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Area Forecast Discussion
Issued by NWS Burlington, VT
Issued by NWS Burlington, VT
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362 FXUS61 KBTV 300603 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 203 AM EDT Sun Jun 30 2024 .SYNOPSIS... Steady rain is expected to continue through the afternoon hours with a break in rainfall early this evening. Several rounds of convection are expected tonight and again on Sunday which could produce a few strong to locally severe storms. Very localized flash flooding also remains possible with any convection tonight that moves over the same area. A cold front on Sunday will put an end to rain and thunderstorm chances with a dry start to July on tap for the North Country. && .NEAR TERM /THROUGH MONDAY/... As of 203 AM EDT Sunday...Challenging forecast early this morning with dynamic weather and mesoscale features driving showers and thunderstorms. Overall, forecast is playing out well with minor changes needed. A lull in precipitation was expected during these late night hours with coverage becoming widely scattered. Precipitation has settled mainly along a line from the Adirondack High Peaks east north-eastward through northern Caledonia and adjacent Essex counties in Vermont. Instability here is low, so rainfall rates have been only moderate. In contrast, potential downpours will exist with activity farther northwest, with showers near the International Border in northern New York. This environment is much more unstable, and we are watching upstream thunderstorm complexes in Ontario for triggering organized thunderstorms in our region towards 4 or 5 AM. This could result in strong thunderstorms across a swath of northern New York into northern Vermont. PREVIOUS DISCUSSION...Infrared satellite shows cloud top cooling occurring across northern NY with GLM detecting a couple of cloud flashes moments ago across southern Clinton county. After an unseasonably cool late June day for North Country standards, temperatures are not expected to drop all that much and actually even rise a few degrees as warmer air aloft is advected into our region in the next few hours. Surface analysis shows the actual surface warm front still well south of our area. While there are some subtle signs of environmental destabilization, the expectation is that thunderstorms that do develop this evening into the first half of the overnight hours should be of garden variety, with potential for torrential downpours given that the PWATs of 1.9 inches is near max of SPC sounding climatology for the Albany, NY upper air site. As a warm front pushed through the North Country, there was a quick burst in isentropic forcing west of the Adirondacks earlier that allowed for a period of heavy rain which has since subsided. So far, rainfall totals are on track with anywhere from a quarter of an inch in Vermont upwards to an inch in northern New York. Rainfall associated with the warm front will continue into the late afternoon or early evening hours before we see a brief break in rainfall. Some gradual destabilization is expected within the warm sector, especially across northern New York, this evening which is expected to act as a catalyst for additional showers and thunderstorms. At this time, flooding looks unlikely given the additional showers and storms will be rather scattered and rainfall totals thus far haven`t been anything excessive. However, we continue to monitor the potential for a wet microburst or two this evening across northern new York. There are some pros and cons for this potential which we will highlight. Pros: DCAPE near or in excess of 1000 J/kg, strong low level shear profiles, and mid-level dry air entrainment. Cons: Potential capping due to rain cooled air and lack of SBCAPE and MUCAPE. With that all said, there could be enough negative buoyancy associated with the DCAPE to punch through any cap briefly. It`ll be something to watch for sure but remains a very conditional set-up. As the sun begins to rise on Sunday, so will our severe probabilities. We have an unusual set-up on Sunday where our severe potential is expected to peak by late morning and end by mid- afternoon. Our airmass tomorrow will be highlighted by a very moist low level environment with a decent bit of dry in the mid and upper levels. This dry air remains a concern as it is expected to limit areal coverage of thunderstorms. A pre-frontal trough is expected to act as a trigger Sunday morning but just how many thunderstorms develop remains very questionable. We will have a very favorable environment tomorrow morning which will be highlighted by 1500+ J/kg of CAPE, 50-60 knots of 0-6 km shear, and upwards of 800-1000 J/kg of DCAPE. There is a potential nothing happens tomorrow while there is also the potential for several strong to severe thunderstorms. All of the high-res CAMs show 1-3 storms in the aforementioned environment which could be long-tracked and could be strong to severe but disagree on the placement of the thunderstorms. It`s more important, in this case in particular, to understand the forcing mechanisms and the background environmental fields rather than trying to see where any single CAM is trying to depict a storm. Timing also remains a bit of uncertainty as the last several runs have an early pre-frontal trough although some ensemble guidance suggests a later passage. A later passage would increase severe potential while earlier passage would decrease it. We will be monitoring this and should have a much better idea come Sunday morning. Following the pre-frontal trough, we will see a cold front lag around 3 hours behind which should help scour out the low level moisture and humid conditions. Decreasing rain chances will be seen late Sunday afternoon through the overnight hours with a few gusts of winds from the northwest following the FROPA. Temperatures will return to more seasonal values Sunday night following the FROPA which will be appreciated as no one really likes 70 degree dewpoints. && .SHORT TERM /MONDAY NIGHT/... As of 318 PM EDT Saturday...A cooler, drier airmass moves into the region for Monday. Dew points will fall into the 50s and 40s by the end of the day. There is the chance of an isolated shower over the higher terrain of Vermont during the day, but with dry low levels and marginal conditions, anything would be very light. There is no threat for thunder. Temperatures drop back into the 40s and 50s overnight, in what will be the coldest night of the week. The boundary layer should decouple in many areas and the coldest hollows could drop into the low 40s or maybe even upper 30s. && .LONG TERM /TUESDAY THROUGH SATURDAY/... As of 318 PM EDT Saturday...Ridging builds in from the south for Tuesday and Wednesday, sending temperatures above normal and bringing the humidity back. Dew points eventually rise into the 60s and temperatures will rise into the 80s. A weakening cold front passes into the region Wednesday night and will bring some showers and possible embedded thunderstorms. The most likely solution is that it passes through northern New York in the evening and through Vermont around and after midnight. However, there is still decent ensemble spread so put a lot of chance PoPs on either side for now until confidence grows. While there is the chance of thunderstorms, there is no severe risk. While the front will not bring in a dry airmass behind it, it will shunt the ridge back to the south before it can become entrenched over the area send the temperatures and humidity to high levels. The formal front generally dissipates but there will still be a bit of a boundary in place on Thursday. There is the possibility that a few showers fire along this but anything would be light and scattered. Overall, the shower threat for the Fourth of July is decreasing and thunderstorms are also not expected regardless. A region of low pressure looks to move into the region for the weekend and bring more chances of showers. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/... Through 06Z Monday...A mix of VFR and MVFR ceilings are in place with widely scattered showers. More organized showers and thunderstorms become more likely over the next several hours, a given location is relatively unlikely to be directly hit. Have indicated vicinity thunderstorms across the SLK-PBG-BTV area in the 10-14Z time frame where/when predictability is a bit higher. Thunderstorm chances peak between 14 and 18Z ahead of a cold front, generally shifting south and east with time, although coverage of storms may remain sparse. Heavier rain will produce IFR conditions. Winds will trend westerly during this time. Then a secondary boundary will gradually shift through the airspace between 21Z and 01Z with scattered showers which will lead to more widespread MVFR conditions and northwesterly winds mainly 6 to 10 knots with a brief period of 20 knot gusts possible. Outlook... Monday: VFR. NO SIG WX. Monday Night: VFR. NO SIG WX. Tuesday: VFR. NO SIG WX. Tuesday Night: VFR. NO SIG WX. Wednesday: VFR. Chance SHRA. Wednesday Night: Mainly VFR, with local IFR possible. Chance SHRA, Slight chance TSRA. Independence Day: Mainly VFR, with local IFR possible. Slight chance SHRA. && .HYDROLOGY... The threat for localized heavy downpours and isolated flash flooding continues across our region although looks less likely than before. The Weather Prediction Center Day 1 Excessive Rainfall Outlook has been reduced to just a 5% chance of rainfall exceeding flash flood guidance within 25 miles of a given point. We`ve generally seen between 0.25" and 1" of rain thus far with the highest amounts of rain across St. Lawrence County. An additional 0.25" to 1" of rain is expected through Sunday afternoon but rainfall rates overall will likely not support flash flooding. However, several round of showers and thunderstorms are expected tonight and again on Sunday. Should one location see several rounds of convection, the additional rainfall coupled with what has already fallen may lead to localized flash flooding. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Clay NEAR TERM...Clay/Kutikoff SHORT TERM...Myskowski LONG TERM...Myskowski AVIATION...Chai/Kutikoff HYDROLOGY...Clay