|Above: Tropical Depression Harvey as seen by the GOES-16 satellite at 9:50 am EDT Wednesday, August 23, 2017. Image credit: NOAA/CIRA/RAMMB. NOAA’s GOES-16 satellite has not been declared operational and its data are preliminary and undergoing testing.|
A Hurricane Watch and Storm Surge Watch is posted for much of the coast of Texas, as Tropical Depression Harvey treks northwestwards at 9 mph over the Gulf of Mexico. An Air Force hurricane hunter plane found that Harvey had reorganized into a tropical cyclone on Wednesday morning, with a large, disorganized closed surface circulation and top winds to 35 mph. Harvey is expected to intensify into a strong tropical storm or Category 1 hurricane that will bring dangerous flooding rains in excess of 10” to Texas late this week. This Hurricane Watch is the first for any part of Texas since Hurricane Ike in 2008. Wednesday also marks the first time the National Hurricane Center (NHC) has issued an operational Storm Surge Watch. The product was not yet official in 2016, when prototype storm surge watch and warning graphics were produced for Hurricane Matthew along the southeast U.S. coast.
Conditions in the Gulf of Mexico on Wednesday morning were favorable to very favorable for intensification. Satellite images showed that Harvey was slowly developing, with a modest amount of heavy thunderstorm activity that was growing in areal coverage and organizing into low-level spiral bands. These thunderstorms were not very intense yet, with cloud tops that were relatively warm, indicating that they did not extend high into the atmosphere. There was some dry air on the west side of Harvey, but the storm was beginning to wrap spiral bands laden with moisture into this area, which should allow the storm to wall itself off from any dry air intrusions. High cirrus clouds streaming to the north and northeast of the center showed the presence of a respectable upper-level outflow channel, which was ventilating the storm and helping intensification. Wind shear was light to moderate, 5 - 15 knots, which is favorable for intensification. The atmosphere had a high mid-level relative humidity of 70%, and the ocean was very warm, with sea surface temperatures (SSTs) of 30°C (86°F.)
|Figure 1. Intensity forecasts made for Harvey at 8 am EDT Wednesday, August 23, 2017. Most of the intensity models predicted Harvey would be a strong tropical storm or Category 1 hurricane by Friday, when landfall is expected on the coast of Texas. Image credit: NOAA/NCEP/EMC.|
Intensity forecast for Harvey
The current favorable conditions for development will remain in place through Friday, according to the 12Z Wednesday run of the SHIPS model. Wind shear will remain light to moderate, and the atmosphere will remain moist. SSTs will remain near 30°C (86°F), and warm waters will extend to considerable depth, with a total ocean heat content of 50 – 80 kilojoules per square centimeter. This is moderately high, but not as high as what Hurricane Katrina encountered in 2005 when it exploded into Category 5 strength in the Gulf of Mexico. Harvey will likely intensify only slowly now through Thursday morning, as it struggles to overcome its current poor structure. After that time, it should be able to intensify more quickly, becoming a Category 1 hurricane by Friday afternoon, when it will be near the Texas coast. None of the intensity forecasts or ensemble forecasts are predicting that Harvey will intensify into a Category 2 hurricane, but I don’t think we can rule that out yet.
Track forecast for Harvey
The 0Z and 6Z Wednesday operational runs of our five best models for predicting tropical cyclone tracks—the GFS, European, HWRF, UKMET, and HMON (which replaced the GFDL model this year)—all predicted that Harvey would make landfall along the central Texas coast within about 100 miles of Corpus Christi on Friday afternoon or evening. However, the ridge of high pressure steering Harvey will weaken at that time, resulting in a collapse of the steering currents and a slow and erratic motion for the storm on Saturday and Sunday. As a result, Harvey is expected to remain stalled out over coastal Texas through the weekend, then move slowly eastwards, dumping torrential rains along the way. One particularly worrisome possibility is that the center of Harvey will re-emerge over the Gulf of Mexico early next week, allowing the storm to re-intensify and dump even more heavy rains along the coast. This was the solution of the 0Z Wednesday run of the European model and the 12Z Wednesday run of the GFS model, our two top models.
All of our top models are predicting that Harvey will dump more than ten inches of rain in coastal Texas near where the center makes landfall, and flooding from heavy rains is the main damaging threat from the storm. Near the center of Harvey, precipitable water (or PW, the amount of moisture in the atmosphere above a point on the ground) is projected to approach 3.00" as the storm nears the Texas coast. Only once has a PW reading of 3.00" been recorded in 63 years of radiosonde launches at Corpus Christi, TX.
NHC is predicting a peak storm surge of 4 - 6 feet on the Texas coast from Harvey. Storm surge expert Hal Needham gave his thoughts on the storm surge forecast in a Wednesday morning blog post.
|Figure 2. Five-day rainfall forecast for Harvey from the 6Z Wednesday, August 23, 2017 run of the HWRF model. The model predicts that most of the Texas coast will receive 8+ inches of rain, with more concentrated rains of 16+ inches (orange colors) focused inland from Austin to College Station. Image credit: NOAA/NCEP/EMC.|
|Figure 3. Five-day rainfall forecast for the period 8 am EDT Wednesday, August 23 through 8 pm Sunday, August 27, 2017 from the 6Z Wednesday, August 23, 2017 run of the GFS model. The model predicts that a swath of 10+ inches (pink colors) of rain will fall in a region between Houston and Corpus Christi, Texas.|
|Figure 4. Five-day rainfall forecast for the period 2 am EDT Wednesday, August 23 through 8 pm Sunday, August 27, 2017 from the 0Z Wednesday, August 23, 2017 run of the European model. The model predicts that a swath of 10+ inches (pink colors) of rain will fall in a region between Houston and Corpus Christi, Texas.|
Pesky 92L unlikely to develop; heavy rains still on tap for South Florida
The long-lived but weak tropical wave known as Invest 92L continues to loiter near Cuba, the Bahamas, and South Florida, but there is very little chance it will develop into a tropical cyclone before it’s eventually pushed out to sea toward the northeast later this week. Of the 70 total ensemble members from the 0Z Wednesday runs of the GFS and European models, none develop 92L, nor do any of our top operational models for cyclone development (GFS, Euro, and UKMET). However, the disorganized storminess around 92L could dump widespread 3-5” rains across South Florida, and localized flooding may develop over the next several days. In its 8:00 am tropical weather outlook, NHC gave 92L a 10% chance of developing into at least a tropical depression by Friday, and a 30% chance by Tuesday, at which point it should be well east of the U.S. East Coast.
|Figure 5. A man is splashed by the swell from Victoria Harbour during heavy winds and rain brought on by Typhoon Hato in Hong Kong on Wednesday, August 23, 2017. The powerful typhoon forced offices and schools to close and left flooded streets, shattered windows and hundreds of canceled flights in its wake. Image credit: Anthony Wallace/AFP/Getty Images.|
Typhoon Hato slams the Hong Kong area, killing at least 3
At least 3 deaths and more than 100 injuries have been reported in the vicinity of Hong Kong from Typhoon Hato. Already a quickly fading tropical storm as it moved further inland, Hato intensified from weak tropical storm to Category 2 hurricane strength in just 24 hours prior to landfall. Hong Kong’s most dire warning for tropical cyclones, signal #10 (denoting hurricane-force winds near sea level), was in place for five hours. It was the territory’s first signal #10 since Category 4-strength Typhoon Vicente passed roughly 60 miles southwest of Hong Kong in 2012.
According to the China Meteorological Administration, Hato came ashore at around 12:50 pm local time Wednesday (12:50 am EDT Wednesday) in the city of Zhuhai, just north of Macau and about 40 miles west of Hong Kong across the Pearl River delta. The Joint Typhoon Warning Center rated Hato at Category 3 strength near landfall, based on satellite-derived wind estimates as well as a 10-minute average wind speed of 92 knots (106 mph) recorded at the hillside Ngong Ping station on Hong Kong's Lantau Island. The 10-minute average of 92 kts corresponds to 1-minute top sustained winds of 100-105 kts (115-120 mph). Update: A gust of 217 km/hr (135 mph) was recorded in Taipa Grande, Macau, beating the previous record of 211 km/hr from Sept. 5, 1964. Thanks to Jerome Reynaud for this information.
The landfall trajectory put Hong Kong squarely in the dangerous right-hand side of the storm. According to weather.com and the Associated Press, the three deaths associated from Hato were all in densely populated Macau, where electricity was knocked out in most areas. The city's all-time strongest official wind gust, 152.3 kilometers per hour (95 mph), was recorded at the Macau-Hong Kong ferry pier just before power went out (thanks to Charlie Lok for this information).
|Figure 6. A tree felled by Typhoon Hato lies at a street in Hong Kong on Wednesday, Aug. 23, 2017. Image credit: AP Photo/Vincent Yu.|
Hato intensified so quickly that its storm surge was not as large as it could have been, but the storm arrived near high tide and widespread surge-related flooding was reported. AFP has dramatic YouTube video of Hato's storm surge. When the damage is combined with economic disruption, such as hundreds of canceled flights, the total cost of Hato to Hong Kong could run between $500 million and $1 billion US dollars, according to experts from Swiss Re and Chinese University cited in the South China Morning Post.
In advance of Hato, the hottest day in Hong Kong history
The strong subsidence (sinking air) ahead of Hato warmed the tropical air mass enough to produce a reading of 39.0°C (102.2°F) at Wetland Park in Hong Kong. If confirmed, this will be the hottest temperature ever recorded in Hong Kong, eclipsing the 37.9°C (100.2°F) notched at Happy Valley on Aug. 8, 2015, and July 9, 2016. Several other readings above 38°C were recorded across Hong Kong on Monday, according to world weather expert Maximiliano Herrera and climatologist Jérôme Reynaud.
Bob Henson co-wrote this post.