Parameters for Ocean Effect Snows on Cape Cod, MA

[USCAPEWEATHERAF]

As the winter season is quickly approaching, many people on Cape Cod wonder, how do we get snow, while no one else in SNE gets snow?  The answer is simple yet complex.  The simple answer is, the ocean water north of Cape Cod, in the Bay and north of the Bay.  As cold air outbreaks happen during the hardcore months of winter, even in the Fall season and spring season these outbreaks happen.  As they overspread the region with extremely cold air aloft, roughly 2500 feet above the ground level is the 850mb level, a crucial level leading to Ocean Effect Snow production.  Right now, we are forecasted to see 850mb temps on Wednesday after 7 pm through 7 am on Friday this week of around -20C, this combined with the temperatures of the water just north of Cape Cod around 50F (10C), this should allow extreme instability to develop over the region of the Cape via NNW winds from Hyannis eastward to Provincetown, including the towns of Yarmouth, Dennis, Harwich, Chatham, Brewster, Orleans, Eastham, Truro, Wellfleet, and Provincetown, MA to receive flurries the further west you are in Hyannis, and snow squalls further east you go.  This extreme delta ts of around +30C would allow convection to develop within the banding of snows.  However, if we were to get convergent winds in the surface to 850mb from Sandwich to Chatham a single band of intense, extremely intense snow rates could develop and with northerly winds would focus from Harwich to Sandwich a 20 mile wide band of snowfall rates between 2-4", commonly seen in BUF or the Tug Hill Plateau of New York.  While this extreme nature is super rare for Cape Cod, I don't expect that to happen, but a few inches is certainly possible if all things come together.  Here's to faith, but I give this scenario about a 24% chance of happening altogether, wind shear of inversion heights will likely preclude a major event, but I have seen miracles happen, via the Blizzard of 2005 and Blizzard of 2015 where we received over 30" each.  There is always a chance!

Parameters needed for a foot of snow:

North winds from surface to 850mb NW winds over Sandwich, N winds over Hyannis and NE winds over Chatham, not going to happen

850mb temps below -10C

ocean temps above 5C

delta ts around +15C likely around +18 to +22C

presence of CAPE at the surface, ocean induced CAPE leading to convection like banding

inversion heights above 800mb, or 3000 feet, to 2500 feet is ok, but not optimal, inversion is the atmosphere warming with height, happens at the surface to 700mb and at the tropopause

 

Some of these are missing, and therefore a major event is unlikely, but could we see a few SN showers, sure.

[USCAPEWEATHERAF]

Our first snowfall looks to occur Overnight Wednesday night into Thursday, Thanksgiving could be white for most of Cape Cod.

[USCAPEWEATHERAF]

Parameters increasing for a good Ocean Effect Snow event, eastern Cape Cod, east of Hyannis, MA.  850mb temps are decreasing to around -22C, and surface to 850mb winds turn more NNW or N for a time on Thursday, this is when the accumulations will be, right now NWS forecasts .5 to 2" of snow, but again this could increase later on in forecasts.  Delta ts reach +30C extreme instability should make up for the presence of dry air, stay tuned!

[USCAPEWEATHERAF]

NWS mentions accumulations likely HYA eastward on Thursday

[USCAPEWEATHERAF]

I will issue my first call snowfall map at around 11pm

[USCAPEWEATHERAF]

18z RGEM is going gangbusters on the OES threat, over 2" of snow likely, WWA could be issued

[USCAPEWEATHERAF]

I decided to wait until 7am or later for the first map tomorrow morning, I will update you once at 7 am and then again after 2 pm tomorrow.

[LongBeachSurfFreak]

Great thread! All joking aside ocean effect snow is awesome. I think the coolest event of all time was produced my the Chesapeake bay down I. Virginia 

[USCAPEWEATHERAF]

Yeah it is not like the Great Lakes though unfortunately, one day we will get ours though

[USCAPEWEATHERAF]

Here is my map, better late than never right?

[cny rider]

You're a good man.

I hope you get to visit Tug Hill some time and enjoy the real thing in all its glory.

 

[USCAPEWEATHERAF]

5 minutes ago, cny rider said:

You're a good man.

I hope you get to visit Tug Hill some time and enjoy the real thing in all its glory.

 

thanks

[Baroclinic Zone]

Hopefully this pans out and you enjoy a white thanksgiving day.

[dendrite]

Yeah...RGEM and Euro are giving the elbow to the fist a tenth or two of liquid. Nammy still isn't interested and keeps the precip formation just offshore. I'd probably lean toward a little snow at least...your map doesn't look outrageous like I thought it would when you posted that the RGEM went "gangbusters". lol

Hopefully you see some white tomorrow other than the breast meat.

[USCAPEWEATHERAF]

1 hour ago, dendrite said:

Yeah...RGEM and Euro are giving the elbow to the fist a tenth or two of liquid. Nammy still isn't interested and keeps the precip formation just offshore. I'd probably lean toward a little snow at least...your map doesn't look outrageous like I thought it would when you posted that the RGEM went "gangbusters". lol

Hopefully you see some white tomorrow other than the breast meat.

THanks Dendrite

[USCAPEWEATHERAF]

17z HRRR model show the omega around -40 units of lift occurring in the Dendrite Snow Growth Zone ard 10-12z in the morning tomorrow

[USCAPEWEATHERAF]

HRRR brings 850mb temps down to -24C tomorrow morning leading to delta ts around +34C, very extreme instability, if those bands reach Cape Cod, rates could exceed 2"/hour

[USCAPEWEATHERAF]

I would bet that the amounts over the Cape east of HYA could see higher than forecasted amounts, HIRES models coming into better agreement on an impactful day tomorrow.  Watch they won't fully capture the event until it is happening, just watch.

[CoastalWx]

It’s too bad winds weren’t more NNW. Seems like the elbow is the best place. Gonna be a wintry T-day there.

[USCAPEWEATHERAF]

We can never get the winds to work out when the instability is so high

[USCAPEWEATHERAF]

NWS snow map looks like mine.

[USCAPEWEATHERAF]

Clouds beginning to filter back onto the Outer Cape from Provincetown to Harwich.  This is happening as wind direction shifts to more northerly direction, more like NNW or 330-340 degrees.  Winds will eventually shift to 360 degrees later this evening into the overnight hours into Friday morning.  Snow showers should develop towards 14-16z later this morning.

[USCAPEWEATHERAF]

Thanks, NWS upped the ante for the Outer Cape, 70% likely probs that snow will occur this afternoon into the evening hours.  1-2" forecasted.

[USCAPEWEATHERAF]

Saw my first flakes of the season, flurries in Chatham, MA this morning.

[USCAPEWEATHERAF]

Confidence in northerly winds this evening and overnight are increasing substantially, while the HRRR model shows little to no precip, it does show a high lift in the SGZ for Chatham this evening.  This will lead to substantial accumulating snows, stay tuned!

[USCAPEWEATHERAF]

Watch the satellite image below, there is like a mesolow present in the OES streamers or something causing the northerly flow of clouds?  Could it be the overall circulation of the high pressure center?

[USCAPEWEATHERAF]

Something is pushing the OES streamers from Northeast to southwest over the ocean south of the Maine coastline.   I don't know what could cause this to happen as the winds are not changing direction at this time.  This could push the clouds onshore across the mid Cape later this morning into the afternoon.

[dendrite]

Just now, USCAPEWEATHERAF said:

Something is pushing the OES streamers from Northeast to southwest over the ocean south of the Maine coastline.   I don't know what could cause this to happen as the winds are not changing direction at this time.  This could push the clouds onshore across the mid Cape later this morning into the afternoon.

Winds will definitely back somewhat as the high approaches from the west, but as Will said, you'll trend toward more sinking air and the wind will also weaken too as the gradient diminishes. Also, winds are a little more northerly with height so you're probably just seeing clouds moving in different directions at different levels. The offshore echoes on radar still seem to be coming from the WNW. There's nothing earth shattering going on. I'd hope for a few tenths later on and call it a win if that verifies.

[USCAPEWEATHERAF]

1 minute ago, dendrite said:

Winds will definitely back somewhat as the high approaches from the west, but as Will said, you'll trend toward more sinking air and the wind will also weaken too as the gradient diminishes. Also, winds are a little more northerly with height so you're probably just seeing clouds moving in different directions at different levels. The offshore echoes on radar still seem to be coming from the WNW. There's nothing earth shattering going on. I'd hope for a few tenths later on and call it a win if that verifies.

Dendrite, the cloud streets themselves are moving NE to SW, that is odd.

[dendrite]

1 minute ago, USCAPEWEATHERAF said:

Dendrite, the cloud streets themselves are moving NE to SW, that is odd.

Not odd at all. The winds were always forecast to back to a more northerly direction as the high approaches. Keep an eye on radar and keep hoping those NW-SE moving echoes start moving westward over time.