872 FXUS61 KBTV 191815 AFDBTV AREA FORECAST DISCUSSION National Weather Service Burlington VT 215 PM EDT Sun May 19 2019 .SYNOPSIS... A warm front will produce a band of rain across northern New York into Vermont this morning. Much warmer temperatures with some humidity will create an environment favorable for additional scattered showers and thunderstorms this afternoon and evening. A series of surface fronts on Monday will interact with heat and moisture to produce strong to locally severe thunderstorms. The primary threat will be isolated damaging winds, along with frequent lightning and brief heavy down pours. Cooler and drier weather returns for midweek. && .NEAR TERM /THROUGH MONDAY/... As of 159 PM EDT Sunday...A Severe Thunderstorm Watch has been issued for much of our forecast area, in effect until 9 pm this evening. Convection associated with an approaching upper shortwave has developed across western NY...and this activity will push eastward this afternoon and evening. It will be aided by convergence along a surface boundary that currently stretches from southern St Lawrence County, NY eastward into central Vermont. Locations south of this boundary are in the upper 70s/lower 80s, with dewpoints in the 60s, resulting in CAPEs of 600+ J/kg. Meanwhile, PWATs of 1-1.3 inches are spreading into the region. With 30-40 kt of SW shear and steepening lapse rates due to daytime heating, any stronger thunderstorms will be capable of producing damaging winds, along with marginally severe hail. Heavy downpours will be possible, as well. Have updated the forecast to incorporate the watch and made necessary adjustments to temps, PoPs, and weather to match the latest trends/guidance. Previous discussion...Active period of weather anticipated with the threat for strong to locally severe thunderstorms increasing on Monday. Water vapor this morning shows embedded 5h vort across central Lake Ontario with regional VAD profiles indicating breezy southwest flow of 45 to 55 knots associated with 925mb to 800mb waa. This favorable low level jet is helping to enhance a pocket of elevated instability with some lightning activity noted over central Lake Ontario attm. This band of rain with embedded rumbles of thunder is progged to lift across northern NY between 09z-15z this morning. The axis of best 850 to 700mb fgen forcing and associated moisture convergence is over northern NY, but some moisture/lift streaks into central/northern VT. Have continued with chc/likely pops thru 15z, with qpf generally a 0.05 to 0.25 across this region. Guidance shows a break in the action from roughly 15z-20z, before additional showers and greater threat for thunderstorms develop. Still some question on forcing along with amount of instability. Crnt thinking weak 5h vort rides along northern periphery of building ridge provides enough upper lvl support, while trrn driven sfc boundary enhances low level convergence for additional precip chcs. Latest HRRR/NAM 3km and BTV 4km show showers with embedded storms developing across southern SLV/Western Dacks around 20z, then spreading into central/southern Vt where sfc based instability will approach 1000 j/kg. Deep layer shear is limited, as best 850 to 500mb flow is located northwest of our cwa, while best instability parameters are across central/western NY. Thinking better chc for strong/severe storms will be across this region, as southwest mid/upper lvl flow may advect high lvl debris clouds into our fa and also limit instability. Bottom line additional showers are expected with some embedded storms and maybe a localized wind gusts in the 40 to 45 mph range if enough instability can materialize this aftn. Have mention likely pops along and south of a Potsdam to BTV to White River Junction line, with chc north. Temps are challenging today, with sharp boundary developing associated with very warm 925mb temps of 18c to 20c across our southern fa, but values in the 12 to 14c range near the international border with some ne sfc winds across the slv and northern cpv developing. Given thermal profiles thinking highs range from mid 60s slv to l80s southern cpv. Would not be surprised to see BTV near 80f while PBG is m/u 60s with se/ne winds off the cold Lake Champlain waters. Tonight areal coverage of precip should decrease around sunset with best upper level support shifting east and instability quickly dissipating. However, have noted in some meso models the threat for additional showers and embedded storm aft 06z as short wave energy and weak pre-frontal trof approach fa. Have continued to mention at least chc pops overnight with temps in the mid 50s to mid 60s with some humidity. Monday...another active day with the threat for strong to severe thunderstorms increasing, as forward progression of boundary is slightly slower in the latest guidance allowing for more sfc heating/instability to develop. Still some question on regards to amount of clearing as soundings show deep moisture and potential for clouds, but most meso scale models show sfc temps warming into the mid/upper 70s with dwpts in the 60s, creating sfc based cape values in the 1200 to 2000 j/kg range. In addition, a ribbon of 40 to 55 knot winds btwn 850mb and 700mb will produce 0 to 6 km shear values in the 40 to 50 knot range. In addition, as sfc temps warm and profiles aloft cool associated with height fall from approaching trof, lapse rates in the lowest 3km will steepen, creating favorable environment for winds to transfer toward the sfc. Furthermore, dcape values are in the 600 to 900 j/kg, with some evidence of a dry layer btwn 700-500mb, helping with dry air entrainment for enhanced wind potential. Given the thermodynamics and mainly unidirectional wind flow, the convective mode will be multiple cellular with some embedded bowing segments, capable of producing localized damaging winds. How robust the convection becomes will be largely depend upon amount of instability that can develop, as multiple trof and boundary in the progressive southwest flow move across our cwa. PW's values near 1.50 and warm cloud depths near 10kft, so any convection will have the potential to produce localized very heavy rainfall rates, but storm motions of 35 to 45 mph will limit hydro concerns. The flash flood threat is minimal, but not zero given the potential for multiple storms. Temps on Monday should warm mainly into the 70s with some lower 80s possible. && .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY/... As of 240 AM EDT Sunday...Cold air advection regime for Monday night and Tuesday as departing cold front continues to push east of the area. Will have continuing chance for showers on both Monday night and Tuesday as upper level low still centered to our north will bring some wrap around moisture into our area. Late in the day Tuesday surface and upper level lows will both finally begin to push east of the area and drier weather will return for Northern New York and Vermont. Temperatures Monday night will dip into the 40s and then return to the 50s on Tuesday. && .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/... As of 240 AM EDT Sunday...The long term will start out quiet with dry weather expected from Tuesday night through early Thursday. Will have our next chance for showers on Thursday and into Thursday night as a broad low pressure area crosses the region. A return to dry weather for Friday, then a low pressure system passes well to our north Friday night and Saturday but spreads some showers across the area as a cold front passes early Saturday morning. The GFS is currently showing a slower solution than the ECMWF and has showers continue into Sunday. The ECMWF favors the faster solution which clears us out with a surface ridge building in for Saturday night and Sunday and drier weather. Lots of uncertainty for next weekends forecast at this time. Tuesday night and Wednesday start out the period with below normal temperatures, but then temperatures warm above normal headed towards the end of the week. && .AVIATION /18Z SUNDAY THROUGH FRIDAY/... Through 12Z Monday...Mainly VFR conditions early this afternoon, trending to MVFR as scattered showers and thunderstorms move into the region. Any thunderstorm activity will be capable of producing gusty winds, locally heavy rainfall, and perhaps some hail. MVFR conditions, possibly briefly IFR, are expected in any convection. The shower activity should decrease in areal coverage late this evening, but still some threat for additional showers after 06z, so continued with VCSH wording. Given the anticipated rainfall and developing south to southwest flow of 10 to 20 knots, soundings indicate low level moisture will increase with some stratus likely. Would not be surprised IFR cigs develop at SLK after 06z, with MVFR possible at most other sites. Winds will be variable today with NE at MSS and becoming south at other sites, except southeast to northeast at PBG thru the afternoon hours. Outlook... Monday Night: Mainly VFR, with local MVFR possible. Chance SHRA. Tuesday: Mainly VFR, with local MVFR possible. Chance SHRA. Tuesday Night: Mainly VFR, with local MVFR possible. NO SIG WX. Wednesday: VFR. NO SIG WX. Wednesday Night: VFR. NO SIG WX. Thursday: VFR. Chance SHRA. Thursday Night: VFR. Chance SHRA. Friday: VFR. Chance SHRA. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Taber NEAR TERM...Hastings/Taber SHORT TERM...Neiles LONG TERM...Neiles AVIATION...Hastings/Taber