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WHAT HAS CHANGED

As of 217 PM EDT Sunday...

Winds may be breezy to locally gusty, especially on mountain summits tomorrow afternoon. A strong thunderstorm is also possible, mainly in eastern Vermont late tomorrow morning into early tomorrow afternoon. Otherwise no significant changes to the current forecast.

KEY MESSAGES

As of 217 PM EDT Sunday...

1. Showers and embedded thunderstorms will increase across the region today and tonight. Locally heavy rainfall possible, especially in the St. Lawrence Valley.

2. A strong thunderstorm is possible late tomorrow morning into the early afternoon, with breezy conditions along a cold front, before conditions trend calmer and drier into mid week.

3. Seasonably warm and unsettled weather expected for the latter half of this into next weekend.

DISCUSSION

As of 217 PM EDT Sunday...

KEY MESSAGE 1: A slow moving upper level trough with associated closed surface low is beginning to enter the St. Lawrence Valley with the axis currently over eastern Ontario. Open wave showers and embedded thunderstorms within an area of 1000J/kg MUCAPE are tracking parallel to the St. Lawrence River, mainly west of the US-11 corridor. This baroclinic zone will serve as the focal point for multiple rounds of showers throughout the day in the St. Lawrence Valley. Recent model guidance has continued the trend of keeping the heaviest of the convection and rain just across the International Border into Ontario. Nonetheless, given PWs to 1.8-2", and deep warm layer cloud depths, heavy rain is still possible in western St. Lawrence County where 1 to 1.5 inches of rain are expected through tonight. Locally up to 2 inches will be possible, especially under any convective elements in western St. Lawrence County. The WPC continues to highlight the St. Lawrence Valley with a Slight Risk (level 2 out of 4) for a 15% chance of exceeding flash flood guidance. While isolated urban flash flooding cannot be ruled out, significant enough rainfall capable of producing flooding is not anticipated over the prevailing flatter terrain. Outside of the North Country showers, channeled flow in the Champlain Valley will lead to intermittent gusts up to 25 mph near Lake Champlain. These gusts will taper down with the arrival of showers this evening.

The deformation boundary, and associated showers, will shift east into the Champlain Valley and Vermont this evening into tonight. A few spot light showers may be possible across Vermont this afternoon, but most will remain dry until later this evening. There is good agreement that overnight Sunday should have the better rainfall coverage and contain the bulk of the rainfall. A few areas of brief 0.25-0.5 inch/hour rates will be possible across northern New York within the boundary as it shifts east towards Vermont. Rainfall across Vermont is expected to range between half an inch and and an inch, with the higher amounts across the central and northern Greens.

KEY MESSAGE 2: A clearing cold front will sweep across the entire region from west to east, early to mid tomorrow morning, reaching Vermont by late tomorrow morning. Wrap around moisture along the convergent deformation band in northern New York will likely lead to some continued light shower chances (only 30-40%) through tomorrow afternoon. In Vermont, modest instability and shear could lead to an isolated strong thunderstorms in the late morning to early afternoon hours, primary east of VT-100. SBCAPE increases to 500-1500J/kg, with 30-40kts of shear would be sufficient for potentially an isolated strong thunderstorm. However, lingering low clouds from overnight rains, and competing updrafts along the front, may lead to lesser chances. The SPC does place most of eastern Vermont in a Marginal Risk (level 1 out of 5) for an isolated severe thunderstorm tomorrow afternoon. Expectations are that most storms if they develop would be sub-severe with plenty of cloud cover and stratiform rain around, but an isolated strong thunderstorm cannot be ruled out if an area can break out of the clouds and tap into some diurnal heating. Associated with the cold front, breezy, to locally gusty, winds are expected along the higher terrain and Champlain Valley tomorrow. As the front swings east, and associated strengthening 40-50 knot low level jet will move along the northern portions of the region. Gusts on summit levels could gust up to 30- 35 MPH, with valley winds around 20-30 MPH, mainly between 2 and 8 PM. Winds will temper down with sunset and the development of a nocturnal inversion.

Showers and thunderstorms will taper off for most by late Monday afternoon. Lingering northwest wrap around flow could lead to some continued stubborn shower chances along the higher terrain of the northern Greens and Adirondacks Monday night. Across eastern Vermont, fog development appears likely in the valleys of the region with brief high pressure nudging in from the west, and calming winds. Tuesday and Wednesday will be quieter and calmer with temperatures in the upper 70s to mid 80s and mostly clear skies. Dewpoints will slowly creep up by mid week however, approaching the mid 60s in some places.

KEY MESSAGE 3: No significant changes to the forecast for later this week/next weekend, as we look to remain squeezed between a strong Bermuda high to our east and troughiness across the Northern/Central Plains. This will place us square under broad W/SW flow, with upper shortwaves occasionally rippling overhead. Humidity will increase as moisture streams northward into our region; dewpoints look to approach 70F with PWATs of 2+ inches Thursday and Friday. Models differ on how long this persists, with a potential frontal passage sometime over the weekend, but this is highly uncertain at this time. In the meantime, we can expect rounds of showers and a few thunderstorms through the end of the week. With no real strong forcing and relatively warm air aloft limiting available CAPE, don't anticipate any severe weather at this time. Locally heavy rain will be possible, but flow should be enough to keep showers/storms moving, limiting any flood threat.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Through 18Z Monday...VFR conditions to mainly prevail through 00z Mon with isolated to scattered showers to occasionally lower visibility and/or ceilings to MVFR. The bulk of the activity through this afternoon and evening will be over northern NY, particularly KMSS, where a few thunderstorms will also be possible. Showers become more widespread from 00z Mon onward, spreading from west to east ahead of an incoming front. Ceilings will lower to 1200-2500 ft at all terminals by 10z, with some indications of IFR ceilings will be possible. Visibility generally 5-6+SM, though conditions could be briefly lower in any heavier rain. Frontal passage occurs Monday morning, bringing showers to an end, but ceilings will likely remain MVFR through the end of the TAF period, with additional rain possible Monday afternoon. S/SW winds to remain 6-12 kt with gusts to around 20 kt through 00z, then gusts diminish overnight. However, LLJ will result in LLWS of 35-40 kt at all terminals overnight, as well. Winds will trend more toward the W after the frontal passage on Monday morning.

Outlook...

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

MARINE

As of 217 PM EDT Sunday... 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 winds on Lake Champlain 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.

CLIMATE

As of 217 PM EDT Sunday... Possible heavy rainfall tomorrow into Monday could approach daily rainfall record levels. Below are the current daily precipitation total records that may be broken at our area climate sites.

Record Precipitation:

August 2: KMSS: 1.31/2020

BTV WATCHES/WARNINGS/ADVISORIES

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