Marine Weather and Tide Forecast for Vineyard Haven, MA

KEY MESSAGES

- First multi-day stretch of hot temperatures (in the mid to upper 90s away from the South Coast) with a moderate degree of humidity Tuesday and Wednesday. Near record heat possible.

- Isolated thunderstorms possible Tuesday north and west of Interstate 95. Better chances for strong to severe thunderstorms on Wednesday with strong straight line winds the main risk.

- Seasonable and dry Thursday and Friday.

- Cooler and unsettled weather is possible Memorial Day Weekend, but confidence remains low.

DISCUSSION
KEY MESSAGE 1...First multi-day stretch of hot temperatures (in the mid to upper 90s away from the South Coast) with a moderate degree of humidity Tuesday and Wednesday. Near record heat possible.

A spell of anomalous heat (e.g. 850 mb temps running in the upper teens Celsius, which are 2-3 standard deviations above climatological normals for late May) will build into Southern New England on Tue, only slightly cooler on Wed. SW breezes should also mitigate the risk for cooling seabreezes but will keep the immediate South Coast, Cape and Islands considerably cooler than further inland (highs mid 70s/low 80s). Away from the South Coast, highs on Tuesday should reach the mid 90s in most areas with upper 90s possible due to westerly downsloping in the CT and Merrimack Valleys. With a southward-sagging frontal boundary nearby on Wed, highs project to be a few degrees cooler in the upper 80s/low 90s north of the Mass Pike and low-mid 90s in the CT-RI-SE MA corridor, with still cooler temps 70s/near 80 South Coast/Cape and Islands.
Even though the NAM-based guidance shows dewpoints in the mid 60s to low 70s, it usually demonstrates a moist-bias in these strong-mixing days and values should be closer to the mid 50s to mid 60s Tue and Wed. That's also a factor which will be alluded to in the next Key Message to follow.

We're looking at a spell of hot temperatures but a generally drier heat, not necessarily oppressively humid like we typically see with forecasted temperatures this hot. With dewpoints mid 50s to lower 60s, it's more of "drier" heat and the difference between the air temperature and the apparent temperature (heat index) may not be that much different. Framing this setup a little more, if this type of setting were deeper in the summer months, it might not rise to the level of heat headlines. However NWS HeatRisk graphics outline portions of the CT and Merrimack Valleys in the Major HeatRisk classification. Given this datapoint, forecasted heat indices which are borderline (mid to upper 90s), and that there may be individuals who are not yet accustomed to this degree of heat as of yet, we opted to issue a Heat Advisory for Tuesday and Wednesday.

KEY MESSAGE 2...Isolated thunderstorms possible Tuesday north and west of Interstate 95. Better chances for strong to severe thunderstorms on Wednesday with strong straight line winds the main risk.

The warm airmass Tuesday and Wednesday will create a maginally to moderately-unstable convective setting, with some risk for thunderstorms both days. I'm skeptical of the NAM's inflated instability parameters given the above moist-bias (upper 60s to low 70s) and the highest CAPE values stem from the NAM and its downscaled 3-km version, but even the drier GFS shows about 1000 J/kg on Tuesday and again on Wednesday. Soundings show a well-mixed PBL favorable for downdraft and gust-front production if any showers or storms can pop up.

What may trigger storms on Tuesday is debatable, as the best mid- level winds/stronger wind shear and strong cold front is to our north and west. A passing sfc trough could be enough to generate isolated storms north and west of I-95; one or two could become strong if storms develop with strong winds the main risk; the lack of sufficiently strong wind shear would lead to more pulse- type storms. Wednesday may offer a better chance at strong to severe storms with the approaching cold front interacting with moderate instability. The front's timing is in some question but somewhat better chances for thunderstorms near to south of the Mass Pike. Both days feature a Level 1 of 5 (Marginal Risk) for severe weather.

KEY MESSAGE 3...Seasonable and dry Thursday and Friday.

Thursday and Friday are fairly straightforward, with a 1030mb high pressure system across the Great Lakes on Thursday before shifting east on Friday, leading to dry weather. What is most noticeable is the cooler air mass. 850mb temperatures lower to +2C to +5C on Thursday, a far cry from the +18C to +20C observed just a few days prior. With a dry and well-mixed boundary layer, mixing up to 850mb seems likely, tapping into those cooler temperatures aloft and yielding highs in the middle 60s to around 70F both days.
Considering climatological normal maximum temperatures for mid-May are generally in the upper 60s to lower 70s, this is more typical for the time of year versus the early taste of summer that challenged daily record highs. With clear skies Thursday night and nearly calm winds as surface high pressure moves overhead, good radiational cooling is expected, allowing temperatures to lower into the 40s, with perhaps some upper 30s across the higher elevations of northern and western Massachusetts. While slightly cooler than normal, temperatures will be nowhere near record-setting.

KEY MESSAGE 4...Cooler and unsettled weather is possible Memorial Day Weekend, but confidence remains low.

The unofficial start to summer this weekend brings the potential for unsettled weather, though there remains a good deal of uncertainty due to model variability at this range. However, it bears watching given the numerous outdoor activities planned for Memorial Day Weekend. Global deterministic and ensemble guidance show the potential for rainfall, particularly Saturday night into Sunday, though there remain large differences in timing among models and runs. That said, it is worth noting both the AIGFS and ECMWF AIFS depict a 1035mb surface high pressure system situated east of New England, suppressing precipitation to the west. This solution would favor a drier, albeit cooler, weekend with easterly flow and highs generally in the lower 60s. For now, the NBM PoPs remain the best first estimate for the upcoming weekend, but do not write it off just yet, there is still pounds of time for forecast adjustments.

AVIATION /19Z MONDAY THROUGH SATURDAY/
Forecaster Confidence Levels...

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

18Z TAF Update:

Mainly VFR conditions through Wed. However, stratus and fog will likely develop over the ocean as SW flow brings higher dewpoints over cooler ocean and this may impact portions of the south coast and Cape/Islands tonight. LLWS risk for all terminals tonight heading into early Tue morning. Isolated t-storms possible Tue afternoon, but uncertain on if they will impact any terminals. E-NE wind becoming S 10-20 kt this afternoon. S-SW wind 5-10 kt tonight increasing Tue with gusts to 25 kt developing.

KBOS...High confidence in TAF. Seabreeze today with wind shift to S late today or early evening.

KBDL...High confidence in TAF.

Outlook /Tuesday Night through Saturday/...

Wednesday: VFR. Breezy. Chance SHRA, slight chance TSRA.

Wednesday Night: VFR. Breezy.

Thursday through Friday: VFR.

Friday Night: VFR. Chance SHRA.

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

MARINE
Forecaster Confidence Levels...

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

Through Tuesday Night: High confidence.

Issued small craft advisories valid from 12z Tuesday through Wednesday evening. SW winds will be increasing to around 15-25 kt Tuesday and remain so through Wednesday, higher over the southern waters. Seas will also be building to around 4-6 ft, with some 7 footers possible Tuesday night over the southern waters.

Mainly dry weather prevails through Tuesday night, although fog may develop both tonight and Tuesday night.

Outlook /Tuesday Night through Saturday/...

Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Chance of rain showers, isolated thunderstorms.

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

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.

Thursday Night through Friday: Winds less than 25 kt.

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

Saturday: Winds less than 25 kt. Rain showers likely.

CLIMATE
Record Highs for Tue May 19...

BOS 90/1949 BDL 94/1962 PVD 91/2017 ORH 92/1962

Record Highs for Wed May 20...

BOS 91/1996 BDL 99/1996 PVD 95/1996 ORH 91/1903

BOX WATCHES/WARNINGS/ADVISORIES
CT...Air Quality Alert until 11 PM EDT this evening for CTZ002.
Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for CTZ002>004.
Air Quality Alert from 11 AM to 11 PM EDT Tuesday for CTZ003- 004.
MA...Air Quality Alert until 11 PM EDT this evening for MAZ008>011.
Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for MAZ003>007-010>019-026.
RI...Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for RIZ001>004.
Air Quality Alert from 11 AM to 11 PM EDT Tuesday for RIZ001>008.
MARINE...Small Craft Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for ANZ254>256.