textproduct: Boston / Norton

WHAT HAS CHANGED

Increased rainfall amounts for tonight into Friday over southeastern Massachusetts and Rhode Island.

KEY MESSAGES

- Rainy conditions develop tonight, tapering off to drizzle or intermittent light rain and onshore flow overnight. Cooler than normal temps Friday with best chances for steady rain in eastern MA.

- A wet and unusually cool weekend on tap.

- Drier and warmer next week.

DISCUSSION

KEY MESSAGE 1...Rainy conditions develop tonight, tapering off to drizzle or intermittent light rain and onshore flow overnight. Cooler than normal temps Friday with best chances for steady rain in eastern MA.

Weak wave of low pressure over the central Alleghany Mtns is bringing a large shield of steady light rains across central and southern/southeast NY as of early this afternoon. Meanwhile in a narrow warm/moist sector, deeper convection was beginning to develop as of early this afternoon over the Delmarva Peninsula region. This wave of low pressure will bring increased cloud cover this afternoon, with light rains breaking out in western MA/CT around the PM commute period, and into the 6-8 PM window for central and eastern MA and RI. These rains should persist into the late evening before moving offshore early overnight. Winds then turn onshore/northeasterly overnight tonight as low pressure circulates over the southern offshore waters, which would support a scenario indicated by a loose model consensus of a low-cloud-overcast and either very light rain or even drizzle, with the better chances for this being across central MA, eastern CT to the eastern/southern coast. Pretty sharp gradient in QPF amts thru 12z Friday, ranging from less than a quarter-inch along the Route 2 corridor southward to about the Mass Pike, but then ramp up substantially to around a half/three-quarters of an inch for CT and most of RI and southeast MA. Lows mainly in the 60s given the increasing moisture.

Friday's forecast is less than straightforward, with a larger than usual degree of uncertainty at this time range. This stems from how the low pressure mentioned above is affected by the deeper convection now getting going in the Delmarva area. Something of a loose majority of models keep a lingering/stalled frontal boundary with weak low pressure meandering near Cape Cod Friday, pulling northward late in the day into the evening. Official forecast follows that idea ... which keeps a lot of overcast skies around central and eastern sections of Southern New England, and the onshore flow and limited opportunity for diurnal heating forcing temperatures on the cool side of guidance, maybe a couple degrees higher than nighttime lows in the upper 60s to lower 70s. There's some opportunity for breaks in the clouds and relatively drier conditions in western New England and temps reaching the mid/upper 70s there. Couldn't rule out rain at virtually any time in that solution, but the area that looks more poised for a washout Friday is southeast and eastern MA. What we view as an outlier outcome is the 12z NAM and its 4-km variant, which seems to struggle with a convective feedback problem, in closing off the weak low near Long Island and then very slowly meanders it northward. This leads to a slug of enhanced heavy rainfall totals over RI and southeast MA, amounts which could lead to localized flooding if taken verbatim. It seems like a spurious solution though as its 06z runs were closer to the above loose consensus the official forecast will follow; later shifts will need to monitor this for changes, but currently view it as not very likely to pan out. All told, have an additional tenth of an inch for most of eastern MA and RI, but upwards of a half-inch of rain for the Cape and Islands. Friday night should feature increasing rain chances again though, ahead of the next wave of rain for the weekend.

KEY MESSAGE 2...A wet and unusually cool weekend on tap.

Bottom line up front: this weekend will feature cool, cloudy, and periodically wet conditions. The large scale pattern will be driven by an increasingly amplified mid level trough and cyclonic flow overhead. Sitting downstream of the trough axis through most of the weekend southern New England will be in the cross hairs of quite the anomalous plume of subtropical moisture; PWATs surge to 2+ inches or 2-3 SD above normal for late August. Temperatures will be cool for this time of year as cool northeast flow takes hold at the surface; expect high temperatures each day in the low 70s (inland) to upper 60s for parts of the eastern shoreline. This results in limited instability and majority of the rain will be stratiform in nature. However, dewpoints in the upper 60s to low 70s will lead to several hundred J/kg of CAPE so some isolated embedded thunder is possible. Otherwise forcing is plentiful with several embedded shortwaves rounding the base of the trough, favorable placement beneath the RRQ of a 300mb jet, and the surface low and associated frontal boundaries slowly moving overhead. Overall, while it won't be raining all weekend everywhere, rain showers will continue off-and- on until the cold front ushers in a drier airmass Sunday night/Monday morning. When all is said and done the combination of rich tropical moisture and good dynamics will lead to rainfall totals of a quarter of an inch to one and a half inches with the higher totals south and east, lesser north and west.

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

Monday should be wonderful comparatively (at least if you prefer sun) as the drier post frontal airmass spreads over SNE with dewpoints falling from near 65-70F Sun to 45-50F Mon. Heights will be steadily rising as more zonal flow takes hold for the first half of the week with no significant disturbances on the horizon. Temperatures remain comfortable ranging from the mid 70s to low 80s.

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:

Through 00z Friday: High confidence.

VFR, though gradual lowering trend to ceilings through this afternoon. Mainly VFR/6SM -RA develops around 21z at BDL and moves eastward across the remaining TAFs thru 00z, with OVC035-050 type bases in RA. Light northerly winds turn east around 5-10 kt for eastern airports, but trend WSW at similar speeds at BDL.

Tonight: Moderate confidence, especially on the timing of sub- VFR conditions.

VFR categories should generally prevail in -RA through 03-06z, with winds becoming a light N/NE (could be variable at times). Light rain then transitions to either very light rain or drizzle after 03-06z, and ceilings then deteriorate to MVFR-IFR levels, with NE winds 5-10 kt.

Friday and Friday Night: Moderate confidence.

MVFR-IFR ceilings (perhaps MVFR/VFR out at BDL) most of the day. Initial either very light rain or drizzle could turn to steady light to moderate rain from south coast to BOS-ORH corridor by mid-morning Friday but uncertain how heavy this second slug of rain might be and when it begins. It does look like rain resumes in eastern/northeast MA by Friday night again. NE winds around 10 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Friday Night through Tuesday/...

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

Saturday Night: Mainly VFR, with areas IFR possible. SHRA likely, slight chance TSRA.

Sunday: Mainly VFR, with local IFR possible. Chance SHRA, slight chance TSRA.

Sunday Night: Mainly VFR, with local IFR possible. Slight chance SHRA, slight chance TSRA.

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 Friday Night: High confidence.

Winds and seas should remain below SCA criterion through Friday night. Easterly winds 10-15 kt with southerly winds at similar speeds through this afternoon, then winds turn northerly tonight/overnight to around 15-20 kt in gusts. These conditions generally continue into Friday as well. Seas mainly 3 ft or less for all waters.

Main issue for mariners is reduced visibilities in rain and mist tonight into Friday.

Outlook /Friday Night through Tuesday/...

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. Rain showers likely, slight chance of thunderstorms.

Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain showers, slight chance of thunderstorms.

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

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

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

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

Tuesday: Winds less than 25 kt.

BOX WATCHES/WARNINGS/ADVISORIES

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