textproduct: Boston / Norton

WHAT HAS CHANGED

No major changes to the current forecast.

KEY MESSAGES

- High heat and humidity return Tuesday and Wednesday. Heat Advisories in effect Tuesday morning through Wednesday morning. Gusty southwest winds Tuesday and Tuesday night.

- Periods of unsettled weather possible later this week.

DISCUSSION

KEY MESSAGE 1...High heat and humidity return Tuesday and Wednesday. Heat Advisories in effect Tuesday morning through Wednesday morning. Gusty southwest winds Tuesday and Tuesday night.

Marginal instability and daytime heating have allowing a few isolated showers to develop across western MA this afternoon. Expect a few spot showers or perhaps a weak storm through the evening.

An strong ridge aloft will advect higher heights and anomalously warm temperatures aloft into the region Tuesday into Wednesday. 925 mb temperatures rise to +26-28C and with efficiently mixed gusty southwesterly to westerly winds. This will allow temperatures to rise well into the 90s both days for much of southern New England. Southwesterly flow will also help advect in a plume of higher dewpoint air with values in the upper 60s to near 70. The added humidity factor will make temperatures feel more like upper 90s and low 100s. Relief from the heat will be minimal with overnight temperatures in the mid 70s to near 80 Tuesday night into Wednesday AM. This has elevated HeatRisk into the Major category likely due to two days of abnormal heat and humidity with little overnight relief. This can impact a larger population of people including those who are heat sensitive and also those who do not use caution with the heat. On Tuesday, the CT Valley and eastern MA have the higher probability for temperatures in the upper 90s/heat index in low 100s. As a trough shifts south into Wednesday the heat shifts toward the south coast while still remaining hot across other parts of southern New England. Heat Advisories will be in effect Tuesday through Wednesday morning for much of southern New England with the exception of the Cape and Islands and Berkshires. Cape Cod was borderline for Wednesday, but kept them out due to the lower confidence in how many locations will hit 90 and onshore SW winds. If confidence increases they may be added to the existing Advisory.

This is a growing risk for gusty southwesterly winds Tuesday into Tuesday night. Model soundings show sufficient mixing down of higher winds aloft; however, there is uncertainty in how strong the winds are aloft and to what degree they get mixed to the surface. It is likely we will see gusts increase late afternoon with gusts 20-30 mph. It can't be ruled out for localized higher gusts 35-40 mph across the south shore and Cape/Islands, late afternoon through Tuesday night as a LLJ strengthens over the region.

KEY MESSAGE 2...Periods of unsettled weather possible later this week.

The period of elevated heat comes to an end for the latter half of the week as the upper level ridge starts to sag more south of the region. 850 mb temperatures start to fall back below 20C and by Friday fall to between 10-15C across southern New England. With the ridge's influence over the region weakening, a strengthening upper level trough moving across Quebec will move in during this period as well. Guidance has continued to show the possibility of shortwaves moving along the base of the trough over southern New England starting as early as Wednesday night, continuing into the weekend. These could bring the risk for more unsettled conditions to round out the week, especially for the weekend.

CSU ML probs are hinting at the risk for damaging winds and possible severe hail for Wednesday into Thursday, however, these probabilities remain low at this time. This would be associated with an incoming cold front that may bring an increased risk of showers and storms, though coverage is looking more isolated based on the latest guidance.

AVIATION /20Z MONDAY THROUGH SATURDAY/

Forecaster Confidence Levels...

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

Today and Tonight: High Confidence. Moderate for shower chances.

VFR conditions are expected across all terminals today. SW winds at 10-15 kts with gusts 20-25kt this afternoon. Winds decrease after 00Z to some degree, but there is some question if some terminals may not decouple overnight with winds staying elevated. If winds do decouple, LLWS may be possible for a few hours across the SE MA terminals overnight. Due to lower confidence this has been kept out of the TAF for now, but may be added for the 00Z TAF update. A few pop-up showers may be possible across the area, however given the low coverage of showers has been messages as VCSH or PROB30 for showers this afternoon through around 00Z.

Tuesday: High Confidence.

VFR. SW winds 10-20 kts with gusts 20-30 kts.

KBOS Terminal...High Confidence.

KBDL Terminal...High confidence.

Outlook /Wednesday through Saturday/...

Wednesday: VFR. Breezy.

Wednesday Night: VFR.

Thursday through Friday: VFR. Breezy.

Friday Night: VFR. Slight chance SHRA.

Saturday: Mainly VFR, with local IFR possible. 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 Wednesdsay...High confidence.

Gusty southwest winds expected Tuesday and Tuesday night. SCA have been issued Tuesday, with gusty southwest winds 25-30 kts across the southern and eastern waters. A SCA has now been added for Boston Harbor and Mass Bay for a period of gusts 25 kts late Tuesday afternoon through the evening. Seas will be quite choppy Tuesday into Wednesday morning with wave heights 5-8 ft possible across the southern ocean waters and 3-5 ft along the coastal waters, including Vineyard and Nantucket Sounds.

Agitated seas will raise the risk to "moderate" for rip currents Tuesday and Wednesday. The southcoast and south-facing beaches of Martha's Vineyard and Nantucket have the higher risk for due to prolonged gusty southwesterly flow.

Outlook /Wednesday through Saturday/...

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas.

Wednesday Night through Friday: Winds less than 25 kt. Seas locally approaching 5 ft.

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

Saturday: Winds less than 25 kt. Chance of rain showers.

BOX WATCHES/WARNINGS/ADVISORIES

CT...Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for CTZ002>004. MA...Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for MAZ003>007-010>021-026. Air Quality Alert from 11 AM to 11 PM EDT Tuesday for MAZ017>024. RI...Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for RIZ001>007. Air Quality Alert from 11 AM to 11 PM EDT Tuesday for RIZ001>008. MARINE...Small Craft Advisory from 4 PM to 11 PM EDT Tuesday for ANZ230- 231-250-251. Small Craft Advisory from noon Tuesday to 11 AM EDT Wednesday for ANZ232-233-235-237-254>256. Small Craft Advisory from noon Tuesday to 5 AM EDT Wednesday for ANZ234.