Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 231143 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 643 AM EST Thu Feb 23 2017 .SYNOPSIS... High pressure centered offshore southeast of New England will bring dry and unseasonably warm conditions today and tonight. A cold front stalls over Southern New England tonight and then moves north as a warm front Friday. A strong cold front will cross the region late Saturday afternoon and night, bringing areas of rain and patchy fog. There will be a low chance for isolated thunderstorms as the front passes. Near seasonable temperatures will return early next week, with blustery conditions on Sunday and Monday. There may be another chance for precipitation around the middle of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 630 AM Update... Areas of dense fog continues over the immediate south coast as moisture continues to move into the area. Anticipate fog to begin to dissipate over the next hour and move offshore as diurnal heating begins mixing out the atmosphere. Aside from the fog, made a few adjustments to the current forecast. Mainly on sky and surface temps. Appears the Merrimack Valley will be the hot spot today with a low chance for upper 60s. Otherwise the bulk of the forecast remains on track for today. Previous Discussion... High pressure centered offshore is bringing a light southwest flow to the region. This moved higher dew point air onshore along the South Coast and brought low clouds and dense fog to the region. After sunrise the clouds and fog should retreat out over the water. A Dense Fog Advisory will continue along the South Coast of RI and MA through the morning rush hour. Low pressure passes across Ontario and Quebec today, while the high pressure remains just offshore. This will create a better defined gradient to aid southwest winds. A 50 knot southwest jet is expected to move up the St Lawrence Valley, while a secondary southwest jet will move across Srn New England at 2000-3000 feet. Speeds in the secondary jet are forecast at 30-35 knots, and may spin up gusts of 20-25 knots at the surface. Warm advection in the southwest flow should limit the mixing depth, but temps at 925 mb reach 9-12C which would support max sfc temps in the lower 60s, roughly 60-64. If we mixed all the way to 850 mb we would tap 7-8C, which would support max sfc temps in the lower 70s. A mixed layer up to either 950 or 925 mb looks more reasonable. The southwest flow off the ocean should keep readings cooler along the South Coast. Ocean temps are in the low 40s, so daytime highs should be in the upper 40s and lower 50s. Note the Climate section at the end of this discussion. Based on expected temps, PVD and ORH stand a chance of tying/breaking the max temp record today. BOS has an outside chance, but we think they will fall just short. BDL would need to mix above 900 mb to have a chance, and this would seem too much. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight... Cold front trailing the Canadian low will drift into or near Southern New England tonight, but becomes parallel to the upper flow by midnight and should stall by that time. Digging shortwave over the Plains will eventually turn the upper flow more southwest and push the stalled front north as a warm front. Although placement of the front when it stalls will determine our overnight weather, it seems likely that most of our area will remain south of the front...certainly the areas south of the Mass Pike. This will mean another night of moist air with fog and low clouds again moving ashore from the south. Dense fog may again be a concern overnight. Dew points in the 40s point to min sfc temps in a similar range. Friday... Cold front slowly lifts north of our area during the day. Southwest flow will increase through the day, eventually generating lift across the front and resulting in light rain or showers. The best chance of this will be in areas north of the front, northern MA. Clouds will work against similar temps to Thursday. But temps at 1000 mb continue to support max sfc temps near 60. The southwest flow will again work to keep south coast areas several degrees cooler. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Mild with widespread showers likely Sat into Sat night * Blustery and cooler Sun/Mon * More active pattern for next week Pattern Overview... 00z guidance is in general agreement synoptically, still have some several mesoscale features to resolve. Synoptically potent Plains trough will move towards New England on Saturday pushing a cold front through by Saturday night. Chilly and blustery conditions for Sunday and Monday as zonal flow takes shape. Guidance diverges beyond Sunday leaving a low confidence forecast. Zonal flow to start but the GFS is more progressive and develops a quick moving system on Monday while the EC is dry until Tuesday. Timing issues need to still be resolved. Next system to watch is Wednesday/Thursday timeframe but model sensitivity to this system is high as the EC is more progressive than the GFS. Regardless, looks like precip during the mid-week with cooler temperatures once again by Thursday. Trended the next weeks forecast towards a blend of the Ensembles as well as the EC which appears to be inline with the UKMET. Details... Saturday into Saturday night... Potent shortwave will move through the flow and approach southern New England on Saturday. This wave will drag a cold front across the region ending the chances for above average temperatures. During the morning, appears to be enough low level moisture trapped under an inversion to keep low clouds and drizzle across a portion of the area. Highest confidence is across the south coast, but this area can be greater as surface southerly winds will continue to advect higher dewpoint into the region. Ahead of the front, anomalous southerly flow with LLJ increasing to 35-45 kts as 925 mb temps warm to 11C. This will result in above average temperatures for the region as well as gusty winds. PWAT values also increase near 2 STD above normal as southern stream moisture is fed into the system. Along the front, a secondary low will develop but appears it will be to far north and east and will to keep southern New England out of the heavy qpf. This appears to be inconjunction with the GEFS as well as the EC Ensembles which has just a 20 percent prob of qpf over 0.5 inch. Will need to continue to watch this secondary low and see where it will eventually develop during this system. Otherwise appears that the bulk of the heavy precip will be across upstate NY and VT including the east slope of the Berks. This area is where the better dynamics will be as well as enhancement due to upslope. May need to watch river levels after this system because of the heavy rainfall potential and snow melt up north. Right now ensembles are pointing towards a few sites near action stage. As the cold front approaches, does appear that profiles turn moist adiabatic. This may help mix some of the higher winds to the surface. So cannot rule out a few gusts above 40 MPH. Also guidance continues to show some elevated instability with TT above 50 and LI below 0. Because of the steep pressure falls with this system, cannot rule out the potential for a rumble of thunder and have continued to mention the potential in the forecast. FROPA will be quick as temperatures plummet behind the front. Could see some wet flakes on the backside depending on how much moisture is left as front moves through. Right now cannot rule out an inch across far NW Mass. Sunday into Tuesday... Chilly and blustery NW flow will take over the region by Sunday into Monday. Temperatures difference between Saturday and Sunday could be close to 15-20F. Wind gusts could also increase close to 30 MPH as the region mixes well above 850mb. This will result in wind chill values in the 30s. A stark contrast from the previous several days. Guidance mesoscale issues come to light by Monday as a weak wave appears to move through the flow. 24 hour timing difference between the GFS and EC with the EC pushing the system through on Tuesday. The latest ECENS and GEFS are leaning closer to the EC keeping the high pressure over the region on Monday. However there are still a few members that are on the fast side. Whether this system comes in on Monday or Tuesday we could see some rain/snow showers for the region. Low confidence on timing. Wednesday and beyond... Large spread in the guidance for this period. Synoptically mean upper level ridge over the Atlantic will help temps warm above average. However potent shortwave moving through the Great Lakes will push a warm and cold front towards southern New England. A lot of issues to be resolved in exact placement of the low and if any secondary low pressure systems develops. But right now models are in agreement on the timing with this system coming through Wed PM/Thursday with another cold snap behind it. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/... Today...Moderate confidence. IFR and LIFR cigs/vsbys along the South Coast and Islands, while VFR conditions farther north. Patchy MVFR vsbys possible in patchy fog before sunrise away from the coast. The south coast fog will retreat offshore this morning, with all areas becoming VFR. Tonight...Moderate confidence. Low clouds and fog will again move ashore along the South Coast and Islands. Expect conditions to return to IFR/LIFR in fog. Potential for MVFR and patchy IFR in lighter fog farther north. Friday...Low-Moderate confidence. A cold front will stall over Southern New England by Friday morning and then slowly lift north. Still quite a bit of uncertainty as to where this will stall. Expect IFR cigs and MVFR vsbys in morning fog. Areas of MVFR cigs/vsbys expected in showers Friday afternoon near or north of the the best chance for this would be in Northern MA. KBOS TAF...Moderate confidence in TAF. KBDL TAF...Moderate confidence in TAF. Outlook /Friday Night through Monday/... Saturday...Moderate confidence. Fog and Drizzle to start the day in MVFR/IFR conditions. Potent cold front will move through Sat PM brining heavy rain, gusty winds near 30 kts and the potential for thunder. Behind the front could see some wet flakes across the NW terminals. All of this could drop conditions to IFR/LIFR overnight, but anticipate a rapid improvement near dawn Sunday. Sunday...Moderate confidence. Improving conditions with blustery NW wind gusts near 30 kts. Monday...Low confidence. Still a large spread in guidance. Mainly VFR conditions with NW gusts near 25kts. Could see some sct showers across the south coast. && .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. Today... Areas of dense fog will linger through mid morning and then shift farther offshore. This means poor vsbys early, but then improving. Southwest winds will increase today with gusts of 20 to 25 knots, especially on the Eastern outer waters. Seas remain below 5 feet. Winds will need to be monitored, but currently look to remain below Small Craft thresholds. Tonight and Friday... Cold front moving south will stall overnight or Friday morning, then move slowly north. The Massachusetts North Shore may spend a few hours north of the front with winds shifting from the east or southeast. Areas south of the front will maintain southwest winds through the period. Outlook /Friday Night through Monday/... Saturday...Moderate confidence. Increasing southerly flow with the potential for SCA gusts. Waves will also increase in response. Vsbys reduced in developing rain and fog. Low potential for thunder late in the day. Sunday...High confidence. Northwest winds will keep seas up and increase wind gusts near 30 kts. Low probability for gales across the outer waters. Monday...Low confidence. Large spread in the guidance but gusty NW winds will continue keeping seas up for sometime. && .CLIMATE... Record Highs for Thu Feb 23 BOS...65/1990 BDL...68/1990 PVD...60/1990 ORH...61/1990 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Dense Fog Advisory until 9 AM EST this morning for MAZ020>024. RI...Dense Fog Advisory until 9 AM EST this morning for RIZ006>008. MARINE...None. && $$ SYNOPSIS...WTB/Dunten NEAR TERM...WTB/Dunten SHORT TERM...WTB LONG TERM...Dunten AVIATION...WTB/Dunten MARINE...WTB/Dunten CLIMATE... is the U.S. government's official web portal to all federal, state and local government web resources and services.