Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 202329 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 729 PM EDT Sun Aug 20 2017 .SYNOPSIS... High pressure will be the main influence on the region`s weather through Monday, with clear to partly cloudy skies and seasonable temperatures. An approaching low pressure system will increases chances for showers and thunderstorms Tuesday, with showers lingering into Wednesday. The remainder of the week is shaping up to be dry, with plentiful sunshine and below normal temperatures. && .NEAR TERM /THROUGH MONDAY/...
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As of 725 PM EDT Sunday...for the early evening refresh, very little was changed. just synced up the next few hours based on current observations. looking high and dry overnight, with just the outside chance of patchy fog for those few areas (primarily eastern VT and `Dacks) where locally cooler areas drop below crossover temperature (roughly 54-57F) for a few hours. Quick learning lesson, "crossover temperature" refers to when the air temperature drops below the lowest dewpoint reading during the prior afternoon. It`s a good fog forecasting technique under fairly quiet weather regimes where radiational cooling is expected. Tonight comes close to that type of scenario, except that we`ll be having 15-20 knots of wind about 500-1000 feet above the surface. That wind may keep just enough mixing going on in the boundary layer to keep fog at bay, well, at least thicker fog. In any case, if I`m spending this much time writing about a little patchy fog, you know it`s a very quiet weather night :) Rest of discussion from earlier this afternoon... Upper low retreating east and high pressure building tonight with clearing and subsidence. Surface high centered over the mid Atlantic states will move east during the period, and surface ridge extending north into the forecast area will shift east by Monday. Light northwest to west wind tonight will become southerly by Monday morning. Warm air advection on southerly flow, abundant sunshine and rising heights result in the potential for max temps on Monday to reach well into the 80s. 925mb temp of 22- 23C around BTV supports max temp near 89. Excellent prospects to see the partial solar eclipse on Monday with little in the way of cloud cover.
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&& .SHORT TERM /MONDAY NIGHT THROUGH TUESDAY NIGHT/... As of 332 PM EDT Sunday...Active weather during the short term time period. Deepening 500 mb trough to the west sweeping through the great lakes strengthening low level flow from the southwest, especially in the St. Lawrence valley. Precipitable water begins to climb Monday night, reaching nearly 2 inches by Tuesday. Surface dewpoints rebounding into the mid to upper 60s areawide, and around 70 not out of the question in the St. Lawrence valley. Low level jet of 30-40 knots at 925 through 850 mb increases to 50 knots at 500 mb mainly over the St. Lawrence Valley, provides for increasing 0-6km shear, however upper level jet dynamics don`t quite line up, with right rear/left front couplet lining up further west and north over southern Ontario. Surface boundary, mainly evidenced by wind shift and lowering dewpoints, will reach St. Lawrence valley Tuesday evening and sweep eastward through the forecast area by Wednesday morning. Storms to fire ahead of the boundary Tuesday afternoon in the St. Lawrence valley and move east through evening. Overall POPs increase Tuesday morning west to east, and locally run BTV4 based pops from model reflectivity were blended in to show categorical probabilities with a forecast line of storms. Storms to be strongest northern New York where low level jet dynamics and shear line up with surface based CAPE generally 1500- 2000 j/kg. As storms move east they will move out of best shear environment but continue to feed on moisture rich air and afternoon temperatures again into the 80s. Heavy rainfall rates possible with efficient warm rainfall processes, however storms should be moving quickly enough to preclude a flood threat. Added gusty wind/heavy rainfall wording to likely and higher POP Tuesday afternoon into evening. Overnight lows will not cool off much especially Monday night in high dewpoint air, and cooling/drying will not arrive in time Tuesday night to be of much help. Max temps Tuesday well into the 80s once again with warm air advection, and will be quite a muggy day. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... As of 253 PM EDT Sunday...The extended forecast will feature large scale pattern change as deep full latitude trof develops across the eastern Conus and ridge builds out west. This will result in temps returning to at or below normal values with much less humidity. The forecast challenge will be timing individual short waves in the southwest to eventually northwest flow aloft and potential areal coverage of showers. Overall...the atmosphere will have much less available moisture as pw values drop below 1.0 and instability is very limited. On Wednesday...first embedded 5h vort arrives around 18z...along with secondary surface convergence line with some additional weak low level cold air advection behind boundary. Instability is driven off sfc heating and developing cool pool aloft with deepening trof...so 30 to 40% chance of showers with maybe a rumble or two of thunder...but cape values are weak. Soundings show high cloud bases with deep dry layer so areal coverage of showers will be limited...but some localized gusty winds possible based on sounding profiles. Progged 850mb temps around 10c support highs upper 60s mountains to upper 70s warmest values. Low level cold air advection continues overnight into Thurs with progged 850mb temps dropping btwn 5-7c by 12z Thurs. The gradient flow will limit fog development and keep temps from completely bottoming out with lows mainly 40s mountains to mid/upper 50s valleys. Thursday...another embedded 5h vort in the west/northwest flow aloft and associated pocket of -20c air moves across our cwa. This energy will limited moisture will help to produce isolated/scattered showers during on Thurs aftn. Once again areal coverage and amount of showers will be limited...because of pws around 0.50 to 0.70. Progged 850mb temps near 6c support highs upper 50s/lower 60s mountains to lower 70s warmest valleys. Friday into next weekend...will feature building 1025mb surface high pres while aloft remains active with embedded 5h vorts in the northwest flow. Each piece of energy will have less impact with limited moisture...so will keep Friday dry at this time...with temps mainly 60s mountains and lower/mid 70s warmer valleys. Lows mainly in the mid 30s to upper 40s depending upon locations...with coldest values at SLK/NEK...especially on Saturday morning. Next weekend is looking mainly dry with slightly below normal temps and low humidity values. Highs in the 60s mountains and 70s valleys with lows ranging from the lower 40s to mid 50s. && .AVIATION /23Z SUNDAY THROUGH FRIDAY/... Through 18Z Monday...Aviation challenge tonight will be fog/br potential at slk/mpv...followed by increasing surface winds on Monday afternoon. Crnt surface map shows 1020mb high pres over central PA with ridge axis across northern NY. This has created clear skies and light winds with vfr conditions. Tonight...expecting vfr to prevail with exception of periods of ifr in fog/br possible at mpv/slk btwn 07-11z. Couple of factors limiting widespread fog include 950mb winds of 15 to 20 knots...two days removed from rainfall...and surface temps only briefly going below cross over values from earlier this aftn. Have mention tempo group at both mpv/slk for potential ifr in shallow fog/br. Otherwise...surface gradient increases on Monday with localized gusts to 20 knots possible at slk/mss/btv aft 16z with vfr prevailing at all sites. Outlook... Monday Night: VFR. Slight Chance SHRA...Slight Chance TSRA. Tuesday: VFR/MVFR. Breezy. Likely SHRA...Slight Chance TSRA. Tuesday Night: VFR. Breezy. Chance SHRA...Chance TSRA. Wednesday: VFR. Chance SHRA. Wednesday Night: VFR. NO SIG WX. Thursday: VFR. Slight Chance SHRA. Thursday Night: VFR. NO SIG WX. Friday: VFR. NO SIG WX. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Hanson NEAR TERM...Hanson/Nash SHORT TERM...Hanson LONG TERM...Taber AVIATION...Taber

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