Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 200155 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 955 PM EDT Wed Apr 19 2017 .SYNOPSIS... Weak low pressure area will cross New England late tonight. A weak area of high pressure will then bring a brief period of dry conditions Thursday. Low pressure from the Plains redevelops along the New England coast Friday afternoon/evening. This means rain Thursday night and Friday. Weak high pressure builds drier weather for Saturday and early Sunday. Another low pressure area moves to the Carolina coast and passes well offshore of Nantucket Sunday night. High pressure in Canada brings dry weather early next week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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10 PM Update... Short wave trough moving across southern Quebec this evening. WAA precip associated with this feature impacted the region earlier this evening with the steadiest precip almost offshore at 10 pm. Rain shield much more fragmented upstream as mid level flow becomes anticyclonic and associated subsidence begins to dry the column from top down. Thus a drying trend overnight however periods of light rain may linger along the south coast. Cool night ahead with SSE winds off the chilly ocean waters. Previous forecast captures these details nicely so no major changes planned. Previous discussion... Weak, flat H5 short wave will work eastward across the region. Short range models signaling H85 jet at 40-45 kt moving across helping with decent lift. With the fast flow aloft, the best dynamics (which is not all that much of to begin with) will remain N of the region as it weakens further overnight. QPF amounts will range from a tenth of an inch of less N of the Mass Pike, up to around 0.2 inches across southern areas where precip will slow overnight especially along the S coast. With clouds across the region and S-SW winds in place, temps will only fall back to the lower-mid 40s by around midnight before slowly rising during the pre-dawn hours.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Thursday... Leftover showers across S coastal areas should push offshore by mid-late morning. Weak ridge at the surface and aloft moves quickly across, but low level moisture will be trapped in light wind field so expect clouds to linger through the day. 12Z MOS and model guidance continues to signal rather mild temps considering the cloud cover. Could realize the milder readings across the interior with light winds. However, will be cooler along the coast as winds, which start off below 10 mph as front moves across during the morning, will shift to N-NE by afternoon then eventually to E. Carried temp ranges from the 50s along the immediate coast to lower-mid 60s inland. With fast flow aloft, leading edge of light rain will start to approach western areas by late afternoon. Carried chance POPs with this approaching system moving into the CT valley by evening. Thursday night... Models signaling rather strong H5 short wave moving across the Great Lakes late Thu night, along with good H5 difluence moving across ahead of this system as the H5 ridge moves offshore by around 06Z. Noting decent PWAT moisture plume working across the region mainly after midnight, on order of 1.1 to 1.2 inches. So, could see some downpours. Best shot of precip will be across central and northern areas as low passes across northern New England. Also noting decent instability moving across N CT into W RI after midnight, with K indices around 30 and TQ values in the upper teens. So, can not rule out some isolated thunder with some of the rain as it moves across. Have mentioned it in the forecast for those areas, even though it looks like better instability works across after 12Z. Expect temps to bottom out in the mid-upper 40s. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Big Picture... Longwave flow pattern trends from zonal late this week to weakly cyclonic over the Central and Eastern USA by early next week. Contour heights remain at or above normal values. The shortwave scale shows one shortwave moving across Canada and passing north of New England Friday night and Saturday. A southern stream shortwave moves ashore from the Pacific today, forms a closed low over the Plains Friday night, and then shifts to the Florida coast late Monday. A portion of this shortwave eject east through the Mid Atlantic states Monday. Mass fields are similar among the models through early next week, which increases confidence. Details... Friday... Northern stream shortwave starts the day over the Great Lakes with supporting 135-knot jet aimed at Buffalo. These move east, with the upper jet reaching the coastline in the afternoon. Meanwhile low level southeast jet feeds in off the ocean into Eastern MA, riding up over the stalled front south of New England. All models show the surface system redeveloping near NYC and Long Island late afternoon and moving off to our east Friday night. The resulting lift combined with precip water values over an inch and southeast flow should bring rainy weather to most of Southern New England until the system moves past on Friday evening. The high precip water values suggest potential for locally heavy downpours embedded in the rain area. Stability parameters are marginal (TT is 46-48) and mostly favor CT. We note that guidance pop values diminish quickly by midday while the moist southeast flow lingers through the afternoon and evening. We have manually maintained likely pops through the afternoon and diminish west-to-east as the surface system moves past. Winds shift from the north- northwest bringing in partial clearing during the night. One other change regards the temperatures. With a steady onshore flow over water temps in the mid 40s, we lowered temps a couple of degrees with max values along the coast in the upper 40s and low 50s and temps inland in the mid to upper 50s. Saturday... Weak high pressure in Canada builds south with drier air. Meanwhile, a second trough axis associated with the northern shortwave moves through New England. Most of the forcing and moisture with this system is focused over Northern New England, but close enough to us to expect areas of daytime clouds, especially in Northern MA. Sunday... Southern stream upper low and trough feed moisture up the coast and generate another area of overrunning south and southwest of New England. The GFS and ECMWF show the surface level easterly jet and low level southwest jet remaining south of our area. This brings into question whether we would get any rain. Meanwhile cross sections show deep lift and deep moisture over southern areas such as Providence and less moisture and lift over northern areas such as Boston. Worcester and Hartford are in between. We will feature chance pops for most areas with the bulk of pcpn Sunday afternoon and evening. South Coastal areas, closest to the offshore low, will feature likely pops at the height of the storm. The shortwave Monday through Wednesday... Canada high pressure reasserts control from the Maritimes much of this period. The ECMWF and GGEM take the low off Florida and eject it northeast and pass it just off Nantucket Tuesday night or Wednesday. The GFS keeps the system well offshore. Not much confidence in the solutions at this time. We will feature slight chance pops in RI and Eastern MA Tuesday and await a consensus to form. && .AVIATION /02Z THURSDAY THROUGH MONDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 10 pm update... A mix of MVFR and VFR cigs with isolated IFR cigs in the Worcester Hills. Mainly VFR vsbys in light rain becoming more spotty overnight, except south coast. Not much change from previous 00z TAFs. Earlier discussion below. ================================================================ Short Term /through Thursday Night/...Moderate confidence. Tonight...VFR early, but MVFR cigs develop in showers near or just after midnight. Low chance for brief IFR in any heavier showers, mainly through 04Z-06Z. MVFR CIGS will likely linger overnight. Thursday...Areas of MVFR to local IFR conditions early Thu morning, then should improve to VFR. Sct -SHRA may linger along S coast through midday before moving off the coast. As next system approaches, will see spotty light rain move into western areas around or after 22Z. Should remain VFR. Thursday night...Generally VFR in -RA early, then CIGS lowering to MVFR-IFR from W-E mainly after midnight. Low risk of -TSRA across N CT/W RI around or after 08Z. E-SE winds KBOS TAF...Moderate confidence due to timing of the onset of MVFR cigs. Low risk for period of IFR CIGS/MVFR VSBYS toward daybreak Thu. CIGS improve to VFR Thursday. KBDL TAF...High confidence. CIGS lower to MVFR around 03Z. May see brief IFR CIGS after 06Z-12Z Thu. Conditions should improve to VFR CIGS by mid morning Thu. Outlook /Friday through Monday/... Friday...Moderate confidence. IFR in rain and fog, improving to VFR from west to east Friday night. East to southeast winds may gust near 30 knots at times along the east coast of Massachusetts Friday. Winds shift from the north and diminish during Friday night. Saturday...Moderate-high confidence. VFR Saturday with potential for MVFR cigs along the east coast of Massachusetts. Sunday-Monday...Low-Moderate confidence. VFR cigs Sunday morning lower to MVFR by Sunday afternoon. Areas of MVFR/IFR in rain and fog late Sunday and Sunday night. Best chance of lower conditions will be near the South Coast and Islands. Conditions improve to VFR Monday.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/...High confidence. Tonight...Small crafts continue on the eastern waters, mainly for seas E and S of Cape Cod and Nantucket and for both wind and seas E of Cape Ann to Plymouth where gusts will be up to around 25 kt. Seas around 5 ft mainly on the outer waters. Conditions should diminish after midnight on Mass and Ipswich Bays as winds diminish, but seas may linger through the remainder of the night on the eastern outer waters. Thursday...Expect light winds and seas below 5 ft Thu as weak high pres builds across the waters. Thursday night...Winds shift to E-SE as low pressure passes N of the waters. Winds and seas will increase after midnight mainly on the eastern waters, with gusts up to 25-30 kt and seas building to around 5 ft. Small crafts will likely be needed. Outlook /Friday through Monday/... Friday...Moderate confidence. East to southeast winds to the north of a warm front Friday. Some southern waters may slip south of the front, with winds becoming more from the south. Wind speeds will increase east of Massachusetts during the day with gusts 25 to 30 knots and a low potential for 35 knot gusts. These winds will help build seas east of Massachusetts with 5-8 feet in the afternoon/evening. Poor visibility at times in rain and fog during the day. A cold front swings across the waters early Friday night, with winds shifting from the north and northwest behind the front with speeds to 20 knots. Seas will diminish overnight as winds become offshore. Visibility will improve with the wind shift. Saturday...Moderate confidence. Winds from the north-northwest with speeds up to 20 knots. Low potential for 25 knots. Lingering 5 foot seas on the eastern outer waters will diminish later in the day. Winds diminish Saturday evening. Sunday-Monday...Moderate confidence. Winds turn from the east Sunday as low pressure passes well offshore. Winds up to 20 knots are possible with the best chance on the southern outer waters. Seas remain below 5 feet on our waters, but build to 5 feet just south of our southern outer waters. Winds turn from the north on Monday but remain below 20 knots. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 AM EDT Thursday for ANZ250. Small Craft Advisory until 2 AM EDT Thursday for ANZ251. Small Craft Advisory for hazardous seas until 8 AM EDT Thursday for ANZ254. Small Craft Advisory for hazardous seas until 5 AM EDT Thursday for ANZ255. && $$ SYNOPSIS...WTB/EVT NEAR TERM...WTB/Nocera/EVT SHORT TERM...EVT LONG TERM...WTB AVIATION...WTB/Nocera/EVT MARINE...WTB/EVT

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