After ravaging eastern Cuba as a strong category 2 hurricane last night, hurricane Sandy is zigzagging her way through the central Bahamas with sustained winds of greater than 100MPH. Rain bands from Sandy are also brushing up against the southeastern coast of Florida as the hurricane passes by to the east. Although Sandy is still purely tropical with a tight core of intense winds over the central Bahamas, the hurricane is beginning to interact with an upper level trough which is why the hurricane has such a large mass of clouds associated with it.

Through the weekend, Sandy is expected to pass close to or over the western Bahamas, which remain under a hurricane warning. The hurricane will then turn towards the north-northeast off the southeast US coast, with large, battering waves and a high risk of rip currents occurring through the weekend from southern Florida north through the Carolinas.

As Sandy snakes its way off the southeast coast this weekend, it is still expected to merge with a mid to upper level trough (shown above), causing the cyclone to begin taking on sub-tropical characteristics.  This means that while Sandy will still draw energy from the warm waters of the Gulf Stream, and still maintain strong winds near its center, the system will begin to also draw energy from non-tropical sources such as the jet stream and warm and cold fronts. While this happens, the winds from Sandy will also spread farther away from the tight core, which means that although Sandy will likely weaken to a category 1 hurricane this weekend, it will remain a large and dangerous storm.

Today, two clusters of modeling have developed with relation to Sandy’s track and eventual landfall:

1. The Euro and NOGAPS models show Sandy staying closer to the US coastline and phase the system more aggressively with the diving central US trough, resulting in a landfall into the Delmarva region.
2. The GFS, GGEM (Canadian) and UKMET models forecast Sandy to wander a little farther north before being captured by the incoming trough, resulting in a landfall in the NYC, Long Island or Southern New England area.

There appear to be couple of differences in the modeling that result in these two camps of possible landfall locations:

1. The main difference seems to be the strength of ridging stretching between the ocean low over the open Atlantic and Sandy. The camp that takes Sandy farther north show slightly weaker ridging in this region. The camp that takes Sandy into the Delmarva region show stronger ridging. The GFS (left) and Euro (right) are posted above to illustrate this difference.
2. The GFS, GGEM and UKMET all show Sandy resisting phasing a little bit initially…which can be seen on this graphic below:

This is the GFS model valid Monday morning. As can be seen, Sandy has been captured by the diving central US trough, but instead of being captured and violently phasing with the trough right away, Sandy resists this phasing. This is likely because the model is picking up on Sandy being a much stronger hurricane right now than originally thought, and maintaining a stronger warm core farther north. Strong warm core cyclones are known to initially resist phasing with a cold core trough, because the systems are not alike.

In the end however, all models do eventually show a phasing of Sandy with this northern trough, resulting in a NW motion towards the northeastern US coastline.

Which camp is ultimately correct? Right now, I’d lean towards the camp that takes Sandy into the coast a little farther north, mainly due to the possibility that Sandy will initially resist phasing a little bit, allowing it to get farther north before ultimately sling-shotting towards the northwest. The GFS and European ensembles generally agree on this type of scenario, with a likely landfall zone from NJ northward towards the Boston area, with some risk of a landfall as far south as northern Virginia, with the time-frame for landfall being Monday into Tuesday:

Now that we have established that landfall is likely, and should occur somewhere from Virginia points north Monday into Tuesday, it is time to discuss the possible strength of Sandy as it impacts the northeast US:

There are a few factors that will allow Sandy to go from a sub-tropical storm with winds near hurricane strength to a very deep hybrid storm as it nears the coastline.

Jet Structure:

Two potent jet streaks, shown above by the blue color-fill, will be positioned to provide for a huge amount of lift over the entire Mid Atlantic, Northeast, and near-shore waters as Sandy approaches.

Potential warm-seclusion nature of Sandy near landfall:

If Sandy is able to maintain a strong warm core as it heads northward into early next week, the warm core will be caught under and upper level trough and surrounded by cooler air aloft, as shown above on the GFS. This will allow for a tremendous amount of lift with Sandy due to the buoyant nature of warmer air.

Mid-level height falls, positive vorticity advection:

As Sandy is captured, the low pressure aloft associated with Sandy will combine with the lower pressure aloft created by the incoming trough, to result in pressures, or “heights” aloft to fall dramatically near and over Sandy as it approaches the US coastline. In addition, the mid-level spin, or “vorticity” within Sandy and the trough will combine and create a large amount of vorticity aloft as shown above by the color-fill. This vorticity aloft will also enhance lift.

When you combine the upper level lift caused by the jet streaks and the remaining warm core of Sandy providing for tremendous lift under a very strong low aloft, you have the ability for the storm to really strengthen as it approaches the US coastline on Monday and Tuesday. It is estimated that Sandy’s pressure will hold steady as it moves northward this weekend due to a baroclinic assist, as discussed earlier. Thus, Sandy will likely have a pressure of somewhere around 965-970mb before it begins to interact with the trough. As trough interaction occurs, Sandy will have 12-24 hours of intensification before the pro-intensification factors max out. Thus, it is reasonable to assume Sandy will strengthen 10-20mb after phasing occurs. Doing the math, this results in a potential 945-960mb low pressure system hitting the US coastline on Monday or Tuesday—and many models remain near the strong end of that if not stronger. This will result in a large and strong wind field as Sandy makes landfall.

It is too early to say who will see how much wind; however as a general idea of the wind potential Sandy will carry as it nears the US coastline:

This is the European model’s representation of winds about 5,000 feet above the ground Monday morning in knots. 1 knot equals 1.15 miles. Winds at this speed at 5,000 feet above the ground (80-90 knots) would likely translate to 60-70 knots on the surface, or about 70-80MPH over the water. These types of wind speeds are consistently showing up on our models over the water, with weaker but still dangerous winds potentially spreading well inland with Sandy.

This wind representation will change as the models continue to hone in on a final landfall destination, however there are some key takeaways from this situation:

1. Sandy is likely to impact the Mid-Atlantic and Northeast as a category 1 hurricane Monday and Tuesday, with residual impacts lasting through Wednesday.
2. Sandy will feature an exceptionally large area of tropical storm force winds, likely several hundred miles across, as it makes landfall.
3. Damaging winds are likely along the coastline near and potentially hundreds of miles north of Sandy’s landfall point along the coast. Some wind damage may occur inland as well, although the strongest winds will be near the ocean.
4. Large battering waves and significant storm surge will occur near and north of Sandy’s landfall destination.
5. Significant rainfall will occur over much of the Mid-Atlantic and Northeast, with some rainfall reaching as far west as Ohio and Michigan.
6. A crippling wet snow event remains possible over the Appellations from central PA south through the mountains of WV, Maryland and western VA.

As this event continues to draw closer I will have further updates with more specific impact information.