Area Forecast Discussion
Issued by NWS Boston, MA

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345 FXUS61 KBOX 252000 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 400 PM EDT Sat Mar 25 2017 .SYNOPSIS... A cold front lingers offshore well south of New England tonight. The front will then push northward as a warm front Sunday. A weather system lifts through the Great Lakes Sunday and Monday bringing wintery weather inland and wet weather near the coast. Below average temperatures return by mid-week with another shot of precip by the end of next week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... Shortwave ridge aloft builds east into our area tonight. This turns our upper flow from the northwest. This allows surface high pressure centered over Quebec to build south into New England. Observed precip has been mainly rain, but cold air advancing southeast created a rain/snow mix...even all snow in the hills...in northern MA this afternoon. Expect mostly light rain farther south but with a potential for a brief rain/snow mix as the precip area moves southeast late afternoon/evening. Trailing edge of pcpn on radar is trending southeast and should move across Southern New England...roughly to the Mass Pike about 5 pm and to the South Coast about 630 pm. The trailing edge of the clouds is roughly along the Canadian border. This trends across our area between 8 pm and midnight. Per composite guidance, the cloud shield motion stalls over SW CT and Long Island between midnight and 3 AM, then slowly builds north again. So expect clearing or partial clearing most places. A northeast marine wind will not carry any low dew points, but we should see a small lowering trend through early tonight. Min temps expected to be a little above dew point, in the mid 20s to lower 30s. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Sunday... High pressure shifts east into the Maritimes. The pressure builds southwest through New England with a typical cold air damming signal. Ageostrophic winds at 1000 mb turn from north Sunday morning at 10-15 knots...would like to see that a little higher, but the direction is good. The GGEM is most aggressive in bringing precip north into New England...and even that is west of Hartford at 12Z Sunday. Most of the others are not even in New England. The strongest low level winds and resulting lift will be in NY/PA, with little wind over here. Expect most of the precip to be to our west, with light QPF in Western New England...about 0 to 0.03 inches. Temperature profiles suggest that if there is any precip, it would involve rain or a rain/sleet mix and mainly for the western and central hills. Consensus timing would bring any chance for the precip into Western New England late morning. If anything reaches farther east it would be in the afternoon/evening, by which point temperatures should be above freezing. Model high temps show upper 30s in the hills and low-mid 40s lower down. With clouds, cold air damming, and the chance of pcpn, we opted to undercut this by 2-4F. Forecast max temps will be in the mid 30s to low 40s. Sunday night... The stronger low level south winds shift farther east Sunday evening and bring a greater potential for lift and precip production. The upper ridge also shifts east at this time, bringing under the influence of the approaching Great Lakes shortwave and associated upper jet. This will mean an increasing chance of precip during the night. Temperature profiles show a layer of above-freezing air between 800-900 mb with max temps around 3C. This indicates a sleet potential, especially overnight as cooling surface temps may allow the precip to refreeze below the warm layer. A 3C temperature is on the low side for freezing rain production, but close enough to consider the possibility. Precip amounts start the evening light, but will build up overnight as the shortwave and low level lift increase. This will be a concern just as the early Monday morning commute gets started. We are anticipating the need for winter weather headlines starting sometime during the night. && .LONG TERM /MONDAY THROUGH SATURDAY/... Highlights... * A period of wintry precip including sleet/frz rain Sunday night * Unsettled weather pattern will continue into Tuesday * Improving conditions for mid-week * Active weather returns late in the week into the weekend Pattern Overview... 00z models and ensembles continue to show an active weather pattern for the region with split flow aloft and several shortwaves ejecting in from the Pacific. First wave is the closed 500mb low currently over the southern Plains. This low deamplifies into an open wave as it approaches the Northeast on Monday. Surface high pressure persists over northern New England keeping surface temperatures cool, resulting in mixed precip at times into early next week. Heavier precip possible on Monday as first open wave moves overhead. A second wave will follow a similar path from the Plains towards the Northeast before interacting with the northern jet stream on Tuesday bringing another shot of precip. Lots of uncertainty with the next southern stream wave and its potential interaction with the northern stream late in the week. Details... Monday...Moderate confidence. Surface temps will eventually warm above freezing during the later half of the day, with the chance that some sites may struggle to get into the 40s due to cold air damming. Models continue to advertise that Monday morning into the early afternoon will see the highest QPF thanks to synoptic lift from passing open wave and the development of a secondary low over the region. PWAT values also increase above an inch, so widespread rainfall is possible. Precip chances will begin to wind down by the evening hours. Overall it appears that Monday will be a chilly, raw, damp day especially north of the Pike. South of the Pike has the chance of getting into a warm sector with temps near 50F. Monday night into Wednesday...Low confidence. Weak ridging in the mid-levels will result in a break in the precip. Focus then turns to the secondary Pacific wave which is just now moving onshore. This wave will interact with the northern stream but the question is how soon and will this slow the precip chances down per previous runs. EC has become less amplified and more progressive compared to the GFS which could result in a more zonal flow during this period. This difference could result in a slow down to precipitation timing and temperature differences Tuesday into Wednesday. However both models continue to show arctic front passing through by the second half of Wednesday. Overall expect near seasonable temperatures for this period with precip chances sometime between Tuesday and Wednesday. Confidence will increase once models have a better sampling of onshore Pacific wave. Thursday...Moderate confidence. Upper level trough overhead as a few shortwaves moves through the flow. Canadian high pressure will build into the Northeast leading to a break in the precip chances. Friday into the weekend...Low confidence. Active pattern remains for the region with the next southern stream wave interacting with the northern stream. The EC continues to be more amplified than the GFS stalling the timing of any precip on Friday. Latest ensembles appears to be trending this way as well. Overall this portion of the forecast will depend on the strength and timing of the upper level wave and thus surface reflections. Because of the large spread in the guidance will continue with low confidence forecast. && .AVIATION /19Z SATURDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/...Moderate confidence. This evening... Area of rain/snow/sleet moving southeast through Massachusetts at 330 pm and should move off the South Coast around 630-730 pm. MVFR cigs/vsbys are expected during this passage, with embedded areas of IFR in the precipitation. Tonight... High pressure builds from the north. This will bring clearing skies between 8 pm and midnight, as well as an improvement to VFR. Light East-Northeast flow through the night. Sunday... Mainly VFR. Areas of MVFR cigs/vsbys in possible precipitation, especially in CT and Western MA. Some sleet or freezing rain possible Sunday morning. Sunday night... Conditions lower to MVFR, with IFR in the Central Hills and the Berkshires. Increasing chance of precipitation with time. Increasing chance of sleet or freezing rain in areas north and west of Boston-Providence. Best chance of precipitation will be late at night, as the Monday morning push starts to gear up. KBOS TAF...High confidence in TAF trends. Moderate confidence in timing. KBDL TAF...High confidence in TAF trends. Moderate confidence in timing. Outlook /Monday through Thursday/... Monday-Monday night...Moderate confidence. Any leftover -FZRA/-PL Monday morning will end across higher terrain. Otherwise MVFR-IFR CIGS in rain. Tuesday into Wednesday...Low confidence. MVFR conditions within any passing rain showers with improving cigs to VFR by Wednesday. N/W winds continue. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/... This evening... High confidence. Area of light rain moves southeast across the waters, possibly accompanied by brief snow with lower vsbys as it moves through. Northeast winds less than 20 knots. Lingering 5 foot seas on the southern outer waters. A Small Craft Advisory continues there. Seas less than 5 feet all other waters. Tonight... High confidence. East-Northeast winds less than 20 knots. Seas will remain less than 5 feet. Sunday... High confidence. East winds less than 20 knots. Seas less than 5 feet. Sunday night... Moderate-High confidence. Approaching weather system from the west will spread rain across the waters from west to east. Winds will remain below 20 knots and seas below 5 feet through the period. Outlook /Monday through Thursday/... Monday...Moderate confidence. Approaching upper level system and passing warm front will increase precip as well as wind and seas across the waters. SCA may be needed. Tuesday into Wednesday...Moderate confidence. Frontal system will pass over the waters increasing them to above 5 feet. SCA may be needed. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ254>256. && $$ SYNOPSIS...WTB/Dunten NEAR TERM...WTB/Dunten SHORT TERM...WTB LONG TERM...Dunten AVIATION...WTB/Dunten MARINE...WTB/Dunten

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