Tropical Storm Nicholas, which formed Sunday morning in the southwest Gulf of Mexico, is on track to be a prolonged and prodigious rainmaker for much of coastal Texas and Louisiana. At midday Sunday, a tropical storm warning extended along the western Gulf of Mexico coast from Barra el Mezquital, Mexico, to Port Aransas, Texas. A tropical storm watch extended northward to High Island, Texas, including Galveston Bay. Flash flood watches included the entire coastal region of Texas and southwest Louisiana, including Houston.

At 2 p.m. EDT Sunday, the center of newly formed Nicholas was located in the Bay of Campeche about 550 miles south of Houston. Nicholas was moving north-northwest at roughly 15 mph with top sustained winds of 40 mph, as confirmed by an Air Force hurricane-hunter flight on Sunday morning. Satellite imagery showed a disorganized but expanding field of showers and thunderstorms (convection), mostly toward the north side of Nicholas.

Figure 1. Official NHC forecast for Nicholas as of 11 a.m. EDT Sunday, September 12, 2021. (Image credit: NHC)

Forecast for Nicholas

Forecast models are consistent in bringing Nicholas on a gently arcing track from north-northwest to north-northeast over the next several days. By late Monday, Nicholas is expected to be paralleling the south Texas coast, and on Tuesday it is expected to slide ashore over the central Texas coast.

The main track question with Nicholas is how far east it goes on its path through the Gulf. Because of Nicholas’s angle of approach, a minor deviation in the projected track could make a major difference in Nicholas’s strength. Only a slight eastward departure could lead to a landfall further north, allowing more of Nicholas’s core circulation to remain over water and which would give it more chance of strengthening. Sea surface temperatures over the northwest Gulf of around 30 degrees Celsius (86°F) and a very moist mid-level atmosphere (relative humidity around 80%) will support intensification through Monday, and wind shear of around 15 knots should not be too much of an impediment.

At midday Sunday, the National Hurricane Center (NHC) was predicting Nicholas’s top sustained winds will reach 65 mph prior to an expected landfall. If Nicholas’s track ends up displaced a bit to the east, it could approach hurricane strength before reaching Texas on Tuesday, as suggested by the 12Z Sunday run of the HWRF model. Conversely, a more westward track displacement could bring Nicholas just inland over South Texas on Monday as a weak tropical storm.

Storm surge from Nicholas will hinge on its exact track and landfall location. Catastrophic surge is not expected given Nicholas’s modest strength and its angle of approach. NHC is predicting a possible 2- to 4-foot surge over most of the Texas coast, including Galveston Bay. Similarly, most of Nicholas’s strongest winds will remain offshore, but tropical storm-force winds capable of bringing down trees and power lines could occur with Nicholas as it moves inland around Tuesday.

Rainfall the big concern with Nicholas

Regardless of how much Nicholas strengthens, its most potent impact likely will be its rainmaking ability. Nicholas is hauling a large swath of tropical moisture toward the Texas and Louisiana coast, and it will interact with a frontal zone expected to sharpen near the coast. Moreover, upper-level steering currents are expected to weaken over the next several days as Nicholas reaches the upper Texas coast. By later in the week, Nicholas may be a tropical depression inching across southeast Texas at 5 mph or less. All these factors will enhance the storm’s rainmaking ability.

Figure 2. 7-day rainfall forecast from 8 a.m. EDT Sunday, September 12, 2021, to Sunday, September 19. Specifics of timing and location will vary depending on the track of Nicholas. Localized amounts may be higher than shown here. (Image credit: NOAA/NWS Weather Prediction Center)

The heaviest rains from Nicholas will most likely be close to the storm’s eventual path into southeast Texas and southwest Louisiana. Localized rainfall amounts along Nicholas’s immediate track could hit 10 to 15 inches or more over the next several days. Other pockets of 10 inches or more can be expected in bands of showers and thunderstorms (convection) that may be scattered along the coast within the storm’s broader circulation.

Southeast Texas and southwest Louisiana are all too familiar with colossal rainfall amounts, as evidenced by the record-smashing 30- to 60-inch-plus totals that hit the region during 2017’s Hurricane Harvey. Nicholas is expected to be much weaker than Harvey at landfall, but it may exhibit a similar slow pace of post-landfall motion, so the potential for flooding will need to be watched closely.

Further east, there could be several inches of rain across southeast Louisiana, where Ida struck, but the heaviest amounts likely will remain west of this region.

Nicholas the fifth-earliest 14th storm since 1966

Nicholas’s formation date of September 12 comes very close to the midway point of hurricane season, which is typically around September 10. The Atlantic averaged 14 named storms per year during the 1991-2020 period, so we’ve already had a full year’s worth of storms with half the season to go. According to Phil Klotzbach of Colorado State University, only four other seasons since accurate satellite records began in 1966 have had as many as 14 named storms by September 12: 2020, 2012, 2011, and 2005.

Another rainmaker could approach the mid-Atlantic late this week

NHC was monitoring four other areas of interest in the Atlantic, any one of which could develop into a tropical depression or tropical storm this week. The system of most immediate concern is an area of disturbed weather over the Bahamas. Multiple runs of the GFS model have been developing this area as it heads northwest toward the Carolinas, though the system remains weak in all but a few GFS ensemble members and is absent from the vast majority of European ensemble members. Regardless of any tropical development, this disturbance could bring heavy rains to the Carolinas as early as Thursday, and rainfall may extend northward along the mid-Atlantic coast by late in the week or the weekend. As of 2 p.m. EDT Sunday, NHC gave this system 2-day and 5-day odds of development of 0% and 50%, respectively.

A tropical wave expected to move off the coast of Africa on Monday has strong model support for development, with NHC givig it 2-day and 5-day odds of development of 20% and 60%, respectively. This system will move westward across the tropical Atlantic, and it will be a threat to develop into a long-track Cape Verdes-type hurricane by late in the week. It is too early to tell if this system will recurve out to sea or potentially be a threat to the Caribbean and/or the U.S. in a few days.

Figure 3. Radar image of Typhoon Chanthu as it skirted the northeast end of Taiwan at 13:10 local time, September 12, 2021. (Image credit: Taiwan Central Weather Bureau)

Chanthu skirts Taiwan, heads towards Shanghai

After making landfall as a category 5 storm on the municipality of Ivana on Philippines’ Batan Island (population 13,000) at 0Z Saturday, September 11, Typhoon Chanthu spared Taiwan a direct hit, passing about 40 miles from the northeast end of the island on Saturday night. Chanthu brought heavy rains of up to five inches to Taiwan, but no major flooding has been reported thus far.

Dry air, increased wind shear, and decreasing ocean heat content have worked together to significantly weaken Chanthu, which was a category 2 storm with 110 mph winds headed north-northeast at 12 mph at 11 a.m. EDT Sunday. Further weakening is expected this week as Chanthu heads north toward China and slows its forward speed to 5-10 mph. On Monday through Wednesday, Chanthu is expected to bring very heavy rains to the Chinese megacity of Shanghai, which has already suffered from one other slow-moving typhoon this year: Typhoon In-fa caused $1.1 billion in damage July 22-26, dumping 37.4 inches of rain at Dalan Town, China.

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Bob Henson

Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...

Jeff Masters

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...