textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes. Although it remains very warm into the weekend, the level of heat does begin to drop off so the current thought is to not extend the Heat Advisory beyond Friday.

KEY MESSAGES

- Heat and humidity continues into Friday, with only a few afternoon showers or thunderstorms.

- Cool front brings scattered storms Saturday. Less humidity levels and dry weather Sunday but daily shower/storm chances resume early to midweek.

DISCUSSION

KEY MESSAGE 1...Heat and humidity continues into Friday, with only a few afternoon showers or thunderstorms.

Latest model guidance does not show much need for changes to the forecast for the remainder of today and tomorrow. As typical with convection, even hi-res models struggle with timing, placement and coverage. We have already seen a few showers develop across areas further east (ie: Boston area to the North Shore) than was previously anticipated. With the SW flow, the south coast sea breeze is moving northward and thus expect shower chances to remain north of this boundary as the afternoon/evening goes on. We do have a decent amount of Mixed Layer CAPE (MLCAPE) on the order of 2000 J/kg, but mid level lapse rates are very meager, so not expecting any substantial thunderstorm development. Convection will wane with sunset. With the very moist airmass, we will again see the development of stratus and fog later tonight, especially across the southern half of SNE. Like today, those clouds and fog will "burn" off tomorrow morning. No real airmass change, though if anything, it will be a degree or two warmer. 925mb temperatures today were in the 23-24C range, and that rises to 25-26C on Friday. When combined with dewpoints in the 70s, heat index (feels like) temperatures will top out well into the 90s, if not low 100s. Thus the Heat Advisory will continue. There will be a warm layer aloft (above roughly 850mb/5000ft) that will act as a lid to keep convection at bay. Though an isolated afternoon shower or thunderstorm is possible, they should be more the exception and likely contained to far western areas where the hilly terrain will help focus convective development. Guidance is a little unclear for Friday night, but we might see some late evening convection move in from the west. Just kept with NBM PoPs showing roughly 20-40% PoPs overnight for the CT River Valley and points east.

KEY MESSAGE2...Cool front brings scattered storms Saturday. Less humidity levels and dry weather Sunday but daily shower/storm chances resume early to midweek.

All in all a general sameness to the weather pattern day-by-day through Saturday, with warm temperatures in the 80s with very high humidity levels, to go along with daily shower and thunderstorm chances following the diurnal cycle. Saturday offers a better shot a a bit more thunderstorm coverage with a cool front, although the risk for organized severe weather looks minimal, kept in check by weak windfields aloft leading to deep layer shear magnitudes in the 25-30 kt range and generally slower SWly storm motions. This front will offer somewhat lower humidity levels, basically knocking dewpoints down from current "oppressive" values in the low to mid 70s to "humid" mid/upper 60s levels. Storms would be capable of lightning and heavy rain, with the strongest of those storms capable of frequent lightning and localized downdrafts capable of sporadic wind damage, but those should be a very small subset of the storms.

Weak subsidence and marginally lower dewpoints could limit or mitigate shower/storm chances on Sunday, although still with the usual summer warmth. The risk for daily shower and thunderstorm chances also increases early to mid next week as the Northeast states become more influenced by a somewhat stronger belt of westerly midlevel flow. With stronger winds aloft with that comes a somewhat better chance for storms becoming strong/severe but the timing is still uncertain.

Stayed fairly close to NBM forecast values during this period.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

18z TAF update. Moderate Confidence.

No big changes to the forecast. Dealing with mainly VFR, although stratus is resulting in some IFR for the Islands. Sea breezes have developed along both coasts, but the east coast sea breeze is pinned pretty close to the coast. Isolated showers dot the area, and a TSRA can't be ruled out later this afternoon. Though the better chances are across the western areas. Placed some PROB30s in TAFs where the better chances exists. Overnight expect a similar situation to unfold as last night. Basically low clouds/stratus and some fog to produce IFR conditions, especially across the southern half of the area. Could also have some fog in the CT River Valley. Those clouds/fog should dissipate fairly quickly Friday morning, except places like ACK and parts of the Cape where it will take a little longer. Could be a few SHRA/-TSRA across the interior Friday afternoon. Looking forward to Friday night, additional low clouds/fog should develop near the south coastal areas. Could also see additional TSRA activity lingering well into late evening across western areas. Later forecasts will fine tune those details.

KBOS Terminal...Moderate confidence. Sea breeze expected to push out a little earlier than the typical 23-00z time. Expect BOS to be on the northern edge of stratus that develops late tonight across southern areas.

KBDL Terminal...Moderate confidence. Monitoring TSRA across eastern NY as to whether they will make it to BDL later this afternoon. Some uncertainty as to the coverage of any stratus and fog late tonight.

Outlook /Friday Night through Tuesday/...

Saturday: VFR. Breezy. Chance SHRA, chance TSRA.

Saturday Night through Sunday: VFR. Breezy.

Sunday Night: VFR.

Monday: VFR. Breezy.

Monday Night: VFR. Slight chance SHRA.

Tuesday: VFR. Breezy. 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 Friday... High Confidence.

No significant changes to the forecast. Overall weather pattern shows a large area of high pressure off the east coast. This will continue to produce a SW flow across the entire region into the weekend.

Outlook /Friday Night through Tuesday/...

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

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

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

Monday: Winds less than 25 kt.

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

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Heat Advisory until 8 PM EDT Friday for CTZ002>004. MA...Heat Advisory until 8 PM EDT Friday for MAZ003>007-010>021-026. RI...Heat Advisory until 8 PM EDT Friday for RIZ001>007. MARINE...None.