Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 261050 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 650 AM EDT Sat May 26 2018 .SYNOPSIS... High pressure southeast of New England will bring another day of warm temperatures today. A cold front will push south across the region late today into this evening, bringing scattered showers and a few thunderstorms. Much cooler and unsettled conditions will linger Sunday, with improving conditions by Monday. High pressure brings mainly dry and warmer weather Tuesday through Thursday. An approaching weather system brings showers Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 650 am Update... Some thin cirrus overhead, otherwise a sunny morning. Satellite imagery shows thicker clouds in VT-NH-Maine, with more upstream over Northern NY and Ontario. Expect the thicker clouds to move in during the afternoon/evening, but dry weather for much of the day. No significant changes to the forecast. Massachusetts joined in this morning with an Air Quality Alert for areas adjacent to CT and RI and extending across SE Mass. RI and CT continue with their existing AQA. Time is 11 AM to 11 PM. Previous discussion... Skies were mainly partly cloudy across the region at 08Z as seen on latest GOES-East Nighttime Microphysics satellite imagery and on observations. Most of the heavier clouds looked confined across central NH/VT northward as the front sits across far N Maine. Scattered showers and a few thunderstorms seen on the NE regional 88D radar crossing N NY into central and N NH/ME heading to the Gulf of Maine at 0820Z. Will see dry conditions for most areas through the day. Expect to see clouds increase across N Mass by around midday, then will shift S through the afternoon. The majority of the 00Z model suite signaling a slow movement to the front, but should start to approach this afternoon. Current timing suggests that the front should reach NE Mass by around 18Z-20Z, then will push to around the Mass Pike by around 00Z or so. Noting rather good instability associated with the front, with SLIs in the zero to -2 range, TQ values around 20 and K indices in the lower 30s. However, the lapse rates are not that great, on the order of 6 to 6.5C/km, but still expect scattered thunderstorms embedded with the showers as the front approaches, but appears the majority of the activity will arrive tonight. Have carried chance POPs across N central and NE Mass by around 20Z-21Z, with slight chance POPs further S and W. What will help with the instability will be the warm summer like temps, which are expected to reach the mid and upper 80s away from the immediate S coast. A few spots may even touch 90. As the front crosses NE Mass, temps will start to fall quickly by evening as winds shift to E-NE && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Tonight... The cold front continues to slowly push southward across the remainder of the region, then will start to stall near or just off the S coast by around 12Z Sunday. Winds will shift to E-NE and temps will fall quickly with the surge of colder air down the coast behind the front. Still some instability lingering, so can not rule out a few thunderstorms near and ahead of the front as it passes. Another impulse moves along the passing front as it reaches the S coast through around 03Z or so, so will see a better shot for showers and a few thunderstorms there. Have carried high CHC POPs there, along with the thunder. As the colder air moves in, will see the convection diminish, but will can not rule out an isolated t-storm through the night across N CT/RI/SE Mass. Temps will fall back to the lower-mid 50s. Sunday... With the front nearly stationary S of the region, will see continued chance for showers. Still can`t rule out a rumble of thunder along S coastal areas through midday as marginal instability lingers. Will see gusty E-NE winds moving in across E coastal areas with gusts up to 20-25 kt, highest across outer Cape Cod and Nantucket. Temps will be coolest across eastern Mass with highs only in the mid-upper 50s, ranging to the lower 60s across the CT valley, where the precip will slowly taper off around midday or early afternoon. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Big Picture... Today`s dramatis coelus...weather of the drama?...features a zonal upper flow along the northern tier of the USA, a closed low over the Western USA, another closed low along the Alaska coast, closed low over the Gulf of Mexico held in place by ridges centered over Mexico and the West Indies. One shortwave in the zonal flow moves east of New England Monday, allowing some drying over our area. A sharper shortwave in the zonal flow moves across Eastern Canada Tuesday, with model consensus showing a windshift over our area as the shortwave moves past. The closed Western low gets kicked out of position as the Alaska low moves down the coast and ashore. The Western low then ejects into the zonal flow, draws the Gulf of Mexico low north and merges, then moves through New England at the end of the week. Model solutions show above normal contour heights through the forecast period, and thermal fields also indicate above normal temperatures. Model solutions remain similar through Thursday morning, then show a similar broad pattern with differences in detail. This gives us high confidence in the forecast through midweek and moderate confidence late week. Concerns... Sunday night-Monday... Wave along the stalled cold front moves out to sea while ridgeline from the Maritimes holds over Southern New England. Cross sections show plenty of moisture below 700 mb but shallow and diminishing lift. The positioning of the ridge line should maintain east- northeast winds through Monday maintaining a moist low-level flow. Will continue to indicate chance pops Sunday night, then mostly cloudy skies Monday. Tuesday through Thursday... Second, sharper shortwave passes to our north on Tuesday. This brings a windshift from westerly to northerly, but lots of dry air in place at the time. High pressure surface and aloft brings subsidence, and dry air lingers through Thursday. Expect mostly clear skies and light wind. Light flow should support sea breezes on Wednesday. Thursday night and Friday... Moisture fields increase at the cirrus level Thursday, then closer to the surface Thursday evening and night. Timing of onset of showers is dependent on the ejecting shortwave and its interactions with the Gulf of Mexico low. Current info suggests showers starting sometime Thursday night and continuing into Friday. && .AVIATION /12Z 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. Today... Mainly VFR conditions through most of the day. Clouds lower across N Mass to near the Mass Pike during the afternoon. ISOLD SHRA/TSRA after 19Z-20Z mainly from the Mass Pike northward, best chance across NE Mass. W-SW winds 5-10 kt except up to 15 kt along S coast. Wind shift to E-NE after 21Z across NE Mass. Tonight... Mainly VFR through 05Z-06Z, then CIGS lower to areas of IFR from NE-SW. Patchy LIFR CIGS across higher terrain and along S coast toward daybreak. VSBYS lower to MVFR-IFR across E Mass/RI in areas of fog. Winds shift to E-NE from N-S overnight with gusts to around 20 kt along E coastal areas after 08Z. Scattered SHRA, chance TSRA. Sunday... MVFR-IFR CIGS linger through most of the day. Scattered SHRA through the day. Isolated TSRA mainly S of the Mass Pike by midday, then offshore late in the day. VSBYS improve to VFR N of Mass Pike around midday, but MVFR-IFR in patchy fog along S coast through the day. KBOS Terminal...High confidence in TAF, except moderate confidence in timing of wind shift today. SW wind around 10 kt. Back door cold front moves across around 19Z-21Z with a wind shift from W to E-NE. Wind gusts to around 20 kt after 03Z tonight. KBDL Terminal...High confidence in TAF. Light S-SW winds become W 5-10 kt by midday. Back door cold front should hold off until around 00Z-02Z when wind shifts to NE. Outlook /Sunday Night through Wednesday/...Moderate to High confidence. Sunday Night: Mainly IFR, with local MVFR possible. Breezy. Areas BR, slight chance SHRA, isolated TSRA. Memorial Day: Mainly VFR, with local IFR possible in the morning. Monday Night: Mainly VFR, with local IFR possible. Patchy BR. Tuesday: VFR. Breezy. Tuesday Night through Wednesday: VFR. && .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. Today... Small Crafts will end by mid morning across BI/RI sounds, otherwise will continue for the southern outer waters through the day. SW winds diminish ahead of the approaching back door cold front, which should push across the eastern waters during the afternoon, then approaching Cape Cod by 00Z. Will see chance of showers/isolated thunderstorms move across the eastern waters after 18Z, with a wind shift to E-NE as the front passes with gusts up to 20 kt. Tonight... Winds shift to E-NE across the southern waters this evening. Will see winds gusting up to around 25 kt across the eastern waters to E of Cape Cod overnight. Gusts up to 25-30 kt and seas build to 3-5 ft across the eastern outer waters toward daybreak, where Small Crafts have been issued. Scattered showers/isolated thunderstorms linger through the night. Areas of fog with reduced visibility. Sunday... E-NE winds increase across all waters as the cold front shifts S. Gusts up to 25-30 kt, highest on the eastern open waters. Seas build up to 5-8 ft on the eastern waters as well. Small crafts will be needed on the southern waters during the day. Showers continue, with best chance along the southern waters. Reduced visibility in areas of fog. Outlook /Sunday Night through Wednesday/...Moderate confidence. Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain showers, areas fog, slight chance of thunderstorms. Visibility 1 to 3 nm. Memorial Day: Winds less than 25 kt. Seas up to 5 ft. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday Night: Winds less than 25 kt. Local visibility 1 to 3 nm. Wednesday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for CTZ002>004. MA...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for MAZ011>013-016>021. RI...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory until 8 AM EDT this morning for ANZ232>234. Small Craft Advisory until 10 AM EDT this morning for ANZ235- 237. Small Craft Advisory from midnight tonight to 6 PM EDT Sunday for ANZ250. Small Craft Advisory from 1 AM to 6 PM EDT Sunday for ANZ251. Small Craft Advisory until 6 PM EDT Sunday for ANZ254. Small Craft Advisory until 11 PM EDT this evening for ANZ255- 256. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/EVT SHORT TERM...EVT LONG TERM...WTB AVIATION...WTB/EVT MARINE...WTB/EVT

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