textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes to the forecast.

KEY MESSAGES

- Dangerous prolonged heat and humidity continues through Sat with max afternoon heat indices in the 100 to 115 degree range with little relief at night. Peak of the heat remains Thu into Fri.

- West/Northwest flow aloft may bring a low convective/severe weather risk through Fri. However, the lack of a trigger and some capping may keep us mainly dry. Greatest risk for thunderstorms/severe weather potential might wait until Independence Day with the actual cold front.

DISCUSSION

KEY MESSAGE 1...Dangerous prolonged heat and humidity continues through Sat with max afternoon heat indices in the 100 to 115 degree range with little relief at night. Peak of the heat remains Thu into Fri.

Ridging over the Mid South will slowly move eastward towards the Carolinas and VA Thursday and Friday. Heights rise across the area, with 500mb heights in the 590-592 dam range by Thursday. This ranks well above the 90th percentile among the KBOS RAOBs, per the SPC Sounding Climatology page. EFI data also continues to show plenty of evidence for an anomalous heat wave.

Heat indices continue to hover between 99-110 away from the south coast. The heat and humidity combination will be at its worst in the CT and Merrimack River Valleys, where heat indices may get up to 110- 114F in some spots Thursday. The temperatures themselves will top out at 100-103F. Extreme Heat Warning in effect through 8 PM EDT Saturday for all of southern New England except the Cape and Islands.

Along the Cape and Islands, cooler waters will help keep the land slightly cooler, but that doesn't stop the heat/humidity combination. Highs Thursday in the mid-to-upper 80s, but the heat index will shoot up towards 95-98F. Heat Advisory in effect for Barnstable and Dukes Counties until 8 PM EDT Saturday.

At least similar mid level temps are forecast Fri, around +22 to +23C at 850 mb. That puts upper 90s on the table still, and for some of the typical westerly wind hot spots like the Merrimack Valley triple digits is still possible. With greater convective chances though temps may top out in the mid to upper 90s. With the forecast dewpoints into the 70s it will still feel like well over 100 degrees for most locations. Into Sat with heights falling, mid level temps also tick down. Temps at 850 mb still support mid 90s with some lingering humidity/high heat indices and headlines look good.

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, Boston may not drop below 80 degrees during the peak of the heat and humidity Thu night!

KEY MESSAGE 2...West/northwest flow aloft may bring a low convective/severe weather risk through Fri. However, the lack of a trigger and some capping may keep us mainly dry. Greatest risk for thunderstorms/severe weather potential might wait until Independence Day with the actual cold front.

The overall upper pattern supports continued west to northwest flow aloft thru the end of the week. Within that flow there is little synoptic forcing for ascent that is likely to contribute to thunderstorms development. In addition, most afternoons will feature at least a small cap as the influences of the Ohio Valley high pressure creates a little subsidence inversion locally.

For certain we are looking at robust heat and humidity that will contribute to high CAPE values Fri and possibly even Sat. Given the overall lack of synoptic forcing if any thunderstorms do develop they should remain isolated. However they could quickly become severe due to the instability levels. With shear being a limiting factor, any storms are likely to collapse as quickly as they intensify. The main hazard would be downburst winds. Upper level heights begin to fall Fri and thus weaken the capping more than other days. This could provide the best opportunity for thunderstorms to be more widespread in coverage. NBM PoP increases a little Fri, especially along the coast and thru the CT River Valley, and even more into Sat. I would not be surprised to see that need to come up a bit as CAMs start to get in range. Shear is forecast to ramp up Fri into Sat and as a result this is likely the best window for organized severe weather chances. Machine learning guidance is in agreement with Fri/Sat being the most likely days to center the severe weather threat over southern New England. With extreme heat still ongoing, hail will be a lower risk than typically, and largely uniform wind direction will also limit any tornado potential. So still expecting that damaging winds are the primary hazard to watch.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Forecaster Confidence Levels...

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

18z TAF Update...

Today through Thursday...High confidence outside of duration of the seabreeze at KBOS through this evening, in addition to fog over eastern MA and the Cape after 06Z tonight.

VFR conditions will dominate this afternoon through Thursday. SW winds will generally gust 15 to 20 knots later today and again Thu...while parts of the Cape/Islands gust to 25 knots and perhaps briefly around 30 knots in the typically prone spots.

KBOS Terminal...High confidence in TAF but will need to watch for the duration of the seabreeze this afternoon. Current thinking is that the winds should transition back to the SW by 21-22Z.

KBDL Terminal...High confidence in TAF.

Outlook /Friday through Monday/...

Friday: VFR. Breezy.

Friday Night: VFR.

Independence Day through Sunday: VFR. Slight chance SHRA, slight chance TSRA.

Sunday Night: VFR. Chance SHRA, slight chance TSRA.

Monday: Mainly VFR, with local IFR possible. 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 Thursday...High confidence.

Bermuda high pressure will result in a persistent SW flow across our waters. SW winds will gust to 25 knots at times today through Thursday and long fetch will result in seas building to 3 to 6 feet across our southern waters. Therefore...we have continued small craft headlines for our southern waters into Thursday.

Outlook /Friday through Monday/...

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.

Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers.

Independence Day: Winds less than 25 kt. Slight chance of rain showers.

Saturday Night: Winds less than 25 kt. Slight chance of rain showers, slight chance of thunderstorms.

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

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.

CLIMATE

Record High Temperatures:

July 1: KBOS: 98/1872 KBDL: 101/1964 KPVD: 97/1964 KORH: 94/1913

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 1: KBOS: 73/1958 KBDL: 73/1968 KPVD: 71/1968 KORH: 72/2018

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. Air Quality Alert until 11 PM EDT this evening 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 MAZ004>024- 026. 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 6 PM EDT Thursday for ANZ231>235- 237. Small Craft Advisory until 11 PM EDT this evening for ANZ236. Small Craft Advisory until 1 AM EDT Friday for ANZ254>256.