759 FXUS61 KBTV 072311 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 711 PM EDT Tue Jul 7 2020 .SYNOPSIS... An isolated storm will be possible this evening with additional chances overnight into Wednesday, as an approaching mid-level trough will bring a higher potential for showers and thunderstorms on Wednesday. A few strong to locally severe storms will be possible Wednesday afternoon. Otherwise, heat and humidity will gradually build through the remainder of this week, with valley high temperatures and heat index values in the low to mid 90s for the Thursday through Saturday period. Cooler air is expected to move in for the start of next week with additional chances for precipitation possible. && .NEAR TERM /THROUGH WEDNESDAY NIGHT/... As of 705 PM EDT Tuesday...There was a bit more shower and thunderstorm activity this afternoon than originally expected but things are beginning to quiet down as the environment continues to stabilize. Nevertheless, the thinking remains the same as before that we could see some showers and a few elevated thunderstorms through the evening and overnight hours with a weak shortwave moving across the North Country. These should be more of the garden variety of thunderstorms versus the strong ones observed this afternoon which could provide some much needed rainfall as we continue to see drought conditions build across the region. Previous Discussion...Moist, southerly flow prevails today as we undergo a change in airmass with muggy air in addition to the already warm air in place as an advancing shortwave ridge crests overhead this evening. This airmass will reach the St. Lawrence and Champlain Valley towards peak heating, but will take more time to reach eastern Vermont. Thus, upper 80s to near 90 are expected for our western valleys with mid 80s across the remainder of the region. The most unstable air today will be across New York today, where sufficient CAPE, lake breeze, and orographic forcing could produce an isolated shower or thunderstorm over the Adirondacks. Activity will quickly decline after sunset. However, we will probably see activity redevelop overnight as a vigorous, upper shortwave trough moves east across Ontario and Quebec and spawns a weak surface reflection near Montreal. The combination of modest elevated instability and strengthening southerly inflow should allow a few showers and thunderstorms to develop at least until the flow turns more southwesterly ending favorable low-level convergence and causing activity to wane again generally around sunrise. Based on low-level winds and isobars, a surface trough axis slowly progresses east, allowing for convection to redevelop (mainly over Vermont) while 0-6km bulk shear increases. With temperatures again in the mid 80s to lower 90s, there will be plenty of thermal instability (approaching 1500-2000 J/kg of MUCAPE) to produce afternoon showers and storms. SPC's severe weather outlook places us in a marginal (Level 1 out of 5) risk outlook, suggesting localized, short-lived severe potential. Lapse rates are not particularly favorable, but with 30-35 kts of 0-6km shear and 20-30 kts of 0-3km shear suggest there could be some brief organization. In fact, several high res models (NAM3, local BTV2 WRF and other WRFs, and the 12Z extended HRRR) show a broken line developing across Rutland/Windsor County and progressing south. Stronger storms could produce strong downburst winds and small hail. While convection quiets down during the evening/overnight hours, the muggy airmass in place will prevent significant cooling. Lows are forecast to remain around 63-71 degrees. && .SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... As of 319 PM EDT Tuesday...Hot and sunny conditions still on tap for Thursday due to surface high pressure and a deep ridge oriented from northwest to southeast centered just to our south. With 925 mb temperatures mostly between 24 and 25 Celsius, we should see maximum temperatures near 95F in the St. Lawrence and Champlain Valleys. When combined with moderate dew points in the mid to upper 60s, the heat index will reach the upper 90s for at least a few hours in these areas. Unfortunately, the air will become fairly stagnant despite deep mixing as winds throughout boundary layer will be under 12 MPH based on NAM-12 forecast soundings during the afternoon hours. With dry air present above the boundary layer, a strong cap will be in place preventing thunderstorm development. Thursday night will be mild with lows in the upper 60s to mid-70s with light breezes and partly to mostly clear skies. && .LONG TERM /FRIDAY THROUGH TUESDAY/... As of 319 PM EDT Tuesday...Friday will be hot and muggy areawide, with higher humidities as you head south and east. More heat headlines are likely as apparent temperatures will likely again reach above 95 in valley locations. A trend towards cloudy skies will begin with the approach of a tropical system or low pressure system with tropical characteristics heading northward along the coast. This increased cloud cover may produce a more pronounced temperature gradient where southern and eastern parts of Vermont are a bit cooler, but thinking that with higher dew points it will still feel similarly hot across the North Country. While it looks like eastern New England is the main target for heavy rain and breezy conditions, the approaching storm will draw highly anomalous moisture into our region. This will support another mild night. By Saturday, precipitable water approaching 2 inches supports heavy rain. Will continue to keep higher precipitation chances in eastern Vermont with greater potential for heavy rain, but at this time there is enough model variation to suggest showers are possible anywhere throughout the day. Scattered thunderstorms would be capable of developing by Saturday afternoon, as well. Sunday looks like a transition day from the hot and humid weather into a somewhat cooler and drier air mass. A weak surface front will eventually slide through the area from the west, providing another chance of thunderstorms. Behind the front some drier air will advect into the region on northwest winds. The main Canadian high pressure area will remain well to our north so the amount of cooling looks limited. High temperatures early next week should remain a few degrees above normal but at least humidity will be low enough to allow temperatures to drop back into the 50s and lower 60s at night. Somewhat unsettled weather with chances for showers on Monday and Tuesday. While there will be potential for thunderstorms, at this range will not include this detail in the forecast given inherent uncertainty. && .AVIATION /23Z TUESDAY THROUGH SUNDAY/... Through 18Z Wednesday...Currently VFR with sustained south winds of 6-14 knots with gusts up to 20 knots from at KBTV. Winds slowly decrease after 23Z tonight. Scattered showers are expected to shift east overnight, and have mentioned VCSH around 09Z-13Z to account for that. Maritime air mass advects from the south, bringing ceilings down to 1500-2500ft agl at KSLK and KRUT, with it falling to 700ft agl at times for KMPV around 06Z-13Z. During the afternoon, scattered showers and thunderstorms possible, with the greatest chance at KRUT, so have mentioned VCTS there starting 16Z. South winds at 5 to 10 kts become more southwesterly. Outlook... Wednesday Night: VFR. Slight chance SHRA, Slight chance TSRA. Thursday: VFR. NO SIG WX. Thursday Night: VFR. NO SIG WX. Friday: VFR. Chance SHRA, Slight chance TSRA. Friday Night: Mainly VFR, with local MVFR possible. Chance SHRA. Saturday: Mainly MVFR, with areas VFR possible. Chance SHRA, Slight chance TSRA. Saturday Night: Mainly VFR, with local IFR possible. Chance SHRA, Slight chance TSRA. Sunday: Mainly VFR, with areas MVFR possible. Chance SHRA, Slight chance TSRA. && .MARINE... Southerly winds of 15 to 25 knots will continue through this evening and overnight period with the stronger winds likely to persist into Wednesday. We should see waves of 2 to 4 feet across the northern end of the broad lake through at least Wednesday morning. A Lake Wind Advisory remains in effect until Wednesday morning and may need to be extended further. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Haynes NEAR TERM...Clay/Haynes SHORT TERM...Kutikoff LONG TERM...Kutikoff AVIATION...Haynes MARINE...Team BTV