textproduct: Boston / Norton

WHAT HAS CHANGED

The Extreme Heat Watch has been upgraded to an Excessive Heat Warning for the east slopes of the Berkshires and the South Coast. A Heat Advisory has been posted for Cape Cod and Martha's Vineyard.

KEY MESSAGES

- Dangerous prolonged heat and humidity Wed through Sat with max afternoon heat indices in the 98 to 112 degree range with little relief at night. Peak of the heat Thu into Fri.

- A cluster of showers and a few thunderstorms may impact parts of our region very late today into tonight. Activity likely weakening with main severe weather risk to our northwest, but cannot rule out a low risk across western MA/CT.

- W/NW flow aloft may bring a low convective/severe weather risk Wed-Fri. However, limited forcing and cap 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 Wed through Sat with max afternoon heat indices in the 98 to 112 degree range with little relief at night. Peak of the heat Thu into Fri.

High confidence forecast in dangerous prolonged heat and humidity Wed through Sat. Focused the headline messaging on the peak threats rather than trying to slice the bologna too thin. Thus, Extreme Heat Warnings for most of southern New England. Heat Advisories for the Cape and Vineyard. The peak of this heat looks to be in the Thursday-Friday time frame.

Heat indices will rise to between 98 and 108 degrees away from the south coast. The worst of the heat and humidity will likely be in the CT and Merrimack River valleys. High temperatures will likely reach between 97 and 103 away from the south coast and any very localized sea breezes. Heat indices will reach dangerous levels in the 102 to 113 degree range. It will remain hot and humid for Independence Day as well. While it may be a few degrees lower than Thu/Fri, dangerous heat and humidity may continue and will be determined by the timing of a cold front.

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!

KEY MESSAGE 2...A cluster of showers and a few thunderstorms may impact parts of our region very late today into tonight. Activity likely weakening with main severe weather risk to our northwest, but cannot rule out a low risk across western MA/CT.

The other concern for late today and especially tonight is a shortwave that will be dropping southeast in northwest flow aloft around the developing ring of fire. Effective shear of 30-40 knots is favorable for this activity to work into our region...but instability will be rather marginal. So expect a cluster of weakening scattered showers and perhaps a few thunderstorms to impact portions of our region. The greatest risk is across western MA/CT. The overall severe weather threat is low given the limited instability in our region...but given northwest flow aloft and lingering momentum can not rule out a low probability for some severe weather in western MA/CT. However...currently thinking greatest risk is northwest of our region.

KEY MESSAGE 3...W/NW flow aloft may bring a low convective/severe weather risk Wed-Fri. However, limited forcing and cap may keep us mainly dry. Greatest risk for thunderstorms/severe weather potential might wait until Independence Day with the actual cold front.

Anomalous heat and humidity will result in rather high instability across our region Wed through Fri. In addition, a remnant EML in northwest flow aloft is present. If somehow a few thunderstorms do develop, they could quickly become severe, even during the overnight/morning hours. However, a lack of overall synoptic forcing and a mid level cap should keep this risk low. This potential will still need to watched. Greatest risk for convection and potentially some severe weather may wait until Independence Day when a cold front and stronger forcing arrives.

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

Tonight...High confidence.

VFR. Still monitoring a cluster of weakening showers and perhaps a few TSRA moving SE form Ontario. Greatest risk for this activity will be across western MA/CT. Not expecting fog to redevelop towards the south coast, but that will also need to be watched.

Wednesday...High confidence.

Mainly VFR but cannot rule out widely scattered showers/t-storms later Wed into Wed night. SW winds becoming gusty, especially towards the Cape and islands.

Wednesday Night...High confidence.

VFR. Gusty SW winds continue.

KBOS Terminal...High Confidence in TAF.

KBDL Terminal...High Confidence in TAF. Showers approach the terminal 21-23z, with some rumbles of thunder possible around 00z tonight.

Outlook /Thursday through Sunday/...

Thursday through Friday: VFR. Breezy.

Friday Night: VFR.

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

Saturday Night through Sunday: VFR. Slight chance SHRA, slight chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Through Wednesday Night...High confidence.

Bermuda high pressure will result in a persistent SW flow across our waters. Winds become gusty tonight and continue into Wednesday night across the southern waters. In addition, long southwest fetch should build seas to 3-6 feet across our southern waters.

Outlook /Thursday through Sunday/...

Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft.

Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft.

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

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

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

Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight 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...Air Quality Alert until 11 PM EDT this evening for CTZ002. Air Quality Alert from 11 AM to 11 PM EDT Wednesday for CTZ002>004. Extreme Heat Warning from 10 AM Wednesday to 8 PM EDT Saturday for CTZ002>004. MA...Extreme Heat Warning from 10 AM Wednesday to 8 PM EDT Saturday for MAZ002>021-026. Heat Advisory from 10 AM Wednesday to 8 PM EDT Saturday for MAZ022-023. RI...Air Quality Alert from 11 AM to 11 PM EDT Wednesday for RIZ001>008. Extreme Heat Warning from 10 AM Wednesday to 8 PM EDT Saturday for RIZ001>007. MARINE...Small Craft Advisory from 8 AM to 11 PM EDT Wednesday for ANZ231>237. Small Craft Advisory from 5 AM Wednesday to 1 AM EDT Friday for ANZ254>256.