textproduct: Burlington

WHAT HAS CHANGED

As of 241 PM EDT Saturday...

Have increased the rainfall totals across the St. Lawrence Valley and portions of the Adirondacks to range between 1 to 1.5 inches, with locally higher amounts possible within any convective activity. An isolated flash flood threat across northern New York remains for late Sunday into early Monday with slow-moving showers and thunderstorms possible.

KEY MESSAGES

As of 241 PM EDT Saturday...

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

2. A chance of showers and thunderstorms persists over the second half of next week.

DISCUSSION

As of 241 PM EDT Saturday...

KEY MESSAGE 1: A slow moving upper level trough with associated closed surface low is currently centered across the Ohio Valley and will track to the east northeast today into tomorrow. A baroclinic zone is also set up across the Great Lakes and eastern Ontario bringing some isolated shower chances today across the St. Lawrence Valley. As this system tracks east, cloud cover will increase areawide into tonight with south to southwest channeled flow just above the surface, which should help limit, or prevent altogether, the coverage of any potential fog tonight.

Into tomorrow, the system will approach the St. Lawrence Valley and the North Country with shower chances increasing by midday Sunday. Thermodynamic profiles denote some skinny SBCAPE between 500- 1000J/kg which will allow for at least some isolated convection and thunderstorm activity. This activity will likely be confined to near the St. Lawrence River and along US-11 in St. Lawrence County. Modest to weak mid level flow between 20-30 knots with Pwats approaching 1.8-2", and deep warm layer cloud depths, locally heavy rain within any convection will be possible. WPC has kept the St. Lawrence Valley in a Slight (Risk level 2 out of 4) for a at least a 15% chance of exceeding flash flood guidance. The axis of these showers remains somewhat variable from run to run across the Hi Res CAMs. The NAM3 keeps much of the convective activity during the day tomorrow over the border in Ontario, whereas the HRRR rides the axis of instability over western St. Lawrence County. Where this deformation boundary sets up will have a large impact on the total rainfall from this system. Right now have kept the convective activity on the US side of the border with a more stable run to run consistent HRRR solution. Should an area see repeated rounds of showers, especially with antecedent rainfall, flash flooding cannot be ruled out. However, the St. Lawrence Valley at the present time is not expected to see significant enough rainfall to produce flooding over the prevailing flatter terrain.

Towards Sunday evening and Sunday night, the deformation boundary will shift east into the Champlain Valley and Vermont by Monday morning. 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. Showers will slowly taper off by Monday afternoon across eastern Vermont with fog development likely in the valleys of the region with brief high pressure nudging in. Currently have increased rainfall totals across the St. Lawrence Valley, with new guidance. Total rainfall for most of the region will be around 0.5 inches. Locally higher amounts between an inch to an inch an a half will be possible from the foothills of the Adirondacks into the St. Lawrence Valley, especially under any convective activity. Tuesday will be drier and calmer with temperatures in the upper 70s to low 80s and clearing skies.

KEY MESSAGE 2: A slow moving to stationary front will be draped across our region for the second half of next week bringing repeated rounds of showers and thunderstorms. While the showers will linger through multiple days, the thermodynamic profiles don't look to support severe potential as we are seeing 250-500 J/kg of CAPE each afternoon with weak shear. These values will support air mass/popcorn thunderstorms with a few rumbles of thunder and short lived localized heavy rain. Consistent with a stationary front, temperatures will be remarkably consistent over the period. Daytime highs in the warmest valleys will be in the mid-80s daily with overnight lows falling into the mid-60s. The stationary front will finally move off at the end of next week as a low pressure will move in from the southeastern US bring more organization to the thunderstorms and the possibility of stronger convection.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

Through 18Z Sunday...VFR conditions will persist through the day today. A scattered to broken deck of fair weather Cu will continue to develop into the afternoon hours with a cirrus deck above. Last night saw the development of fog in many of our favored locations, and the low level moisture remains available. However, the low pressure to our west will bring increased winds and clouds overnight inhibiting pure radiational effects. For now, I'm including some shallow fog development at MPV and SLK near sunrise but don't anticipate widespread formation like we had last night.

Outlook...

Sunday Night: Mainly VFR, with local MVFR possible. Likely SHRA, Slight chance TSRA. Monday: Mainly MVFR, with local IFR possible. Definite SHRA, Slight chance TSRA. Monday Night: Mainly VFR, with local MVFR possible. Slight chance SHRA. Tuesday: VFR. Slight chance SHRA. Tuesday Night: VFR. Slight chance SHRA, Slight chance TSRA. Wednesday: VFR. Chance SHRA, Chance TSRA. Wednesday Night: VFR. Chance SHRA, Slight chance TSRA. Thursday: Mainly VFR, with areas MVFR possible. Chance SHRA, Slight chance TSRA.

CLIMATE

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.