503 FXUS61 KBOX 271507 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1007 AM EST Sun Jan 27 2019 .SYNOPSIS... Departing high pressure maintains dry weather most of today. A cold front then moves through New England this evening and may bring light snow showers across NW Massachusetts. Cold and dry air then returns Monday. There is the potential for a winter storm late Tuesday into Wednesday. Much colder air expected late next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10 am update... Snow showers have come to an end across the Cape as mixing has increased with sunrise. Currently focus of snow showers is over New York. These will approach western MA by early afternoon but downloping east of the Berkshires and lack of moisture should limit them to western/central MA. Previous forecast is on track. Previous Discussion... A weakening, stalled frontal boundary has been the focus for very light returns on WSR-88D this morning across the Cape/Islands. Have noted a few obs reporting SN above 10sm, suggesting a few flurries are reaching the surface. With increased mixing this morning, the S flow should lead to faster erosion of the sfc convergence that are spawning these, so POPs will only last for a couple hours longer. Otherwise, noting enough above BL moisture and localized near sfc instability across W and NW portions of the forecast area for late afternoon SHSN or even an isolated squall to cross the Berkshires. May need to watch these as a brief burst of localized heavier SN cannot be ruled out. Otherwise, the combination of downsloping and drier air should mitigate this risk further E. Light variable flow turns from the southeast this morning and south and southwest this afternoon. This will draw warmer...or at least less cold...air into Southern New England. Temperatures at 900 mb climb to -4C, and if mixed this would support max sfc temps in the low to mid 40s. Low level jet moves up across the region midday and afternoon. Mixing max tap winds of 25 kt, possibly 30 kt. Closed upper low over Northern Ontario swings a shortwave across the Great Lakes and up the St Lawrence Valley. Southern stream shortwave lifts up the East Coast and moves its left exit region over Southern New England. This will generate lift over the region, most favorable area would be closest to the northern shortwave. Moisture is very limited with PW values around 0.35. Expect scattered snow showers, with best chance over NW and N Central Mass. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Tonight... Upper trough over Canada will push an arctic cold front through Southern New England early tonight. Winds turn out of the west and then northwest, drawing colder air and lower dew points back over our area. The drier air will support clearing skies overnight. Temperatures upstream suggest min sfc temps by Monday morning will be in the teens and lower 20s. Monday... High pressure over the region will maintain dry weather through the day. Mixing should reach to 925 mb, with temperatures at that level around -15C. Expect max sfc temps in the upper 20s and lower 30s. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Highlights... * Seasonably cool but mainly dry into early Tue * Monitoring a potential winter storm late Tue into Wed * Coldest temperatures of the season possible for Thu into Fri as arctic air moves across southern New England Overview and model preferences... Beginning to see some of the first vestiges of meso-scale agreement regarding the second lobe of the polar vortex and its sfc reflections Tue-Wed. Although the mean longwave trof remains well W, this lobe will force an eastward shift, and may even allow for a slight negative tilt as the vortex approaches New England. While this forces the parent low pres at the sfc back to the N-NW, to also does so with a developing frontal wave, something that most operational and ensemble guidance now agrees upon. This frontal wave track will ultimately determine thermal profiles, and whether there is a second p-type change with the comma-head. These are details that will converge over-time, but for now, feel that a general blend of non-GFS guidance will work. The GFS has trended slower, closer to the more consistent ECMWF/CMC, but remains the progressive outlier. Beyond this system, with the longwave trof shifting E, still anticipate an arctic intrusion, with bitter cold conditions. Will continue with the same blend for this period as well. Details... Mon night... Low-mid lvl warm advection ahead of developing low pres well to the W. However, mass fields suggest weak pres gradient and upper lvl moisture should hold off enough to warrant at least modest radiational cooling, yielding mins in the teens to mid 20s. Tue into Wed... Generally dry start, as mid layer shows nearly 20C dewpoint depressions to overcome throughout the day. Feel that isentropic induced precip shield will hold off until mainly late day Tue. POP update this morning will reflect this thinking. Initial round of precip late Tue and Tue night is based on warm advection, which, based on mean lower lvl temp profiles starts as SN. However as the secondary wave develops along a strengthening (40-50 kt) LLJ. The final track and strength of this secondary wave will then determine a potential change to RA, especially E of the Worcester Hills. Areas W and NW may remain mostly SN. Then, will need to monitor comma head development early Wed. A slower/more amplified pattern favors a change back to SN, with a risk for heavy SN if comma-head instability can be established. As noted by the previous forecaster, the latest trends do not yield as much moisture (peak PWATs generally below an inch) and although rapid deepening is occurring the transition N is rather progressive, especially given that leading high pres is oriented more E of N, limiting the blocking until the frontal wave gets into N New England. However, where mainly SN is observed there is the risk for warning level snows. At the very least, the latter portions of the Tue evening commute, as well as the Wed morning commute could be impacted in some way by wintry wx. Regarding wind risk, with strengthening ongoing downstream, the strongest LLJ may be over N New England rather than S New England. But breezing conditions cannot be ruled out as the frontal wave develops. Wed night into Thu... Secondary arctic cold front introduces the modified arctic airmass into New England. Modified, as the initial H85 temps were on the order of -40C, but will be closer to -25C to -20C by the time it reaches New England. This is still nearly 3 std deviations below normal. So anticipate bitter cold with highs in the teens to low 20s, and lows falling back into the single digits, potentially below 0F for some. Wind chills, particularly Wed night through Thu night could be near advisory levels. This arctic influence could bring about a modest risk for SHSN or even squalls associated with the arctic frontal passage Wed night into early Thu, and dependent wind trajectories, some ocean effect as the delta-T values between H85 and SSTs could be around -20C. Fri into the weekend.... As cutoff low pres pivots toward Labrador, noting modest warm advection and height rises through the period suggesting a gradual moderating trend. Conditions are likely to remain below normal into at least Sat based on current projections, especially as pres gradients slacken enough for overnight radiational cooling. && .AVIATION /15Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...High confidence. Today... VFR. Winds shift to the S with gusts 20-25kt late morning and afternoon. Some late day SHSN mainly W and NW MA. Tonight...VFR. Monday...VFR. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Monday Night through Thursday/... Monday Night: Mainly VFR. Tuesday: MVFR/IFR conditions possible. Breezy. Chance SN. Tuesday Night: Mainly IFR, with areas MVFR possible. Breezy. RA, SN likely. Wednesday: Mainly VFR, with areas IFR possible. Windy with gusts to 35 kt. SN likely. Wednesday Night into Thursday: VFR. Windy with areas gusts to 35 kt. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...Moderate confidence. Today... High pressure moves off through the Maritimes. A weak disturbance passes well offshore, but may generate a few light rain/snow showers over the waters east and southeast of Nantucket this morning. South to southwest winds will increase by afternoon with gusts of 25 to 30 kt. This wind will build seas, with 5-6 ft heights east of Massachusetts. Patches of 5 ft seas possible on the southern waters. Small Craft Advisory is in effect. Tonight... Cold front sweeps across the waters the first part of the night. Drier air moves in the second part of the night on west to northwest winds. Winds gust 25-30 kt the first part of the night, then diminish overnight. Seas near 5 feet on the outer waters. Monday... Weak high pressure builds over the waters. North winds less than 25 kt. Seas less than 5 feet. Outlook /Monday Night through Thursday/... Monday Night into Tuesday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain Tuesday. Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 9 ft. Rain. Local visibility 1 to 3 nm. Wednesday into Wednesday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 12 ft. Chance of freezing spray, slight chance of snow showers. Visibility 1 to 3 nm. Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of freezing spray, slight chance of snow showers. Areas of visibility 1 to 3 nm. && .HYDROLOGY... Run off from Thursday's 1-3 inch rainstorm continues to cause numerous rivers to remain in flood, however most have crested and are experiencing only minor flooding. Other sites of concern...most are above flood stage Taunton River near Bridgewater - falling Pawtuxet River at Cranston - falling Pawcatuck River at Westerly - falling Pawcatuck River at Wood River Junction - crested Connecticut River at Middle Haddam - rising slowly but with tidal influence River Flood Warnings continue at these locations. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 1 AM EST Monday for ANZ231>235-237. Small Craft Advisory until 10 PM EST this evening for ANZ230- 236. Small Craft Advisory until 7 AM EST Monday for ANZ250-251-254. Small Craft Advisory until 6 AM EST Monday for ANZ255-256. && $$ SYNOPSIS...Doody/BW NEAR TERM...WTB/Doody SHORT TERM...WTB LONG TERM...Doody AVIATION...WTB/Doody MARINE...WTB/Doody HYDROLOGY...