Marine Weather and Tide Forecast for Cambridge, MA

NEAR TERM /UNTIL 6 AM FRIDAY MORNING/
Key Messages:

* Isolated to widely scattered aftn showers/t-storms this afternoon, some which may contain a localized heavy-downpour risk.

* Low clouds and areas of fog overnight, locally dense in some location.

Details:

Radar this afternoon (2:30PM) shows scattered convective showers and a few storms developing from the south pushing northward across RI, CT, and mainly central MA. Expect this to continue through the afternoon hours. There is plenty of moisture around with around 1.2-1.4" of precipitable water across southern New England and marginal elevated instability. Given the warm cloud depths and skinny CAPE sounding profiles, there is still a risk for a storm or two to be capable of localized heavy rainfall.
Showers and storms decrease this evening.

Main event for tonight will be the low stratus and fog. The moist airmass, warm temperatures aloft, and calm winds will support development of areas of fog and low stratus tonight.
Impacts will be lowered visibilities to as low as 1/2 to 1/4 of a mile in localized spots tonight through early Friday morning.

SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY/
Key Messages:

* Gradual clear of low clouds and fog Friday morning through the afternoon.

* Potential for scattered showers and storms in the afternoon, with the higher chances for western and central MA.

Details:

Friday morning will start with low clouds and areas of fog.
Subtle height rises work into the region in the morning which will help bring breaks in the clouds in the afternoon. The Cape and Islands will likely be slower to improve than the interior. Model soundings show gradual improvements after 8 AM Friday morning with further improvements in the afternoon. As clouds break, we'll get more sunshine in that timeframe which will help build a bit more instability, mainly across western MA along with help of the approaching front to bring weak lift. Will a moist airmass prevailing, this will support scattered showers and storms in the afternoon, with the higher chances for western MA, central MA, and parts of CT. A few stray showers or storms may move into central and northeast MA.

LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/
Key Messages...

* A round of strong to severe storms expected on Saturday afternoon and evening.

* Drier and cooler to start next week followed by more unsettled weather.

A mid level trough and surface low digging into the Great Lakes on Saturday will bring a round of strong to severe storms to southern New England Saturday afternoon and evening. Ahead of this system a warm front moves north and brings dewpoints well into the 60s by Saturday afternoon on warm and moist southerly flow. This will make for a warm (upper 70s to near 80) and relatively humid day on Saturday and help to generate a good amount of instability in the warm sector, on the order of 500-1500 J/kg of MUCAPE. A stout inversion around 900 mb looks to keep the majority of convection elevated, but as the cold front approaches there is potential to break that cap, especially northwestern/interior SNE where the strongest instability lies. Meanwhile, 0-6 km bulk shear around 25- 30 kts would be enough to prolong some of the updrafts and allow for a few strong to severe storms, of which the main threat would be damaging winds and secondarily large hail. However, 0-1km shear (25- 30kts) and 0-1km helicity (~150m2s2) are enough to mention a non zero chance of a tornado in western MA. While CSU machine learning probs of severe weather have decreased a bit in the last 24 hours, they continue to indicate a 2-5% chance for tornadoes, 5-30% chance of severe wind, and 5-15% chance of severe hail. The SREF also indicates a 20-50% chance of overlapping 500+ J/kg CAPE with 30+kts 0-6 bulk shear. This is a good indicator of chance of severe weather. We're just getting into the hi-res guidance at this point, but timing-wise the storms look to move through from west to east, generally noon to 8PM. So, Saturday shouldn't be a washout but expect generally a 3-4 hour period of showers and storms at any one location.

The cold front then sweeps through Saturday night so Sunday will feel cooler (upper 60s and low 70s) and less humid with dewpoints back in the 40s and low 50s. Expect a mix of clouds and sun with scattered diurnal showers thanks to the cold pool aloft and slow to exit low. Mid level ridging then pushes in with weak surface high pressure to start the week so we'll see slightly more sun and mostly dry weather for Monday and Tuesday. However, uncertainty is higher, especially as we get into Tuesday and mid week; a nearby mid level system over eastern Canada may be in close enough proximity to bring some scattered showers Tuesday. This is followed by signs of a deeper, move organized low bringing more widespread rain to the region mid week into the latter half of the week.

AVIATION /23Z THURSDAY THROUGH TUESDAY/
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Tonight: Moderate confidence.

Any residual showers will dissipate after 00z. Areas of IFR to LIFR fog and stratus develop after 00z. Showed most airports around 1/2 to 4SM, but fog could become locally dense and visbys could be as low as 1/4SM, especially western airports and Cape/Islands. Light to calm winds.

Friday: High confidence in trends, but moderate on timing.

IFR-LIFR from overnight should start to improve out of IFR 13-16z then toward VFR in the afternoon (around/after 18z). The Cape/Islands will be slower to improve, but should see gradual improvements after 15/16z. Some risk for SHRA/TS possible west of ORH after 18z but stronger activity seems more likely west of the Berkshires. Not high enough confidence to include in the TAF at this point. Light S winds 5-9 kts.

Friday night: Moderate confidence.

VFR for terminals away from the south coast. On the south coast, starting VFR and lowering to IFR in fog/low stratus. Winds light and variable.

KBOS TAF...Moderate confidence. MVFR starts to lower to IFR- LIFR levels around and after 01z. Conditions gradually improve after 12z through the day Friday. Lower confidence on timing of VFR, but more likely mid to late afternoon.

KBDL TAF...Moderate confidence. IFR-LIFR in fog/stratus tonight, locally dense fog possible.

Outlook /Saturday Night through Tuesday/...

Saturday Night: VFR. Chance SHRA.

Sunday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA.

Sunday Night: Mainly VFR, with local MVFR possible. Breezy.
Slight chance SHRA.

Monday through Tuesday: VFR. Breezy.

MARINE
Forecaster Confidence Levels:

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

High confidence overall.

E/SE winds remain below SCA levels tonight and Fri. Seas will contineu to diminish. Areas of marine fog develop tonight and could linger into a good part of Fri, returning Friday night.

Outlook /Saturday through Tuesday/...

Saturday: Winds less than 25 kts. Seas 2-4 ft. Scattered showers and storms on southern waters moving from west to east in the afternoon and evening.

Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers.

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

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

Monday Night through Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 1 AM EDT Friday for ANZ254>256.