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.
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.
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.