Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 232010 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 410 PM EDT Tue May 23 2017 .SYNOPSIS... Dry conditions prevail this evening. Another wave of low pressure tracks south of New England late tonight into early Wed, bringing a chance for rainfall to the south coast. This will be followed by dry and seasonable conditions Wed afternoon and night. Low pressure brings showers and cooler temperatures Thursday and Friday. Weak high pressure will bring a dry start to the holiday weekend Saturday. Another coastal low brings late Sunday into early Monday. Leftover showers are possible Monday and Tuesday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... Tonight...Clouds today were limited mainly to a cirrus cloud deck that allowed filtered sunshine at times. Surface high pressure was in control. During tonight, a digging shortwave across the Midwest pushes a weak disturbance over the Mid-Atlantic Region. In response to this, an area of low pressure develops this evening on the mid Atlantic coastline, along a stalled frontal boundary. This low traverses northeastward tonight. Clouds are expected to thicken and lower as the night progresses, in advance of this low pressure system. ECMWF/NAM/GFS/GFS ensemble mean are fairly similar on the track and forward speed of this low, with the low center arriving southwest of 40N/70W for 12Z Wed. This system looks to offer a chance for rain at least across southern/southeastern portions of our area late tonight. The remainder of southern New England, while cloudy late tonight, may stay dry thru daybreak. Have chance pops extending roughly from IJD-PVD-TAN late tonight, and high chance to likely pops for Cape and Islands. Where it does rain, QPF would be limited to about 0.10 inch or less prior to 12Z. With prevailing winds becoming light from the east, some low clouds and patchy fog may develop in portions of the coastal plain tonight. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday... Low pressure tracks further northeastward during Wednesday, with the low center likely passing near 40N/70W in the morning. Precip chances are at their highest early in the morning, with chance pops IJD-SFZ-near OWD and southeast, then high chance to likely pops Cape and Islands. Where it does rain, QPF should be limited to around 0.25 inch or less. Otherwise, chances for rain diminish for late morning, with a dry afternoon, as the low moves further offshore and away from our region. Northeast winds will gust during the morning hours across the Cape and Islands due to tight pressure gradient from building high pressure to our west, and passing coastal low to the east. Gusts will be near 15 to 25 MPH. As a surface high pressure builds into the area, should see increasing sunshine, partly sunny skies. The east/northeast onshore winds will limit high temps to some extent across the coastal plain. In addition there is a chance of sea breeze development across south coastal MA/RI. Forecasting high temps in the mid 60s to mid 70s, except lower highs in the upper 50s to mid 60s along east and south coastal MA/coastal RI. Wednesday Night... Increasing clouds expected in anticipation of another low pressure system to affect our region. The low emerges from the vicinity of Virginia and North Carolina, starting to make its way to the Mid Atlantic Coastline during the overnight. 12Z model runs look to provide a slower approach of this low, so Wed night should be predominantly dry. However as we advance towards daybreak Thu, have slight chance pops for northern CT/southern RI. In addition, could see some low clouds and patchy fog within the coastal plain as a light east wind continues. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Big Picture... The longwave scale retains cyclonic flow over the Eastern USA much of the period while a ridge builds over the Western USA. Deep trough over the Eastern USA evolves into a closed low that sweeps across New England Friday. A weak ridge brings drier weather Saturday, then a shortwave races through on Sunday. A northern stream closed low over Canada drops south over the Great Lakes, then guides a southern stream shortwave across New England early in the week. Upper mass fields are similar through Saturday. The fields thereafter show similar projections but with different timing through the early week. We used a blend of model data for the extended range grids with low-moderate confidence for the Sunday night-Monday system. Details... Thursday-Friday... Shortwave coming around the upper low will generate a coastal low near Chesapeake Bay Thursday. That low will then move up the coast Thursday night and cross our area Friday. The 12Z models show a 40- 50 knot south jet between 850 and 1000 mb moving up the coast Thursday. This shifts well offshore over Thursday night as the upper shortwave kicks east to the coast. Meanwhile a 20-30 knot east jet at the surface will feed surface level moisture into our area. The presence of these low level jets suggest a good inflow of moisture into our area, while the shortwave and upper jet will provide sufficient dynamics for lift. Two questions in the data are the jet shifting well offshore as it does and a mid-level dry zone overhead Thursday afternoon which fills in Thursday night. Either of these could interfere with the expected rainfall. For now, we will slow the onset of rain into Thursday morning...which may still be too soon if that dry layer has anything to say about it...with the major portion of rain taking place Thursday night and Friday morning. The coastal low and upper low move past Southern New England Friday morning. This will bring winds around out of the north-northwest and cut off the inflow of moisture. Expect the leading precip to taper off, but with leftover showers through the day Friday. Low level flow will be from the east Thursday, coming over water temps in the 50s. Thursday max temps in eastern Mass will be buffered by this water with max values upper 50s and lower 60s. Farther inland, low level temps suggest a max of 60-65. With dew points in the upper 40s to mid 50s and rain, expect min temps to be close to dew point. Temps aloft on Friday suggest max temps around 60 or low 60s in the east and mid to upper 60s in the CT Valley. Saturday... High pressure brings a day of rest from the wet weather. Temps of 8C to 9C at 850 mb suggests max temps in the low to mid 70s. If the surface flow is light enough, a sea breeze may occur along the east Mass coast and hold max temps there in the 60s. Sunday through Tuesday... The models show a dance of sorts in the atmosphere. Closed low from Canada dips south into the Great Lakes area. In doing this, it pulls a shortwave in the southern stream farther northeast. This will push surface low pressure through the Ohio Valley and to the New Jersey coast on Sunday. The ECMWF is faster with the surface low, with less resulting interaction between upper systems. The GFS is farther north and shows more interaction. In any case, expect showers for our area for Sunday afternoon and night. For Monday and Tuesday the northern closed low shifts closer to us and should provide a continued chance of clouds and showers. && .AVIATION /20Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... 20Z update... 18Z update... Rest of this afternoon...High confidence. VFR across the region. Lower clouds w/IFR cigs may linger just E of outer Cape Cod (just offshore) this afternoon. Sea breezes anticipated along coastlines. Tonight...Moderate confidence. Mainly VFR with MVFR/IFR across the south coast, Cape and Islands in low clouds and fog, as well as passing showers from coastal low later tonight. Also areas of fog south coastal MA/Cape/Islands. Gusty NE winds to near 20 kts possible near daybreak. Chance for showers mainly Cape/Islands later tonight. Wednesday...High confidence. Improving trend to VFR, with the exception of Cape/Islands, where IFR/vsbys cigs may linger thru Wed am, and IFR cigs may last into the afternoon. East wind persists. Passing showers Wed am, mainly across Cape/Islands. Wed night...Across interior southern New England, VFR likely and dry. Along east coastal and south coastal MA including Cape/Islands, IFR possible in low clouds and fog. Light NE/E wind persists thru the night. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Thursday through Sunday/...moderate to high forecast confidence. Thursday... IFR cigs and vsbys in rain and fog with east wind gusting to 20 knots along the shoreline. Friday... Conditions improve to mixed VFR/MVFR in showers as winds shift from the northwest. Saturday... VFR. Diminishing northwest wind. Sunday... VFR early, trending to MVFR/IFR in showers and fog by Sunday night. Winds from the southeast. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High confidence. Tonight and Wednesday... Another coastal low will develop across the Mid-Atlantic and move northeastward towards the waters tonight, then pass northeast of the waters on Wednesday. Visibilities could drop in patchy fog and light to moderate rainfall late tonight and Wed morning. Improving visibility and rainfall ending by midday Wednesday as high pressure builds in the area. However, due to the pressure gradient between the offshore low and high pressure, a period of gusty northeast winds is expected during Wednesday and Wednesday evening. Seas will begin to build during Wed as low moves further offshore. Highest confidence of SCA thresholds for winds/seas being met during Wed/Wed night are along the southern outer coastal waters, and RI/BI Sounds. A Small Craft Advisory has been issued for this area. Outlook /Thursday through Sunday/... Thursday-Friday... Poor vsbys in rain and fog Thursday and Thursday night. Easterly winds around 25 knots with strongest winds during the afternoon and evening. These winds then lift north of the waters tonight. Seas 5 to 6 feet on the outer waters Thursday and Thursday night. Low pressure moves past the waters Friday morning, at which point the winds shift from the NNW with speeds 20 knots or less. Expect seas 5- 7 feet on the outer waters and RI Sound. Small Craft Advisory may be needed on some of the waters both days. Saturday-Sunday... Winds remain below 25 knots through the period. Seas subside below 5 feet early Saturday. Seas may return to 5 feet on the outer waters late Sunday night. Poor vsbys possible in showers and fog late Sunday and Sunday night. && .TIDES/COASTAL FLOODING... High astronomical tides will occur over several tide cycles along eastern Massachusetts coast from Thursday into Memorial Day, with tides around 12 feet in Boston and 4 feet on Nantucket. At the very least, minor nuisance flooding will occur in the most vulnerable locations such as Morrissey Blvd in Boston. Any surge on top of these tides would lead to more widespread, but minor, coastal flooding, including on Nantucket. Right now, this looks to be a possibility Thursday into Friday due to expected onshore winds. Model surge guidance (ESTOFS) shows a potential 0.6 ft surge, which would give a storm tide near 13 feet Thursday night in Boston and just under 5 ft on Nantucket. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 10 AM Wednesday to 6 AM EDT Thursday for ANZ254>256. Small Craft Advisory from 2 PM Wednesday to 6 AM EDT Thursday for ANZ235-237. && $$ SYNOPSIS...WTB/NMB NEAR TERM...NMB SHORT TERM...NMB LONG TERM...WTB AVIATION...WTB/NMB MARINE...WTB/NMB TIDES/COASTAL FLOODING...Staff

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