Marine Weather and Tide Forecast for North Falmouth, MA

NEAR TERM /UNTIL 6 PM THIS EVENING/

115 PM update...

A dry but chilly airmass remains over the region with dew pts in the teens all the way down to the coastline and islands. Highs mainly in the 30s, which is just about on track for early Feb.
Although, somewhat pleasant given the dry weather, dim/filtered sunshine and light winds. Earlier discussion below.

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1015 AM update...

Quick glance at 12z guidance coming in supports previous forecast on a widespread 5-9" snow event. Depending on duration and intensity of snow bands, there is a low probability of 10-12" across northern MA to the VT/NH border and possibly as far south of the Greater Boston area. Snow begins this evening, 8 pm to 11 pm from west to east and quickly becomes moderate to heavy at times in response to strong mid level WAA. Heaviest snow falls from about 10 pm to 4 am from west to east, however possibly lingering until 7 am across RI and eastern MA, as cold conveyor belt (CCB) gets going in response to low level moist easterly jet.

Forecast confidence/uncertainty centered around duration, intensity and location of heavier snow bands (1-2" per hour)
from mid level Fgen in the DGZ vs arrival time of mid level dry air (dry slot), which will end the heavier snowfall rates. As of snow best banding signal is across northern MA to the VT/NH border, possible extending southward to Boston. Then a secondary snow band develops 09z-15z from CCB off the ocean into RI and central/eastern MA. This band will not be as intense as northern band, given forcing is just below the DGZ, nonetheless will yielding accumulating snow up to 1 inch per hour a times from 09z-15z west to east. Other uncertainty is timing of northward extent of snow/mixed ptype line from HFD- PVD-PYM to the south coast, but how far north and duration? In this region, per model soundings, could be some sleet/ice pellets with warm nose around 800 mb and then freezing rain/drizzle as the dry slot invades the ice/snow growth region.
More this afternoon as Well evaluate the reminder of the 12z guidance.

SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/

Key Messages:

* Snowstorm moves across southern New England tonight into Sunday morning.

* Medium confidence in snow totals, with a most likely range of 4-8" across southern New England.

* Potential for freezing drizzle/snizzle on back end of storm Sunday AM.

Latest guidance suite trended slightly slower with the timing of this event. While surface temperatures across most of our region should not be a problem for supporting snowfall, there are still some uncertainties that lower confidence in these snowfall totals. The first is the duration of this storm. It's moving fast, and should persist for only about 12 hours or so. As it looks now, we have about a 4-5 hour period where the conditions line up best to produce higher snowfall rates (from about 10 PM to 3 AM). A shorter duration would cut back on snowfall amounts.

Another issue is the expectation of drier air moving into the dendritic growth zone (DGZ) after midnight. This can have the effect of making smaller snowflakes or freezing rain instead.
Neither is good for accumulating snow. As of this writing, still have the greatest confidence this will impact areas south of the Mass Pike, but there is a risk for it moving farther north.
The 08/00Z NAM is the slowest to do this, but even it brings the dry air in the DGZ to the NH/VT/MA border by daybreak Sunday.
This will need to be monitored through today.

Considering all of this, have the highest confidence in 4-8" of snow across most of southern New England, which is enough to convert Winter Storm Watches to Warnings. Towards the Cape and islands, am thinking there are just too many issues to have confidence in averaging 6" of snow. Winter Weather Advisories posted there instead. Subtle changes over the next 24 hours may alter this forecast further.

LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/

Key Messages:

* Quiet to start the week through Thursday, slightly colder than normal but dry

* Low pressure moving off the mid-Atlantic coast Tuesday night being monitored for possible light snow, but details remain uncertain

* Stronger system possible Thursday, but the track and precipitation types remain uncertain this far out

Sunday night through Tuesday...

High pressure builds in and remains over southern New England into Tuesday. Forecast soundings across the region Monday indicate good mixing, and with 925 mb temperatures sitting around -10C for much of the day, slightly colder than normal temperatures can be expected. Highs for Monday will likely be in the low to mid 30s across the region with some spots in the higher elevations sitting in the 20s. Lows Sunday night will likely be in the teens and low 20s, but are expected to sink to single digits and teens for Monday night with the cold airmass settling over the region. Since these temperatures aren't expected to climb much higher during these few days, not much snowmelt can be expected to start the week.

Tuesday night...

A shortwave disturbance from the Rockies is expected to progress eastward and a surface low will likely develop ahead of it across the central US. This will likely reach the mid-Atlantic by Tuesday night. Precipitation and track are still quite uncertain at this time. Ensemble guidance at this time keeps the highest probabilities for QPF >0.1 inches over 24 hours (ending 00z Thurs) to the South Coast, with probs ranging from 50% to 75% up into RI and along the Cape. Probs are below 50% towards Boston and north. The one ensemble that has notably low probs is the GEFS, which also tracks the system well to the south and east offshore. This is also evident in the GFS compared to the other deterministic models. If this system does pass along/just south of the South Coast, some light snow could accumulate, especially with the colder temperatures aloft still in place.

Wednesday through Friday...

Brief high pressure looks to return following the passage of Tuesday night's low. A notably stronger shortwave from the Rockies moving east is expected to produce a stronger surface low out ahead of it that may reach the Northeast by Thursday morning. As it is still quite far out, timing, track, and ptype are not nailed down at this time. Ensemble solutions are leaning towards taking this low's center along the South Coast or just south of it, and when coupled with possible warmer temperatures aloft (925 mb temperatures reaching 0C and slightly above), this could encourage a mix of winter precipitation types. The extent that the low's associated WAA extends north will be a key factor here, and with the spread in possible tracks across all models, this remains uncertain. The ECMWF shows a primary low passing to our northwest with a secondary low developing over southern New England before undergoing more intense cyclogenesis over the Gulf of Maine as it moves off into the Maritimes, but as of the time of this writing, this is the one model with that track. PoPs were reduced to Likely at the highest across the extended period.

AVIATION /18Z SATURDAY THROUGH WEDNESDAY/
Forecast Confidence Levels:

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

18z update...

This evening and overnight...High Confidence on trends, but some uncertainty on exact timing of details.

Snow quickly overspreads the region between 02z western CT/MA to 05z eastern MA. Snow quickly becomes moderate to heavy at times with snowfall rates of 1-2 inches possible at times from about 04z to 10z west to east. Snow may mix with some sleet/ice pellets and freezing rain after 09z. Mixing may include rain across the islands after 09z. IFR will become common overnight, with LIFR at times in heavier snow. Not much wind with this event, generally 10 kt or less overnight.

Sunday...High Confidence in trends, some uncertainty how departure of lingers snow bands.

Snow or mixed PL/FZDZ ends 09z-12z across CT and western MA.
Snow or a mix PF/FZDZ ends 12z-15z across RI and eastern MA with a mix of rain possible south coast and islands. Very light snow may linger along the eastern MA coast until 18z, then improving conditions.

Snow totals for most terminals, 5-9 inches, but low probability (10-20% chance) of 10-12" across northern MA terminals, including KBOS.

Sunday night...high confidence.

VFR, dry weather and light winds. Thus, not expecting any blowing/drifting snow.

KBOS TAF...High Confidence in TAF trends. Mainly snow event, however, low probability (10-20% chance) of PL/FZDZ mixing in at times after 09z Sunday. High confidence for 5-9" snow totals, with a low prob (10-20%) of 10-12". Heaviest snow with 1-2" hourly rates from about 04z-09z.

KBDL TAF...High Confidence in TAF trends. Snow may end as a mix of snow/PL/FZ after 07z-08z. High probability for snow totals 5-9". Much improved conditions after 12z Sunday.

Outlook /Monday through Wednesday/...

Monday: VFR. Breezy.

Monday Night: VFR.

Tuesday: VFR. Slight chance SN.

Tuesday Night: Mainly VFR, with local IFR possible. Chance SN.

Wednesday: Mainly VFR, with local MVFR possible. Slight chance SN.

MARINE
Forecaster Confidence Levels:

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

Overall high confidence through Saturday.

Winds and seas continue to subside today. As such, let the Small Craft Advisories expire this morning. A low pressure should move east around the 40N parallel. Rough seas should redevelop across the outer coastal waters Sunday. Will likely need Small Craft Advisories again.

Outlook /Sunday Night through Wednesday/...

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

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

Tuesday: Winds less than 25 kt. Chance of snow.

Tuesday Night: Winds less than 25 kt. Chance of snow. Local visibility 1 nm or less.

Wednesday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of snow.

BOX WATCHES/WARNINGS/ADVISORIES
CT...Winter Storm Warning from 7 PM this evening to 1 PM EST Sunday for CTZ002>004.
MA...Winter Storm Warning from 7 PM this evening to 1 PM EST Sunday for MAZ002>021-026.
Winter Weather Advisory from 7 PM this evening to 1 PM EST Sunday for MAZ022>024.
RI...Winter Storm Warning from 7 PM this evening to 1 PM EST Sunday for RIZ001>007.
Winter Weather Advisory from 7 PM this evening to 1 PM EST Sunday for RIZ008.
MARINE...None.