Area Forecast Discussion
Issued by NWS Boston, MA

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862 FXUS61 KBOX 142054 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 354 PM EST Sat Jan 14 2017 .SYNOPSIS...
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High pressure moves E of the region today with a weak low pres sliding well S of the coast overnight. Some light snowfall is possible mainly across Connecticut, Rhode Island and Southeast Massachusetts late today and tonight. High pressure maintains dry and seasonable conditions Sunday and Monday. Low pressure from the Plains approaches Southern New England with rain and mild air for Tuesday and Wednesday. The low then redevelops to our east and moves off Wednesday night. High pressure then builds in with dry and continued mild weather for the end of the week.
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&& .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 1035 AM Update... Areas of mid and high clouds were pushing across the region at mid morning as seen on latest visible satellite imagery as well as 14Z observations. Temperatures were running up to 3 degrees below forecast at both 13Z and 14Z. Noting extensive cloud cover across NY/PA through the mid Atlantic states heading eastward, so clouds will continue to increase through the afternoon. Large 1040 hPa high pres crossing the region this afternoon, with light/variable or calm winds through midday, then will shift to light S-SW during the afternoon. Previous forecast had this well covered, but did update to incorporate current trends. Reminder of forecast in pretty good shape, but again updated to extend current trends especially for cloud cover. Temps/dewpts looked OK as well, though did tweak temps up a bit across S coastal areas as winds become more onshore off the milder ocean. Previous Discussion... The high clouds will give way to a BKN-OVC AC band in advance of weak low pres developing near the Delmarva. Have highs reaching the upper 20s and low 30s. Dry wx prevails everywhere through mid afternoon thanks to 1040+ high pres slowly sliding offshore. Winds remain light, but will back to the S-SW by afternoon. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Late today and overnight... Weak low pres developing off the Delmarva will slide well S of the 40/70 benchmark just after midnight tonight. With center of high pres more E of S New England overnight than N, this will allow weak mid lvl f-gen band N of the developing low pres to just brush S Coastal New England during the evening hours. Sounding data suggests moisture loading from H7-sfc, but mainly S of a line through the border of MA with RI/CT and continuing E. Areas N remain under the subsidence of high pres and are not able to moisten enough to be of impact. S of this line, where PWATs reach about 0.75 inches just offshore light snowfall is possible, mainly from late afternoon into the early overnight hours. F-gen is weak and omega from this is below the primary snow growth regime but within the region saturated in respect to ice. Therefore, some light accumulations are possible below an inch, and likely even below a half inch most locations, especially as one heads N, where only flurries can be expected. Overnight mins dip mainly into the 20s, as the cloud cover associated with the wave will limit the ability for radiational cooling until late. Sunday... Secondary high pres/ridging will allow for clearing and weak pres gradient/winds and dry wx after clouds clear from exiting low pres. H92 temps hover between -6C and -8C, but once again subsidence inversion could limit mixing. In spite of sunrise, will be limiting highs to the low-mid 30s primarily. && .LONG TERM /MONDAY THROUGH SATURDAY/...
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Big Picture... Hudsons Bay upper low retreats north, leaving a large-scale zonal flow for much of the long term. Toward next weekend the large scale a developing Pacific Coast trough and West Atlantic Ridge. In the shortwave scale, Baja California upper low ejects through the Southern Plains Monday and Great Lakes Tuesday, then New England Wednesday. East Coast ridge then builds late week. Contour heights are mostly forecast to be above the climatological average. GFS does show heights briefly near normal Wednesday as the upper trough moves past, but the ECMWF and GGEM are much warmer. Overall this looks like a dry mild week, punctuated by a period of rain Tuesday-Wednesday. Model mass fields are similar through Wednesday morning, then diverge in handling the midweek weather system. They then show a general agreement on high pressure arriving for Friday and Saturday. Confidence for the long term is moderate. Details... Monday... High pressure settled offshore, bringing a light west to southwest flow. The mixed layer reaches to about 950 mb, where temps support max sfc temps of 35 to 40. Some increase in cirrues at night, especially across CT and RI, but otherwise fair skies and light winds. This will allow radiational cooling for a portion of the night. Dew points 15 to 30 should allow min temps mainly 20-30. Tuesday-Wednesday... Models are trending slower with the onset of clouds and precipitation. The lower airmass remains dry through midday while mid and high clouds are on the increase. Low level ageostropic flow turns from the north but remains 20 knots or less. Meanwhile temps will have time to climb above freezing in most places. If cold air does linger, it would be most likely in some of the East Slope communities of Northwest MA. There also remains a chance that the combination of slightly stronger drainage flow and evening evaporative cooling could bring a period of freezing rain in Northwest MA. This will need to be monitored. Aside from that potential icing scenario, temperature profiles suggest rain for our area from onset to finish. Models are showing a coastal redevelopment of the system on Wednesday, but disagree on the location. The GFS says Gulf of Maine while the ECMWF says Long Island. The GGEM tries to do a little of both. Still plenty of time for these details to sort out. Thursday through Saturday... The ECMWF shows a second shortwave sweeping down from Canada Thursday while the GFS shows, at best, a very shallow version of one. This may be good for snow showers in the hills, but moisture profiles are sufficiently dry to maintain a dry forecast. High pressure then builds in for Friday and Saturday. Temps in the projected mixed layer support max sfc temps in the 40s. Dew points are projected in the upper 20s and 30s, supporting lows in the 30s.
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&& .AVIATION /21Z SATURDAY THROUGH THURSDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/... 15z update... Through this afternoon...High confidence. VFR. BKN-OVC AOA 12Kft. Light/variable or calm winds become S-SW, but remaining light. This evening and overnight...Moderate confidence. VFR remains N of a line along the MA border with CT/RI into SE MA. S of this line, light snowfall is possible, with some light accumulations (less than an inch) possible especially near the coast and over the Islands. This could yield a period of MVFR conditions in lowered vsbys and CIGS. Conditions clear during the early morning hours Sun. Winds S-SW, but shift back to the NW after snows end. Sun...High confidence. VFR. NW flow, mainly 10 kt or less. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF overall. Low risk for very light snowfall this evening, but VFR likely to dominate. Outlook /Monday through Thursday/... Monday... High confidence. VFR. Tuesday-Wednesday... Moderate confidence VFR Tuesday morning, lowering to MVFR cigs/vsbys in rain Tuesday afternoon/evening, then IFR in rain and fog Tuesday night and Wednesday. Conditions may improve Wednesday afternoon. Variable winds Tuesday becomine S-SE Tuesday night, S-SW Wednesday, and West by Wednesday afternoon. Thursday... Moderate confidence... VFR.
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&& .MARINE...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/... 15z update... Through this afternoon...High confidence. Winds and seas have diminished so allowed small crafts to expire at 10 AM. Expect light/variable, mainly W-NW winds to back to S-SW during the afternoon but remain AOB 10 kt. Seas 4 ft or less. Tonight into Sunday...High confidence. S-SW winds remain below small craft criteria into the overnight hours before they turn back to the NW late tonight and early Sunday morning. Even with this shift, gusts should remain below small craft. Seas should remain below 5 ft. Some rain, even mixing with snow in spots tonight will yield brief vsby restrictions. Outlook /Monday through Thursday/... Moderate confidence. Monday-Tuesday... Winds less than 20 knots and seas less than 5 feet. Tuesday night-Wednesday... Southeast winds reach 25 knots on the outer waters and possibly RI Sound, with seas building to 5-7 feet. Best chance is on the southern outer waters. Small Craft Advisory may be needed on these waters. Visibility will be reduced in periods of rain Tuesday night, with rain tapering off as winds shift from the West Wednesday afternoon. Thursday... Northwest winds will gust to 25 knots on some of the waters. Seas of 5-7 feet will slowly diminish during the afternoon.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 6 AM to 9 PM EST Sunday for ANZ250- 254. Small Craft Advisory from 6 AM to 1 PM EST Sunday for ANZ251.
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&& $$ SYNOPSIS...WTB/EVT NEAR TERM...EVT SHORT TERM...EVT LONG TERM...WTB AVIATION...WTB/EVT MARINE...WTB/EVT

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