Area Forecast Discussion
Issued by NWS Burlington, VT

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732 FXUS61 KBTV 291924 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 324 PM EDT Wed Jun 29 2016 .SYNOPSIS... An upper level low pressure system moving from west to east across the North Country this afternoon and evening will bring scattered showers and thunderstorms. Locally heavy downpours and small hail will be possible with the stronger storms. High pressure then builds into the Northeastern United States for Thursday with drier and mostly sunny conditions. High temperatures will generally range from the upper 70s to lower 80s, with lower humidity levels. Another frontal system brings active weather for Friday, with afternoon showers and thunderstorms expected. Some storms may become strong to locally severe, along with heavy downpours. The Independence Day weekend outlook looks mostly dry and favorable for outdoor plans. && .NEAR TERM /THROUGH THURSDAY/... As of 221 PM EDT Wednesday...Slow-moving upper low with H5 vort axis is continuing a slow progression across the Champlain Valley at 18Z, and will translate east of the CT river valley around 00Z this evening. Associated mid-level cold pool with 500mb temps -15C to -16C contributing to low wbz heights (7-8kft) and surface- based CAPE values of 500-1000 J/kg ahead of main convective line extending from eastern Franklin County VT swd across the VT side of the Champlain Valley. Mid-level forcing ahead of 500mb trough and sufficient instability will maintain line of showers and thunderstorms ewd across central/ern VT through early this evening. Main threat will be small hail and locally heavy downpours. It appears that developing mesoscale cold pool will maintain ewd progression of 10-15kts, and minimize duration of heavy rainfall. May still see localized rainfall amounts 1 to 1.5", but generally below flash flood guidance values. Heavier rainfall footprints yesterday were across portions of Orleans/ Caledonia/Essex counties, and can`t rule out some isold flooding issues in that region late afternoon into early this evening. Once mid-level trough axis moves to the east late this evening, will see diminishing PoPs and gradual clearing from west to east, especially after midnight. Given widespread rainfall last 24-48 hours, and light winds late as weak high pressure builds in from the west, locally dense fog is anticipated across the typically favored valleys of central/ern VT, and within the valleys of the Adirondack region, mainly 05-11Z. Overnight lows generally low-mid 50s, except a few 40s in the nrn Adirondacks. Weak ridge of high pressure in place surface and aloft on Thursday with generally light winds and mostly sunny conditions. High temps generally upr 70s to lower 80s. && .SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/... As of 320 PM EDT Wednesday...Weak ridging on Thursday will continue into Thursday night, shifting east by Friday morning. For Friday southerly gradient winds increase in advance of next surface trough and vigorous 500 mb trough moving through the great lakes. Rain begins Friday afternoon in the warm air advection pattern and continues until a cold frontal passage Friday night. Some ingredients present for heavy rainfall Friday and added it to the forecast 18-24z Friday. 850 mb flow from the south at 20-30 knots will bring in low level moisture along/ahead of the boundary. Model sounding shows tall skinny CAPE profile, with relatively low CAPE value and saturated conditions. Warm cloud depth just a little shy of the preferred 3km depth, and Precipitable Water values near 1.4-1.5" are in the 75th percentile. Playing down the severe threat now with surface max temps forecast in the upper 70s to near 80 which would reduce CAPE. 0-3 km shear less than 30kts, and highest shear values coincide with the frontal passage later Friday and do not line up well with the thermodynamics. && .LONG TERM /SATURDAY THROUGH TUESDAY/... As of 320 PM EDT Wednesday...Saturday becomes our typical post frontal regime with the upper low settling in over the area with lowering heights, residual low level moisture and west to northwest uplslope flow. As a result chance showers during daytime heating Saturday, mainly across the northern portion of the area closer to the upper low centered over Quebec. Cool through the weekend until the upper low begins to pull away on Monday, with a dry 3rd & 4th of July forecast. Broad ridge over the central US sets up for the rest of the week. a shortwave trough moves west to east in westerly flow but will remain mainly south of the forecast area across southern NY and PA on Tuesday. Weak ridge builds in for mid week with seasonably warm temperatures. && .AVIATION /19Z Wednesday THROUGH Monday/... Through 18Z Thursday...Scattered shower and thunderstorm activity mvg eastward across the North Country this afternoon and early this evening. Will see localized MVFR to IFR conditions with heavy downpours, with small hail also possible in strongest convection. Convective activity generally clears the region to the east after 01z Thursday. Aside from the tstm activity, winds are generally light W-NW at 5-8kts, and will become light and variable tonight. Expect to see patchy dense fog development tonight, especially for MSS/MPV/SLK between 04-11Z. Generally VFR for the daylight hrs Thursday with light westerly winds. Outlook 18z thursday through Monday... 18z Thursday through 12z Friday...VFR except localized LIFR radiational fog likely at MPV and SLK 06-12Z Friday morning. 12z Friday through 12z Saturday...VFR trending MVFR/IFR as a cold front brings scattered showers and possible strong thunderstorms during Friday afternoon. Convective activity should move east of the region and diminish Friday night. 12z Saturday through Monday...Mainly VFR, except few -SHRA possible nrn mountains Saturday aftn/early evening. && .HYDROLOGY... Scattered shower and thunderstorm development ongoing this afternoon as 500mb low and associated thermal trough (-15 to -16C) shifts slowly ewd across the North Country. NAM soundings suggest slow-moving activity, with NEWD propagation/MBE vectors of 5-10kts. Consistent with that, have seen training thunderstorms with radar estimated rainfall of 1 to 2" across Grand Isle county and also across Eastern Essex NY on wrn shore of Lake Champlain. Air mass remains unstable ewd across VT this aftn into this evening with SBCAPE values generally 500-1000 J/kg. PW values are a bit lower today, generally 1.1 - 1.2" and 3-hr flash flood guidance of 2 to 3" most sections. Can`t rule out some localized flooding concerns this afternoon and evening, but mainly across the areas that observed heavier rainfall yesterday (namely portions of srn Franklin NY into wrn Clinton counties, and also across Orleans, Caledonia, and Essex counties in VT). Will monitor convective and radar trends through the afternoon and evening hours. For Friday, heavy rainfall potential raises local flood concerns. Factors arguing against flood threat is rainfall will be a quick hit between 18z and 00z, and despite recent rainfall overall antecedent conditions remain fairly dry. Biggest concern would be for heavy rainers over small areas to produce flash flooding, especially if rain falls over the isolated spots where we`ve had rain in the past 24 hours. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Banacos NEAR TERM...Banacos SHORT TERM...Hanson LONG TERM...Hanson AVIATION...Banacos HYDROLOGY...Banacos/Hanson

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