Hurricane Sandy came ashore on the evening of Monday, 29 October, just south of Atlantic City, NJ. It was definitely a unique, “storm of the age” kind of event as it transitioned from a tropical system while it was offshore to a non-tropical (or “extratropical,” as meteorologists like to say) super storm as it crossed the coast and moved inland. The combination of tropical energy (in the form of moisture and heat) and winter storm energy (in the form of jet stream winds and a sharp temperature gradient between very cold air to its west and warm, tropical air to its east) led to the lowest sea level (barometric) pressure ever recorded off the northeastern U.S. Just before landfall, a hurricane hunter aircraft measured a sea level pressure of 940 mb (27.76″) in the center of Sandy and when it came ashore a weather observing site near Atlantic City measured a pressure just above 945 mb (27.91″). Both values broke the previous record of 946 mb (27.94″) measured at the Bellport Coast Guard Station on Long Island during the Great New England Hurricane of September 1938. The surface map showing contours of sea level pressure looked like this just as Sandy was crossing the coast and transitioning to an extratropical super storm on the evening of Monday, 29 October:
Even more remarkable than the storm itself was the accuracy of the weather model forecasts of the storm as much as 8 1/2 days in advance. The forecasts showed how much progress has been made in weather forecasting as computers have become more and more powerful. The model from the European Center for Medium Range Weather Forecasts (ECMWF) gave a surprisingly accurate depiction for the location and intensity of Sandy at landfall from both its morning and evening runs on Sunday, October 21. That’s 8 days before the storm hit New Jersey and a couple days before it had even become very well organized in the Caribbean. Truly incredible! Here is what the model predicted on Sunday evening, 21 October:
Unfortunately, most of the stories about this excellent forecast have focused on how much better the European model forecast was with Sandy compared to the output from the primary American global weather forecast model, the Global Forecast System (or GFS), at the same time. For instance, see these stories:
The forecast from the GFS model was noticeably poorer as evidenced by this map showing its output from the same time as the ECMWF model run on October 21:
While the GFS did develop the storm, it clearly was taking it out to sea into the central Atlantic and entirely missed the intensity of the cold air diving into the eastern United States and its interaction with the storm. The GFS needed a few more days before it finally started bringing the storm closer to the coast and had it deepen when encountering the cold trough in the eastern U.S. It definitely was the poorer model for this storm.
On average, the ECMWF model beats the GFS, as it has for decades, because the Europeans have focused their energies and resources on improving one thing: medium-range weather forecasting. They have a a higher resolution model with a better scheme for ingesting initial observations and satellite data which runs on a more powerful computer system than what the U.S. has. The U.S. agency containing the National Weather Service and responsible for developing weather models, NOAA, has limited resources and must allocate them across a broad spectrum of needs to protect life and property in the U.S.—the needs range from short-term forecasting of severe weather events, such as tornadoes, to the long-term issues such as summertime drought or wintertime cold. And, while the ECMWF model is better on average than the GFS, it is far from perfect. Just a month before Sandy, the ECMWF model was consistently trying to bring another storm, hurricane Nadine, into southern Europe as a destructive extratropical storm. The GFS, on the other hand, generally kept Nadine out at sea with a looping track south and southwest of the Azores. The GFS was the much better model in this case as the storm got nowhere near southern Europe and threatened the Azores twice.
Back to Sandy, for a moment—I would hate for us to get mired in a controversy about ECMWF vs. GFS and miss the incredible skill shown by the ECMWF model more than eight days in advance. Billions of dollars will be saved or losses prevented if such forecast skill becomes commonplace. This forecast gives us an example of what is possible as we continue to advance the science of meteorological modeling. Now if we can just find the money to continue funding the research in this all-important area…
Here’s a second interesting view of Isaac, now well inland over northern Louisiana. This time it’s an animation of the surface wind field around Isaac at 11:00 a.m. CDT. The creative and wonderfully imaginative people at Google’s Big Picture data visualization group created the surface wind visualization site (http://hint.fm/wind/index.html) which was the source for this animation. The hypnotic swirl is mesmerizing.
Here’s an interesting 22-hour radar loop from the New Orleans NEXRAD site (the National Weather Service’s weather radar site) showing hurricane Isaac very slowly crossing the coast and heading inland:
Quiz of the day: What do all the following symbols depict?
Answer: Well, you might think each is a different way to represent a surface low pressure system like what you might see on your favorite daily weather map (for example, from The Weather Channel). [Note: Northern Hemisphere only. Down south of the Equator, the winds spin clockwise around these things.] It turns out, though, one is an outlier. This symbol:
might look vaguely familiar—for it is the “History” button from the iPad Google Search App (Note: If you don’t have an iPad or the Google Search App, you can see a screen shot of what I’m talking about here. The History icon is at the lower-left of the main screen.) It looks like this:
Whoa! Where did that come from? And, when did the symbol for a low pressure system become an image to represent search history? My best guess is one of the Google button-icon graphics artists secretly is a fanatical weather weenie, and this icon was a perfect way to subtly expose that interest. Very clever! Even more so, I bet if I were to hop on a plane to go taste a Barossa Valley Shiraz, the app would automagically respond to my hemispheric orientation and convert the icon to: