textproduct: Burlington

WHAT HAS CHANGED

As of 219 PM EDT Wednesday...

Confidence in isolated severe thunderstorms has increased for Thursday afternoon and evening.

KEY MESSAGES

As of 219 PM EDT Wednesday...

1. Thunderstorms are increasingly likely Thursday afternoon and evening, with greater chances in Vermont and particularly southern areas. Some storms will be capable of strong winds and heavy rain.

2. Lingering and decreasing shower and thunderstorm threat for Friday followed by beautiful weather to start the weekend.

3. Unsettled weather with increasing chances for precipitation expected for early next week.

DISCUSSION

As of 219 PM EDT Wednesday...

KEY MESSAGE 1:

Likelihood of numerous showers and thunderstorms for Thursday afternoon and evening continues to increase today in portions of northern New York and Vermont. Big picture: isolated severe thunderstorms areawide remain possible east of the St. Lawrence Valley. If confidence in location and intensity of thunderstorms increases tonight, this outlook could very well change. Strong moisture advection will occur ahead of a cold front with well aligned southwesterly flow and mainly southerly winds at the surface. Plenty of solar heating to start the day concurrent with the incoming trough cooling temperatures aloft will aid in blossoming instability. MLCAPE will surge into the 1000-1500 J/kg range across the Champlain Valley and central Vermont during the afternoon as surface temperatures and dew points will likely be in excess of 80 and 60. About 30-35 knots of deep layer shear and similar effective shear will likely be present. Overall, the CAPE/shear balance appears to be sufficient for some stronger variety thunderstorms and possible low-topped supercells to form.

In our region, the most likely timing for thunderstorms, including isolated severe storms, appears to be from about 4 to 8 PM. The greatest risk area is somewhat uncertain, but favors the region from the eastern Adirondacks into at least the western half of Vermont. Storm initiation may be upstream in central or eastern New York earlier in the afternoon where the best instability will be located, with cells moving parallel to the flow towards the northeast into our region coincident with a warm front. This boundary will help winds turn to more of a southeasterly flow near the surface across the southern Champlain Valley into portions of central Vermont; with some low level convergent flow along with terrain-generated lift, storms may tend to strengthen during the evening as 0-1 km storm relative helicity increases. The low level spin/veering winds will aid in organized, stronger storms that could be more long lived than a typical thunderstorm. All hazards with these storms are on the table, with the most likely one being straight line winds. The greater the instability, the more likely these hazards come to fruition. At this time, a greater risk/more coverage of stronger storms is favored south of our region per consensus of both the physically-based and machine learning model guidance.

There is also a localized flash flood threat with this event, particularly in the evening and in south central/western portions of Vermont. While precipitable water will not be particularly high, values in excess of 1.5" and/or 85th percentile for the time of year will be sufficient for heavy rain. Additionally, integrated vapor transport from the southwest coincident with deep moisture will be present. Some CAMs show a plausible, but unlikely, scenario in which multi-cell thunderstorms that had been propagating southeastward stall out for a couple of hours, repeatedly moving over the same areas and producing locally 2+ inches of rainfall within three hours. Some slight pressure rises and pause in height falls are noted during this period, which can contribute to stalling of convection that was previously moving along. There remains mixed signals among CAMs during the latest model cycle on this potential, so confidence is fairly low on flash flooding, but note training thunderstorms are possible if ingredients line up.

KEY MESSAGE 2: Deep moisture will be shunted to our east by Friday morning. As such, the only limited thunderstorm potential will be tied to the lingering surface cold front which will most likely be passing uneventfully through the region during the morning. It appears it will get hung up just enough over southern Vermont that a small window during the afternoon hours where showers develop along the boundary exists such that a thunderstorm is possible in south central to southeastern portions of the state, especially if these areas break out into some sunshine. Abundant dry air aloft points to widely scattered showers, so only a very select few locations will see hazardous weather as showers will struggle to produce lightning. Thunderstorm probabilities have fallen into the 10 to 13% range in southern/eastern portions of Vermont (contrasted with 30-40% in much of the area on Thursday). Following any convective activity during the afternoon, a very pleasant air mass will continue advecting in from the northwest Friday night setting the stage for a gorgeous, sunny day with low humidity for Saturday as ridging builds in at the surface and aloft. High temperatures should be very seasonable, ranging through the 70s.

KEY MESSAGE 3: High pressure will slide to our east on Sunday, and at this point, expect the second half of the weekend to be mainly dry. Uncertainty grows thereafter as an upper trough swings across Quebec and into northern/eastern New England, sweeping a cold front along its wake. There remains considerable spread in model solutions with this system, particularly how far southward the front makes it before possibly lifting back north as a warm front as low pressure slides to our north. For example, the 12z GFS solution sets up the front directly over our region, with waves of low pressure to slide along it from later Sunday into mid week. The result is a period of particularly wet and cooler weather. Do note the GEFS ensembles don't support the operational solution, with 24-hr probabilities of greater than a half inch of rainfall remaining fairly low. Meanwhile, the 00z ECMWF isn't so dire, lifting the cold front back to our north Monday into Tuesday, keeping rounds of showers more transient and allowing us to warm back up into the 80s by mid week. Have not made changes to the NBM from Sunday onward given the uncertainty, but it'll be interesting to see how trends play out as we draw closer to next week.

AVIATION /00Z THURSDAY THROUGH MONDAY/

Through 00Z Friday...Mainly VFR conditions will continue during the overnight hours, with the exception of some valley fog that will develop over portions of eastern Vermont...affecting KMPV and possibly KEFK between 08Z and 12Z. Fog is less likely at other TAF sites, including the climatologically favored KSLK. This is due to increasing flow just off the surface as high pressure shifts to the east and return southerly flow develops. Any fog that develops will lift by 13Z, and conditions will trend widespread VFR. Showers and some thunderstorms will develop mainly after 18Z, but cloud bases are expected to remain above 3000 ft. Some showers and thunderstorms may contain briefly heavy rainfall, potentially reducing visibility to 2-4SM at times. However, have left mention on storms and reduced visibilities out of the TAFs at this point until we can better pinpoint the timing.

Winds will start off light overnight, but will increase from the south/southwest on Thursday after 12Z. Expect daytime gusts 16-25 kt.

Outlook...

Thursday Night: VFR. Chance SHRA. Friday: Mainly VFR, with local MVFR possible. Chance SHRA, Slight chance TSRA. Friday Night: VFR. NO SIG WX. Saturday: VFR. NO SIG WX. Saturday Night: VFR. NO SIG WX. Sunday: VFR. NO SIG WX. Sunday Night: Mainly VFR, with local MVFR possible. Chance SHRA. Monday: Mainly VFR, with areas MVFR possible. Chance SHRA, Slight chance TSRA.

BTV WATCHES/WARNINGS/ADVISORIES

VT...None. NY...None.