Friday, October 26, 2012
Hi everybody. I just finished a hectic week. Two midterms. A ton of homework. I even finished an entire package of Omega-Three gummy bears from Costco. Maybe it’s a stress thing.
But honestly, the thing that made this week the most hectic, was this.
This, my friends, is a wonderful satellite image that shows the evolution of Hurricane Sandy. I’m not sure where the loop starts, but I believe it starts at 22:15 UTC October 24 and ends at 18:46 UTC October 26. The loop is a composite image of very high resolution infrared imagery at night and visible imagery during the day. On October 24, when the storm legitimately looked like a hurricane, there were some serious signs that it could impact the U.S. One model (the ECMWF) was bringing it straight into New York City as a 950 mb megastorm. The other model, the GFS (which is the model I use the most and the model that the UW uses to initialize their WRF forecasts), showed the storm weakening and harmlessly moving out to sea. One thing I like to do on this blog is give people advanced warnings of historic storms that may be in the making. However, I just didn’t have the time to do so.
I did, however, let some East Coast friends know that a potentially historic storm could be heading their way. I told them it probably wouldn’t happen, but that it might, and it could be very serious if it does.
Before I show you the models and give you all a heart attack, let’s look at some of the atmospheric and oceanographic conditions that have contributed to the formation of this once-in-a-lifetime storm. And no, folks, I am not exaggerating.
For any tropical system to form, water temperatures need to be hot. Take a look at the pictures above. The top one is from “My Fox Hurricane.” And no, these water temperatures are not conservative estimates (props to all of you who got that joke.) They are real measurements from buoys. The picture below is taken from satellites, and it shows the sea surface temperatures throughout the Western Atlantic basin. They are hot. The standard “rule of thumb” for hurricane formation is that they need water that is greater than 80 degrees Fahrenheit do a depth of at least 150 feet. If you take a look at the SSTs measured by the buoys above, they are well above 80 degrees. And the satellite picture shows that these warm temperatures extend up the East Coast thanks to the Gulf Stream. The water temperatures off the northern East Coast are up five degrees Fahrenheit above normal, and Sandy is predicted to perfectly follow the Gulf Stream, picking up more steam (literally) and becoming more powerful in the process. See the picture below for a map of the SST anomalies as of October 25. This picture was taken from NOAA’s Office of Satellite Data Processing and Distribution.
But if you look at the last satellite image (below), the storm doesn’t really look like a hurricane. It doesn’t have an “eye” and is rather asymmetrical. It looks more like a mid-latitude cyclone than a hurricane.
Why is this? The storm is clearly over warm enough waters to be a hurricane. The answer, my fellow meteorological connoisseurs, is because of strong horizontal temperature gradients in the mid-latitudes. In terms of temperature, the transition from autumn to winter is much quicker than the transition from winter to spring. Even though the air temperatures to the northwest of this storm aren’t incredibly cold, they contrast sharply with the tropically influenced temperatures to the southeast. This temperature difference forms fronts. Just a little meteorological 101 here… fronts are boundaries between two different types of air masses. And the air masses on either side of this storm are very different indeed.
I have a detailed blog post I wrote last (2011) summer about the differences in tropical storms vs. extratropical storms. I highly recommend reading it here
This is clearly shown in the models. Take a look at the 18z GFS 500mb vorticity and height model below. Don’t worry if you don’t know what it means… just follow my lead and look at what I’m telling you to look at.
See that super red thingy over the Western Atlantic? That’s Sandy. Pretty obvious. But now, look to the northwest of Sandy. See that big “trough” in the 500mb height level? It looks like a V. That represents colder air coming down from Canada. This chart is valid 00 UTC Monday October 29, and is 54 hours out.
And then, something extraordinary happens.
This image is valid 30 hours later, at 06 UTC on the 30th. Look at how far the trough has come, and look at how there is the slim line stretching all the way up into northern Canada connected to Sandy. That line represents increased vorticity at the 500mb level, which is simply defined as “the tendency for an air parcel to spin.” High levels of vorticity are associated with foul weather and moving air parcels. What this diagram shows is how much cold, Canadian air Sandy is drawing into itself. This contrast between air masses will help transition Sandy from a tropical cyclone, which is built on a uniform mass of warm humid air, to a mid latitude cyclone, which gets its strength from horizontal temperature and humidity differences. As you can see in this picture, the differences will be absolutely massive, and this will lead to the formation of an extremely powerful storm.
I’ve got to head to a band practice now, but I will be extremely active on this blog and I will post more model forecasts in the future. As of now, it looks like a 940-950 mb cyclone will hit somewhere between North Carolina and Massachusetts, with 80 mph sustained winds on the coast and a very large gale-force wind field that will extend for hundreds of miles. Areas to the northwest of the storm will experience blizzard conditions, and areas along the immediate coast will experience a storm surge of at least 10 feet, nearly a foot of rain, and insane amounts of wind. This storm has already killed 42 people, and it will inevitably kill some more and cause billions of dollars of damage.
If you have any relatives or know anybody on the East Coast, WARN THEM NOW and let them know that the strongest extratropical storm they may ever see will be upon them in several days.