Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 182312 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 712 PM EDT Fri May 18 2018 .SYNOPSIS... A low pressure system will bring widespread rain showers to the North Country Saturday. Rain will continue overnight and then gradually become limited heading into Sunday as a frontal system tracks through the region. High pressure quickly builds back in and the outlook for next week is above normal temperatures with dry conditions. && .NEAR TERM /THROUGH SATURDAY NIGHT/...
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As of 704 PM EDT Friday...Surface analysis places 1030mb high pres just north of Sherbrooke, Quebec with mainly clear skies across our fa. This high pres and very low sfc dwpts in the upper teens to upper 20s will combined with mainly clear skies through midnight to allow sfc temps to drop quickly after sunset. Thinking overnight lows will range from the upper 20s nek/slk to l/m 30s parts of central VT to l/m 40s cpv/slv with some patchy frost possible in deeper/protected valleys. Otherwise...clouds increase toward morning, along with increasing south winds associated with developing low level jet of 20 to 30 knots at 1000 feet. This jet will help to increase moisture with clouds lowering and thickening on Saturday Morning. Have delayed arrival of rain by several hours, based on latest trends...which support rain arriving btwn 10-noon south and noon and 2pm central and btwn 2 pm-4pm near the International border. Some downslope shadowing is likely over the cpv with developing 850mb southwest jet of 40 to 50 knots off the dacks. Temps warm into the 50s to lower 60s, but cool back into the l/m 50s when the rain arrives. All covered in crnt forecast. Previous discussion below: WV/Vis satellite imagery paints the picture for today. Its been largely clear skies with little in the way of active weather. Dry weather has allowed RH`s to fall into the low 20% range today. With large dewpoint depressions and high pressure still impacting the region anticipate lows this evening falling quickly. Lows will generally be in the 40s for most areas with low 30s likely in the normal cold hollows of the Northeast Kingdom and the Adirondacks. Southerly winds pick up rapidly in the morning on Saturday as a low pressure digs to the west. Warm air advection precip should begin tracking into southern Vermont near daybreak with the rain overspreading the region by mid day. Biggest change with this forecast package was to delay the onset of precipitation in the northern Champlain Valley and for parts of northern Vermont. Even though we`ll be under warm air advection, we`ll see widespread rain and cloud cover so temps will be on the cooler side in the mid to upper 50s for highs. The widespread rain showers will put a damper on outdoor activities on Saturday. At some point overnight guidance is keying in on a lull in the precip as a dry slot will move into but as the upper level trough sweeps through anticipate another round of showers during the late night hours heading into daybreak on Sunday. The thunder chances will be mainly during the late night hours on Saturday night but are minimal at best and SPC calibrated thunder progs give us less than a 5% chance. Lows Saturday night will be in the mid 50s as the warm air advection continues overnight.
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&& .SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/... As of 308 PM EDT Friday...System cold front will be passing through the area Sunday morning before clearing well east by later in the afternoon. Model consensus largely supports this scenario and as such have maintained higher pops from the Adirondacks east into Vermont through the morning hours before showing sharp drying by afternoon. Weak elevated instability lingers ahead of the main boundary across eastern and southern zones during this early period and will also introduce an outside shot of a few non-severe storms, mainly across southern VT accordingly. By afternoon expect increasing amounts of sunshine on modestly gusty northwesterly flow as the post- frontal boundary layer deepens to 850 mb. High temperatures will occur toward the later half of the afternoon given morning rainfall and clouds, generally from 66 to 73. Deep layer drying along with fair and dry weather then continue for Sunday night as ridging builds east from southern Canada and the Great Lakes states. Some patchy fog will will possible in favored mountain hollows and river valleys as lows bottom out from the upper 30s to mid 40s with some customary variability. Winds light. && .LONG TERM /MONDAY THROUGH FRIDAY/... As of 308 PM EDT Friday...A weak surface high will be in place Monday with highs in the 60s across most mountain areas with 70s in the valleys. Tuesday will start off with increasing clouds during the day, then with rain chances increasing in the evening hours. Between yesterday and today, the ECMWF and the GFS have almost entirely traded places in how they handle a weak system that will push through Tuesday night into Wednesday morning. Run-to-run inconsistency aside, there does appear to be better overall consensus than yesterday. Instability is marginal, with mainly elevated CAPE values of 100 J/kg or less across our area. Mid-level lapse rates of 7 to 8 C/km certainly leaves some wiggle room for convection, though. Most of this convection will be capped near 700mb by a weak warm nose, which should prevent cloud graupel from forming and thus keep thunder potential to a minimum. Additionally, the overall lack of strong positive vorticity advection and warm air advection still has me concerned that forecast models are still too bullish on height falls and the resulting return flow of warm, moist air. A cold front pushes through Wednesday night, but most moisture will be advecting away with the initial shortwave, so not much activity is anticipated with the front at this time. On Thursday, A longwave trough will amplify along or just off the US East Coast, which should scour out any remaining moisture and bring temperatures down towards seasonal norms. && .AVIATION /23Z FRIDAY THROUGH WEDNESDAY/... Through 18Z Saturday...Generally VFR through the overnight hours before lowered ceilings and rain starts to move into the region. Winds will be largely orographically forced through the afternoon as channeled flow continues through both the Saint Lawrence and Champlain valleys with light and variable elsewhere. Winds become light overnight but will rapidly pick up out of the south southeast in the morning hours Saturday as mid clouds move in with a ceiling around 6000-7000 feet by 12z. Rain showers will spread from the southwest to the northeast with rain expected at SLK and RUT by 14-16z Saturday. Within any of the showers there`s potential for some MVFR visibilities. Outlook... Saturday Night: Mainly VFR, with local IFR possible. Likely SHRA, Slight chance TSRA. Sunday: Mainly VFR, with areas MVFR possible. Chance SHRA. Sunday Night: VFR. NO SIG WX. Monday: VFR. NO SIG WX. Monday Night: VFR. Slight chance SHRA. Tuesday: VFR. Chance SHRA. Tuesday Night: VFR. Chance SHRA. Wednesday: Mainly VFR, with local MVFR possible. Chance SHRA. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Deal NEAR TERM...Deal/Taber SHORT TERM...JMG LONG TERM...Haynes AVIATION...Deal

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