Harvey Slams Ashore in Texas; Catastrophic Flood Threat Still to Come

Hurricane Harvey ripped into the central Texas coast as a Category 4 storm late Friday night. Harvey’s center made landfall just before 10 pm CDT about 4 miles east of Rockport with top sustained winds estimated by the National Hurricane Center (NHC) at 130 mph, which is minimal Category 4 strength. A parachute-borne dropsonde instrument package deposited by a Hurricane Hunter flight around 10 pm found that Harvey’s pressure had dropped to 938 millibars. Since the surface winds at this point were 10 knots, the dropsonde did not sample the true center of the storm, so the actual central pressure is probably a millibar or so lower. Microwave measurements from the aircraft estimated that surface winds were 111 knots (128 mph).

Harvey is the first Category 4 storm to strike the U.S. since Hurricane Charley in 2004, and the first major hurricane (defined as Category 3 or stronger at landfall) since Hurricane Wilma in 2005. Several more recent hurricanes that reached land without Category 3 winds were still large and powerful enough to deliver a devastating storm surge, including Ike (2008) and Sandy (2012).

Figure 1. NWS/NEXRAD radar loop of Hurricane Harvey as its western eyewall pushed ashore between 0142Z and 0242Z Saturday (8:42 and 9:42 pm CDT Friday night).

Harvey’s main eyewall ground slowly through the regional resort town of Rockport late Friday as the storm crept onshore, moving northwest at about 7 mph. Rockport mayor pro tem Patrick Rios expressed concern on Friday afternoon that several thousand of the city’s 9,500 residents may have stayed in place, and he said the city was preparing for as many as 10 fatalities. Storm chaser Josh Morgerman reported structural damage, including at the Rockport hotel where he was positioned. The eyewall also sat squarely over Aransas Pass and nearby Port Aransas, a few miles southwest of Rockport. NHC reported that a Texas Coastal Ocean Observing Station at Aransas Pass recorded sustained winds of 111 mph with gusts to 131 mph, and at 9:00 pm CDT, Port Aransas reported sustained winds of 95 mph with gusts to 109 mph.

Rockport and Fulton, TX have been hit repeatedly by several smaller scale vortices within the eye wall. pic.twitter.com/0Iox9PheOd

— Taylor Trogdon (@TTrogdon) August 26, 2017

Only about 15 miles west of Port Aransas, the city of Corpus Christi got blasted with high winds late Friday night, and more than 40,000 power outages were reported across the surrounding county. However, Corpus Christi narrowly escaped a brush with the eyewall—which is especially fortunate given that mandatory evacuation orders were never issued for the city. Meanwhile, in the other direction from Harvey’s core, a cluster of intense thunderstorms battered western parts of the Houston area, with at least one tornadic circulation evident on radar late Friday night.

Harvey’s storm surge was still increasing late Friday night. Here are the highest totals as of 10 pm CDT:

Port Aransas, 5.3’
Port Lavaca, 4.8’
Corpus Christi, 4.4’

Figure 2. Enhanced infrared satellite image of Harvey as of 1652Z (11:52 am CDT) Friday, August 25, 2017. Image credit: RAMMB / CIRA@CSU.

Light upper-level winds gave Harvey room to grow

Harvey is the first Category 4 hurricane to be tracked with the new GOES-16 satellite, launched in November 2015. Imagery from GOES-16 showed Harvey to be a remarkably well-structured  hurricane as it approached land, with outflow evident in all directions. This symmetry was partly the result of a lack of strong upper-level winds close to Harvey that would have distorted the storm’s outflow, according to Greg Tripoli (University of Wisconsin–Madison). Tripoli and colleagues at UW created maps of potential vorticity near the top of the hurricane, where outflow is maximized. On Friday, he said, “There was an annular ring of zero potential vorticity surrounding [Harvey], probably produced by the storm over the last few days but not carried away from the storm by the environment. This suggests that there was little or no environmental resistance to outflow.” Sometimes a strong upper-level jet close to a hurricane can provide an outflow channel, reducing the resistance to outflow in that direction, but this wasn’t necessary today, said Tripoli: “Harvey simply has no resistance in all directions!”

Figure 3. WU depiction of NHC 5-day outlook for Hurricane Harvey as of 10:00 pm CDT Friday, August 25. The concentric circles indicate how slowly Harvey is expected to move over the next few days.

Catastrophic inland flooding possible as Harvey lingers for days

As of late Friday night, there was no major change to the prognosis for extreme rainfall and potentially devastating flooding from Harvey from this weekend through at least part of next week. Harvey's core and its eye remained impressively well structured even as it moved almost completely onshore late Friday night, and the hurricane may take longer than usual to spin down given its expected proximity to the coast. The 10 pm CDT forecast from NHC slows Harvey to a crawl on Saturday, still at hurricane strength, then keeps the storm within about 100 miles of its landfall location as a tropical storm from Sunday until Wednesday, when it is predicted to creep toward Houston. Because steering currents will be so weak and difficult to analyze over the next several days, various models have different tracks for Harvey within Texas, but they generally agree any motion will be very slow to occur.

Figure 4.  Precipitation outlook from NOAA/NWS for the period from Friday evening, August 25, to Wednesday evening, August 30, 2017. If Harvey remains stalled in Texas beyond Wednesday, additional rain can be expected. Image credit: NOAA/NWS/WPC and NHC.

The latest 5-day precipitation outlook from the NOAA/NWS Weather Prediction Center (see Figure 4) projects that an area larger than the state of Massachusetts, including Houston and Galveston, can expect more than 20” of rain between now and Wednesday. Amounts of more than 10” cover an even larger area, extending into parts of the Austin-San Antonio urban corridor and including Corpus Christi and Beaumont-Port Arthur, TX, and Lake Charles, LA. Local rainfall amounts could be far above 20", as suggested by multiple models. Very serious flooding over the next several days can be expected well inland from the areas immediately at risk from Harvey’s initial landfall and storm surge. For example, Austin/San Antonio NWS office notes the potential for life-threatening flash flooding, especially from San Antonio south and east.

There is virtually no precedent for such a slow-moving system maintaining at least tropical storm strength along the Texas coast for five days. Whether or not Harvey maintains tropical storm strength will depend in large part on how close its center drifts to the coast. Regardless of its status, Harvey’s slow movement and huge amounts of moisture will lead to enormous rainfall and will likely produce vast areas of flooding. Official NHC storm surge guidance, which extends out to 72 hours (currently through Monday evening, August 28), is not intended to depict the longer-term threat—perhaps extending well beyond Monday—posed just inland by huge amounts of freshwater blocked by the surge from flowing to the sea. The multi-day onshore flow and storm surge coupled with extreme inland rainfall would push enormous amounts of water from several directions into Galveston Bay and the Houston Ship Channel. The result could be widespread, possibly unprecedented flooding early next week across southeastern parts of the Houston metropolitan area along and near the bay and the ship channel. See our post from Friday morning for more on this serious threat.

We’ll be back with our next update on Saturday.

Jeff Masters contributed to this post.

Figure 5. The largest storm-total rainfall on record associated with tropical cyclones (including tropical depressions, tropical storms, and hurricanes) in each of the 48 contiguous U.S. states, 1950-2012. Image credit: Tweeted by NWS/Jackson, KY.