textproduct: Boston / Norton

WHAT HAS CHANGED

An Extreme Heat Watch has been issued for most of Southern New England from 10 AM Wednesday through 8 PM Friday.

KEY MESSAGES

- Scattered showers and a rogue thunderstorm or two in interior Southern New England dissipate by sundown. Fog and low clouds return tonight into early Monday morning, but giving way to mostly sunny conditions and summerlike temperatures for Monday away from the coasts.

- Warming trend this week with a period of dangerous heat likely by mid to late week.

- Rounds of showers and storms possible mid to late week.

DISCUSSION

KEY MESSAGE 1...Scattered showers and a rogue thunderstorm or two in interior Southern New England dissipate by sundown. Fog and low clouds return tonight into early Monday morning, but giving way to mostly sunny conditions and summerlike temperatures for Monday away from the coasts.

Weak high pressure near the eastern MA coast will continue to drive our weather pattern through Monday. Starting to see scattered pop-up showers develop in a weakly-unstable airmass mainly in interior Southern New England and along the seabreeze frontal boundary in eastern MA as of Sunday afternoon. Wind fields aloft are on the weak side and steering flow looks rather erratic/slow; while some of these showers could grow to produce a couple lightning strikes, the risk for storms rising to severe weather potential is nil. The slow- moving, erratic nature to the showers means that those who experience a direct hit could see locally heavy downpours. In general the best chance is across northern CT, western and central MA and into RI, but there will be several hrs of dry weather in between. Expectation is that these showers/weak thundershowers will tend to diminish in coverage and strength by sunset.

The next item to watch for is for re-developing low clouds and fog to again work their way back landward overnight. Daytime satellite imagery shows a continued marine moist layer in the eastern MA waters, and though this has tried to return landward as was the case yesterday, it's been struggling to do so. It seems more likely that once the sun goes down and the boundary layer starts to cool off and decouple that fog and low clouds end up returning. It's a little uncertain exactly when this comes back in, but the odds start to increase after sundown and especially late tonight. So the potential exists for some areas of reduced visibility for early Monday morning commuters. Otherwise, Monday itself turns out to be a dry day with fog dissipating by mid morning and seabreezes keeping coastal MA cooler (lower 70s) compared to mid 80s away from the coast.

KEY MESSAGE 2... Warming trend this week with a period of dangerous heat likely by mid to late week.

Confidence continues to increase for a period of extreme heat mid to late week. As a result, an Extreme Heat Watch has been issued from 10am Wednesday to 8pm Friday.

Overall, the meteorological picture for next week is fairly clear; a sprawling ridge of high pressure over the Tennessee Valley moves E over the Northeast by late week. There is lower confidence in the eventual track/strength of shortwaves riding around the periphery of the ridge but this will be addressed in the third key message. Ensembles continue to highlight a broad area for 95+ degree temperatures and have increased the chances of >100 temperatures Greatest signal for extreme heat continues to be in the CT and Merrimack River Valleys. Here, chances for highs above 100 degrees have surged to 50-60 percent Wednesday through Friday. Wednesday will feature the least extreme actual air temperatures with cloud cover from lingering storms. Even so, lingering boundary layer moisture should help keep dew point temps above 65 degrees, and with highs in the upper 90's apparent temperatures likely soar into the 105-110 range for much of the CWA. Thursday and Friday will be downright hot with 925mb temps soaring to +30C as the center of the ridge crests over the northeast CONUS. While the increased heat may result in better mixing and slightly lower dew point temperatures, apparent temperatures with dew points in the lower to mid 60s combined with air temperatures between 98 and 103 degrees will have no issue exceeding High Heat Warning criteria in the hottest locations. These values continue to push HeatRisk values to a Major (3/4) level for most of the CWA. In Urban areas, the Merrimack Valley, and the CT River Valley forecast impacts are still extreme (4/4), especially Thursday and Friday.

To reiterate from the previous forecast update, this heat event will feature extremely high overnight temperatures offering very little in the way of relief from the extreme heat. Those without access to adequate cooling and hydration will be especially vulnerable to these elevated nighttime temperatures.

Some relief may arrive by next weekend as the ridge aloft breaks down. Still, the NBM shows temperatures in the lower to mid 90s with 850 mb temperatures around +20C so heat statements may be needed even through next weekend.

KEY MESSAGE 3...Rounds of showers and storms possible mid to late week.

Disturbance over central Canada will act as a reservoir for shortwaves as they make their way up and over the expansive subtropical ridge that develops over the eastern CONUS. As one of the shortwaves moves into New England Tuesday night there could be some showers and storms with perhaps a more organized MCS originating from southern Canada. Confidence is admittedly low, but increasing overnight instability would provide plenty of fuel for any storms that form. Once the ridge has built up into southern New England for Wednesday and Thursday, chances for showers and storms drop significantly. Chances increase for the end of the week as the ridge breaks down. More refined details will come into the picture as we get closer to both events. Could see severe weather closer to the end of the week as guidance shows an elevated mixed layer advecting over the region Thursday night through Friday night. Best chance of severe weather would likely be as the ridge breaks down later Friday.

AVIATION /19Z SUNDAY THROUGH FRIDAY/

Forecaster Confidence Levels...

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

18Z TAF Update:

Through 00z Monday: Overall high confidence, but moderate near the eastern MA coast.

Mainly VFR, though a couple things to monitor which could offer at least brief restrictions. One is SCT slow-moving SHRA/possible ISO TS mainly affecting BDL, ORH, PVD and perhaps BED 19-00z, with brief MVFR visby restrictions if a direct hit. Coverage of SHRA/TS decreases with sunset. The other is a potential return of marine stratus and fog at IFR levels at BOS, which were TEMPO'd in through 00z. Light winds, easterly near the coast.

Tonight: High confidence in trends but moderate on timing.

VFR early, but we should see stratus/fog return at IFR-LIFR levels for most airports, lowest categories Cape and Islands, PVD and BOS. Exact onset timing is uncertain but the risk increases after sundown, and especially after 02z. Light S/SW winds, calm at times.

Monday: High confidence.

IFR-LIFR stratus and fog disperse 12-13z to VFR conditions thereafter. Light winds, with seabreezes developing at BOS by 15z and PVD by 16-18z.

KBOS Terminal...Moderate confidence in TAF. Showed prevailing VFR but with TEMPO IFR ceilings/visbys thru sundown to account for stratus/fog over the water returning inland. Better chance IFR-LIFR stratus after 02z Monday, dissipating by 12-13z Monday. Seabreeze starting around 15z Monday.

KBDL Terminal...High confidence in TAF.

Outlook /Monday Night through Friday/...

Tuesday through Tuesday Night: VFR. Breezy. Chance SHRA, slight chance TSRA.

Wednesday: VFR. Breezy. Slight chance SHRA, slight chance TSRA.

Wednesday Night through Thursday: VFR. Breezy.

Thursday Night: VFR.

Friday: VFR. Breezy. 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 Monday Night: High confidence.

Winds and seas remain below small craft advisory thresholds, with light east/southeast winds through Monday night and seas 3 ft or less all waters. Winds near shore/port on Monday could turn east around 10 kt due to sea/bay breezes. Main issue for mariners tonight into Monday is fog and mist, which could reduce visibility to less than 1 mile; it is less certain if mist/fog develops again Monday evening.

Outlook /Monday Night through Friday/...

Tuesday: Winds less than 25 kt.

Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Chance of rain showers, slight chance of thunderstorms.

Wednesday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain showers.

Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain showers.

Thursday: Winds less than 25 kt. Seas up to 5 ft.

Thursday Night: Winds less than 25 kt. Seas locally approaching 5 ft.

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

BOX WATCHES/WARNINGS/ADVISORIES

CT...Extreme Heat Watch from Wednesday morning through Friday evening for CTZ002>004. MA...Extreme Heat Watch from Wednesday morning through Friday evening for MAZ002>021-026. RI...Extreme Heat Watch from Wednesday morning through Friday evening for RIZ001>007. MARINE...None.