textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes to the forecast.

KEY MESSAGES

- Dangerous prolonged heatwave continues through Sat July 4th. Afternoon Heat Indices of 100 to 115 degrees today & Fri but still 95 to 100+ on Sat. Little relief at night either.

- Low convective/severe weather probability today and tonight given the lack of forcing...but if isolated storms are able to develop they could become severe. The thunderstorm and severe weather potential increases late Fri and/or Sat ahead of an approaching cold front.

- Relief from the heat with cooler/seasonable temperatures returning for the first half of next week.

DISCUSSION

KEY MESSAGE 1...Dangerous prolonged heatwave continues through Sat July 4th. Afternoon Heat Indices of 100 to 115 degrees today and Fri but still 95 to 100+ on Sat. Little relief at night either.

The peak of this dangerous heatwave will be today and Friday and the model guidance continues to indicate some very impressive parameters for extreme heat. 925T are indicated to be in the +28C to +30C range today and Friday. Given westerly flow thinking that near record high temperatures in the 97 to 103 degree range are reasonable today with plenty of sunshine and a very warm start. More importantly...very high dewpoints will result in dangerous heat indices of 105 to 115 degrees this afternoon! Near record high temperatures are expected again on Friday. Perhaps a degree or two lower in some spots...but generally in the 96 to 102 degree range. Heat Indices will again be at extreme/dangerous levels on the order of 100 to 110 degrees. While Sat July 4th should not be as extreme as today and Friday given height falls and somewhat lower mid level temperatures...still expecting high temperatures into the 90s with heat indices of 95- 100+ degrees when many have outdoor activities planned. So in a nutshell...the dangerous heat and humidity will continue through Sat July 4th.

Not only will this be a dangerous prolonged period of heat, there will be little relief at night. Low temperatures will only drop into the middle 70s in most locations and 75 to 80 degrees in the urban centers. In fact, the urban heat island of Boston may struggle to drop below 80 tonight and Friday night.

KEY MESSAGE 2...Low convective/severe weather probability today and tonight given the lack of forcing...but if isolated storms are able to develop they could become severe. The thunderstorm and severe weather potential increases late Fri and/or Sat ahead of an approaching cold front.

Most of the model guidance has struggled in capturing the convective complexes that have impacted parts of the northeast the last couple of days. Thinking the risk for today and tonight is somewhat lower than the last two days...but an isolated thunderstorm risk still exists. The upper level forcing is quite meager today and there will be a mid level cap in place. Nonetheless...very high instability will develop for southern New England standards coupled with a remnant EML. So if we are able to get a few storms to develop from elevated heating/localized convergence they could become severe in this environment. However...thinking if any do develop they will be rather isolated with a much more limited coverage than the past two days.

We do think the thunderstorm and severe weather potential will increase late Fri and/or Sat with height falls associated with a cold front. This is also indicated by the machine learning guidance...but will have to wait until we get more inside the high resolution model window to have any more specifics. The machine learning guidance does indicate that if we do get severe weather...greatest risk would be straight line wind damage...but high Pwats also will bring a concern for localized torrential rainfall/street flooding as well as dangerous cloud to ground lightning.

KEY MESSAGE 3...Relief from the heat with cooler/seasonable temperatures returning for the first half of next week.

Long range ensemble guidance is in good agreement in the upper level ridge breaking down. This will bring and end to the extreme heat. Cooler/seasonable temperatures return Sunday and continue into next week!

AVIATION /12Z THURSDAY THROUGH MONDAY/

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

12z TAF Update...

Today through Friday...High confidence.

Mainly VFR conditions into Friday afternoon. Some low clouds have developed along the south coast with webcams indicating that they sit just offshore of terminals like ACK. With SWrly winds, its possible that IFR conditions develop intermittently at ACK through 15Z. Fog/Stratus development again likely late this evening and overnight for terminals across the Cape and Islands. Otherwise...while we can not rule out an isolated thunderstorm risk later today and tonight thinking areal coverage/duration will be rather limited if even any are able to develop. Greater risk for some thunderstorms probably waits until late Fri and/or Sat.

Westerly winds may gust to 20+ knots this afternoon with some SW wind gusts around 25 knots towards the Cape and Islands. Another round of westerly 20+ knot gusts will develop again during the day Friday.

KBOS Terminal...High confidence in TAF. Thinking gradient remains strong enough today to prevent a sea breeze.

KBDL Terminal...High confidence in TAF.

Outlook /Friday Night through Monday/...

Friday Night: VFR. Scattered SHRA, scattered TSRA.

Independence Day: VFR. Breezy. Scattered SHRA, scattered TSRA.

Saturday Night: VFR. Breezy. Isolated SHRA, isolated TSRA.

Sunday: VFR. Slight chance SHRA, slight chance TSRA.

Sunday Night: VFR. Chance SHRA.

Monday: VFR. Chance SHRA, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

Today through Friday...High confidence.

SW winds will gust to 25 knots at times today and long fetch will result in 3 to 5 foot across our southern waters. Therefore...we have continued small craft headlines for our southern waters today. Winds should diminish a tad and turn to more of a westerly component tonight into Friday...allowing us to drop the small craft headlines by that time.

Outlook /Friday Night through Monday/...

Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Isolated rain showers, isolated thunderstorms.

Independence Day through Saturday Night: Winds less than 25 kt. Scattered rain showers, scattered thunderstorms.

Sunday: Winds less than 25 kt.

Sunday Night: Winds less than 25 kt. Chance of rain showers.

Monday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, slight chance of thunderstorms.

CLIMATE

Record High Temperatures:

July 2: KBOS: 98/1963 KBDL: 99/1966 KPVD: 98/1941 KORH: 97/1901

July 3: KBOS: 102/1911 KBDL: 102/1966 KPVD: 98/2002 KORH: 96/1911

July 4: KBOS: 104/1911 KBDL: 99/1911 KPVD: 99/1919 KORH: 102/1911

Record High Minimum Temperatures:

July 2: KBOS: 76/2002 KBDL: 73/2018 KPVD: 75/1941 KORH: 73/1963

July 3: KBOS: 80/2002 KBDL: 73/2018 KPVD: 78/2002 KORH: 72/2002

July 4: KBOS: 77/2002 KBDL: 74/2018 KPVD: 77/2002 KORH: 73/2018

July 5: KBOS: 81/1999 KBDL: 77/1999 KPVD: 78/1999 KORH: 73/1999

BOX WATCHES/WARNINGS/ADVISORIES

CT...Extreme Heat Warning until 8 PM EDT Saturday for CTZ002>004. MA...Extreme Heat Warning until 8 PM EDT Saturday for MAZ002>021- 026. Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for MAZ006-007-013>024. Heat Advisory until 8 PM EDT Saturday for MAZ022-023. RI...Extreme Heat Warning until 8 PM EDT Saturday for RIZ001>007. Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ231>235-237. Small Craft Advisory until 1 AM EDT Friday for ANZ254>256.