Area Forecast Discussion

WHAT HAS CHANGED

As of 250 AM EDT Thursday...

A Heat Advisory was issued for today in eastern Windsor County from 12 to 8 PM.

KEY MESSAGES

As of 250 AM EDT Thursday...

1. Building heat and humidity will impact the region with potentially dangerous conditions today in Windsor County and more widespread for Friday.

2. Isolated to scattered thunderstorms are expected today with a cold front potentially bringing stronger variety storms Friday afternoon.

3. Strong cold front to bring widespread rainfall and thunderstorms to the region late Sunday with more seasonal, yet unsettled, weather for next week.

DISCUSSION

As of 250 AM EDT Thursday...

KEY MESSAGE 1: Temperatures are slow to fall this morning with more expansive cloud cover limiting radiational cooling. As such, temperatures will start warm today with 925mb temperatures ranging 23-25C. Mixing supports high temperatures in the upper 80s to around 90 degrees. Conditions will be a little uncomfortable this afternoon with heat index values pushing 90 for most locations and mid/upper 90s in Windsor County. A heat advisory was issued for portions of southern Vermont from 12 to 8 pm today. The saving grace for the Champlain Valley could be cloud cover associated with convection. Some showers and a few thunderstorms may form this afternoon limiting max temperatures enough to be excluded from higher heat related concerns. However, it will still be an uncomfortable day and caution is advised since most are not acclimated to these conditions.

Friday, temperatures warm further as 925mb temps rise in the 23-27C range. Ridging will be sufficient to cap morning/early afternoon convection from the Champlain Valley eastward, so heat indices are expected to climb into the mid/upper 90s for the Champlain Valley and portions of the Connecticut River Valley. Timing of the front will play a part in heat stress for the Champlain Valley, but current model output favors a post-4 PM passage. Those in northeastern Vermont may be included in the Advisory later if model warming trends continue. There is a non-zero chance that heat indices could approach/exceed 100 for a few locations, but have held back a little since some guidance pushing these values has had recent tendencies to run a little too warm. Should more of a consensus build in other model output later today, excessive/extreme heat concerns will increase.

KEY MESSAGE 2: With heat building and a weak trough moving through the region today, some isolated to scattered thunderstorms will be possible. Coverage of thunderstorms have increased from previous forecasts mainly due to a more consolidated consensus on the trough timing. This trough is somewhat of a ridge runner, but won't benefit from much capping to keep skies clear; so convection will likely self limit given skinny CAPE profiles. Still, moisture/temperatures will be sufficient to allow for 1000+J/kg CAPE this afternoon and triggering should be sufficient for thunderstorms. A few of these storms could be heavy rainers with 1.5+inches of PWAT and expected slow movement.

Friday is a different story. The ridge axis builds more over the region with temperatures warming further. CAPE will likely build beneath a capping inversion until a cold front sweeps through the region. Shear will increase to around 30kts supporting more favorable dynamics for organized convection. Shear coupled with 1500 to potentially 2500 J/kg CAPE will support at least isolated stronger variety thunderstorms. SPC also has the region under a Slight (15%) risk of severe thunderstorms given heat, moisture, and a good triggering mechanism. While storms are expected to be moving, some locations that have received heavy rainfall may be susceptible to flooding. This will be especially true for areas where roads have washed out.

KEY MESSAGE 3: Global deterministic and ensemble guidance is coming into better agreement depicting a sharp cold front moving across the region Sunday afternoon and evening. There will likely be some discrepancies in subsequent model runs on when exactly the front is expected to move through but there should be some decent instability in place. Thunderstorms appear to be a good bet along the frontal boundary as we will see daytime temperatures soar into the upper 80s with dewpoints in the the 60s. Even if the front is delayed and comes through overnight, elevated instability would still be present and produce scattered to numerous thunderstorms. The latest machine learning algorithms are showing the potential for some strong to severe storms on Sunday but the timing of the front will be the key to whether we see generic thunderstorms or those of the stronger caliber.

Following the frontal passage, temperatures appear to drop back closer to seasonal normals through the first half of next week. We will be under the influence of a weak upper level trough that will shunt the warmer air to the south while we see temperatures in the 70s and lower dewpoints than what we have seen as of late. However, with the influence of an upper level trough and a pocket of cold air aloft, a few diurnally driven showers and possibly a thunderstorm or two cannot be ruled out each day. It doesn't appear like any day will be a wash-out by any means but some passing showers/storms will remain a possibility each day.

AVIATION /06Z THURSDAY THROUGH MONDAY/

Through 06Z Friday...Mid-level clouds are building in the wake of the precipitation which has surprisingly helped stunt the potential for fog development this morning. We will continue to see some rain showers move across the region through 9Z with localized MVFR visibilities possible within any showers. Ceilings will slowly drop from 3000-5000 ft to 1000- 3000 ft between now and 12Z with a quick improvement to high VFR by mid-morning. Additional showers and thunderstorms are expected again this afternoon but will likely be less widespread than those observed on Wednesday. Still, MVFR with some very localized IFR visibilities could see seen with these showers between 18Z and 00Z. Winds will be light from the west today and become light and variable overnight.

Outlook...

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

CLIMATE

Record High Temperatures:

June 12: KBTV: 94/2017 KMPV: 88/2017 KMSS: 89/2005

Record High Minimum Temperatures:

June 11: KBTV: 71/2017

June 12: KBTV: 71/2017 KPBG: 67/2017 KSLK: 62/1996

BTV WATCHES / WARNINGS / ADVISORIES

VT...Heat Advisory from noon to 8 PM EDT Friday for VTZ001-002-005- 009-011-021. Heat Advisory from noon today to 8 PM EDT this evening for VTZ021. NY...Heat Advisory from noon to 8 PM EDT Friday for NYZ028-035.