Nantucket Sound Marine Forecast
|Today...N Winds 10 To 15 Kt With Gusts Up To 20 Kt, Increasing To 15 To 20 Kt With Gusts Up To 30 Kt This Afternoon. Seas 2 To 3 Ft. Patchy Fog. Snow And Rain Showers Likely This Morning, Then A Chance Of Snow Showers This Afternoon. Vsby 1 To 3 Nm.|
|Tonight...Nw Winds 20 To 25 Kt With Gusts Up To 30 Kt. Seas 3 To 5 Ft. Freezing Spray. A Chance Of Snow Showers.|
|Fri...Nw Winds 20 To 25 Kt With Gusts Up To 30 Kt, Increasing To 25 To 30 Kt With Gusts Up To 40 Kt In The Afternoon. Seas 3 To 5 Ft. Freezing Spray. A Chance Of Snow Showers. Vsby 1 Nm Or Less.|
|Fri Night...Nw Winds 25 To 30 Kt, Diminishing To 20 To 25 Kt After Midnight. Gusts Up To 40 Kt. Seas 4 To 6 Ft. Freezing Spray. A Chance Of Snow Showers. Vsby 1 Nm Or Less.|
|Sat And Sat Night...Nw Winds 15 To 20 Kt With Gusts Up To 25 Kt. Seas 2 To 3 Ft.|
|Sun And Sun Night...N Winds 5 To 10 Kt, Becoming E 10 To 15 Kt. Seas 2 To 3 Ft.|
|Mon And Mon Night...Ne Winds 20 To 25 Kt. Gusts Up To 35 Kt. Seas 4 To 7 Ft. A Chance Of Rain And Snow Showers. Vsby 1 To 3 Nm. Seas Are Reported As Significant Wave Height, Which Is The Average Of The Highest Third Of The Waves. Individual Wave Heights May Be More Than Twice The Significant Wave Height.|
| Area Forecast Discussion|
National Weather Service Boston/Norton MA
651am EST Thu Jan 28 2021
Northerly onshore flow will produce intermittent periods of light ocean-effect showers today across Eastern Massachusetts with minor snow accumulations possible. A surge of arctic air will bring the coldest temperatures of the season by Friday morning with widespread sub- zero wind chill values. A period of heavy ocean- effect snow showers or intense snow squalls could impact parts of the Cape Friday afternoon but where the band sets up remains to be fine tuned. Cold will continue through Saturday with moderating conditions Sunday. There is the potential for a winter storm to affect the area Monday and Tuesday, followed by drier more seasonable weather Wednesday.
Near Term - Until 6pm This Evening
Radar shows rain/snow showers moving ashore in Eastern MA. BOS reported a brief period of freezing drizzle, but then changed over to snow. Northeast-to-southwest movement of the showers is supported by the 925-mb flow around the storm passing well south of New England. Overall this lines up with the forecast of clouds and snow showers in Ern MA/RI. Continue to forecast partial clearing in Northern CT and Western/Central MA.
Previous discussion... * Ocean-effect showers today with potential for 1-2 inches accumulation over parts of Eastern MA. * Sunshine returns for the interior today as the leading edge of Arctic air arrives.
Surface low exits the Carolina coast early Thursday and undergoes rapid cyclogenesis. While it will pass too far south of the 70W/40N benchmark to give us any synoptic precipitation, winds will build from the NNE as the low moves off the coast into the Atlantic. Winds are likely to gust to 30 kts. The NNE wind will be accompanied by a region of enhanced moisture that may generate more ocean effect snow across Cape Cod, the islands, and the South Shore of Massachusetts. There remains a lower probability that these snow showers will expand to the I-95 corridor or even further. Locations under the most persistent bands could pick up 1 to 2 inches, especially over Cape Ann, Plymouth County and into the Boston metro.
Temperature forecast is a little more tricky but there is a model consensus that the leading edge of the Arctic air will reach Western MA by mid to late morning and Eastern MA/RI by the afternoon hours. So we could see midnight highs in the 20s for our Western zones, while further east, there could be enough time for highs to rise into the 30s before falling. Also, sunshine returns by Thursday afternoon albeit with falling temperatures.
Short Term - 6pm This Evening Through 6pm Friday
* Possible brief period of intense convective snow showers or snow squalls across the Outer Cape Friday afternoon. 1-2 inches per hour snowfall rates likely. * Snow accumulation amounts are highly dependent on where the banding sets up, but current thinking is 2-4 inches for the Outer Cape with upwards of 6 inches not out of the question. * Travel could be highly challenging to near impossible with near white-out conditions but details still have to be hammered out. So be sure to monitor the forecast if you have travel plans on the Cape Fri afternoon. * Otherwise, brutally cold day with wind chill values below zero. Wind Chill Advisory has been issued for much of interior MA and northern CT.
While the aforementioned main storm passes well to our south, we will have a rather active weather day across S New England. A piece of the polar vortex heads our way on Friday, sending 850mb temperatures to between -21C and -25C. 850mb winds increase to 45 to 50 kt, which given deep mixing with lapse rates approaching dry adiabatic, this would lead to 25 to 30 mph winds at the surface, leading to widespread sub-zero wind chill values. Overall, brutally cold conditions with widespread highs in the teens and wind chill values starting out as low as -20F with strong NW winds. A Wind Chill Advisory has been issued for much of interior MA and CT.
While much of Southern New England will be dealing with quiet weather albeit brutally cold conditions, it looks increasingly likely that at least part of the Cape (particularly Outer Cape) could be dealing with a high-end ocean-effect snow event. An inverted surface trough extending outward from low pressure well to our east will swing from north to south just to the east of Cape Cod during the day. The retrograding inverted trough will coincide with strong low-level frontogenesis and the passing of the upper trough.
00z NAM Bufkit shows surface CAPE of 200 J/kg, almost dry adiabatic lapse rates and up to 40 units of omega in the favorable snow growth region. HREF shows 40 to 50 percent chance of exceeding 2 inches per hour snowfall rate for the Outer Cape. The big question will be how close the band of heavy snow showers and snow squalls will get to the Cape and SE MA. The other question is how strong the convective snow showers will get. But upon collaboration with WPC, the thinking is that the corridor from Provincetown down to Chatham could pick up 2 to 4 inches with 6 inches plus not too far away. Will let the later shift evaluate the 12z guidance before issuing any winter- related headlines, given that this is a convective type event. In addition, with temps in the 20s and 30 to 40 mph gusts, blowing snow could occur quickly and result in near white-out conditions and near impossible travel for a few hours. So while the details still have to be hammered out, be sure to monitor the weather if you have travel plans across the Cape Fri afternoon because road conditions could deteriorate really quickly.
Long Term - Friday Night Through Wednesday
Big Picture... Five-wave GFS shows a trough over the Gulf of Maine early in the weekend, with a ridge over the Plains and another trough along the Pacific Coast. Shortwave charts are similar. The Gulf of Maine cold low will eventually be bumped east out to sea later in the weekend as the Pacific system ejects east. Pacific system develops a deep amplitude Monday as it reaches the Eastern USA.
The deep amplitude suggests a slower forward motion and is supported by the GFS 5-wave charts. So it is interesting to see the GFS with the most progressive motion while the ECMWF (European Centre for Medium-Range Weather Forecasts) continues to hang back. The GFS brings precipitation into CT by Sunday evening, the GGEM is about 6 hours slower, and the ECMWF precipitation moves up into the South Coast about 24-hours after the GFS. These differences are similar to the the forecasts for the past two nights. While there is confidence that precipitation will move in early this week, confidence in timing the precipitation remains low.
Upper contours remain well below normal much of the weekend, recovering to near normal Sunday night, then below normal again Monday (GFS) or Tuesday (ECMWF). Expect below normal temperatures over the weekend, trending to seasonably cold early next week.
Details... Friday night... Dry and very cold inland. Strong winds and 17-19C temp difference over water suggest continued ocean-effect snow showers over Cape Cod with potential for snow accumulations. Northwest winds will diminish as the pressure gradient diminishes a little. This will diminish the ocean snows overnight. Low level airmass temps suggest min temps near zero in NW Mass and 10-15 along the coast.
Wind chills will be below zero through the night, mostly in the single numbers below zero.
Saturday and Sunday... High pressure builds east, maintaining mostly clear skies much of the period. Some increase in high-level moisture Sunday suggests increasing high clouds. Big surprise, the GFS grids show lots of clouds during the afternoon, while the ECMWF is mostly clear through Sunday evening. Will use a blend of NBM grids with one of the less pessimistic sources for the two grid fields. This keeps most areas dry Sunday night, with low-end chance probability of precipitation in CT late Sunday night.
Temps at 925-mb suggest max temps upper teens to mid 20s Saturday, and 20s to low 30s Sunday. Light wind Saturday night will allow some radiational cooling. Min temps Saturday night based on dew points show single numbers inland with possibly a few just below zero west of Worcester...and teens along the coast.
Monday-Tuesday... The Pacific system arrives in the Eastern USA, generating a coastal storm along the Carolina coast. GFS is faster, the ECMWF is slower. Both bring plenty of precipitation to Srn New England, just a little different on timing. As with the Sunday skies, and based on the stronger amplitude aloft, we favored a stronger blend of the NBM with slower guidance. Both camps of models bring roughly 1-2 inches of water to the region, with snow over interior sections while the coastal plain converts to rain Monday afternoon...then mixes with snow Monday night and Tuesday. Interesting to see IVT guidance clearly showing the moisture feeding in from an atmospheric river from the southern Gulf of Mexico and waters east of Florida.
Wednesday... The weather system lifts out through the Maritimes, drawing clouds/precipitation off to the northeast. Expect seasonable temperatures.
* Gale Warnings for all waters and Freezing Spray Advisory in effect.
Scattered rain showers will continue over the eastern waters today. Winds will continue to prevail out of the north/northwest.
By tonight, a strengthening pressure gradient will result in gusty northwest winds over the coastal waters. As a result, Gale Warning has been issued for all waters. Additionally, a surge of cold arctic air in from the north will work in tandem with the gusty winds to create light to moderate freezing spray conditions. Currently have a freezing spray advisory in effect from Midnight Thursday through about 3PM Friday.
Outlook /Friday Night through Monday/... Friday Night: Low risk for gale force winds with gusts up to 45 kt. Rough seas up to 17 ft. Freezing spray, slight chance of snow showers. Areas of visibility 1 to 3 nm.
Saturday: Winds less than 25 kt. Areas of rough seas. Freezing spray. Local visibility 1 to 3 nm.
Saturday Night: Winds less than 25 kt. Local rough seas.
Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Sunday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of snow showers. Areas of visibility 1 to 3 nm.
Monday: Moderate risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Rough seas up to 12 ft. Rain likely, chance of snow. Areas of visibility 1 to 3 nm.
NOAA Boston MA Office: Watches - Warnings - Advisories
CT...Wind Chill Advisory from midnight tonight to noon EST Friday for CTZ002>004. MA...Wind Chill Advisory from midnight tonight to noon EST Friday for MAZ002>004-008>012-026. RI...None. MARINE...Freezing Spray Advisory from 1am to 3pm EST Friday for ANZ231>233-235-237-250-251-254>256. Gale Warning from 1am Friday to 7am EST Saturday for ANZ230>237-250-251-254>256.