textproduct: Burlington

WHAT HAS CHANGED

As of 230 AM EDT Thursday...

Strong to possibly severe thunderstorm chances this afternoon have increased across southwestern Vermont. A Slight Risk has been introduced to Essex County, NY and Addison and Rutland Counties in Vermont for this afternoon. There is also increasing confidence in potential localized flash flooding across Rutland County and southern Vermont this afternoon/evening from potential training thunderstorms.

KEY MESSAGES

As of 230 AM EDT Thursday...

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

2. Some lingering showers will be possible early on Friday, before high pressure, and drying conditions lead to a quiet weekend.

3. An active weather pattern will prevail early next week.

DISCUSSION

As of 230 AM EDT Thursday...

KEY MESSAGE 1: Confidence in afternoon showers and thunderstorms, some of which could be strong to severe, remains on track. SPC has nudged the Slight Risk (level 2 out of 5) for severe thunderstorms northeast to include Essex County NY, and Rutland and Addison Counties in Vermont for this afternoon. The areas included in the previous Marginal Risk for severe thunderstorms for today has remained unchanged. Synoptically, an occluding surface low with associated warm front over western New York this morning will lift through the region early to mid this afternoon. Showers will be possible along the warm front as early as mid to late this morning along the southern Greens and St. Lawrence Valley. Water vapor depictions upstream into western New York and Pennsylvania show decent dry air ahead of a stout boundary of showers in western Pennsylvania. This clearing will help our region destabilize this afternoon as MLCAPE surges to 1000-1500J/kg, with some pockets of 1500-2000J/kg across the eastern Adirondacks and Champlain Valley. While there will be good instability in the warm sector this afternoon as temperatures climb to near 80, with dewpoints in the low 60s, deep layer shear will be less than ideal for strong organized storms. 20-30knots of bulk 0-6km shear is expected this afternoon, which is on the lower side of sufficient shear for severe storms; the better shear will be well to our east near the main cold front. Regardless, numerous showers and scattered thunderstorms, with some isolated storms becoming strong to severe, are anticipated, mainly in the 3-8 PM timeframe, moving west to east.

The main risks associated with our storm potential this afternoon will be gusty winds, isolated small hail. Although shear is on the lower bounds for favorable severe conditions, storms modes look to favor some mini bowing segments and multicells which can drive strong cold pools and thus isolated gusty winds. Given decent mid level lapse rates to 6-7C/km, and good elevated instability in the hail growth zone, small hail will be possible within stronger updrafts. While the probabilities are low, there is a low risk for some isolated rotation within storms this afternoon. As the warm front lifts through the region this afternoon, winds in the low level will turn more southeasterly, and paired with prevailing southwesterly flow aloft, veering flow could lead to an increase in low level spin. 0-1km SRH will be low around 100-150 m2/s2, though could be enough to lead to some isolated rotation, especially should any storms develop along the warm front, and out ahead of the main boundary this afternoon. All hazards will be possible this afternoon, though the main concerns will be winds in the form of straight-lined winds. 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.

Heavy rain will also be a concern with WPC placing the entire area in a Marginal Risk (level 1 out of 4) for a 5% chance of exceeding flash flood guidance within 25 miles of an area. Pwats will be around 1.5-1.7 inches, the 85th percentile for this time of year, with modest warm layer cloud depths around 10,000 ft. While these do not favor flash flood potential, the flash flood risk today will be driven by conditional training storms. Cells may become aligned to the background flow across southern Vermont and lead to a localized chance for flash flooding. Probabilities for 2" of rain through 8 AM Friday highlight western Rutland County in areas like Fair Haven, and Rutland itself in a 20% probability. HREF probabilities denote a 10% chance for this area to see upwards of 3". These high rainfall amounts will be highly conditional on training thunderstorms and does still appear unlikely, though there is some better confidence than previous forecasts for this area to see some localized flash flooding. Have increased rainfall amounts to reflect this in southern Vermont to around an inch, to locally 1-2" should a few thunderstorms move over the same areas. Interests in southern Vermont, should pay close attention to our forecast and radar trends this afternoon, as conditions could change quickly.

KEY MESSAGE 2: A cold front will gradually move from west to east early Friday spurring some lingering showers across Vermont. Given the timing of the frontal passage, instability will be low, so anticipating just some light showers. Though should the front slow as it nears the VT/NH border, some isolated rumbles of thunder may be possible. Showers will taper off Friday early to mid afternoon, with clearing skies from west to east. Temperatures will be a few degrees cooler than Thursday in the low to mid 70s, with perhaps some upper 70s, should the cold front slow as it moves east allowing for more diurnal heating in the deeper valleys. Lows Friday night will cool off under strong radiational cooling into the upper 40s for most locations, outside of areas near the St. Lawrence River and Lake Champlain where low 50s are expected. Some pocket low 40s, to near 40 will be possible in the more sheltered locations of the Adirondacks. With the strong radiational cooling, valley fog following the recent rains will be possible. Saturday looks to be pleasant with temperatures in the mid to upper 70s under high pressure. Some light southwest channeled flow in the St. Lawrence Valley will be possible, though by and large winds will remain light under 10 MPH. Clouds will increase Saturday night.

KEY MESSAGE 3: Sunday into Monday, a vigorous vort will dig towards the base of a longwave trough pattern positioned in Quebec Province. Meanwhile, a strong upper high in the Desert Southwest will weaken and shift east. This will open up a stream of tropical Pacific air that tracks along the international border. This will result in accelerating flow over our region with systems. Surface high pressure will build across Quebec Province once the vigorous vort pushes east. The northeasterly flow from that high and the attempt for warm, humid air eking closer to us will place us near along a stationary moisture gradient through at least next Wednesday.

There's plenty of spread, but perusing various ensemble probabilities of greater than 0.5" shows some potential throughout Monday through Thursday. Taking in a view of a 72-hour time frame, NBM probabilities for at least 2" in the Monday-Thursday time frame is about 10-15% and 30-40% for an inch of rain. So a period of above normal precipitation appears likely heading into next week. Will fine tune the timing as we draw a little closer. Temperatures will be run relatively close to seasonal norms (mid to upper 70s at day, 50s at night, but 40s for the Adirondack Regional Airport).

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Through 18Z Friday...VFR conditions will likely prevail through the afternoon and evening hours outside of thunderstorm activity. A round of showers and thunderstorms will be possible between 18Z and 21Z which will move from west to east with an additional round of showers and thunderstorms between 21Z and 00Z. We will likely see brief visibility reduction to 3 to 5 SM if any terminal is impacted by a thunderstorm this afternoon or evening. Winds will be gusty at 20 to 30 knots from the south ahead of the cold front through this evening with winds going light and variable after 6Z with winds shifting to the WNW on Friday. IFR stratus seems likely at KSLK between 6Z and 12Z and has been included with this TAF package. In addition, patchy fog will be possible through the Winooski and Connecticut with a TEMP for IFR fog added at KMPV.

Outlook...

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

MARINE

As of 230 AM EDT Thursday...

A Lake Wind Advisory is in effect for the broad and northern waters of Lake Champlain until 8 PM today. South winds ahead an approaching frontal system will lead to increasing channeled winds on Lake Champlain by late this morning enhancing gusts and wave conditions. Winds and gusts on the lake will increase to 15 to 25 knots through this afternoon. Waves will be generally 1 to 3 ft. Gusts will weaken back to around 10 to 15 knots after sunset this evening with the passage of a boundary. Thunderstorms will be possible this afternoon primarily between 2-6 PM, which can further enhance gusty winds. Within any thunderstorms, small hail, and heavy rain will also be possible.

BTV WATCHES/WARNINGS/ADVISORIES

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