Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 260759 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 359 AM EDT Sat May 26 2018 .SYNOPSIS... High pressure centered south and east of New England will allow for mild temperatures overnight, and another day of warmth for much of Saturday. A cold front drops south across the region Saturday afternoon and evening, bringing scattered showers and a few thunderstorms. This is followed by much cooler and unsettled weather Sunday with some improvement by Monday. High pressure brings mainly dry and warmer weather Tuesday through Thursday. An approaching weather system brings showers Friday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 155 AM Update... Mid level clouds starting to move across central and N Mass as seen on latest observations as well as GOES-East Nighttime Microphysics Composite satellite imagery. Bands of showers remains across N NY into N VT/NH into western ME well ahead of approaching cold front near the N ME/Canadian border. Did note a few showers moving across S VT around 0430Z, but dissipated as they moved into the drier air across S NH into N central Mass with T/Td spreads from 8 degrees at KEEN to 12 degrees at KAFN at 05Z. Expect mid level clouds to continue to slowly shift S through the remainder of the night. Will see dry conditions continue as the leading edge of the precip will remain well N. Temps running a few degs warmer than forecast especially across N central and NE Mass where SCT-BKN clouds have moved across. Otherwise, readings are pretty much on track at 05Z. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/... Expecting a nearly zonal mid level flow over the USA/Canadian border into early next week, with a few small amplitude waves migrating through the longwave pattern. Of greater significance would be a strong mid level ridge off the SE USA coast near the Bahamas. This ridge should maintain an above normal stream of moisture with tropical origins across the eastern USA. The question for southern New England is whether any northern stream disturbances during the coming week can alter the flow pattern enough to pull this moisture farther north into our area. Model solutions are similar enough to favor a consensus approach through this holiday weekend to smooth over the less predictable details. More significant detail differences for the rest of next week, mainly due to timing issues. Have higher confidence in the forecast through Wednesday, than the latter half of next week. Details... High pressure building over the Maritimes brings increasing east winds and a cool moist marine flow to Southern New England. Gusty winds along the east coast, too. At least a chance of showers Sunday, with a more early spring-like raw feel. Greatest risk for showers toward the eastern half of southern New England. High pressure moves east of the Maritimes on Monday. Surface dewpoints start to slowly rise, and clouds should gradually diminish. Looking mainly dry through Wednesday, as another high pressure moves SE across our region from central Canada. This high pressure should be offshore by Thursday, leading to an increasing risk for showers late Thursday into Friday. Near to above normal temperatures expected Monday through Friday.Saturday... There is some uncertainty in exact timing of the back door cold front, which will cross thru the area sometime Sat afternoon and evening, aided by a short wave pushing thru the area. 12Z NAM has slowed down the timing of fropa, holding off until towards 00Z for NE MA. However the 12Z GFS shows the front pushing thru a large portion of southern New England between 18Z and 00Z, and hi-res guidance leans towards the GFS solution. As a result have leaned towards the faster solutions for this forecast package. Temps will tumble behind the front. Anticipate mostly sunny skies in the morning, humid with dew points in the upper 50s to low 60s. Should have enough time prior to fropa for temps to climb into the mid and upper 80s for much of the region, even some highs around 90 possible in the lower CT River Valley. Exception will be across south coastal MA/RI including the Cape/Islands, where SW flow will bring marine influenced air. Then increasing clouds as the back door cold front approaches. Scattered afternoon/evening showers are expected, with a few thunderstorms possible. Low level lapse rates are steep south of the front, but model soundings suggest warm air in the mid levels that hinder the extent of instability. Precipitable water peaks out around 1.25 inches, could see brief locally heavy rainfall. Winds aloft are weak but dry air intrusion in the mid levels could allow for some gusty winds with any thunderstorm. SPC has our area highlighted for general thunder. Saturday night... The cold front progresses thru the remainder of southern New England Sat night, bringing a continued chance for showers and isolated thunderstorms. Wind shift to the N/NE allows cooler marine- influenced airmass to continue pressing for much of the area thru the area, having a noticeably cooler feel. Models show potential for gusts around 20 kts along a portion of the east coastal shoreline. Expect overnight lows for much of the area to dip into the low to mid 50s except possibly parts of SW MA and northern CT where lows in the upper 50s to around 60 are probable, farther away form the ocean influence. && .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-Friday... Moisture fields increase at the cirrus level Thursday, then closer to the surface Thursday evening and night. Timing of onset of showers is dependant 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 /08Z 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. Through 12Z... VFR. SW winds diminish across the CT valley to S around 5 kt or light/variable, and SW 5-10 kt elsewhere, except up to 15 kt on the outer Cape and Nantucket where gusts to around 20 kt possible through 10Z. 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. Overnight... Have re-issued Small Craft Advisories for Nantucket Sound, Buzzards Bay and Vineyard Sound through the remainder of the night. SW winds continue to gust up to 25-30 kt with seas up to 3-5 ft from the late evening ferries as they crossed. Have extended it through the night, and may need to extend into Saturday ahead of the front. Will revisit for the early morning update. Remainder of forecast in good shape. Saturday... Small Craft Advisory continues into the morning for RI/BI Sounds and into the afternoon for the southern/eastern outer coastal waters. Increasing chances of late day showers and thunderstorms ahead of a NE wind shift associated with a back door cold front. Gusty winds immediately behind the front may approach 25 kts, which may produce SCA conditions over the northeastern coastal waters. Saturday Night... NE winds (for southern coastal waters SW winds becoming NE during the evening), increasing with gusts to 25 kt possible with seas 2 to 4 feet. SCA headlines may be needed. Chance for showers and isolated thunderstorms. 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...None. 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/NMB NEAR TERM...EVT SHORT TERM...NMB LONG TERM...WTB AVIATION...WTB/EVT MARINE...WTB/EVT/NMB

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