textproduct: Boston / Norton

WHAT HAS CHANGED

Aviation section updated, but no significant changes to the forecast.

KEY MESSAGES

- Dry today through tonight.

- Unsettled weather returning Thursday with a front passing through, followed by a brief tranquil period Friday.

- Another round of unsettled weather possible for the weekend.

DISCUSSION

KEY MESSAGE 1...Dry today through tonight.

With the low earlier this week now off the coast of Nova Scotia, high pressure has been able to build in over southern New England in its place. Notably aloft, a trough well to our north and west in Canada continues shifting eastward over the southern portion of the Hudson Bay. With where we are in context of this trough, strong SW- ly flow aloft develops today through the column. Diurnal mixing should also aid in mixing down this SW flow, which may inhibit sea breeze development despite the presence of high pressure centered just off to our east. Any early morning fog and lower clouds will diminish shortly after sunrise. 925 mb temperatures will likely remain in the 20-23C range for the afternoon, encouraging highs into the mid to upper 80s.

KEY MESSAGE 2...Unsettled weather returning Thursday with a front passing through, followed by a brief tranquil period Friday.

A shortwave within this slightly SW flow makes its way towards southern New England heading into tomorrow, bringing with it more favorable dynamics and moisture for showers as the jet aloft also becomes more SW and cyclonic. With better ascent and forcing, will need to keep an eye on where the best moisture tracks. The differences between the CAMs and the global guidance remain quite significant, with the NAM showing greater moisture through the column across the whole region, while the Canadian, ECMWF, and GFS all indicate layers of dry air in the lower to mid levels that would greatly affect precipitation. Given the signal for dry air limiting rainfall, I kept the chances for precipitation to around 30% or less and total QPF to up to 0.15". This is in line with what most of the ensemble guidance is indicating as well, as the greater concentration of moisture is shown in this guidance to remain just to our southwest.

Following the passage of this front, much drier air files in behind it. Dew points are expected to drop into the 50s across the region and remain generally in the upper 50s and low 60s Thursday night into Friday. High pressure makes its return Friday as a brief respite from this unsettled pattern, with onshore breezes and highs in the low 80s and upper 70s, with mid 70s along the immediate coastal areas.

KEY MESSAGE 3...Another round of unsettled weather possible for the weekend.

A much larger shortwave moves in from the Great Lakes Saturday into Sunday, greatly amplifying the SW flow aloft. Guidance signals at the possibility of a series of waves tracking NE along within this SW flow, and a surface low may pass along the east coast up into southern New England. Track is still somewhat uncertain, but this would likely bring the return of showers. Depending on the timing of these waves and low pressure, thunderstorms could be possible if they can take advantage of daytime heat.

Heading into the start of next week, it's also worth noting that the troughing associated with the leading shortwave cutting south through the Great Lakes could end up more as a closed off low aloft. Deterministic guidance keys in more on this possibility, but the Canadian ensemble notes the possibility along with the ECMWF ensemble hinting at it slightly; the GEFS keeps it a bit more open as of the 00z run. We may find ourselves in the low's post-frontal airmass for Monday with more W-ly flow keeping the region slightly cooler; 850 mb temps may fall to near 10C. Will be something to continue monitoring in future updates.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

Forecaster Confidence Levels...

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

12z TAF Update - Overall high confidence

Still some spotty low ceilings and MVFR visibilities, mainly across southern RI/southeast MA, however those conditions will be disipating by 14z. Otherwise expecting VFR area wide for the day. Have some clouds in the 6000-8000ft range currently. Those should move to the east with primarily daytime heating cumulus developing by early afternoon with bases 4000-5000ft. Light southwest surface winds should give way to a sea breeze along portions of the east coast. Guidance is a bit inconsistent as to timing and how far inland. With respect to Boston specifically, it does lead to the question "to sea breeze or not to sea breeze?". Currently the expectation is yes, but it just might not happen. Otherwise overnight VFR for most of the area. Should see some MVFR/IFR ceilings develop again late tonight, but guidance suggests it will be limited to just the immediate south coast, Cape and Islands. For Thursday, we might start to see some -SHRA develop prior to 18z, but still looks like primarily VFR for the area during the morning.

KBOS Terminal...Moderate confidence with respect to the sea breeze. Still conflicting guidance, but the trend was for more of the guidance to show a sea breeze. Opted to show it for the 15-20z time period, but if it does form, it may not last that long.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday Night through Sunday/...

Thursday Night: VFR. Slight chance SHRA.

Friday: VFR.

Friday Night: VFR. Slight chance SHRA.

Saturday: VFR. Chance SHRA.

Saturday Night: Mainly VFR, with local IFR possible. Chance SHRA.

Sunday: VFR. 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 Thursday...High confidence.

Light winds generally from the SW, though gusts nearing 20 kt are possible for this afternoon in the eastern waters. Showers possible over the waters Thursday into Thursday night. Patchy fog also a possible tonight, particularly for the southern waters. Seas to 2-3 ft.

Outlook /Thursday Night through Sunday/...

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

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

Saturday: Winds less than 25 kt. Chance of rain showers, slight chance of thunderstorms.

Saturday Night: Winds less than 25 kt. Rain showers likely, slight chance of thunderstorms.

Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers, slight chance of thunderstorms.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...None. RI...None. MARINE...None.