Area forecast discussion National Weather Service Burlington Vermont 733 am EDT Sat may 25 2013 Synopsis... a deep trough and associated coastal low pressure system will produce a widespread 1 to 2 inches of additional rainfall across the north country this weekend. Furthermore...some accumulating snowfall will be possible across the higher summits of the Adirondack and Green Mountains. This rainfall will cause significant rises on local waterways...with some river flooding possible. Temperatures will be much below normal through Sunday...with a warming trend expected by early next week. && Near term /through Sunday/... as of 656 am EDT Saturday...very minor update to fine tune probability of precipitation/temperatures and winds based on current observation. Radar continues to show bands of light to moderate rain rotating across our County Warning Area...with some bright band signatures associated with the melting layer on kcxx radar. Water vapor shows a classic backside deformation zn developing across central Vermont into northern New York at this time. This will produce periods of rain...with some heavy at times through today. Quantitative precipitation forecast amounts look good. Just starting to see rises on local waterways and expect river flooding to become more of an issue by this afternoon. Flood Watch looks very good. Otherwise...temperatures will hold in the M/u 30s mountains to l/m40s valleys with some l/m50s possible in the slv today. Have discarded the lav guidance which shows btv dropping into the middle 30s by noon today...its late may and this seems highly unlikely. As for the snow...Pico web cam shows The Summit is white...with some mixing of snow and rain down to the Killington base area. Current accumulating snow levels are around 3500ft...expect them to drop between 2000 and 2500 feet by this evening...with some accumulation possible. Meanwhile...accumulating snow will continue across the eastern dacks above 3500 feet...as Whiteface Summit temperature is down to 27f. Overall forecast remains on track with expected quantitative precipitation forecast amounts and impacts across our County Warning Area. Still anticipating storm total precipitation of 1 to 2 inches with localized 3 inch amounts across the western slopes...parts of the cpv...and mountains of north-central Vermont by Sunday afternoon. In addition...still expecting some accumulating heavy wet snow above 2000 feet...especially Sat night into Sunday morning across the northern dacks and Green Mountains Forecast challenge is impacts of heavy rainfall on area waterways...followed by potential impacts of snow across higher terrain. Water vapor shows an impressive negatively tilted 7/500 mb circulation across the Middle Atlantic States with a baroclinic Leaf structure across our forecast area associated with rrq of 25h jet over eastern Canada. This closed system will advect plenty of deep Atlantic moisture into our County Warning Area through tonight...with favorable backside deformation developing by this evening. Overall...models in very good agreement with system becmg vertically stacked over the Gulf of Maine...and the associated quantitative precipitation forecast across our County Warning Area. GFS/NAM and European model (ecmwf) all show impressive 7h fgen forcing...strong 850 to 500mb q-vector convergence...and deep 850 to 500mb upward vertical velocities across our County Warning Area through 06z Sunday. This will result in periods of rain...which will be moderate to heavy at times...especially through this evening. By late tonight into Sunday...system will transition from large scale synoptic lift to mesoscale features drive by the terrain. GFS/NAM shows very favorable 850 to 700mb relative humidity and low level winds late tonight into Sunday for enhanced precipitation across the cpv...western slopes...and north/central mountains of Vermont. This favorable moisture/lift is present through 18z Sunday...before finally weakening by evening. Will continue to mention probability of precipitation near 100% for today and tonight...but likely/Cat probability of precipitation on Sunday...especially in the morning. Given...the deep closed circulation feel precipitation/clouds will linger through midday Sunday...but will taper off by afternoon. Also...feel quantitative precipitation forecast/probability of precipitation will be much less across the slv...as models show limited lift/rh. Thinking mostly low likely western dacks to chance probability of precipitation slv. This area will also see the warmest temperatures on Sat and Sunday...highs in the 50s. Still anticipating storm total quantitative precipitation forecast amounts between 1 and 2 inches with localized 3 inches possible...across the cpv...western slopes...and mountains of northern Vermont. Thinking cpv convergence and upslope follow will help to enhance quantitative precipitation forecast amounts across the western slopes and cpv. Much less quantitative precipitation forecast is anticipated across the slv/western dacks...generally <0.25". Also...thinking amounts across the CT River Valley will be closer to 1.0". This will cause significant rises on local waterways...with some main Stem flooding possible. See hdyro section blw for more information. Now for the hardest part of the forecast...trying to determine snow levels and potential impacts. NAM/GFS and local WRF BUFKIT soundings show thermal profiles becmg cold enough by 00z this evening for snow above 2000 feet. Still having a hard time determining where the cold air is coming from...as our low level cold air advection is limited. Thinking dynamic cooling and heavier precipitation will mix colder air aloft toward the surface...as cold core low moves closer. This is extremely tricky to determine...especially in late may...with a very high sun angle. Have noted 28f at Whiteface and 32 atop Mount Mansfield....with Whiteface web cam already showing the ground white...before sunset on Friday evening. Both GFS/NAM show a ribbon of 850 mb temperatures between -1c and -3c...and 925mb temperatures near 0c developing across our County Warning Area today. Thinking snow levels will start near Summit level (3500 to 4000 feet) today...but drop to near 2000 feet by this evening. Expect a sharp increase in accumulating snowfall across the mountains as you increase your elevation above 2500 feet. Thinking between 1 and 3 inches will be possible by Sunday morning between 2000 and 2500 feet...with over 6 inches likely at the summits from Montana Mansfield to Jay Peak. I realize we technically don't forecast for the summits...but given the Holiday weekend and potential for hikers/campers in the mountains and possible isolated power outages will issue Special Weather Statement to highlight this potential. On a side note...if we can clear skies late Sunday afternoon...the snow covered mountain summits should make for some great pictures. The combination of precipitation/clouds and prognosticated 850 mb temperatures near 0c...will support highs only in the M/u30s mountains to l/M 40s cpv/CT River Valley. Warmest temperatures will be across the slv...readings will warm into the 50s. Temperatures will only fall slightly tonight...as winds/precipitation and clouds prevail...thinking l/M 30s mountains to u30s to l40s. On Sunday...temperatures will becm tricky...especially if we break out into some sun late in the afternoon. Expect late day highs for the cpv...with values reaching the l/M 50s...with mainly 40s in the mountains and l60s in the slv. Most of the Marathon will experience light rain with cool temperatures and breezy north winds...especially near the lake on Sunday morning. Conditions will improve by Sunday afternoon with scattered showers and temperatures slowly warming to near 50f...and lighter winds. && Short term /Sunday night through Monday/... as of 440 am EDT Saturday...mid/upper level trough will slowly lift NE into the Canadian Maritimes as surface high pressure builds into our County Warning Area. This will result in a slow clearing trend with temperatures returning to near normal levels by Monday. Latest guidance suggests the potential for some frost across the sheltered valleys of the dacks and parts of Vermont. This maybe difficult given the very saturated llvls and potential for fog development if skies clear and winds decrease. && Long term /Monday night through Friday/... as of 440 am EDT Saturday...pattern change takes place in the extended as drier northwest flow aloft exists Monday night through Tuesday night. We will start to see a gradual warming trend take place during this time period. More pronounced warming will take place during the middle and latter part of the week when a warm front lifts north of the area and we get into the warm sector with building upper ridge moving in. First the warm front will provide a focus Wednesday for convection as sufficient instability moves into the region. Thus have kept a chance of thunderstorms in the forecast for Wednesday with some lingering threat on Thursday as front gradually lifts north of the border. Once it does...southwest flow aloft develops and brings in noticeably warmer low level temperatures and this suggests highs climbing into the upper 70s to middle 80s Thursday and Friday. Lack of forcing suggests convective threat is minimal and have thus kept the forecast dry on Friday. && Aviation /12z Saturday through Wednesday/... through 12z Sunday...expect conditions to remain in the MVFR and IFR categories at most locations due to clouds...fog...and rain. The only exception to this will be kmss where VFR conditions will exist as they will be far enough west from the low pressure system along the coast that is producing all the clouds and precipitation over the remainder of the area. Watch for winds to increase after 14z from the north with gusts in the 15 to 25 knot range expected. After 06z the wind gusts will taper off to around 10 knots...but ceilings will still be in the MVFR/IFR categories and visibilities will improve a bit into the MVFR and VFR categories. Outlook 12z Sunday through Wednesday...the combination of a surface boundary and associated low pressure will keep showers in the taf sites through the weekend. These showers will produce periods of MVFR ceilings/visibilities with some IFR conditions possible...especially at night. In addition...breezy northwest winds can be expected over the weekend as surface low pressure tracks into the Gulf of Maine on Sunday. A slow clearing trend is anticipated on Monday...with mainly VFR conditions Tuesday into Wednesday. && Hydrology... as of 440 am Saturday...a widespread rain event will continue today...storm total precipitation values of 1 to 2 inches with localized 3 inch amounts are possible across the western slopes...parts of the cpv...and mountains of north-central Vermont by Sunday afternoon. Locally run sshp model hydrographs suggest these totals will likely bring numerous small streams to bankfull...and several mainstem rivers into flood by this afternoon into Sunday. Thus a Flood Watch continue for this potential. At this time...the rivers of most concern would be the missisquoi...LaMoille...Otter...and Passumpsic in Vermont...and the ausable river in northern New York. Given the current trends in rainfall timing and amounts...most river crests are expected to occur late tonight or Sunday morning. && Marine... as of 440 PM Saturday...looking to be a pretty miserable start to the prime boating season on Lake Champlain. A storm more typical of winter will bring an extended period of strong northerly winds over the lake. Expect winds in the 20 to 30 knot range into early Sunday. Expect frequent gusts up to 35 knots, especially across the Open Lake. For that reason, a lake Wind Advisory has been posted and will remain in effect until the winds fall below 25 knots. These winds will make for very rough conditions, with waves across the Open Lake ultimately building upwards of 4 to 6 feet. A steady rain and temperatures only in the 40s on today will add to the misery. The outlook for Monday still looks like it will be the best day of the Holiday weekend for boating. Northwest winds will be in the 10 to 15 knot range with mostly sunny skies and milder temperatures. && Climate... with the unseasonably chilly weather expected for today...some record low maximums may be approached or broken. They are listed below. Burlington: 51f (1921,1925) montpelier: 48f (1967) massena: 55f (1969,1979) St johnsbury: 52f (1931,1925) && Btv watches/warnings/advisories... Vermont...Flood Watch through Sunday afternoon for vtz002>012-016>019. New York...Flood Watch through Sunday afternoon for nyz028-031-034-035. && $$ Synopsis...Taber near term...Taber short term...Taber long term...evenson aviation...evenson/rjs hydrology...weather forecast office btv marine...weather forecast office btv climate...weather forecast office btv