textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes.

KEY MESSAGES

- Warming trend takes hold. Confidence continues to increase in dangerous heat Wednesday through Friday.

- Showers and thunderstorms possible mid to late week.

DISCUSSION

KEY MESSAGE 1...Warming trend takes hold. Confidence continues to increase in dangerous heat Wednesday through Friday.

Morning fog that developed along the south coast and parts of the Cape (plus Martha's Vineyard) should erode away after sunrise this morning. High pressure continues to exert its influence over the region ahead of a strengthening upper level ridge to our west. Highs today climb into the mid to upper 80s for most, staying a bit cooler towards the coasts and over the Cape and Islands.

Not much has changed in the overall trends of the forecast overnight. A strong upper level ridge continues building in from the west, shifting over the northeast CONUS late this week. Ensembles remain in strong agreement that we will see temperatures over 90-95 starting Wednesday and continuing through the rest of the week. The highest probabilities of temperatures reaching and/or exceeding 100F over urban areas and the river valleys, especially on Thursday, also continue to increase. GEFS probabilities of 100F+ for Thursday afternoon range now reach up to 60-70 percent with Canadian ensemble probabilities peaking slightly higher at up to 80 percent. All ensembles are in pretty good agreement having the highest probabilities in the CT Valley and towards the Boston metro up into the Merrimack Valley. The continued overall consensus across the different guidance suite aids in increasing confidence in the risk for this dangerous heat overall. Mean 850 mb temperatures across the ensembles continue to reach 20-24C during this stretch, and 925 mb temperatures in the deterministic guidance continue to key in on 25C temperatures and higher, even to 30C. Notably, the GFS has the highest 925 mb temperatures centered over CT and eastern MA. This is all before dew points and overall moisture are being taken into account. Considering dew points are expected to be in the upper 60s and even into the low 70s, heat indices to 110F are not out of the question for some areas. An Extreme Heat Watch will be in effect from Wednesday morning through Friday evening for this reason, along with limited overnight temperature recovery. Major (3/4) to Extreme (4/4) heat risk impacts are very possible, with the Extreme impacts more likely over urban areas and in river valleys.

As briefly highlighted above, another issue with these high daytime temperatures is that not much relief is expected overnight. NBM has low probabilities for temperatures getting below 70F overnight between Wednesday and Friday, generally below 10 percent across much of southern New England. It also has probabilities between 75-90 percent for early Thursday morning and early Friday morning temperatures in eastern MA (especially Boston) remaining above 75F from overnight. The other ensembles are equally pessimistic with recovery; GEFS, ECMWF ENS, and the Canadian ensemble are all essentially showing maximum probabilities that the region stays at or above 70F overnight between Wednesday and Friday night.

KEY MESSAGE 2...Showers and thunderstorms possible mid to late week.

Shortwaves originating from a disturbance in central Canada will track eastward and may impact southern New England before the upper level ridge fully builds up over the region. These waves will track along the eastern periphery, with one likely arriving Tuesday night and another arriving once the ridge starts breaking down towards the end of the week. Showers and storms are a possibility Tuesday night, and some guidance is hinting at increased instability overnight that could fuel any storms that do develop. CSU ML probs and other guidance aren't showing much in the way of a severe signal at this time, but we will continue to monitor for any risk that could develop. With the ridge fully build up over the region Wednesday and Thursday, chances for any showers or storms drop significantly. Chances return as the ridge breaks down towards the end of the week, and the risk for severe weather could be higher towards Friday and Saturday. CIPS analogs are hinting at it particularly for western and central MA. More refined details will come into the picture as we get closer to both events.

AVIATION /18Z MONDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

Through 00Z Today...High confidence.

VFR with light winds. Seabreezes continue through sunset.

Tonight...High confidence.

IFR likely to return with fog overnight across portions of SE MA, RI, CT, and the Cape and Islands. Winds mostly S, remaining light and going calm from time to time.

Tuesday...High confidence.

VFR. Winds pick up in the afternoon from the SSW, gusting to 20 kt across much of SE MA, RI, and CT. Showers possible towards the late evening hours.

Tuesday Night...High Confidence.

VFR. Light southwest winds.

KBOS Terminal...High Confidence in TAF.

KBDL Terminal...High Confidence in TAF.

Outlook /Wednesday through Saturday/...

Wednesday through Wednesday Night: VFR. Breezy. Slight chance SHRA.

Thursday through Thursday Night: VFR. Breezy.

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

Friday Night: VFR. Slight chance SHRA, slight chance TSRA.

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

MARINE

Forecaster Confidence Levels...

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

Through Tuesday Night: High confidence.

Relatively light winds and seas expected through at least Tuesday morning. Increasing southwest winds ahead of a cold front could result in some gusts near 25 kt Tuesday afternoon into Tuesday night. Areas of fog return tonight, resulting in poor visibility less than 1 mile at times.

Outlook /Wednesday through Saturday/...

Wednesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Slight chance of rain showers.

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

Thursday: Low risk for Small Craft Advisory winds with gusts up to 20 kt. Areas of seas approaching 5 ft.

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

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

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

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

BOX WATCHES/WARNINGS/ADVISORIES

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