Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 031432 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 1032 AM EDT Mon Aug 3 2020 .SYNOPSIS... A brief break in active weather will occur today in the wake of exiting low pressure to the northeast and Tropical Storm Isaias approaching from the south. Widespread rain associated with the tropical cyclone will develop across the region early Tuesday and become moderate to locally heavy Tuesday evening into Wednesday morning. High pressure and dry conditions return for the end of the work week and into the weekend. && .NEAR TERM /THROUGH TONIGHT/... As of 1027 AM EDT Monday...Forecast remains on track this morning with clouds beginning to break across the Champlain Valley. A cold front has worked into northern New York and will continue spreading eastward throughout the day. Continued clearing is expected but temperatures will remain steady through much of the day before gradual warming this afternoon. Previous Discussion...Looking at a fairly quiet day to start the work week before active weather returns late tonight into mid- week. Low pressure responsible for yesterdays severe storms continues to pull northeast through east-central Quebec, but a trailing frontal boundary is still poised to our northwest across the Ottawa and St. Lawrence River valleys. A few showers are streaming out ahead of this boundary and will try to work into portions of northwest New York early this morning but not expecting much more than an increase in cloud cover and a few isolated showers or sprinkles through mid- morning. Thereafter, as the boundary moves through drier mid-level air working into the region will scatter out clouds for a partly sunny afternoon with temps rising to near normal values in the mid 70s to low 80s. Dewpoints should drop pleasantly into the upper 50s to low 60s as well, so a it should be a nice afternoon and evening. Tranquil conditions will continue into the early overnight before clouds increase towards the midnight hour and onward as moisture associated with tropical cyclone Isaias will begin to advance into the mid-Atlantic and Northeast. Showers will begin to work into southern portions of the CWA toward daybreak, with lows mild in the 60s for most areas except mid/upper 50s in the NEK. && .SHORT TERM /TUESDAY THROUGH WEDNESDAY/... As of 427 AM EDT Monday...A flash flood watch has been issued for Essex County, NY and all of VT, except Grand Isle county from 18z Tues until 15z Weds for 1 to 3 inches of rainfall and the potential for localized flash flooding due to very high rainfall rates. Latest 00z models and guidance from our national centers WPC/NHC has a slight shift westward in the track of tropical system Isaias, resulting in slightly more precipitation across the Champlain Valley and parts of VT and the eastern Dacks. Large scale pattern shows deep southerly flow with significant moisture transport along the eastern seaboard on Tues into Weds, while mid/upper lvl digs acrs the central Great Lakes. This feature will help capture tropical system Isaias on Tues night into Weds and quickly track the system from eastern MD at 18z Tues to near Lebanon, NH by 06z Weds. This track has shifted slightly further west, resulting in more qpf into the CPV/eastern Dacks of our cwa. Guidance is indicating a few ingredients coming together for a predecessor rain event ahead of tropical system Isaias on Tuesday. The synoptic scale features include rrq of 250mb jet of 100 to 115 knots lifting acrs our cwa, helping to promote deep ascent, ahead of mid/upper lvl trof. In addition, 5h vorticity is streaming acrs our cwa in the fast confluence flow aloft, along with the nose of 850mb jet of 45 to 55 knots. These factors, combined with pws climbing btwn 1.50 and 2.0 supports the development of widespread light rain acrs our cwa on Tuesday, with areas of moderate rain possible where llvl forcing is better. Rainfall generally 0.25 to 0.75 on Tues acrs our cwa. By 00z Weds an area of moderate to heavy rainfall from Isaias will overspread our central/eastern cwa. The exact axis of heaviest rainfall will be determined by the eventual track of the system, but crnt fcst has shifted the heavier qpf axis western into parts of the CPV. Very strong llvl moisture advection is present on Tues evening, associated with nose of a 850mb jet of 55 to 65 knots, along with good ulvl divergence signature and deep available moisture with pws near 1.75". These factors combined with strong lift, will produce localized very heavy rainfall rates of 0.50 to 1.50 inches per hour acrs our fa for a period of time on Tues evening. The heaviest rainfall rates should shift east of our fa by 06z Weds, as well defined mid/upper lvl dry slot develops by early Weds morning. Total qpf will range from near 1.0 SLV to localized 3 inches acrs portions of VT by 12z Weds. Given the slightly westward shift in the track, the threat for stronger winds have developed across portions of eastern and southern VT. Have noted bottom of the mixed layer at VSF shows winds of 30 to 35 knots around 06z Weds, so thinking some localized gusts up to 40 mph will be possible in the heavier convective downpours. The core of strongest 925mb to 850mb winds impacts our eastern/southern cwa btwn 00z and 09z Weds. Some localized gusts up to 40 mph also possible acrs the favorable downslope regions of the southern Greens from Mendon Gap to Danby. Temps generally on the cooler side for Tues with highs in the 70s, with lows in the 60s overnight and back into the mid 70s to lower 80s on Weds. && .LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/... As of 427 AM EDT Monday...A generally quiet period of wx anticipated for mid week into next weekend with building sfc high pres. Still have to watch weak embedded short waves in the flow aloft and if they can advect moisture northward into our cwa, for potential showers over the weekend. Temps start near normal with values in the mid 70s to lower 80s, but warm well into the 80s by Saturday as progged 925mb temps climb btwn 20-22c. Lows mainly in the upper 40s to lower 60s depending upon location. Given the anticipated rainfall, some patchy fog is possible in the deeper valleys for late week, especially with light winds associated with sfc high pres overhead. && .AVIATION /15Z MONDAY THROUGH FRIDAY/... Through 12Z Tuesday...Mix of MVFR across northern New York and VFR across Vermont under BKN-OVC skies will continue this morning before conditions lift to VFR over the entire region after 18Z and skies break to SCT-BKN. Further improvement to mainly clear skies and increasing cirrus is expected late afternoon into early evening before low/mid clouds increase after midnight as Tropical Storm Isaias approaches. Breezy southwesterly winds of 10-15kts with gusts 20-25kt persist through midday and abate to nearly calm by 00Z. Outlook... Tuesday: VFR. Definite SHRA, Definite RA. Tuesday Night: Mainly MVFR, with local IFR possible. Windy with gusts to 35 kt. Definite RA. Wednesday: Mainly VFR, with local IFR possible. Chance SHRA. Wednesday Night: VFR. NO SIG WX. Thursday: VFR. NO SIG WX. Thursday Night: VFR. NO SIG WX. Friday: VFR. NO SIG WX. && .HYDROLOGY... As of 427 AM EDT Monday...A flash flood watch has been issued for Essex County, NY and all of VT, except Grand Isle County from 18z Tues until 15z Weds. Rainfall of 1 to 3 inches is expected, which given the potential for very high rainfall rates, could produce localized flash flooding across complex terrain and poor drainage in urban areas. 1 hour flash flood guidance across our HSA is between 1.5 and 2.0", while 3 hour guidance is in the 2 to 3 inch range. The heaviest period of rainfall will occur between 21z Tues and 06z Weds acrs the watch area. Sharp rises on streams and rivers are anticipated, but the threat for widespread river flooding is low at this time, due to the quick movement of precipitation across our region. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...Flash Flood Watch from Tuesday afternoon through Wednesday morning for VTZ002>012-016>019. NY...Flash Flood Watch from Tuesday afternoon through Wednesday morning for NYZ034-035. && $$ SYNOPSIS...Lahiff NEAR TERM...Clay/Lahiff SHORT TERM...Taber LONG TERM...Taber AVIATION...Lahiff HYDROLOGY...Taber

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