Every once in a while a storm comes along on the computer models we watch that causes us to pause. For many of these instances, such a storm will be seen once on the models, and the simply disappear. This can happen for a variety of reasons, but usually has to to with a problem with the models themselves – whether it be incorrect sampling of atmospheric data, or a defect in model physics. After all, the atmosphere dictates the weather, not computer models. They are at best imperfect tools sampling a continually changing atmosphere, and the farther down the road we go in time, the more uncertainty there is when it comes to using them to forecasting the weather.
So, when we first started talking about the idea of this storm which will have far reaching effects for many along the eastern seaboard later this weekend and early next week, we were somewhat skeptical as to whether such an event could happen. It truly is an unusual occurrence – most hurricanes taking the path Sandy is currently on are whisked out to sea, with only rough surf and maybe some clouds along the majority of the east coast; but not this one – not Sandy.
Why So Confident?
As stated above, most storms seen 7 or 8 days down the road can’t be taken that seriously. The difference here was because of two factors. First, we started seeing it repeatedly come up on the best medium range computer model we have – the European. Second, and more importantly, what the European was showing made sense synoptically. This was the reason why we were able to at least mention the possibility this could happen back on Sunday. You could mention every big storm you see on the models a week out, but you wouldn’t be seen as very credible. So, we liked what we saw – enough to warrant a “heads up” a few days ago. But what exactly was the European showing?
The above image is a snapshot of the upper atmosphere this coming Sunday from the European model run this afternoon. The first feature I want you to look at is the trough (blue U-shaped structure) advancing through the upper plains in the image above. This trough is a deep one – extending from Canada into the southern United States. The depth of this trough is a result of the upstream ridge (upside down U) over the west coast of the U.S. Those of you who have been following Jim Rinaldi’s videos over the last few days already know that. The degree of ridging out west allows for for a deeper trough downstream in the east. Normally, a trough like this would push a hurricane such as Sandy out to sea. However, an extreme blocking pattern to the east of the United States – manifested by a large high pressure sitting over Greenland (H) and a huge Oceanic storm (L) in the central Atlantic – is preventing that from happening. Both prevent an escape path for Sandy. As a result, she’s trapped along the east coast. All of this makes sense synoptically, and hence the higher than normal confidence that an event such as this might occur. Now, as this trough advances toward the coast this coming Sunday, Sandy will interact with, instead of being pushed out by, this trough. The trough will effectively “capture” Sandy – and pull it into the coast, making a rather abrupt left turn.
This scenario, while once felt somewhat unlikely, is supported by nearly every major model we have. The American model, or GFS, has finally come around to the European’s thinking, which is why just today you have been likely hearing about this storm on your local news. Most of the tropical model we have are similar as well, illustrating a left hook pathway to the Mid-Atlantic coast resulting from the capture scenario discussed above (image below courtesy of Weather Bell Analytics, LLC):
Why all the Hype?
This “capturing” of Sandy by the trough will transition the storm from a tropical hurricane (which is a WARM core system with it’s highest winds and greatest rains near the center of the system) to a hybrid type or extratropical storm (or COLD core system, with winds and rain much more spread out). Additionally, this trough interaction will cause rapid deepening, meaning the pressure will fall quickly. This is important because as the pressure falls, the winds with the storm pick up, because of the pressure contrast with an area of higher pressure to its north.
In the image above from the European, also courtesy of Weatherbell, we see Sandy just north of her current position. The colors represent wind speeds in knots about a mile up in the atmosphere. Typically, winds at the ground surface are slightly lower. Her pressure at this point in time is around 953 mb, and the highest winds (just over 100 knots, or around 105 mph), are concentrated around the center of the storm – a warm core or tropical system.
Now notice how Sandy has changed as she’s moved north in the image above – she’s losing her tropical characteristics as she interacts with that trough, and is making the transition to an extratropical, or cold core system. She’s deepening, as her pressure has dropped to 932 mb, and the wind field has greatly expanded – sustained winds of greater than 45 miles per hour extend for over 1000 miles, while winds of greater than 65 mph, with gusts to 90 mph, extend for over 500 miles. Granted this is about a mile up in the atmosphere, so winds will be slightly lower at the surface, but you get the picture – this will truly be a widespread wind event as it roars ashore.
In addition, because of the blocking to the east, Sandy will be a very slow mover, affecting the area from around the Sunday evening through Wednesday morning time frame (give or take 6-12 hours), with the height of the storm likely Monday afternoon through Tuesday evening, based on the current model runs. This will result in a tremendous amount of rainfall, as shown below, courtesy of the Hydrometeorological Prediction Center (HPC):
As you can see, a general 5-10 inches, with local amounts slightly higher, are likely. Couple that rain, which will soften the ground, and sustained winds of 40-70 mph, with gusts to 90, and you have a recipe for extensive tree and property damage, flooding, and widespread power outages.
What to expect:
This setup is truly a historical, once in a hundred year event, if it comes to fruition in the way that nearly all model guidance suggest. As a result, after a cloudy and rather rainy and breezy day on Sunday, conditions will deteriorate rather rapidly from southeast to northwest across the area starting sometime on Monday, and we can expect, in general across the entire area:
- high winds sustained 40-60mph with higher gusts possible at the height inland, 60-80mph+ along the coastal areas… contributing to property, tree, and power line damage.
- flooding rainfall on the order of 6-12″ in a 24-36 hour period, which will cause flash flooding, as well as possibly historic river flooding.
- widespread power outages that can last for days if not weeks.
- similar effects as Hurricane Irene last year for inland residents, if not worse, including all of eastern PA andNew Jersey.
- massive beach erosion and damage, along with destruction to poorly constructed homes and businesses along shore points
- coastal flooding and storm surge for a landfalling tropical system combined with a full moon, which enhances the wave action even further.
Because this storm will be so large, with such a massive windfield, the exact point at which the center crosses the coast won’t be as important if this was a purely tropical system especially away from the coast. However, coastal areas that lie to the north of the storms center will have a much greater coastal flooding threat, due to the wind direction coming off the ocean for a prolnged period of time, over two or three high tidal cycles and during a full moon:
The center may make landfall anywhere from the Delmarva Peninsula (Ocean City, MD area) to eastern Long Island (Islip), with somewhere along the New Jersey coastline being most likely at the moment.
Finally, for those of you who would’ve like to have seen snow with this event, you’ll have to head pretty far west. The mountains of West Virginia and southwest PA seem to be your best bet for a foot or perhaps much more of heavy wet snow.
So, that’s it for now. Sorry for the long read… we’ll have more as we get closer to the storm.