Tuesday Snow

Yesterday I sat here in the weather office looking at the weather maps for Tuesday and thought, “that’s probably going to come west – at least on 1 or 2 model runs” and that’s exactly what has happened. These systems always seem to want to sneak a bit closer to the coast and this one definitely had that look.

gfsNE_sfc_prec_060

 

The actual low pressure that forms is forecast to be well off the coast – far southeast of Nantucket and the Cape but there’s a fairly large swath of precipitation that extends far northwest of the low center. Why is that?

gfsNE_850_spd_054

The GFS closes off an area of low pressure just east of Atlantic City about 5000 feet above the ocean and you can see a band of convergence up through southern New England. In fact, as the large scale trough sharpens there’s a fairly notable zone of frontogenesis that develops from 650mb-850mb across southern New England.

fgen

 

Frontogenesis in the atmosphere forces a thermally direct circulation to develop in order to restore thermal wind balance. That circulation results in upward motion on the warm side of the temperature gradient – and that is what will force precipitation well west of the surface low.

The question is just how far west that frontogenetical circulation will get. The GEFS members show several pretty far west as you can see plenty of precipitation progged in Connecticut. f60

The ensemble mean off the 18z GFS brings >0.25″ of liquid as far west as New Haven, Bridgeport, Middletown, and Willimantic. Additionally, the atmosphere will be quite chilly in the low-mid troposphere. The 18z GFS shows good snow growth with a deep layer of high RH through the dendritic growth zone (-12c to -18c). The one downside to the GFS BUFKIT sounding below is that the best lift (omega) is above the DGZ here in HVN though it is better in GON where the best lift is a bit lower and deeper in the troposphere.

HVN

 

So what’s going to happen? That’s what we all care about of course! At this point I’d say some accumulating snow is likely especially in southeastern Connecticut. The amount of snow is still up in the air and we won’t have a better idea until later tonight. We should be able to throw out some numbers tomorrow morning but from a probabilistic sense here’s what I’m thinking.

chart_3 (4)

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Just a Little Snow Tonight

chart_3

It was a cruel change to the spreadsheet – but I had to replace the >24″  bin with >1″ bin. It’s back to southern New England snow reality after a brief hiatus. Here’s the more typical snow accumulation map.

WVIT Forecast Snow CT

Here are all the dorky details on the snow! A beautiful and impressive period of bombogenesis is underway off the Carolina coast. The synoptic setup is picturesque.

GFS 250 mb Wind Speed

GFS 250 mb Wind Speed

The 250mb GFS forecast shows a powerful coupled jet streak resulting in very strong lift over the Canadian Maritimes and adjacent parts of the North Atlantic by 18z Sunday. Cyclogenesis is already underway with a potent PV disturbance rounding the base of the trough and heading offshore.

With the trough remaining progressive and with a neutral tilt this low isn’t going to hug the coast. In fact, convection firing offshore over the Gulf Stream argues for a farther offshore track. Regardless, as the storm winds up offshore winds are going to ramp up and we will see a period of light to occasionally moderate snow clip Connecticut.

The light snow across the region this evening is due to mid level frontogenesis that is occurring overhead.

frontb12

The NAM and GFS shows a band of frontogenesis from just off the New Jersey shore through western Maine of. I’ve attached the 800-600mb vertically averaged frontogenesis. The tightening of the thermal gradient (frontogenesis) results in a thermally direct circulation (need to restore thermal wind balance!!!).  All of this leads to a band of vertical motion that produces an area of enhanced precipitation within a larger area lighter precipitation associated with the synoptic scale ascent.  You can see on radar the band of precipitation that lines up pretty well with the forecast area of frontogenesis.

LongIsland.rad

While the band of snow overhead looks relatively impressive on radar it’s anything but impressive on the ground. A gusty northerly wind is advecting dry air through the boundary layer. It will be a tug of war all night between snow being produced around 700mb (nice snow growth too!) and a steady stream of dry air coming south in the boundary layer. We’ll sublimate more snow than will fall! Here’s the 00z OKX sounding that really tells the story.

OKX

Bottom line is that this “event” will wind up pretty marginal. The actual low and mid level low centers that close off will wind up too far east to clip us with the comma head. The frontogenesis producing -SN overhead will slowly weaken and scoot east later tonight.

Some Impact From Sandy Appears More Likely

Our computer models are slowly getting into better agreement that hurricane Sandy will pay someone in the northeast a visit by early next week. The threat area for a direct hit stretches from North Carolina all the way to Nova Scotia.

12Z Euro 500 mb Heights/Vorticity 84 Hour Forecast

The giant “block” that we’ve been talking about over the Atlantic Ocean appears strong enough to prevent the hurricane from sliding east and out to sea.

The block itself is what’s called a Rex block. Rex blocks occur when a ridge of high pressure exists poleward of a cut-off low pressure. These patterns are somewhat stable and slow to change. To the west of the Rex block a ridge of high pressure (red dashed line) is nosing up from the Caribbean to the north Atlantic. This ridge is effectively preventing Sandy from losing too much longitude as it heads to the north.

So will the block hold? There was some hope  that a weakness would develop allowing Sandy to sneak out to sea. It does not look like that will happen with only a handful of pieces of available guidance (including the 18z operational GFS and some of its ensemble members) showing an out to sea track.

Here’s a spaghetti plot showing all the different GFS ensemble forecasts. Basically, each line represents a separate forecast using initial conditions that have been tweaked a bit to represent the inherent uncertainty in our ability to capture an accurate initialization of the atmosphere.

12z GEFS Ensemble Members

You can see here that even the ensemble members that appear to have found an escape route near Bermuda get stuck in their tracks and are forced to turn around by the block. This tells me, when coupled with the Euro ensembles and most other computer models, that a trip out into the open Atlantic is unlikely. It’s a fairly good bet (better than 50/50 chance) that this thing is coming toward the U.S.

But just how close it gets to Connecticut is an open question. It’s also unclear when the storm will make landfall. Some computer models bring Sandy ashore Sunday night. Others bring it ashore Tuesday night. Some models target Maine for a direct hit while others like Ocean City, Maryland. There are several that have a bullseye right on Connecticut and Long Island.

Until we have a better idea where the storm will make landfall it’s impossible to get too specific with the exact hazards and threats that Sandy will pose. We are still 5 days from a possible impact which is an eternity when forecasting a hurricane’s track!

It’s also unclear how strong the storm will be when it reaches the northeast.

There’s no question the storm will be “warm core” in nature by the time it reaches our latitude. It will not be an extra-tropical storm. This means the storm’s strength will decrease with height the way hurricanes do! It’s also likely that Sandy will develop frontal systems (or baroclinicity) which is a trait of an extra-tropical storm. Basically, the storm will be a hybrid between a nor’easter and a hurricane.

It’s important to note that a warm-core “hybrid” can be quite dangerous and powerful. The hurricane of 1938 (This is an example folks…. not comparing the two!!!) was likely a hybrid storm as was the 1991 “Perfect Storm”.

Here’s a way to quantify the storm’s forecast hybrid characteristics courtesy of Bob Hart of Florida State University. This cyclone phase-space shows how warm core a storm is (i.e. how a cyclones’s intensity varies with height) and how symmetric a storm’s temperature gradient is. A purely tropical system would have an intensity decreasing with height (can measure this from the thermal wind) and no temperature gradient tangential to its motion.

This indicates the potential (based on 12z GFS) for Sandy to strengthen once it undergoes extratropical transition. It appears as if the storm will become a dangerous warm seclusion. This means it will have an asymmetric thermal gradient yet remain warm core. Warm seclusions are infrequent in our neck of the woods and they’re characterized by a large radius of gale force winds and a tight (tropical-like) core of stronger winds near the storm center.

For what it’s worth the cyclone-phase space indicates that this will be a deep warm core system (can figure this out by comparing the thermal wind in the upper troposphere to lower troposphere) and not a more typical shallow warm core system.

While the pressure readings I’ve seen from some of the global models look far too low (i.e. 930ish mb) there is the potential for this storm to be quite intense. When coupled with an astronomically high tide the concern is there for coastal flooding for anyone in the storm’s path. Wind damage and heavy rain will also be an issue somewhere near Sandy’s path. Will that be here in Connecticut? It’s just too soon to tell.

We’ll have to watch Sandy closely over the next few days but at this point I’m becoming increasingly concerned about a rather serious storm somewhere in New England. There remains an outside chance that this storm could smack Long Island and southern New England directly as a hurricane. There’s also a chance, albeit a small that, that the storm is able to sneak through a weakness in the block and head out to sea like the 18z GFS and some of its ensembles now show.

Even without a direct impact coastal flooding and flooding rains are possible with a storm thats hundreds of miles away as a secondary low pressure system may develop off the coast and absorb some of Sandy’s moisture. We need to keep our guard up and be ready to start preparations when Sandy’s probable path and strength becomes more clear. This is a low confidence but potentially very high impact storm.