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 110 degrees again Friday but still 95 to 100+ on Saturday. Little relief at night either.

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

- Relief from the heat with cooler and more 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 110 degrees again Friday but still 95 to 100+ on Saturday. Little relief at night either.

Dangerous heatwave ongoing across New England today and Friday. As of 2 PM EDT, Boston (KBOS) has already broken the high temperature record today, with 99F recorded a handful of times so far this afternoon (previous record was 98F in 1963). Recording 100F here has yet to occur, but is certainly within reach over the next couple of hours. Time will tell.

925mb temps have been hanging at 28-29C today, and will continue through Friday. Center of the ridge in the Carolina Piedmont sinks ever so slightly Friday, allowing heights to fall a tad. As a result, high temperatures are just a touch "cooler" Friday (not that you'll really feel a difference). Dewpoints are still well into the 70s the whole time, meaning that the heat/humidity combination won't have a problem yielding heat indices well into the triple digits (100-110F) across most of New England.

Still hot but a bit more uncertainty introduced to Saturday based on frontal timing. The spread in 850 mb temps across guidance has increased to anywhere between +15C and +19 or +20C. With westerly downsloping winds and compressional warming ahead of the front adding a couple of degrees that could mean anywhere from near 90 to mid 90s. NBM guidance is probably a little overbaked but not by a meaningful degree. It will still be hot and dewpoints will still make it feel a good 5 to 10 degrees warmer than the air temps. So even if many sites do not hit exact extreme heat criteria, the prolonged nature of the heat plus the holiday outdoor activities mean that the Extreme Heat Warning and Heat Advisory for most of the Cape and Islands should continue thru Sat.

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

Cold front is forecast to be in the vicinity of southern New England Sat. It will still be hot/humid and there will be more than enough shear to organize convection. The limiting factor now appears to be lapse rates, generally around 6C/km. So guidance is struggling to push CAPE north of 1000 J/kg. It is a little long range for some of the CAM guidance, but there is everything from little to no convection to fairly widespread storms from the RRFS. I have a hard time seeing the heat breaking without some showers and storms to clear it out, so much of what happens Sat may depend on Fri/Fri night. Once again primary severe threat would come from damaging winds and perhaps some large hail.

KEY MESSAGE 3...Relief from the heat with cooler and more 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 and a tendency towards lower heights next week. This will bring an end to the extreme heat, with readings more in line with typical summer. Guidance is also increasing PoP towards Tue as a weak upper low drifts or opens up as it crosses the region. Precip chances remain low confidence however, with large spread between the 25th and 75th percentile QPF.

AVIATION /00Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

00z TAF Update...

Today through Friday...High confidence.

Widespread VFR continues and is expected to continue thru Fri afternoon. In the very near term, a south coast sea breeze continues to push north. Winds behind this will shift to a more southwesterly direction before westerly winds gradually take over again after midnight. Isolated SHRA/TSRA have also developed in and around the Hudson Valley and are spilling into the Berkshires at this hour. They are expected to weaken with the loss of daytime heating and are not expected to impact any terminals. Otherwise keeping an eye on fog tonight but with west/southwest winds expected to remain overnight that should keep things away from ACK and more or less relied on persistence in the TAF there.

Fri afternoon will have to keep a closer eye on SHRA/TSRA. Some guidance wants to develop things much closer to the forecast area and it would then have a better chance of holding together into the evening. Right now the greatest risk in across CT, but remains low confidence enough to keep out of the TAF.

KBOS Terminal...High confidence in TAF. Sea breeze from the south will approach the terminal around 02z and wind direction shifts to around 240.

KBDL Terminal...High confidence in TAF thru midday Fri. Some guidance wants to bring SHRA/TSRA into the vicinity Fri afternoon. This is low confidence enough to not include in the TAF at this time.

Outlook /Saturday through Tuesday/...

Independence Day: VFR. Breezy. Chance SHRA, slight chance TSRA.

Saturday Night through Sunday: VFR.

Sunday Night: VFR. Slight chance SHRA.

Monday through Monday Night: VFR. Chance SHRA.

Tuesday: VFR. Breezy. Chance SHRA.

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 until 6 PM EDT this evening. 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 /Saturday through Tuesday/...

Independence Day: Winds less than 25 kt. Chance of rain showers, slight chance of thunderstorms.

Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers.

Sunday: Winds less than 25 kt.

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

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

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers.

CLIMATE

Record High Temperatures:

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 until 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 until 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory until 1 AM EDT Friday for ANZ254>256.