Hurricane Sandy, the storm projected to eventually impact the east coast of the US, has already hit eastern Jamaica and is now eying eastern Cuba as a hurricane. As can be seen above, the hurricane has developed a small eye with a well-organized ring of cold, intense cloud tops wrapping around this small eye. Sandy is becoming an impressive looking hurricane.

Over the next few days, Sandy will begin to have its early US impact. However, first Cuba will deal with a potent category 2 or 3 hurricane early Thursday morning. Thereafter, on Thursday and Friday, Sandy will snake its way through the central and western Bahamas, potentially as a hurricane—the Bahamian government has issued a hurricane warning for that portion of the island chain. Eastern Florida may also be grazed by Sandy, with breezy conditions and heavy rains likely to end the work-week.

As Sandy moves through the Bahamas and off the southeast US coast it will merge with a mid to upper level trough of low pressure. On the map above, the black lines indicate isobars about 18,000 feet off the ground, and one can see the mid-level low that Sandy, represented by the ball of yellow and orange off the coast of Florida, combines with. While this happens, the winds located approximately 40,000 feet off the ground, represented by the wind barbs on this map, will pierce through Sandy from the south at over 50MPH. This will cause Sandy to acquire sub-tropical characteristics off the southeast US coast as we head towards Saturday. What does this mean?

1. Sandy’s wind field will expand, but initially weaken.
2. Precipitation will spread out farther from the center of Sandy.
3. Sandy will retain a “warm-core,” meaning that heat from the ocean will fuel thunderstorms with Sandy.
4. Sandy will also tap “baroclinic” sources of energy, meaning that it will begin to tap sources of energy from the jet-stream, changes in temperature, and will be more resilient to wind shear and lower sea surface temperatures.

These sub-tropical characterizes will become important, because as Sandy moves just off the southeast coast of the US Saturday and Sunday, it will not only tap the warm waters of the Gulf Stream, it will be near or under portions of three jet-streaks, shown above Sunday afternoon, that will ventilate the storm and provide plentiful lift. This will allow Sandy to maintain strong tropical storm or category 1 hurricane intensity through the weekend as it crawls northward off the southeast coast of the US. This much we know…it’s what happens after Sunday that may impact millions of people.

There are a number of players on the field for this potential major east coast storm. I have highlighted them on this evening’s GFS run, valid early Monday morning:

1. First and forecast, a very large –NAO block over the North Atlantic will force the flow over the northwest Atlantic to turn south-southeast, towards the north-northwest, and in combination with a blocking ocean low, will make it difficult for Sandy to escape out to sea.
2. The PNA over the Pacific and western N. America will go “positive,” which translates to ridging over the western US. This will force pieces of energy south over the northern Plains and Great Lakes, which will carve out a trough over the central US.
3. As the pieces of energy attempt to move east, they will be blocked by the massive –NAO block and will have no choice but to carve out a sharper, deeper, and negatively tilted trough. This will further aid in turning the steering currents towards the north-northwest over the northeastern US and far northwestern Atlantic.
4. Assuming the NAO block is as strong as currently projected on essentially all modeling, Sandy will have no choice but to be caught in these NNW steering currents, and come towards the Mid Atlantic or Northeast US coastline.

Let’s take a look at some of our forecast models and see if they track Sandy in this direction:

This afternoon’s UKMET, shown above, captures Sandy late, and pulls the storm towards Maine as a very strong storm system Tuesday morning.

This afternoon’s GGEM (Canadian) model is very similar, taking a very strong storm system into the Boston area Tuesday morning.

This afternoon’s European model is much faster with the whole scenario and takes a still sub-tropical (and potent) Sandy into the Delmarva Peninsula on MONDAY.

The most recent GFS runs have been interesting. They have shown Sandy trying to escape out to sea, only to be kept in the fray by the large blocking high over the N. Atlantic, and eventually pulled back towards the Canadian Maritimes and New England by the Great Lakes trough that is forced to deepen and pull Sandy back in due to the +PNA ridge over the western US and strong –NAO block.

Regardless of western US ridging and central US trough depth and strength, the operational models have all found a way to sooner or later sling-shot Sandy towards the northeastern US due to the extensive blocking not only preventing the cyclone from escaping, but also forcing a trough to sit and deepen over the Great Lakes and Ohio Valley, which also aids in pulling Sandy in. The GFS, European, and GGEM (Canadian) ensembles all more or less pull Sandy in as well:

GFS ensemble tracks:

European ensemble mean valid Tuesday morning:

GGEM (Canadian) ensembles valid Tuesday morning:

So, this essentially has the unmitigated impact of saying the chances of a major impact from a hybrid tropical-nor’easter are increasing by the day over the northeastern US. Due to the strength of the possible system, strong damaging winds, coastal flooding, heavy rains and potential snow in the mountains of western NY, PA, WV and western VA are all impacts that may be felt from Virginia northward towards Nova Scotia, including the I-95 corridor from Richmond through Boston as early as Monday.

However, it is important to note that this forecast is not quite a certainty:

-If Sandy manages to move farther east than forecast over the Bahamas, it may still be difficult for the block and central US trough to draw it towards the US coastline.

-If the western US pattern ends up significantly flatter than shown, the central US trough may have a harder time deepening enough to capture Sandy. This is the potentially the biggest risk for this big storm forecast.

-If the blocking in the Atlantic ends up weaker than currently projected, it would take an extremely deep trough to capture Sandy.

As we head towards the weekend, this forecast confidence will increase. However, right now it is important to state that a strong storm is likely to track into or very close to the Mid-Atlantic and Northeastern US, and Canadian Maritimes Monday into Tuesday. Whether or not the system actually strikes the US or Canada is still a bit uncertain, but impacts, especially to the marine community, appear increasingly likely.

Right now I’d put the odds at Sandy making landfall over the Mid Atlantic or Northeast at about 60%…the chances of it missing but passing within 100 miles of the coast at 30%…and the chances of it missing by more than 100 miles at 10%.

There is also a chance of a “weaker” nor’easter developing and impacting the Northeast if Sandy stays out to sea, due to the depth of the potential trough over the Great Lakes and Ohio Valley.

I will have another post tomorrow afternoon on this situation.