textproduct: Boston / Norton

WHAT HAS CHANGED

No significant changes to the forecast.

KEY MESSAGES

- Cooler than normal temperatures today with steady rain most likely towards eastern MA.

- Wet and unusually cool for the weekend.

- Drier and warmer to start next week.

DISCUSSION

KEY MESSAGE 1...Cooler than normal temperatures today with steady rain most likely towards eastern MA.

Weak low pressure currently centered south of Long Island is expected to slowly shift northeastward today. As it does, eastern MA, particularly in SE MA, Cape Cod, and the Islands, will likely see a continuation of steadier rain. There remains a consistent sharp gradient that starts in the northern portion of the south shore, continues along I-95 to the RI border, then along the northern borders of RI and CT where expected rainfall totals drop from around 0.5" to around 0.3" to the NW, diminishing heading further north. The NAM once again appears to be an outlier with regards to totals, as the other models trend more towards an additional 0.1" or less towards Boston and north and west, then up to an additional 0.25"-0.5" towards the Cape, SE MA, and RI. The latest HREF PMM has also kept the footprint of higher totals to SE MA, Cape Cod, the Islands, RI, and along the south coast. With persistent cloud cover and resultant surface cyclonic flow from the low pressure, onshore winds will keep the region feeling much cooler than normal for late August. Highs today will likely only get to the low to mid 70s in the eastern part of the region, with coastal areas possibly being cooler. Towards the interior, not much in the way of additional precipitation is expected, though cloudier conditions are still expected given this pattern. Temperatures there may get into the upper 70s, particularly in the CT River Valley.

Rain should become much lighter going into tonight. Onshore winds continue, though may start pivoting more northerly as the low continues northeastward. Lows will generally be in the 50s for much of the interior and the 60s further east.

KEY MESSAGE 2...Wet and unusually cool for the weekend.

The low pressure to our south will continue tracking NE heading into Saturday. The cold front associated with it will likely stall and linger over and around our eastern coastline, possibly crossing right through the Cape. With the front just staying put, the more overcast and onshore flow would persist into Saturday. Slight discrepancies still exist within the model guidance regarding the low's exact location, but the general consensus is that it should be just offshore of Nantucket sometime Saturday morning. The NAM has this low tracking further to the northeast at a faster pace than the other models, while the Canadian and the RAP have it tracking much slower, still sitting just south of Long Island by that same time. As the day goes on, the low is expected to track more north and could reach downeast Maine by Saturday night.

The synoptic story for the weekend remains generally unchanged; a much more amplified trough with strong cyclonic flow overhead moves closer from the Great Lakes Saturday into Sunday. This set up in conjunction with an anomalous moisture plume containing PWATs approaching 2.0" favors more widespread rainfall across the region for the weekend. NAEFS guidance indicates specific humidity and PWAT values up to around 2 standard deviations above normal associated with this plume, and other guidance shows favorable IVT right over much of eastern MA and RI within the same timeframe this weekend. Ensembles do have low probs for 1" totals in 24 hours ending Sunday, notably towards the Cape and Islands between the ECMWF ensemble and the Canadian ensemble. Totals reaching the 1" to 1.5" mark are not out of the question.

Temperatures this weekend will likely continue to be below normal for this time of year, given the onshore flow and persistent cloud cover. Highs will likely top out in the 70s both days, with Saturday looking to be the slightly cooler day of the two as Sunday starts to see some drier air move in during the day, helping reduce cloud cover. Given the continued onshore flow and the cooler temperatures, not much in the way of instability is expected to develop. Some embedded thunder can't be entirely ruled out, but predominantly stratiform rain is expected.

KEY MESSAGE 3...Drier and warmer to start next week.

The front finally shifts offshore as the trough aloft progresses east and the surface low exits the east coast area. Post-front, much drier air takes hold with more westerly winds, clearing out clouds and dropping dewpoints through Wednesday. The base of the trough tracks directly overhead into Tuesday and heights rise behind it as it continues eastward. High pressure dominates and high temperatures return to the 70s and 80s.

AVIATION /07Z FRIDAY THROUGH TUESDAY/

Forecaster Confidence Levels...

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

06z TAF Update:

Through 12z...Moderate confidence, especially on the timing of sub-VFR conditions.

Latest guidance has trended more optimistically for terminals aside from the Cape and Islands with regards to the onset of sub-VFR conditions. More northern terminals could end up seeing more borderline VFR/MVFR ceilings tonight. However, ceilings towards the Cape and Islands will likely deteriorate prior to 12z this morning. Winds will remain generally N/NE at 5-10 kt. Lighter -RA becomes more DZ as it exits TAF sites.

Friday and Friday Night...Moderate confidence.

MVFR-IFR ceilings move in through the morning and afternoon, but could see borderline MVFR/VFR areas take a bit longer to go down. BDL may even stay VFR, though periods of MVFR are not out of the question. Very light rain or drizzle towards eastern MA/SE MA/south coast/Cape Cod & Islands could become a steadier light rain with periods of more moderate rainfall also likely. Uncertainty remains regarding just how heavy this round of rain could end up being, though, along with when exactly it starts and how long it may last in different areas. At the latest, it should resume in eastern/NE MA by the nighttime hours. NE winds continue at around 10 kt with gustier winds to 20 kt possible for the Cape and Islands.

Saturday...Moderate confidence.

MVFR-IFR as yet another round of rain may impact the area. Exact timing uncertain during the daytime hours and it should be lighter before picking up in the afternoon/evening. This timing is subject to change with future updates. Continued cyclonic flow will keep winds generally NE to NNE up to 10 kt, occasionally reaching 15 kt in E MA.

KBOS Terminal...Moderate confidence in TAF. Guidance trended more optimistically, however, exact timing for onset of sub-VFR conditions still remains in flux.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday Night through Tuesday/...

Saturday Night: MVFR/IFR conditions possible. Chance SHRA.

Sunday: Mainly VFR, with areas IFR possible. Chance SHRA, isolated TSRA.

Sunday Night: VFR. Slight chance SHRA.

Monday: VFR. Breezy.

Monday Night: VFR.

Tuesday: VFR. Breezy.

MARINE

Forecaster Confidence Levels...

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

Through Saturday...High confidence.

Winds and seas should remain below SCA criterion through Friday night. Winds shift more NE today and may get up to around 20 kt over the southern waters. Elsewhere, winds generally range between 10-15 kt. Winds diminish slightly and turn more N Friday night into Saturday morning. Winds over the eastern waters maintain more of an easterly component to them, ranging from NE to SE. The southern waters shift more W, ranging between NW and SW by Saturday afternoon. Seas mainly 3 ft or less for all waters with localized spots of 4 ft seas Saturday.

Main issue for mariners is reduced visibilities in rain and mist Friday night, especially for the eastern waters.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Winds less than 25 kt. Rain showers likely, isolated thunderstorms.

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

Sunday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers, slight chance of thunderstorms.

Monday through Monday Night: Winds less than 25 kt. Seas up to 5 ft.

Tuesday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

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