FXUS61 KBOX 241536 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1036 AM EST Sat Dec 24 2022 .SYNOPSIS... Cold and dry holiday weekend for most of southern New England, but scattered ocean-effect snow showers will impact the Cape and Islands today. High pressure will bring dry weather next week with a gradual warming trend, then rain showers are possible for New Year`s weekend as low pressure approaches from the Ohio Valley. .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1035 AM Update... * Significant Ocean Effect Snow Band Will Impact Marthas Vineyard at times into Sun AM and may brush Cape/South coast * Potential for another ocean effect snow band near Nantucket especially tonight into Sun AM An extremely rare and significant snow band has been impacting Marthas Vineyard this morning on very cold west to southwest flow. Most of the time our ocean effect snow events are dealing with northerly flow with a shallow inversion layer and often a limited fetch. This results in very minor snow accumulations in most cases. In this situation...rare arctic air invaded the region on a 240/250 degree wind. The soundings indicate a rather deep mixed layer up to the inversion at 4-5 thousand feet with uniform WSW winds. Large temp differentials from the top of the mixed layer to the SST were on the order of 22C/23C. This was resulting in impressive instability with ocean induced Capes of 500+ J/KG and Normalized Capes of 0.25. Given the above...the result has been a significant band of sound effect snow that has been impacting Marthas Vineyard this morning. Much of that region has already received 4-6 inches of snow. It does appear that the brunt of this band is shifting north of the region and may graze parts of the south coast and Cape Cod later today and tonight. However...some of our mesoscale models indicate this band may drop back south later today and especially tonight as low level winds turn a bit more westerly. Even outside the bands...lighter ocean effect snow showers may impact the region. Give the above...we earlier upgraded to a Winter Storm Warning for Marthas Vineyard for 6 to 8 inches of snow with localized 10 inch amounts not out of the question. Given how extremely rare these type of events our confidence in overall amounts is less than we would like it to be...but the mesoscale meteorology supports it. Also...some of the guidance indicates a second band may develop near Nantucket especially tonight into Sunday morning. We opted to include them in a Winter Weather Advisory for 1-5 inches of snow. Nonetheless...ocean effect lighter snow showers will continue to impact them at times into Sunday morning. Lastly...a coating to 2" of snow is possible on some spots of the south coast and Cape. This will depend on how far north any bands of ocean effect snow can get. But many spots in this region may not see more than flurries. Meanwhile...across the rest of the region plenty of sunshine will continue but it will be very cold. High temps mainly in the upper teens to middle 20s with gusty winds. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Pressure gradient continues to diminish tonight into Sunday. Low temperatures tonight expected to be similar to those this morning. High temperatures Sunday should be higher than today as the airmass continues to moderate over our region. This, combined with more of a west wind, should bring the ocean-effect snow showers to an end, although some clouds are likely to linger across the islands into Christmas Day. .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Highlights: * Dry next week with a gradual warming trend. * Mild with rain showers possible over New Year`s weekend. Significant pattern change will get underway next week as closed low that`s bringing our current Arctic blast exits Quebec and is replaced by rising heights across much of eastern U.S. as subtropical ridge builds off eastern seaboard. Models do show one short wave rotating around trough Tuesday, which could bring a few rain or snow showers, but airmass looks fairly dry so it may end up just producing some cloudiness. Looking farther ahead, ensembles show high probability of temperatures reaching 50 degrees or higher over New Year`s weekend. At same time, there is an indication that low pressure will be making its way east from Ohio Valley, bringing an increased chance of rain showers as well. .AVIATION /15Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Through Sunday...High confidence. MVFR/IFR in bands of snow showers near Cape Cod and the Islands and offshore through tonight, otherwise VFR. Gusty W/SW winds persist today but gradually diminish tonight and Sunday. KBOS Terminal...High confidence. KBDL Terminal...High confidence. Outlook /Sunday Night through Wednesday/... Christmas Day: VFR. Windy with local gusts up to 30 kt. Sunday Night through Monday: VFR. Breezy. Monday Night through Wednesday: VFR. .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. High Confidence. Gusty W to SW winds will continue to gradually diminish across the waters through Sunday. Storm Warnings converted to Gale Warnings, which will gradually be converted to Small Craft Advisories with time. Some of the nearshore waters will likely be converted later this morning, but most waters will see gale- force gusts into this evening. Rough seas and areas of light freezing spray expected over the nearshore waters into Sunday. Outlook /Sunday Night through Wednesday/... Christmas Day: Strong winds with gusts up to 30 kt. Rough seas up to 12 ft. Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 10 ft. Monday: Winds less than 25 kt. Seas up to 5 ft. Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday: Winds less than 25 kt. Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Wednesday: Winds less than 25 kt. .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Winter Storm Warning until 1 PM EST Sunday for MAZ023. Winter Weather Advisory until 1 PM EST Sunday for MAZ024. RI...Winter Weather Advisory until 1 PM EST Sunday for RIZ008. MARINE...Small Craft Advisory until 1 PM EST Sunday for ANZ230. Gale Warning until 8 PM EST this evening for ANZ231-233>235- 237-250. Gale Warning until 11 PM EST this evening for ANZ232-254>256. Small Craft Advisory until 7 PM EST Sunday for ANZ236. Small Craft Advisory until 6 PM EST Sunday for ANZ251. $$ SYNOPSIS...Belk/JWD NEAR TERM...Frank SHORT TERM...Belk LONG TERM...JWD AVIATION...Belk/JWD MARINE...Belk/JWD ============================================================= FXUS61 KBOX 241217 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 717 AM EST Sat Dec 24 2022 .SYNOPSIS... Cold and dry holiday weekend for most of southern New England, but scattered ocean-effect snow showers will impact the Cape and Islands today. High pressure will bring dry weather next week with a gradual warming trend, then rain showers are possible for New Year`s weekend as low pressure approaches from the Ohio Valley. .NEAR TERM /UNTIL 6 PM THIS EVENING/... 715 AM Update: Issued a Winter Weather Advisory for Block Island and Martha`s Vineyard. Getting reports of 1-2" so far and seeing visibility down to 1/2SM on both islands. Earlier thinking below... * Ocean Effect Snow is the focus for today Made some adjustments to PoPs for the band of ocean effect snow affecting the Islands, leaning heavily on 06z 3km NAM which has a great handle on its current position and expected trends. This is an unusual situation that we often only see with SW flow when there is a departing arctic airmass. It`s actually rather shallow, only up to around 3,000 ft in depth, which is why it looks unimpressive on radar, but IR satellite shows its origins are in Long Island Sound and it extends across Block Island to Martha`s Vineyard and Chatham. Looking at NAM soundings, there`s roughly 500 J/kg of ocean- induced CAPE and little if any directional shear in the mixing layer. Two things stand out that should limit accumulations - one is the fact that wind speeds are rather strong in the mixing layer which minimizes parcel residence time over the water and the other is the fact most of the lift is occurring in the colder air outside of the maximum dendritic growth zone. Additionally, the band is expected to migrate south of Block Island and the Vineyard this morning, before heading back north toward Cape Cod this afternoon, so it shouldn`t stay over one area too long. We did bump up snowfall totals to 1-3" and it wouldn`t be surprising to see one or two 4" amounts in more persistent bands, but confidence isn`t high enough to issue Advisories just yet. So far it looks like an inch at most from looking at webcams. Otherwise forecast is on track. .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Pressure gradient continues to diminish tonight into Sunday. Low temperatures tonight expected to be similar to those this morning. High temperatures Sunday should be higher than today as the airmass continues to moderate over our region. This, combined with more of a west wind, should bring the ocean-effect snow showers to an end, although some clouds are likely to linger across the islands into Christmas Day. .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Highlights: * Dry next week with a gradual warming trend. * Mild with rain showers possible over New Year`s weekend. Significant pattern change will get underway next week as closed low that`s bringing our current Arctic blast exits Quebec and is replaced by rising heights across much of eastern U.S. as subtropical ridge builds off eastern seaboard. Models do show one short wave rotating around trough Tuesday, which could bring a few rain or snow showers, but airmass looks fairly dry so it may end up just producing some cloudiness. Looking farther ahead, ensembles show high probability of temperatures reaching 50 degrees or higher over New Year`s weekend. At same time, there is an indication that low pressure will be making its way east from Ohio Valley, bringing an increased chance of rain showers as well. .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Through Sunday...High confidence. MVFR/IFR in bands of snow showers near Cape Cod and the Islands and offshore through tonight, otherwise VFR. Gusty W/SW winds persist today but gradually diminish tonight and Sunday. KBOS Terminal...High confidence. KBDL Terminal...High confidence. Outlook /Sunday Night through Wednesday/... Christmas Day: VFR. Windy with local gusts up to 30 kt. Sunday Night through Monday: VFR. Breezy. Monday Night through Wednesday: VFR. .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. High Confidence. Gusty W to SW winds will continue to gradually diminish across the waters through Sunday. Storm Warnings converted to Gale Warnings, which will gradually be converted to Small Craft Advisories with time. Some of the nearshore waters will likely be converted later this morning, but most waters will see gale- force gusts into this evening. Rough seas and areas of light freezing spray expected over the nearshore waters into Sunday. Outlook /Sunday Night through Wednesday/... Christmas Day: Strong winds with gusts up to 30 kt. Rough seas up to 12 ft. Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 10 ft. Monday: Winds less than 25 kt. Seas up to 5 ft. Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday: Winds less than 25 kt. Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Wednesday: Winds less than 25 kt. .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Winter Weather Advisory until 6 PM EST this evening for MAZ023. RI...Winter Weather Advisory until 6 PM EST this evening for RIZ008. MARINE...Small Craft Advisory until 1 PM EST Sunday for ANZ230. Gale Warning until 8 PM EST this evening for ANZ231-233>235- 237-250. Gale Warning until 11 PM EST this evening for ANZ232-254>256. Small Craft Advisory until 7 PM EST Sunday for ANZ236. Small Craft Advisory until 6 PM EST Sunday for ANZ251. $$ SYNOPSIS...Belk/JWD NEAR TERM...Belk/JWD SHORT TERM...Belk LONG TERM...JWD AVIATION...Belk/JWD MARINE...Belk/JWD