Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 202351 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 751 PM EDT Tue Aug 20 2019 .SYNOPSIS... Showers and thunderstorms return Wednesday afternoon with the potential for some organized storms capable of gusty winds, locally heavy rainfall, and frequent lightning. A cold front will push through Thursday morning, followed by high pressure building in and a return to cooler and drier weather for the end of the week. && .NEAR TERM /THROUGH WEDNESDAY NIGHT/... As of 736 PM EDT Tuesday...The forecast remains nearly spot on as of early evening. I did blend current hourly temperatures and dewpoints into our extant forecast later this evening, and incorporated some newer model rh data to tweak arrival time of mid/upper level clouds later tonight. All and all these were essentially noise-level adjustments. Have a great evening. Prior discussion... Period of quiet weather continues tonight as high pressure departs the region to the east. Light southerly winds will continue overnight, which will keep overnight temperatures relatively warm despite the initial clear skies. Clouds will spread over the region after midnight as a warm front approaches the area. Overnight lows will be in the upper 50s to mid 60s. Interesting setup coming together for Wednesday with several ingredients present for the development of some strong to severe thunderstorms. However, as has been the case most of the summer for the North Country, several factors also working against the challenging forecast to weigh out both sides with the result being a conditional severe threat for tomorrow. In the upper levels, synoptic picture features an upper-level low digging upstream over the northern Great Lakes Region during the day Wednesday with southwesterly flow increasing over the BTV CWA ahead of the system. Models continue to indicate a shortwave will move through eastern NY and into western New England during the early afternoon hours on Wednesday...which will be a key player in deep convective initiation. Meanwhile, surface temperatures and dewpoints will increase quickly on strengthening southwesterly flow just ahead of the wave. A warm front will lift through southern and central New England during the morning hours, which will spread some cloud cover over much of Vermont and into eastern New York...limiting development of instability. Thus, greatest axis of instability is progged to develop over the Saint Lawrence Valley (1500+ J/kg), with degree of destabilization further east still in question. However if Vermont is able to destabilize after morning cloud cover, showers and thunderstorms are likely to develop in the early afternoon ahead of the upper wave and along a prefrontal trough. Severe threat: Best deep layer shear will be removed to the northwest closer to the heart of the upper trough, but still looking at 25-35 kts 0-6 km shear in our forecast area...supportive of organized strong to severe convection with organized multicells being the primary convective mode. Forecast hodographs indicating 0-3 km SRH values of over 250 m2/s2 over southern the potential for rotating sustained updrafts will be relatively high with any storms that develop in southern into central Vermont. Low and mid-level shear and helicity values decrease further north through the remainder of the CWA. Lapse rates are not overly impressive, in fact latest model runs have trended back on the amount of dry air available in the mid- levels, instead indicating near moist adiabatic lapse rates. This does work somewhat against the severe wind threat, but overall after evaluating all the factors still seeing enough signals for some storms with potentially damaging gusty winds, so have included mention of this threats in the forecast. Rainfall: With regards to rainfall amounts, storms will be capable of locally heavy rainfall given PW values > 1.75 inches and deep warm cloud layers of upwards of 12k ft. However, Corfidi vectors between 15 and 25 kts suggest storms will be moving fast enough to not pose any widespread flooding threats. Total QPF between Wednesday morning and Thursday morning is generally in the quarter to two thirds inch range, with locally amounts around an inch possible within any thunderstorms. Storms and showers will dissipate Wednesday night as a cold front moves through closer to dawn. Could still get a rumble of thunder or two overnight, but main show will be over shortly after sunset. Wednesday night lows will be in the mid to upper 60s. && .SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... As of 430 PM EDT Tuesday...Cold front will push east of our region early Thursday and chance for showers will decrease through the day lingering longest in eastern VT. Winds aloft will turn out of the west advecting in lower dewpoints but better cold air advection does not arrive until overnight Thursday night. Maximum temperatures will range from the lower 70s to lower 80s Thursday afternoon. Timing of the front unfavorable for thunderstorm development and not a lot of moisture with this feature. Surface high pressure will begin to ridge into the area Thursday night, temperatures will drop into the upper 40s to upper 50s. && .LONG TERM /FRIDAY THROUGH TUESDAY/... As of 430 PM EDT Tuesday...Still looks quiet for the extended portion of the forecast, mainly surface high pressure dominating the weather pattern from Friday through early next week. Lingering upper level trough still pushing across the area Friday into Saturday, though mainly just clouds expected with the trough. Next real chance for showers not until Monday night into Tuesday when next frontal system approaches. Temperatures will trend warmer after upper trough finally pushes east of the area Saturday night. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... Through 00Z Thursday...VFR through 12Z as high pressure moves east of the region. High clouds will overspread the region after 06Z as a warm front moves through, which should limit fog potential overnight. Will see clouds lowering and some showers develop after 12Z especially KMSS/KRUT. Some thunderstorms possible after 16Z especially from KSLK eastward across the Champlain Valley including KBTV around 18z and the rest of VT after 18Z and may be out of the forecast region by 23z. Have left mention of thunderstorms out until timing and location get more specific. Any thunderstorms after 16z may be strong with brief gusty winds, LLWS, and IFR possible. Outlook... Wednesday Night: VFR. Chance SHRA, Slight chance TSRA early. Thursday: VFR. Slight chance SHRA. Thursday Night: VFR. NO SIG WX. Friday: VFR. NO SIG WX. Friday Night: VFR. NO SIG WX. Saturday: Mainly VFR, with areas MVFR possible. NO SIG WX. Saturday Night: VFR. NO SIG WX. Sunday: VFR. NO SIG WX. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Duell NEAR TERM...Duell/JMG SHORT TERM...Neiles LONG TERM...Neiles AVIATION...Sisson is the U.S. government's official web portal to all federal, state and local government web resources and services.