Concern builds as Tropical Storm Sandy provides forecasting headache

European forecast model showing TS Sandy, phased with a trough over the Central US, as a very strong storm at 933mb just off the New Jersey coast. Such a strong storm is almost unprecedented for our area.

Forecast models continue to waiver with track, intensity, and impacts on a potential major storm system developing off the East Coast at the end of this coming weekend and continuing into early next week. Earlier this week we outlined the potential in the long range for a system, but dismissed the hype which was already growing, with some outlets already talking about wind numbers and rain potential. Now that we are a few days closer to the event, and around 5 days away from potential impacts, we have some slightly increased confidence that the storm will at least have a chance of bringing impacts to our area. In essence, the storm isn’t just a “fantasy land” storm on forecast models anymore — it seems to be a legitimate possibility (not a certainty). So, we’ll lay out all details below with a full explanation of the event in which we try to answer any questions you may have in regards to the storm and it’s impact on the weather in the NYC area.

What’s the deal with this threat, why is it developing in the first place? Tropical Storm Sandy formed a few days ago in the Caribbean. The storm is strengthening a bit and is expected to drift northward over the next day or two, into the Northern Carribean and southwest Atlantic. The pattern over the United States and Northern Atlantic ocean is completely blocked up (i.e, the pattern is very amplified, with unusually deep troughs and ridges thanks to a very low NAO and AO index). This means the steering flow (what guides the tropical storm) will bring it out of the Caribbean and then on a north, possibly northeast heading for a while. More often than not, storms in this position head out to sea. But as you can see in this diagram, there is not much room for Sandy to do that. The ridge axis building over the top of her and to her east over the Atlantic remains strong. And to the west of her, over the Central US, you can see a trough beginning to dive south and east.

Keep reading for details, diagrams, and thoughts on the potential threat…

What are some of the model guidance solutions on the table? Model guidance has been all over the place, and that’s putting it mildly. But as meteorologists, to say that some of the solutions they’ve shown haven’t been slightly concerning would be a lie. The myriad of solutions boils down to a few options, all of them involving a potential phase of Tropical Storm Sandy with a trough and jet stream energy over the Central United States. As these two pieces interact, forecast models are struggling with the exact details. The models that phase them quickly and strongly bring Sandy from the Western Atlantic back towards the coast — making landfall somewhere in the Northeast US. Other forecast models have partial interaction (or a partial phase) and the phase occurs later, allowing Sandy to escape north and east for a while — but then as the phase occurs, she quickly is hooked back west towards the coast of Maine and New England. And finally, a few models still show no phase and interaction at all – with Sandy escaping harmlessly out to sea.

How in the world can a phase bring the storm back towards the coast in the other direction?The atmosphere is an amazing thing. One of the most important pieces that we discussed earlier is the

Some of the more impressive model guidance solutions with Sandy show the phase occurring underneath a strong blocking high to the north, forcing her to retrograde to the west towards the East Coast.

“Blocking” pattern, with a series of blocking high pressure systems building into the North Atlantic. In this case, some of the strongest model guidance solutions are picking up on a blocking high building into Newfoundland as Sandy moves up the east coast. At the same time, an energetic disturbance moving through the Central US is sliding towards Sandy. As the two phase, Sandy cannot escape to the north and east because of the block. Instead, the disturbance over the Central US slides underneath her, phases, and a newly strengthened Sandy is forced to retrograde back towards the coast. We can see this in the graphic to the right from last nights European model — which showed a 950mb low pressure system making landfall on the coast of New Jersey.

What are the chances that this phase happens? While we are hesitant to say it is “likely” to happen, the chances of a phase seem to be increasing with each model cycle as more forecast models pick up on the possibility and begin to trend towards the strong solutions. Still, the out to sea solution remains an equal possibility. It should be noted that phases of this magnitude with tropical systems that retrograde back towards the East Coast are incredibly rare and have happened only a few times in history (yes, in history). So, we won’t be at all surprised if things don’t come together as some of the models show to retrograde a powerful storm back towards the coast.

If the phase happens, and some of the strong model forecasts verify, what will happen? This will depend on the exact track of the storm system. Retrograding and phased systems are a completely different beast than your usual tropical cyclone (like Irene for example). When the phase occurs, the storm can rapidly strengthen to very low pressures and the wind field can expand. The storm then becomes a “Hybrid” and no longer a direct tropical storm (but we won’t get into that for now, to avoid confusion). Usually, the strongest tropical-related winds remain on the southeast side of the low pressure…which in this case would be out over the open waters. However, very powerful southeast gales would develop north and east of the storm, and very powerful downsloping northwest winds would develop on the west and southwest side.

The worst case scenario for the NJ/NYC area would be a landfall near or south of our area, which would place our area in position to not only receive strong storm surge and beach erosion, but very heavy rain and powerful southeast gales off the water. A less-concerning situation (but still powerful one) would be if the storm comes inland to our north. This would mean less storm surge, but heavy rain and strong northwest winds.

What do we do from here? Wait for us to give you an update, which will be giving each day going forward — and multiple times a day if it appears the likelihood of the storm occurring is still going up. It is very important to remember that the likelihood of the event occurring is still low. The event is still over 100 hours away — which is a lifetime for forecast models. Things can still change dramatically. So stay tuned here, and to your meteorology sources, for further updates. As soon as we get them, we’ll be passing them on to our Facebook and Twitter accounts.

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