textproduct: Boston / Norton

WHAT HAS CHANGED

The Extreme Heat Watch has been upgraded to an Excessive Heat Warning for all of southern New England except the south coast and Berkshires.

KEY MESSAGES

- Dangerous prolonged heat and humidity Wed through Sat with 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 scattered showers and a few t-storms may work into parts of our region very late today and tonight. Activity likely weakening with main severe weather risk to our northwest, but can not rule out a low risk in western MA/CT.

- W/NW flow aloft may bring a low convective/severe weather risk given high Cape/remnant EML Wed-Fri...but limited forcing and cap may keep us mainly dry. Greatest risk for thunderstorms/severe weather potential might wait until Sat July 4th with the actual cold front.

DISCUSSION

KEY MESSAGE 1...Dangerous prolonged heat and humidity Wed through Sat with 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. This continues to be supported by the NAEFS situational awareness table which indicates many parameters outside the models climatology. The EFI/shift of tails also indicates both min/max temperatures on the edge or outside of the models climatology. 925T peak on Thu and Fri in the +28C to +30C degree range and that is when we expect the worst of the extreme heat and humidity.

Today will be very warm and with 850T climbing to +16C/+17C we should see highs top out in the upper 80s to the lower 90s away from the south coast. The dangerous heat and humidity starts on Wed when 850T climb to +20C/+21C and expect highs to reach the middle 90s away from the south coast and possibly upper 90s in a few spots. This coupled with dewpoints rising to 70+ will result in dangerous heat and humidity. 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 River Valley and Merrimack Valleys. The extreme heat and humidity peak on Thu and Fri when 850T reach between +22C and +24C. 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 on Sat July 4th 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 heatwave...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!

Given the above...we opted to upgrade most of our region to an Extreme Heat Warning Wed through Sat. While some areas may technically not hit warning criteria until Thu and may not on Sat depending on the timing of the cold front...the overall message remains the same. We did hold the excessive heat watch along the Berks and south coast of MA/RI. However...this area will still experience dangerous heat and humidity but wanted to go with the warning where our confidence was highest at this point.

KEY MESSAGE 2...A cluster of scattered showers and a few t-storms may work into parts of our region very late today and tonight. Activity likely weakening with main severe weather risk to our northwest, but can not rule out a low risk in 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 given high Cape/remnant EML Wed-Fri...but limited forcing and cap may keep us mainly dry. Greatest risk for thunderstorms/severe weather potential might wait until Sat July 4th 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 we can get a few thunderstorms going they could quickly become severe even during the overnight/morning hours. However...lack of overall synoptic forcing and a mid level cap may keep this risk low but it will still need to watched. Greatest risk for convection and potentially some severe weather may wait until Sat July 4th when the cold front/strong forcing arrives.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

12z TAF Update...

Today and tonight...High confidence.

Mainly VFR after areas of low clouds/ground fog mainly across RI/SE MA by mid morning. Otherwise...will need to watch for a cluster of weakening scattered showers and perhaps a few t-storms toward evening and into tonight. Greatest risk for this activity will be across western MA/CT. S-SW winds 6-13 knots this afternoon into tonight...but 10-20 knots later tonight across the Cape/Islands.

Wednesday...High confidence.

Mainly VFR but can not rule out widely scattered showers/t-storms later Wed into wed night. SW winds 10 to 15 knots with 20 to 25 knot gusts in the afternoon. Strongest of those gusts toward the Cape/Islands.

KBOS Terminal...High Confidence in TAF. Sea breeze develops by mid to late morning but should be knocked out and turn S-SW by mid to late afternoon.

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

Outlook /Wednesday Night through Saturday/...

Wednesday Night: VFR. Breezy. Slight chance SHRA, isolated TSRA.

Thursday through Friday Night: VFR. Breezy.

Independence Day: VFR. Chance SHRA, chance TSRA.

MARINE

Forecaster Confidence Levels...

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

Today through Wednesday...High confidence.

Bermuda high pressure will result in a persistent SW flow across our waters. Despite the inversion enough mixing especially by Wednesday...particularly near shore should yield SW wind gusts up to 25 knots. In addition....long southwest fetch should build seas to 3- 6 feet across our southern waters. Small craft headlines have been posted for our southern waters and sounds towards daybreak and through Wednesday.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain showers, isolated thunderstorms.

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

Friday: Winds less than 25 kt.

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

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for CTZ002. 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 MAZ003>007-010>019-026. Extreme Heat Watch from Wednesday morning through Saturday evening for MAZ002-008-009-020-021. RI...Extreme Heat Warning from 10 AM Wednesday to 8 PM EDT Saturday for RIZ001>004. Extreme Heat Watch from Wednesday morning through Saturday evening for RIZ005>007. MARINE...Small Craft Advisory from 8 AM to 11 PM EDT Wednesday for ANZ231>237. Small Craft Advisory from 5 AM Wednesday to 4 PM EDT Thursday for ANZ254>256.