textproduct: Burlington

WHAT HAS CHANGED

As of 505 PM EDT Wednesday...

A Severe Thunderstorm Watch has been issued until 11 PM for all of northern New York and the Champlain Valley or New York and Vermont. Damaging winds, frequent lightning, heavy rain, and hail.

KEY MESSAGES

As of 228 PM EDT Wednesday...

1. Major to localized extreme heat risk is expected through Friday with widespread heat index values of 95F to 105F. Some locations could exceed 105F heat index due to very warm air temperatures and dewpoints rising into the 70s.

2. Daily chances for showers and thunderstorms will continue through Friday. Some of these could be strong to severe, but there remains considerable uncertainty in areal coverage and timing.

3. Hot and humid conditions will linger through the end of the weekend with daily chances of showers and thunderstorms into next week.

DISCUSSION

As of 228 PM EDT Wednesday...

KEY MESSAGE 1: Dangerous heat will continue to impact Vermont and northern New York today through Friday. Widespread temperatures in the upper 80s to mid 90s have already been observed with heat indices in the Champlain Valley already near 100F and near 105F in the St. Lawrence Valley. While just a Heat Advisory is in effect for the St. Lawrence Valley with heat indices near or at 105F in Potsdam and Ogdensburg from localized urban heat island processes, increasing clouds from thunderstorms in Ontario will help to limit the potential for any extended Extreme Heat Warning conditions. The high heat index values will be present both tomorrow and Friday afternoon areawide, with the highest occurring Thursday with just about everyone hitting 90F, and some locations could approach 100F. Unfortunately, humidity will also be on the increase; dewpoints will be in the upper 60s to low 70s. This will not only make for dangerous heat index values exceeding 100F in places, it will keep the overnights very warm and muggy, limiting any relief from the hot and humid daytime hours. We'll see a bit of a respite from the humidity Friday and Saturday, but it will still be very warm and muggy. Temperatures on the 4th will be around the low to mid 80s, with some locations in the upper 80s in the northern Champlain Valley and lower Connecticut Upper Valley. Heat Index values for holiday activities will be around the mid to upper 80s.

Heat related impacts will increase through the week due to the cumulative effects from the prolonged heat wave. Please make sure to take proper heat safety precautions, including limiting your time outdoors, drinking plenty of water, and seeking indoor shelter with air conditioning. Check on your relatives and neighbors, especially if they are elderly. And don't forget about your pets; bring them indoors or provide them with ample shade and fresh cool water.

KEY MESSAGE 2: Heat and humidity will combine to create impressive SB CAPE values today between 2500-4000 J/kg today and 1500-2500 J/kg on Thursday, with some indications of values exceeding 3000 J/kg at times. Shear will be less impressive, generally 25-35 kt depending on the model and day, perhaps increasing to 40+ kt by Thursday afternoon. We are currently monitoring a decaying MCS complex near Montreal, which if it survives will move into a more favorable environment as it approaches Vermont. Current track brings it into the northern Champlain Valley around 3 PM with perhaps some gusty winds, heavy rain, and frequent lightning. This threat will be conditional on whether it survive and can tap into the better environment. Otherwise today, with no strong forcing mechanism other than the abundant instability, expect terrain-induced convection today, generally remaining isolated to scattered in coverage. Strong to severe thunderstorms can't be ruled out given the large CAPE values; SPC continues to encompass our entire forecast area in a Slight Risk (level 2 out of 5) for Today. Model guidance during this Ring of Fire setup has been lackluster at best. Convection across Wisconsin and the upper mitt of Michigan will likely persist towards eastern Ontario close to sunset, based on persistence and upper flow patterns, which may pose an additional more organized thunderstorm threat into this evening for northern New York, though confidence is low at this time. Furthermore, additional thunderstorm chances may exist into late tonight into the Champlain Valley and Vermont, though any storms overnight are not expected to be severe. As one would expect with the amount of humidity, we're also in WPC's Marginal Risk (level 1 out of 4) for excessive rainfall. Any storms that pop up today would at the least produce heavy rainfall, though widespread flooding is not anticipated.

Additional thunderstorms are possible Thursday and Friday, though marginal mid-level lapse rates due to loss of the EML will limit instability and likely areal coverage/intensity for any convection on Friday. We are included in a Marginal Risk (level 1 out of 5) in SPC's Day 2 severe outlook. Friday is more uncertain, both in areal coverage and potential storm severity. Still, given the upcoming holiday festivities, we urge everyone to stay tuned to later forecast updates as strong/severe thunderstorms can't be totally ruled out any one day.

KEY MESSAGE 3: A stationary frontal boundary will be draped over northern New York and Vermont Saturday night, drifting southwards through Sunday night as the weekend comes to a close. This indicates lingering chances for showers and thunderstorms, about 25-55% chance, highest on Monday as a shortwave moves through the region. Because of this, nights will be mild in the 60s with highs and apparent temps remaining warm and muggy in the upper 70s and 80s. Although it looks like it won't be quite as hot as we're expecting these next couple of days, it is important to remember to drink water and take frequent breaks if working outside, especially with such an extended period of warmer weather. Next week, deterministic model guidance diverges sharply, increasing uncertainty for the long range forecast. At this time, high temperatures are most likely to range in the 80s and lower 90s with chances for showers and thunderstorms each afternoon through midweek.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Through 18Z Thursday...Daily chances for showers and thunderstorms will continue through Friday. Some of these could be strong to severe, but there remains considerable uncertainty in areal coverage and timing. This afternoon, VFR conditions prevail across all terminals, though this would likely change if a shower or thunderstorm develops or moves over a terminal. At the moment, chances of thunderstorms are around 20-40% for much of the next several hours. Winds are out of the south-southwest at 5-15 knots this afternoon and are expected to increase into the evening with gusts 20-25 knots possible, most likely at MSS with southwesterly channeled flow. After 00Z Thursday, precipitation is more likely to become primarily showers instead of thunderstorms, though some lightning could still persist into the evening and early overnight hours. Brief low level wind shear may also impact a few sites 00Z-06Z Thursday as surface winds decrease while winds aloft remain elevated. After 06Z Thursday, precipitation is much less likely, and attention turns to favored fog locations like SLK and MPV, especially if these sites receive rainfall this afternoon or evening. Patchy fog is not out of the question at any site as surface winds quickly decrease, but SLK and MPV look most favorable for this. Any location that sees fog tonight will likely return to VFR conditions around 12Z-14Z Thursday as fog burns off with the rising sun. Additional thunderstorms are possible Thursday and Friday, though areal coverage and intensity will likely be limited.

Outlook...

Thursday Night: VFR. Chance SHRA, Chance TSRA. Friday: VFR. Slight chance SHRA. Friday Night: VFR. NO SIG WX. Independence Day: VFR. Chance SHRA, Chance TSRA. Saturday Night: VFR. Slight chance SHRA, Slight chance TSRA. Sunday: VFR. Slight chance SHRA, Slight chance TSRA. Sunday Night: VFR. Chance SHRA. Monday: VFR. Chance SHRA, Slight chance TSRA.

CLIMATE

Extreme heat and humidity will affect the region for the latter half of the week, which could lead to some daily records. Below are the current daily high and low temperature records in jeopardy at our area climate sites.

Record High Temperatures:

July 1: KBTV: 96/2018 KMPV: 92/2018 KPBG: 94/1968 KMSS: 94/2018

July 2: KBTV: 97/2018 KMPV: 90/2018 KMSS: 94/2018

July 3: KMPV: 91/2002

Record High Minimum Temperatures:

July 1: KBTV: 76/2018 KPBG: 73/1971 KSLK: 69/2018

July 2: KPBG: 77/2002 KSLK: 68/2002

July 3: KBTV: 76/1911 KPBG: 73/2002

July 4: KPBG: 71/1973 KSLK: 67/1952

BTV WATCHES/WARNINGS/ADVISORIES

VT...Heat Advisory until 7 PM EDT Friday for VTZ003-004-006>008-010- 016>020. Extreme Heat Warning until 7 PM EDT Friday for VTZ001-002-005- 009-011-021. NY...Heat Advisory until 7 PM EDT Friday for NYZ026-027-029>031-034- 087. Extreme Heat Warning until 7 PM EDT Friday for NYZ028-035.