textproduct: Boston / Norton

WHAT HAS CHANGED

The Extreme Heat Warning was replaced with a Heat Advisory for southern New England through 8 pm this evening.

KEY MESSAGES

- Hot and humid today with a few showers & t-storms this afternoon into tonight...but not a washout. Main severe weather/damaging wind gust threat is to our southwest with just a low risk in our region with the best chance for that in southwest MA/CT.

- Much cooler temperatures for the first half of next week and that is especially true if the potential for a soaking heavy rain comes to fruition in the late Sun through Tue time frame.

DISCUSSION

KEY MESSAGE 1...Hot and humid today with a few showers & t-storms this afternoon into tonight...but not a washout. Main severe weather/damaging wind gust threat is to our southwest with just a low risk in our region with the best chance for that in southwest MA

One more hot and humid day of weather is on tap for southern New England today. However...the upper level ridge is in the process of breaking down which will result in somewhat lower mid level temperatures. So while we will not see the extreme heat of the past two days...still expect highs to reach the middle 90s in many locations. Heat indices of 95-102 are expected today...so not to the extreme levels of the past two days. Nonetheless...this will be the 4th day of heat and humidity in our region which is significant. We did collaborate with our surrounding offices to replace the Extreme Heat Warning with an Advisory...but the overall message has not changed.

The other concern for today will be assessing the convective potential. We still expect to generate some modest instability today...but the higher instability/steeper mid level lapse rates will be to our southwest. We often see the convection tend to gravitate towards the axis of better instability. The CAMs seems to want to follow that path as well as they show the main potential for severe weather/damaging wind gusts and most widespread convection to our southwest. That being said...given modest instability and effective shear of 30-40 knots we can not rule out a low probability of severe weather in our region. This is particularly true across southwest MA/northern CT as it is possible they end up on the northern edge of complex of thunderstorms.

So in a nutshell...a few showers & t-storms are possible this afternoon into tonight but not expecting a washout. While a low risk for severe weather can not be ruled out...appears the greater potential will be to our southwest. If any severe weather were to our occur in our region the main risk would be locally strong to damaging straight line wind gusts and brief torrential rainfall.

KEY MESSAGE 2...Much cooler temperatures for the first half of next week and that is especially true if the potential for a soaking heavy rainfall occurs in the late Sun into Tue time frame.

All of the model guidance is in very good agreement in the upper level ridge breaking down bringing much cooler temperatures for the first half of next week. Now just how much we cool will depend upon if a potential soaking heavy rain occurs in the late Sun through Tue time frame. This part of the forecast remains quite uncertain. There are ingredients in place that would support a heavy rainfall with high pressure over eastern Canada and low pressure passing to our south. This sets up a potential over running setup for band of heavy rain and perhaps embedded thunderstorms. The ensembles show a large spread in potential outcomes from very limited rainfall to several inches of rain! Trying to forecast synoptic rainfall during the summer when baroclinicity is weak and convective processes are involved is quite difficult. We will need more time to sort this out. However...given summer Pwats of 1.5-2+ inches if we are able to get sufficient forcing in our region then the potential heavy rainfall would be realized. That remains to be seen but something we will continue to watch.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

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

12z TAF Update...

Today through Sunday...High confidence.

VFR conditions dominate today through Sunday. We will need to watch for a few showers and isolated t-storms mainly this afternoon and tonight. Convective activity is already more active than anticipated across upstate New York. Concerns are increasing that storms will be more widespread than expected given frontal boundary flirting with the region today. High resolution guidance is widely varied regarding areal coverage and intensity of storms this afternoon but have elected to introduce PROB30 for TSRA at most mainland terminals between 17 and 21Z. Potential for a second round of thunderstorms around the 00Z hour will need to be monitored as a potential addition in future amendments.

WNW winds may but to 20+ knots at times this afternoon. Winds diminish tonight and turn light N/calm before coming light E on Sun. KBOS Terminal...Moderate confidence in TAF. Uncertainty surrounding thunderstorm coverage and duration.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday Night through Wednesday/...

Sunday Night: VFR. Chance SHRA.

Monday: Mainly VFR, with local MVFR possible. Breezy. Numerous SHRA, isolated TSRA.

Monday Night: Mainly VFR, with local MVFR possible. Breezy. Numerous SHRA, slight chance TSRA.

Tuesday: Mainly VFR, with local MVFR possible. Breezy. Scattered SHRA, isolated TSRA.

Tuesday Night through Wednesday: VFR. Slight chance SHRA.

MARINE

Forecaster Confidence Levels...

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

Today through Sunday...High confidence.

A relatively weak pressure gradient will keep winds/seas below small craft advisory thresholds this Holiday Weekend. Some 20+ knot WNW wind gusts over the land later this morning and afternoon may result in some near shore chop...But no headlines will be needed for the waters this weekend with seas generally 3 feet or less.

Outlook /Sunday Night through Wednesday/...

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

Monday: Winds less than 25 kt. Scattered rain showers, isolated thunderstorms.

Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Numerous rain showers, slight chance of thunderstorms.

Tuesday: Winds less than 25 kt. Seas up to 5 ft. Scattered rain showers, isolated thunderstorms.

Tuesday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers.

Wednesday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Heat Advisory until 8 PM EDT this evening for CTZ002>004. MA...Heat Advisory until 8 PM EDT this evening for MAZ002>023-026. RI...Heat Advisory until 8 PM EDT this evening for RIZ001>007. MARINE...None.