Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 071721 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1221 PM EST Thu Dec 7 2017 .SYNOPSIS... Dry and seasonably chilly weather continues today and into tomorrow. Low pressure will bring the potential for accumulating snow Saturday and Saturday night across eastern Massachusetts and Rhode Island, possibly farther inland pending exact track of low pressure offshore. Temperatures will average a bit colder than normal into early next week with perhaps a coastal storm bringing the potential for some rain and/or accumulating snow along with some wind Monday night into Tuesday night. Unseasonably cold and windy weather likely follows on Wednesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10pm update... Although drier and cooler air continues to filter in under broad W flow, a connection to lake moisture will still yield a risk for occasional flurries mainly across NW MA. This will be enhanced by a weak open shortwave passage this afternoon, so will maintain the possibility through much of the day. Otherwise, only updates this morning will be to warm temps across SE MA, where downslope will be maximized, and cool temps in the NW, where clouds connected to Lake Effect streamers will limit diurnal warming. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight into Friday... Mid-level shortwave will push through northern New England tonight. This combined with CAA and low level moisture will keep LES going. Overall flow in the low levels will be more West to East which could push some snow showers into extreme western MA. Did include a slight chc for some flurries to indicate this potential, but overall expect very little in the way of impact. Temperatures will fall early tonight as winds turn more calm and skies become mostly clear. Expect lows in the low to mid 20s away from the coast line. Winds will begin to pick up towards morning as 850 mb LLJ will increase to 30-40 kts. This will keep temps from truly falling esp across higher terrain and near the coastlines. Upper level shortwave will sharpen and dig into the northern Great Lakes tomorrow. This will push the region into a more southwest flow however temps aloft remain the same as today. But with more clouds moving into the region thanks to increase mid-level moisture, temps will be similar to today or just a few degrees cooler. Overall expect mainly dry weather for Friday. However towards the evening, precip chances will begin to increase as approaching system from the Carolinas will start to push some moisture towards the Cape and the Islands. Precip should remain liquid on Friday as better forcing will remain well south of the area. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Accumulating snow possible Sat/Sat night w/highest risk E MA/RI * Chilly on Sun with a few brief snow showers/flurries possible * Coastal storm may bring rain and/or snow Mon night into Tue night * Unseasonably cold weather follows Wed into Thu Details... Friday night... Southern stream shortwave may bring some light rain and/or light snow to the far southeast New England coast Friday night. However, the amplification of this shortwave may be limited so it may be a struggle for precipitation to make it much further northwest than that. If some light rain and/or light snow does make it onto the southeast New England coast, marginal boundary layer temps and light precipitation intensity will probably result in little if any appreciable snow accumulation. Saturday and Saturday night... The potential continues for the first accumulating snowfall across portions of the region with the highest risk across eastern MA/RI. It remains uncertain how far northwest main axis of precipitation will make it along with potential boundary layer issues across southeast New England. The big wild card is the strength and timing of northern stream energy dropping southeast towards the Great Lakes. If it becomes amplified enough it will pull moisture off the mid Atlantic coast into southern New England. On the flip side of the coin, if it ends up a little less amplified there will be little if any precipitation that makes it northwest of the Cape Cod Canal. It appears that the best chance for some snow will be Saturday, especially during the afternoon and night. As we talked about in the previous paragraph, there remains considerable uncertainty amongst the models on how far northwest the axis of precipitation gets and boundary layer issues are possible across the southeast New England coast. The 00Z GFS/GGEM fail to get much precipitation northwest of the Cape Cod Canal. The ECMWF/UKMET are the most aggressive bringing measurable precipitation northwest of the Boston to Providence corridor and into parts of the interior. The EPS/GEFS individual ensemble members continue to show a large spread in possible outcomes. These include complete misses, to a plowable snow across eastern MA/RI. There are even a few outlier members that focus the accumulating snow across the interior with boundary layer issues working onto the coastal plain with a more amplified/westward track. In a nutshell, the greatest risk for some accumulating snow will be across portions of eastern MA and RI, but impacts further into the interior can not be ruled out. Boundary layer may be too warm east of the Cape Cod Canal to get much if any accumulating snow. 6Z Guidance Update... The latest 6z model suite mainly the NAM/GFS and its ensembles have shifted the precipitation axis significantly further northwest. We certainly do not want to make any changes based on one suite of model runs, but given the westward support from the UKMET/ECMWF there may be an increasing threat for accumulating snow back into the interior. Again though, no changes until we see how the 12z model suite trends. Sunday... A shortwave crosses the region with perhaps a few brief passing snow showers/flurries. High temps mainly in the 30s to around 40. Monday through Tuesday night... While much of the daytime hours on Monday look dry, potent shortwave energy will carve out a deep trough to our west. This may allow for coastal low pressure to develop and perhaps rapidly intensify. Depending on when/where and how fast this exactly happens will determine just how we are impacted. All we can say is there is the potential for rain and/or accumulating snow Monday night through Tuesday night along with some wind. Wednesday... Unseasonably cold and windy weather may follow this storm on Wednesday, but still quite a ways out. && .AVIATION /17Z 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. Short Term /through tomorrow night/... Through tonight...High confidence. VFR. W winds gust to around 20-25 kt near shore, but remain around 10 kt inland. Flurries possible in extreme NW MA. Tomorrow and tomorrow night...High confidence. VFR continues although with CIGS gradually lowering through the day. Lowest CIGS Nantucket/Cape Cod overnight, but generally only low VFR. Some light SHRA mainly ACK, outer Cape Cod through early Sat morning. Winds slowly shift to SW tomorrow afternoon, but remain at around 10 kt. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Friday Night through Monday/... Friday Night: Mainly VFR, with local IFR possible. Breezy. Chance -RA/-SN across southeast New England. Saturday: Mainly VFR, with areas IFR possible. Chance SN. Saturday Night: MVFR/IFR conditions possible. Chance SN, slight chance SHSN. Sunday: Mainly VFR, with areas IFR possible. Breezy. Slight chance SHSN. Sunday Night: VFR. Breezy. Monday: Mainly VFR, with local IFR possible. Breezy.
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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 Friday/...High confidence. Southerly swell continues to keep ocean waters at of above 5 feet today. Increasing LLJ will result in an increase in winds late this evening into the overnight hours for the southern waters. SCA have been issued/extended because of this. High pressure will pass south of the waters by Friday which will help slacken wind and relax seas to below SCA by mid-day. Outlook /Friday Night through Monday/... Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain. Saturday: Winds less than 25 kt. Rain likely. Local visibility 1 to 3 nm. Saturday Night: Winds less than 25 kt. Local rough seas. Rain likely, chance of rain showers. Local visibility 1 to 3 nm. Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 10 ft. Slight chance of rain showers, slight chance of snow showers. Areas of visibility 1 to 3 nm. Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 9 ft. Monday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 4 AM EST Friday for ANZ231>234-250- 251. Small Craft Advisory until 7 AM EST Friday for ANZ235-237. Small Craft Advisory until 1 PM EST Friday for ANZ254-255. Small Craft Advisory until 10 AM EST Friday for ANZ256. && $$ SYNOPSIS...Frank/Dunten NEAR TERM...Doody/Dunten SHORT TERM...Dunten LONG TERM...Frank AVIATION...Frank/Doody MARINE...Frank/Dunten

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