermal issues but overall confidence is improving. Active pattern for the region with split flow aloft and several shortwaves ejecting in from the Pacific. First wave is a 500mb closed low over the Midwest on Sunday. This low deamplifies into an open wave as it approaches the Northeast Monday. Due to the confluent flow aloft, high pressure persists over northern New England keeping surface temperatures cool, resulting in mixed precip at times for the weekend into early next week. Heavier precip possible on Monday as first open wave moves overhead. A second wave will follow a similar path from the Plains towards the Northeast before meshing or partially interacting with the northern jet stream on Tuesday. This will result in another system for New England Tues/Wed with high pressure to follow. Details... Sunday...Moderate confidence. For Sunday, cold front will stall near the Mid-Atlantic as high pressure moves southeast into the Gulf of Maine. Guidance indicates that low levels will dry out Sunday morning keeping the first half of the day dry. However, with mid-level ridge axis overhead will see and increase in moisture thanks to southwesterly flow aloft. At the surface easterly flow will begin to dominate increasing the low levels throughout the day. This could result in low clouds and drizzle. Surface temps will warm above freezing thanks to mid-level ridge, but some areas may struggle as cold air may be slow to dislodge/cold air damming. Still a lot of uncertainty with this time frame. Sunday night into Monday...Moderate confidence. Stalled front south of the region looks to return back north as a warm front late Sunday into Monday, as surface low pressure moves into the Great Lakes. Although warmer air aloft should move easily back into the region, the low-level cold air may be a little tougher to dislodge this time due to the nearby surface high pressure. Believe this is the timeframe where we will see the heaviest precip due to passage of the open wave and development of a secondary low over the region. Still some uncertainty with the thermal profiles but cross-sections, soundings and even CIPS analogs show this timeframe has the potential for icing especially north of the Pike. Still some uncertainty with this timeframe as a difference of a degree or two in the thermal profiles will change the p-type. Will continue the mentioning of snow, sleet, ice and/or rain. Winter weather advisories may be needed. Tuesday into Wednesday... Continued risk of freezing rain early on, if not at a minimum a cold rain is forecast. Associated deeper mid to upper level low over the S Hudson Bay region with a cascading, deamplifying open wave trough through the S-stream yields an elongated area of low pressure across the E CONUS with secondary low development across the Mid-Atlantic. Initial over-running moist ascent transitioning to frontogenetical, synoptic forcing ahead of a sweeping cold front. However the colder air lags. So altogether, PoP chances through Tuesday night, clearing into Wednesday. A cool, damp day especially for N/NE MA initially, gradually warming, with a decent slug of rain. The cold air lagging along with clearing Wednesday, may end up mild as conditions dry. Thursday... High pressure in control however the crux of the colder airmass is drawn S to the E of New England as the early to mid week low over the Hudson Bay region of Canada undergoes deeper cyclogenesis across the N Atlantic. Given a mix of sun and clouds, and that being a late March sun, leaning mild with more seasonable conditions. Cold air advection proceeding in the low levels along with an enhancement of flow aloft, expect some gusty NW winds the the boundary layer mixing up to around H85. Friday into the weekend... Confluence within the mid to upper levels of N/S streams across the NE CONUS continues. Any closed lows emerging out of the SW CONUS undergo deamplification. Scenario for continued over-running setups to which the thermal profiles across the region require scrutiny. Conflicting signals of low pressure evolution associated with weak wave impulses neighbored against high pressure out of Canada. A lot can change and thus a low confidence forecast. Only confidence is that we`ll continue to see an active weather pattern into the long term period. && .AVIATION /20Z FRIDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...Moderate confidence. Tonight... Mainly VFR but with possible MVFR late at night in northern MA. Southwest winds will gust 25-30 knots early, then diminish overnight. Saturday... A cold front moves north to south across the region during the morning. An area of light rain may develop, with cigs and vsbys lowering to MVFR. Winds shift from the north in the morning and northeast during afternoon/evening. Saturday night... Mainly VFR as high pressure builds from the north. MVFR will lurk to our southwest, preparing to move north again. East flow through the night. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Sunday through Wednesday/... Saturday night...High confidence. Northerly winds with spotty precip overnight. Terminals north of the Mass Pike have the best chance for wintry precip. Sunday into Monday...Moderate confidence. Showery weather to start with possible MVFR to IFR cigs. Precip chances increase into Monday which could result in mixed precip for terminals north of the Mass Pike. Tuesday... MVFR-IFR mix with -RA/RA, low risk FZRA early over N/NE MA. N/E winds turning out of the N/W. Wednesday... Improving. Cigs lifting VFR. N/W winds continue. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... Tonight...High confidence. Southwest winds gusting 25-30 knots early will diminish overnight, and become west toward morning. Rough seas linger across the outer coastal waters, and ocean-exposed south coastal sounds. Gales early on the eastern outer waters. Small Craft Advisories will lower on Boston Harbor and Narragansett Bay, but linger on the remaining waters. Saturday...High confidence. West winds shift from the north in the morning and early afternoon as a cold front moves north to south. Winds then shift from the northeast by afternoon as high pressure moves in from Quebec. Winds will be less than 25 knots through the day. Seas will linger at 5 feet on the southern outer waters and parts of RI Sound much of the day. Small Craft Advisory will be needed where seas linger near 5 feet. Saturday night...Moderate-High confidence. Northeast winds turn from due east overnight. Speeds remain below 20 knots through the night. Seas will remain less than 5 feet through the night. Outlook /Sunday through Wednesday/... Sunday...Moderate confidence. Cold front will be south of the waters as surface high pressure moves northeast of the region. Conditions should remain below SCA but if the front gets closer than seas and winds may be a bit stronger. Monday...Moderate confidence. Approaching upper level system and passing warm front will increase precip as well as wind and seas of the waters. SCA may be needed. Tuesday through Wednesday Rain continuing across the waters with E winds. Could see some visibility restrictions ahead of a cold front sweeping the region late into Wednesday morning. Behind the front winds become more westerly and could see some gusts. Waves over 5 feet possible on the outer waters. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 6 AM EDT Saturday for ANZ232>235- 237. Small Craft Advisory until 8 PM EDT this evening for ANZ230. Small Craft Advisory until midnight EDT tonight for ANZ231-251. Small Craft Advisory until 10 PM EDT this evening for ANZ236. Gale Warning until 10 PM EDT this evening for ANZ250-254. Small Craft Advisory until 8 PM EDT Saturday for ANZ255-256. && $$ SYNOPSIS...WTB/Dunten/Sipprell NEAR TERM...WTB/Dunten SHORT TERM...WTB LONG TERM...Dunten/Sipprell AVIATION...WTB/Dunten/Sipprell MARINE...WTB/Dunten/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.