textproduct: Burlington

WHAT HAS CHANGED

As of 229 AM EDT Monday...

A Heat Advisory has been issued across much of Vermont and northern New York for Tuesday afternoon. The Storm Prediction Center has expanded the forecast region of severe weather in our region, and upgraded areas along the International Border to an Enhanced Risk (Level 3 of 5).

KEY MESSAGES

As of 229 AM EDT Monday...

1. Heat will begin to build, with a Heat Advisory posted for most all of Vermont and northern New York on Tuesday. Some localized fire weather concerns are possible.

2. A sharp cold front is expected to produce strong to severe showers and thunderstorms Tuesday evening and overnight. A few storms may be capable of significant severe weather hazards.

3. Seasonable temperatures are generally expected for the latter half of the week into the weekend, with a few chances for showers possible.

DISCUSSION

As of 229 AM EDT Monday...

KEY MESSAGE 1: Today will be another nice day. Heat is expected to build, but dewpoints will remain low today. A shortwave will progress southeast today, but with significant low level dry air, it may struggle. Still, high res guidance has increased the area of radar returns, and so the forecast continues to indicate a slight chance of some afternoon showers today. Southwest winds will help usher in the warmer weather, and it will bring some breezier weather compared to the last couple days with some winds to 15 mph, mainly across northern New York. Within and showers that may develop, there may be some embedded gusts given the evaporative cooling amidst the dry low-level environment.

Tuesday will be the peak of a brief, but potent, surge of heat. Observations upstream indicate that the NBM forecast for dewpoints may be underforecasting our region, and slightly elevated dewpoints upwards. The latest forecast runs have also nudged their 925mb temperatures up just a bit more, and so much of the forecast area is poised to hit 90s with surface dewpoints in the mid 60s, locally near 70, especially across the St. Lawrence Valley. With this, forecast heat indices have crept up towards 93-103 for the region. Southwesterly winds will increase further as well, with speeds 10-20 mph across the entire region, and gusts 20-35 mph. Sustaining well- mixed conditions will likely be what keeps us below 100, but there is a real possibility a few places might reach triple digits. The heat will be brought to a close by a potentially significant frontal boundary Tuesday evening into Tuesday night. Even with higher dewpoints, the warmer temperatures will push south-central Vermont minimum relative humidity values towards 35 percent. Parts of these areas have escaped recent rainfall, and the combination of sun, winds, and heat could drive localized fire weather concerns.

KEY MESSAGE 2: The range of forecast guidance available has been consistent in depicting a cluster of powerful thunderstorms from about 6PM - 4AM descending north out of Canada and gradually dissipating after passing over the international border. Some of these storms may be capable of significant severe weather.

Much of Tuesday afternoon will be stable, as a warm layer in the mid- levels will effectively cap convection. By evening, height falls will begin to erode that region of mid-level warmth. Given temperatures will still range in the upper 80s to mid 90s, the region will begin to quickly destabilize while a powerful cold front drops south of Montreal about 8 PM Tuesday. CAPE values are high, ranging between 2000-3000 J/kg, and enhanced west-northwest flow channeled between an anomalous high in the central US and an anomalous upper low developing across eastern Quebec will result in 0-6km shear approaching 55-60 kts. Shear is mostly unidirectional across the region, but the development of any mesoscale low, may allow flow to back southwesterly as the boundary shifts south. This potential is indicated in the extended 00z HRRR run, and even shows up within the 00z GFS. Thus, thunderstorms are expected to initially be discrete, but quickly organize into broken line segments. Any bowing that takes place may allow for some rotation to develop as well. Remnants of an elevated mixed layer will be present, but it will be shifting south somewhat before thunderstorms initiate. So the timing of strongest thunderstorms will be in that 6pm-midnight window and along the international border. After becoming briefly intense, they will then gradually weaken as they advance southwards across the rest of our forecast area.

All storm threats may occur with this, but for now the forecast grids indicate heavy rain and gusty winds. Some hail could be favored while the storm mode is more discrete, and a tornado cannot be entirely discounted within the development of any bowing segments. The Storm Prediction Center has forecast an Enhanced Risk (Level 3 of 5) of Severe Storms along the international border. This indicates the potential for widespread strong to severe thunderstorms with significant intensity.

Not to be left out is the heavy rainfall potential. If clusters are limited, then we'll have heavy rainfall within strong to severe storms, but may miss out on any flood potential due to their fast motions. However, if a broader area of thunderstorms develop, some forecast evolution depicts thunderstorms redeveloping on the western periphery of bowing segments in the Northeast Kingdom and resulting in a swath of high rainfall totals as they move southeast across northern Vermont, especially. The present forecast shows a sharp gradient with the higher rainfall totals favored across far northern Vermont, with values of 1-1.5" indicated from eastern Franklin Vermont to Essex County, Vermont.

After Wednesday morning, the front should be positioned near or south of the area. Activity will dwindle, and somewhat cooler air will begin the process of moving in. Still on the warm side with 80s for most, and perhaps some readings in the lower 90s for Rutland and Windsor Counties.

KEY MESSAGE 3: Into the latter portion of this week and the weekend, temperatures will remain relatively seasonable to seasonably warm increasing from the low 70s to near 80 on Friday to the upper 70s to mid 80s by the start of next week. While moisture slowly returns by the start of next week, dewpoints in the upper 40s to mid 50s will make things feel comfortable. Synoptically we will remain under cyclonic northwesterly flow as a cyclonic gyre looks to become stagnant over northern Quebec, keeping ridging across the west, and a favoring the potential for a few shortwaves to pass through the area into the weekend. Timing and spatial patterns of any of these shortwaves remains muddied as these will likely be associated with convection and mesoscale processes. That being said, there is a growing signal that better chances for any precipitation will be Saturday into Sunday with a diving shortwave across the Great Lakes. Questions remain where the shortwave sets up in the base of the trough. GEFS and Euro ensembles are fairly consistent with increasing precipitation during the day Saturday into Saturday night as a trough swings through the region with lingering showers into Sunday. The Canadian model solutions favor a more southerly stream of energy that would keep most of the region dry through the weekend. Trends have favored southern Vermont as a target for better precipitation chances taking the average of the aforementioned model solutions.

AVIATION /12Z MONDAY THROUGH FRIDAY/

Through 12Z Tuesday...Prevailing VFR conditions are expected at all terminals through the TAF period. A few mid to upper clouds are passing through northern New York this morning from convective blowoff associated with decaying showers in Canada, with otherwise mainly clear skies expected thereafter. Winds today will start off light from the south (around 5-10 knots), and trend more southwesterly by early this afternoon. Channeled southwest flow at MSS/SLK could bring intermittent gusts to 15-20 kts through this afternoon. Clouds will briefly increase this afternoon with a passing boundary, before trending back to more scattered to few cloud cover. A brief sprinkle may be possible at BTV/MPV this afternoon, though any precipitation will likely fall as virga due to dry low levels, with no terminal impacts. Winds will trend towards calm with a scattered deck of stratus around 10,000ft agl overnight tonight.

Outlook...

Tuesday: VFR. Windy with gusts to 30 kt. NO SIG WX. Tuesday Night: VFR. Chance SHRA, Slight chance TSRA. Wednesday: VFR. NO SIG WX. Wednesday Night: VFR. NO SIG WX. Thursday: VFR. Slight chance SHRA, Slight chance TSRA. Thursday Night: VFR. Slight chance SHRA. Friday: VFR. NO SIG WX.

CLIMATE

High temperatures on Tuesday will be hot, but we are at the climatological warmest part of the year. Therefore, it is far from a certainty that any records will occur. At this time, PBG, MPV, and MSS are the more likely long-term climate sites to set a record. See below for current records:

Max Temp Records Date BTV MPV MSS PBG SLK 07-14 100|1995 94|1952 92|2012 95|1952 95|1934

High Min Temp Records DatePBG 07-14 70|1974

BTV WATCHES/WARNINGS/ADVISORIES

VT...Heat Advisory from 11 AM to 8 PM EDT Tuesday for VTZ001>011- 016>018-020-021. NY...Heat Advisory from 11 AM to 8 PM EDT Tuesday for NYZ026>029- 031-035-087.