Scientific Forecaster Discussion

000 fxus61 kbox 180919 afdbox Area forecast discussion National Weather Service Boston/Norton Massachusetts 419 am EST Mon Feb 18 2019 Synopsis... low pressure tracking south of New England will bring periods of accumulating light snow today to the entire region with the bulk of the snow accumulation occurring early this morning. As the low exits drier, colder and blustery weather follows tonight into Tuesday. Strong high pressure builds over New England Wednesday providing dry and cold weather but with light winds. Another low tracks up the East Coast Wed night and Thursday with snow overspreading southern New England but likely changing to a wintry mix inland Thursday and to rain along the South Coast. High pressure follows Friday and Saturday with a return to dry, colder weather. Another round of a wintry mix of precipitation is possible in the Saturday night - Sunday timeframe. && Near term /until 6 PM this evening/... *** periods of light to moderate snow continue *** Overall, the few reports received so far this morning are generally at or below 2 inches of snow. This is likely a result of two factors, the slow process of moistening the low-mid lvls in spite of increased moisture through the dgz and the split banding dynamics. The northern most, linked to mid lvl wave convergence has manifest as a somewhat disorganized band which impacted areas up to the Mass Pike through about 2am local. The second was co-located with the strongest convergence along a very meager low level jet. This is mainly a case where even through the overall dynamics were weak and split, the strongest lift resulting was within the fully saturated dgz. Meanwhile, of late, the focus is along a modest mid lvl deformation band coincident the leading edge of the upper lvl warm front which is slowly pushing north from New Jersey and S of Li. Banding has been more wave-like with a west-east crest/base orientation and with some drier air already filtering in from the north the slrs have dropped off as the snow-type is more grainy than dendritic. With this progression, and the overall dynamic forcing shifting toward a lingering inverted trof snowfall rates will likely drop to light-moderate across the board, especially as mid lvl warming leads to a transition to sleet or even rain along the immediate S coast. The only caveat will be, as winds shift to the E, some coastal enhancement along Plymouth Colorado Massachusetts and portions of the cape, which could lead to locally enhanced snowfall rates through the day even as rates become lighter from west-east with deeper dry air entrainment. Low pres passes southeast this afternoon, with winds shifting toward roughly 320, which will, even as the snow ends elsewhere, then force the ocean effect to linger later across Cape Cod, which will likely be the location where snow ends the latest, mainly late evening. Lots of both positive and negative factors for snowfall to consider, but given sn, when observed is producing roughly 1-2in in 3hr with the disorganized banding this am. Given the coastal enhancement possible, higher totals are still possible across portions of southeast Massachusetts or even into Rhode Island. Therefore, will maintain the current winter storm and winter weather advisories as they stand. One other note, there may be a mix of light freezing drizzle especially as snowfall rates drop as drier air filters in from above. Light glaze is possible as snow dissipates. && Short term /6 PM this evening through 6 PM Tuesday/... overnight... any remaining snow/fzdz will dissipate as drier continues to filter through the column to the sfc. Pres rises, as low pres continues to deepen offshore and high pres moves east from the Great Lakes will lead to increasing northwest flow, enhanced by cold air advection. Even with clearing (slowest along Cape Cod due to ocean effect) the pres rises and sfc gradient will limit radiational cooling. Morning mins mainly in the teens and low 20s although wind chills drop into the single digits. Tue... northwest flow continues with 850 mb temps dropping below -12c or about 2 std deviations below normal. This suggests lows, even with full mixing will struggle to reach the low 30s, and this will be the cap based on current guidance. Otherwise dry as high pres moves overhead. && Long term /Tuesday night through Sunday/... * dry and cold conditions Monday night into Wednesday * snow Wed night but likely changing to a wintry mix inland Thu and to rain along the coast * dry and cold weather returns later Thursday into Saturday * another round of wintry precipitation possible Sat ngt/Sunday Overview... Three distinct features stand out in the long wave pattern this period, 1) western Continental U.S. Trough, 2) subtropical ridge near the Bahamas and 3) fairly strong northern stream jet across eastern Quebec into Newfoundland. This pattern will eject two systems into New England with Gulf of Mexico moisture, first event Wed ngt/Thu and another Sat ngt/sun. The subtropical ridge will provide a storm track close to the southern New England coast with parent low pressure traversing into the Great Lakes while secondary low pressure develops along or near the South Coast. This will help to maintain shallow cold air across the interior. However with mid level low tracking well northwest of New England, initial deep cold air over southern New England erodes as mid level warm air invades the region. This setup supports snow changing to a wintry mix inland and snow to rain especially along the South Coast. The challenge to this portion of the forecast will be the amplitude of the northern stream across eastern Quebec and Newfoundland, as this will dictate the duration of cold air over New England. Ultimately this will determine how quickly snow changes over to a wintry mix and eventually rain. Details... Tuesday night and Wednesday ... Dry cold northwest flow across the region this period with deep layer ridge advecting into the area Wed as 1037 mb high crests over the region. This will provide cold temps Tue ngt with lows in the single digits and teens and then only rebounding into the upper 20s and low 30s Wed as strong high yields subsidence inversion/shallow boundary layer. Given the ideal radiational cooling conditions Tue ngt/early Wed leaned toward the colder MOS guidance for lows. While it will be cold Wed high pres overhead will provide light winds and sunshine, taking the edge off the cold. However sunshine will likely fade behind increasing mid/high clouds in the afternoon. Wednesday night and Thursday ... Parent low enters the Great Lakes while secondary low forms along the mid Atlc coast and moves northeast along or near the CT/RI/MA coastline early Thu before moving offshore Thu afternoon. Column is initially cold enough to support snow but with mid level low tracking well northwest of southern New England, mid level warm air invades the region changing snow to a wintry mix inland as secondary low traps low level cold air at least into the morning hours. However along the South Coast should be enough low level warming for snow to rain with temps climbing well into the 40s. Not a major storm by any means but may have enough snow/wintry mix away from the South Coast for some travel impacts. As of now it looks like the bulk of the precip occurs after the Wed evening commute and before the Thu morning rush hour. However given the time range here (72+ hrs out), the forecast timing will likely change by at least a few hours. Given mid level low becomes and open wave system becomes progressive with drying trend later Thu. Friday and Saturday ... Dry northwest flow aloft becomes reestablished over New England along with seasonably cool temperatures. Leaned toward the colder MOS temps Fri ngt/Sat morning given 1030+ mb high cresting over the region. Saturday night and Sunday ... Next system in the continuous southwest flow impacts the region Sat ngt/Sunday. Antecedent airmass not as cold as mid week system, thus risk of some snow to wintry mix on the front end before ending as rain but duration and amounts of snow/ice look to be less than mid week event. && Aviation /09z Monday through Friday/... forecaster confidence levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short term /through Tuesday/...high confidence. Through this evening... mix of predominantly IFR and MVFR in widespread light to moderate snow. The lowest vsbys will occur through about 15z...with mainly +2sm thereafter as snow continues. Light snow accumulations expected all runways. Low risk for a mix of light fzdz mainly late morning into the evening with a trace of ice accumulations in spots. Otherwise, winds shifting to the east but remaining mainly at or below 10 kt. Overnight into Tuesday... any lingering snow ends around 03z, rapid improvement to VFR all sites especially as winds shift to the northwest. Northwest flow and VFR conditions then linger into Tuesday. Kbos terminal...high confidence in taf overall. Timing of heavier snow may be off a bit. Kbdl terminal...high confidence in taf. Timing of heavier snow may be off a bit. Outlook /Tuesday night through Friday/... Monday night: mainly VFR, with local MVFR possible. Breezy. Tuesday through Tuesday night: VFR. Breezy. Wednesday: mainly VFR, with local IFR possible. Chance snow. Wednesday night: mainly IFR, with areas MVFR possible. Breezy. Snow likely, freezing rain likely, pl likely. Thursday: mainly MVFR, with areas VFR possible. Breezy. Chance ra, chance freezing rain. Thursday night through friday: VFR. Breezy. && Marine... forecaster confidence levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short term /through Tuesday/...high confidence. Light easterly flow will gradually increase reaching about 25-30 kt at times just before it shifts to the northwest this afternoon and evening. Small craft advisories continue, and then linger into Tue as seas peak around 8 ft across the southeast ocean waters Tue morning. Light freezing spray is possible late tonight and Tue, mainly offshore of the East Coast. Outlook /Tuesday night through Friday/... Monday night: low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of snow, chance of freezing spray. Tuesday: winds less than 25 kt. Local rough seas. Tuesday night: low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Wednesday: winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of snow. Local visibility 1 to 3 nm. Wednesday night: winds less than 25 kt. Seas locally approaching 5 ft. Snow likely, rain likely, sleet likely, freezing rain likely. Visibility 1 to 3 nm. Thursday: low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Rain likely, freezing rain likely. Thursday night: winds less than 25 kt. Local rough seas. Friday: winds less than 25 kt. Areas of seas approaching 5 ft. && Tides/coastal flooding... high astronomical tides Wed and Thu, with heights around 12 foot in Boston. Most concerned with Thu midday high tide given risk of weak low pressure moving across the region providing onshore flow. Estofs offering a surge of about 0.8 ft and ensemble storm surge from Stevens Institute indicating near 1 ft of surge. Etss not very representative with almost zero surge. Therefore will discount this model and follow estofs and Stevens Institute ensemble surge guidance. With potential storm surge up to 1 ft and if occurring at time of high tide we can expect a round of minor coastal flooding with the Thu midday tide cycle. Still 3+ days away so will have to watch timing of secondary wave. && Box watches/warnings/advisories... CT...Winter Weather Advisory until 4 PM EST this afternoon for ctz002>004. Massachusetts...Winter Weather Advisory until 4 PM EST this afternoon for maz002>016-022>024-026. Winter Storm Warning until 4 PM EST this afternoon for maz017>021. Rhode Island...Winter Weather Advisory until 4 PM EST this afternoon for riz006>008. Winter Storm Warning until 4 PM EST this afternoon for riz001>005. Marine...Small Craft Advisory from 1 am to 7 am EST Tuesday for anz232. Small Craft Advisory from 10 PM this evening to 5 am EST Tuesday for anz233-234. Small Craft Advisory from 10 PM this evening to 10 am EST Tuesday for anz231-251. Small Craft Advisory from 10 PM this evening to 7 am EST Tuesday for anz235-237. Small Craft Advisory from 3 PM this afternoon to 7 PM EST Tuesday for anz250. Small Craft Advisory from 1 PM this afternoon to 1 PM EST Tuesday for anz254>256. && $$ Synopsis...nocera/doody near term...doody

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