Hurricane season heats up
Late last month, Hurricane Harvey inundated not only Houston, but, incredibly, an area the size of Maryland, with no location in that zone receiving less than 30 inches of rain. The hurricane initially came ashore as a Category 4 storm, the first U.S. landfalling Cat 4 since Charley in 2004.
Harvey had intensified rapidly as it moved toward the Texas coast. And the small town of Rockport, which is very near the coast, was ravaged as the storm made landfall. The highest wind gust, near Rockport, was 132 mph. But wind will not be what this storm is remembered for, it will be water — LOTS of water. The hurricane produced rainfall of simply staggering proportions. One location, east of Houston, measured more than 51 inches. This breaks the U.S. record for rain from a tropical weather system. The top four heaviest single site rain totals from a tropical cyclone in the Lower 48 are now Harvey at 51.88″ in Cedar Bayou, Texas; Amelia, with 48.00″ in 1978 in Medina, Texas; Easy, with 45.20″ in Yankeetown, Florida in 1950; and Claudette, with 45.00″ in 1979 in Alvin, Texas;
The astounding amount of rain that fell was especially problematic in Houston, where building over wetlands is allowed. The wetlands are what help to absorb the water and help move it along. But it you pave over them, you suddenly have impermeable surfaces, with FOUR FEET of rain falling on them. That fact exacerbated the flooding in Houston. This storm was able to dump these giant rain totals due to one simple fact: the steering currents collapsed. Just like going sailing and having the wind go calm. Harvey came ashore and simply stalled.
Meanwhile, at this writing, Aug. 31, there is a major hurricane in the far eastern Atlantic. Her name is Irma, and I fear she has bad intentions. The storm is about 2000 miles east of the windward islands of the Caribbean sea. There is a good chance that this storm will make it all the way across the Atlantic and ultimately have a potentially very significant US impact. By the time you read this, it will have likely already happened, though the storm may have recurved out to sea, though at this time there is not a lot of support for that scenario. There is also the possibility that it could encounter the high terrain of Hispaniola and Eastern Cuba, which could disrupt the circulation and weaken the storm somewhat.
Looking at three of the long range models, the Euro, the U.S. GFS, and the Canadian GEM, the European Model has a very intense hurricane near Key West, Florida around Sept. 10th, while the GFS takes a much different view of things, bringing Irma right into New England, with major impacts in Maine. The GEM solution is quite similar to the Euro. The timeline for the GFS and GEM are around the 10th of September as well. Since the paper comes out on the 13th, we should know what became of Irma by then, though if Irma gets into the Gulf of Mexico, landfall could be rather close to publication.
Back to Harvey for a moment; that hurricane underscores the point that rapid intensification of hurricanes close to the coast can and does occur. The storm deepened by 33 millibars in only 24 hours, all the way down to 27.70” (most home barometers only go down to 29.00”). When we meteorologists talk about winter storms “bombing out”, that term means a pressure drop of 24 millibars in 24 hours. So this hurricane exceeded the basic bomb definition by deepening (strengthening) by 9 millibars. That is quite significant. And it did so on final approach to landfall. Strengthening hurricanes at landfall tend to bring the stronger winds aloft (they are stronger because they are not slowed by friction with the ground) down to the surface quite readily, as evidenced by that 132 mph gust.
Now, given that I am in the media business, I always keep my eyes and ears open when significant storms are being covered.
Let me give you two examples of why it’s really important to understand exactly what it is you are hearing. The first example was a report that Harvey (while the rains were still going on in Houston), had dumped the equivalent of 2.5 Mississippi Rivers on Houston. That’s great, except for one problem, the flow rate of the Mississippi was never mentioned. So that stat is kind of meaningless. Were they talking average flow? Current flow? No telling.
The other item that caught my ear was the report that Harvey had dumped far more rain than Katrina did. The problem is, although they both produced disastrous flooding, they did so in two VERY different ways. While Harvey was a stalled tropical cyclone that unloaded tremendous rainfall, Katrina’s main damage was caused by storm surge and failed levees. So comparing rainfall between the two storms and concluding Harvey was worse on the basis alone is very misleading.
Ted Shapiro holds the Broadcast Seal of Approval from both the American Meteorological Society and the National Weather Association. An Alexandria, Va. native, he has been chief meteorologist at WAGM-TV since 2006. Email him at firstname.lastname@example.org.