Area Forecast Discussion
Issued by NWS Boston, MA

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419 FXUS61 KBOX 251120 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 720 AM EDT Sat Mar 25 2017 .SYNOPSIS... A cold front will push south across the region today. This front will stall just to our south tonight, then will try to push northward as a warm front Sunday. Active weather pattern continues into Tuesday, with the potential for wintry precip Sunday night. Below average temperatures return by mid-week with another shot of precip by the end of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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730 AM Update... Cold front continues to sag southward this morning as waves of precipitation moves along it. While precip is fairly light right now, upstream appears to be a good batch that will move through southern New England. Have increased pops and wx accordingly. One thing to watch is when the heavy precip moves through late this morning, cannot rule out a mix of snow/sleet as thermal undergo rapid cooling, especially if temperatures are near 33-34F. Aside from the precip, the overall trend in the forecast remains on track. Previous Discussion... A cold front will work its way south across our region this morning. Winds turn north, then northeast behind this front. Even though this front should be to our south, most of the guidance continues to show an impressive back of Q-vector convergence and upper level divergence to the north of this boundary. Enough moisture should remain where there is a high probability for light rainfall at times, especially from mid morning into this afternoon. Nearly steady temperatures today.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... A high pressure over southeast Canada and northern New England will nudge slightly farther south tonight into Sunday. Still expecting dry weather for much of tonight. The high pressure to the north is a near classic cold air damming signature. It will also provide an excellent overrunning surface. Low level ageostrophic winds become north late tonight at 1000 mb and 925 mb. The ageostrophic winds at 925 mb shift east to southeast Sunday morning, a sign of increasing warm air advection aloft. Depending upon the timing of precipitation tonight into Sunday, there is a window for a period of sleet or freezing rain across the interior. The areas most a risk are central and western MA, northern Ct and northern RI. Expecting temperatures to rise above freezing by noon Sunday, ending the threat of mixed precipitation. Will need to monitor the extent of the low level cold pool very carefully. This timing may change with later forecasts. Near normal temperatures expected tonight, and Sunday. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... 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... Sunday night into Monday...Moderate confidence. *** Increasing chances for a period of sleet/freezing rain across the higher terrain Sunday night into Monday morning *** Stalled front south of the region will begin to return back north as a warm front late Sunday into Monday, as surface low pressure moves into the Great Lakes. Although warmer air aloft should move easily back into the region, the low-level cold air may be a little tougher to dislodge due to the nearby surface high pressure. Both the NAM and high-res versions of the EC and GFS are indicating northerly winds across the CT Valley, a clear sign of cold air damming. While the surface cold air is in place, the focus then turns to precipitation amounts and timing. Spotty light precip may start out during the first half of the night, but as the upper level shortwave approaches, there appears to be enough synoptic lift for higher qpf chances. While there is still some uncertainty with the thermal profiles but cross-sections, soundings and even CIPS analogs show this timeframe has the potential for icing especially north of the Pike. Sleet and freezing rain are the main concerns, with the chance that freezing rain could be ongoing during the Monday morning commute. Keep in mind that a difference of a degree or two in the thermal profiles will change the p-type. Winter weather headlines may be needed. 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 /11Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...Moderate confidence. Today...A cold front moves north to south across the region this morning. An area of light rain should develop, with cigs and vsbys lowering to MVFR. Winds shift from the north this morning, then northeast this afternoon and evening. Tonight...Mainly VFR as high pressure builds from the north. MVFR will lurk to our southwest, preparing to move north again. East flow through the night. Sunday...mainly VFR, with areas of MVFR in precipitation. Some freezing rain or sleet possible Sunday morning. KBOS TAF...High confidence in TAF trends. Moderate confidence in timing. KBDL TAF...High confidence in TAF trends. Moderate confidence in timing. Outlook /Sunday Night through Wednesday/... Sunday night...Moderate confidence. Cigs lower to MVFR/IFR in due to increasing low level moisture. Spotty precip will increase closer to daybreak. Anticipate areas of -FZRA/-PL across the interior overnight. 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/... Today...High confidence. W winds shift to N this morning and early afternoon as a cold front moves north to south. Winds then shift from the northeast by afternoon as high pressure moves in from Quebec. Winds will be less than 25 knots through the day. Seas will linger at 5 feet on the southern outer waters and parts of RI Sound much of the day. Small Craft Advisories continue where needed. Tonight...High confidence. Northeast winds turn from due east overnight. Speeds remain below 20 knots through the night. Seas will remain less than 5 feet through the night. Sunday...High confidence. East winds continue less than 20 kt Sunday. Seas remain less than 5 feet. Outlook /Sunday Night through Wednesday/... 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. Small Craft Advisory until noon EDT today for ANZ235. && $$ SYNOPSIS...Belk/Dunten NEAR TERM...Belk/Dunten SHORT TERM...Belk LONG TERM...Dunten AVIATION...Belk/Dunten MARINE...Belk/Dunten is the U.S. government's official web portal to all federal, state and local government web resources and services.