Heavy Rain, Strong Winds for Southeast U.S. Coast; TD 16-E Remnants Drench Omaha

An elongated area of disturbed weather along the Southeast U.S. coast from Georgia to North Carolina is bringing heavy rains to the coast, but is not a danger to become a tropical storm due to high wind shear of 30 - 40 knots. However, the storm's impact will be similar to that of a weak tropical storm, with heavy rains of up to 5" predicted along the North Carolina coast and strong onshore winds that will bring battering waves and flooding. Long range radar out of Charleston, SC is showing that an area of low pressure (formerly tracked as Invest 97L by NHC) is moving slowly towards the coast, bringing heavy rain. This low will move ashore over South Carolina by Friday. Farther to the north, the Outer Banks of North Carolina are under a High Surf Advisory for waves of 6 - 9 feet; a storm surge of 2 - 3 feet is expected to potentially cause overwash on the only road connecting the vulnerable barrier islands to the mainland, U.S. Highway 12.


Figure 1. Latest regional radar image for the Southeast U.S.


TD 16E remnants help drench Omaha
After bringing heavy rains to parts of the Arizona/New Mexico desert, the remnants of Tropical Depression 16E joined forces with an upper-level trough to produce one of the wettest days in the history of the Omaha, Nebraska area. The heaviest complex of thunderstorms—which developed just northeast of the approaching 16E remnants--parked across the Missouri Valley of Nebraska and Iowa, dumping huge amounts of rain in a fairly localized area over the early morning hours on Wednesday. Omaha’s Eppley Airfield picked up 3.75” in just four hours, and a WU personal weather station in Council Bluffs, Iowa, notched 7.88” in six hours. Lighter showers and storms persisted into Thursday morning along the same corridor, while upper-level energy associated with the 16E remnants evolved into an apparent mesoscale convective vortex across Nebraska and South Dakota. Using NOAA's HYSPLIT trajectory model, which analyzes the source of air masses at various heights, weather.com's Nick Wiltgen found that the upper-level moisture associated with these rains was clearly related to TD 16E, while lower-level moisture came mainly from the Gulf of Mexico and other sources.

Omaha’s calendar-day rainfall on Wednesday totaled 5.74”, making it the fourth-wettest day in the city’s weather history going back to 1871. Impressive amounts of rain also fell in north-central Nebraska and south-central South Dakota, where CoCoRaHS reports ranged as high as 5.95” in southeast Lyman County, SD. It was still raining lightly in Omaha on Thursday morning, with another 0.21” falling between midnight to 10 am CDT. The wettest calendar day on record in Omaha was August 7, 1999, when 6.46” fell, while the total of 10.48” on August 6-7 was the city’s biggest soaking in any 24-hour period. There were no reports of major flood damage in the Omaha/Council Bluffs area on Wednesday, although many residents had to deal with partially submerged vehicles.


Figure 2. The Omaha-area NWS NEXRAD radar at 6:58 am Wednesday, September 22, shows an intense complex of thunderstorms focused across the Omaha metropolitan area. Image credit: NCAR/RAL Real-Time Weather Data.


Figure 3. Rainfall on Wednesday, September 22, at a wunderground PWS in Council Bluffs, Iowa, totaled 8.28”, with another 0.22” (not shown) falling on Thursday as of 10:30 am CDT.


Figure 4. In Ottosen, Iowa, intense thunderstorms extending toward central Iowa from the Missouri Valley on Wednesday morning, September 23, 2015, led to what wunderphotographer Zeman88 dubbed “easily the best sunrise I have ever seen on my way to work as it filled nearly the entire eastern sky.” Image credit: wunderphotographer zeman88.



Figure 5. Thick clouds from heavy thunderstorms to the southeast of Fort Pierre, SD, sweep into the area at sunset on Wednesday, September 23, 2015. Image credit: wunderphotographer Tandistar.

Gulf of Mexico storm next week
A southerly flow of moisture from the Western Caribbean and Southern Gulf of Mexico towards the northern Gulf of Mexico will develop this weekend, and our top three models for forecasting tropical cyclone genesis are showing an area of low pressure capable of becoming a tropical or subtropical depression forming near Mexico's Yucatan Peninsula on Sunday evening. An upper-level trough of low pressure over the Western Gulf of Mexico next week will likely bring high wind shear to the Gulf, limiting the potential for any system in the Gulf to strengthen. The models are currently predicting that this system will get pulled northwards to affect the U.S. coast from Louisiana to the Florida Panhandle by Tuesday. Regardless of whether or not this system develops into a named storm or not, the central Gulf Coast can expect heavy rains from Monday night through Wednesday. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 0% and 20%, respectively.

Ida weakens to a tropical depression
Persistent wind shear has taken its toll on Ida, which is now a tropical depression wandering slowly over the Central Atlantic, well away from any land areas. Satellite images on Wednesday morning showed Ida's center of circulation partially exposed to view by high wind shear, and all of Ida's heavy thunderstorms limited to the southeast side of the center. Ida will continue to move slowly in a region of weak steering currents for the next five days, and it is possible that high wind shear will destroy the storm by early next week, as suggested by Thursday morning runs of the GFS model.


Figure 6. Latest satellite image of Tropical Storm Ida.

Tropical Storm Dujuan a threat to Japan, China, and Taiwan
Tropical Storm Dujuan, located about 620 miles southeast of Okinawa in Japan's Ryukyu Islands at 8 am EDT Thursday, appears destined to become a major typhoon that will threaten Japan, China, and Taiwan early next week. Wind shear has dropped over the past day, and satellite loops on Thursday morning showed the storm was larger and more organized, with increasing spiral banding, the beginnings of an eyewall, and upper-level outflow channels opening to both the north and the south. The Joint Typhoon Warning Center (JTWC) projects that Dujuan will rapidly intensify to Category 4 strength by Sunday, and our top track model forecasts show Dujuan passing between Okinawa and Miyakojimi island in Japan's Ryukyu Islands near 21 UTC Sunday. However, the long-range forecast of where Dujuan might make final landfall is much lower confidence than usual, given a complex upper-level steering pattern the storm is in. Landfall is likely to occur somewhere along the Chinese coast from just north of Taiwan to just south of Shanghai, on Tuesday evening (U.S. EDT time.)


Figure 7. In this image of the Pacific Ocean taken at the exact time of the fall equinox, Wednesday, September 23 at 4:20 am EDT, we see Tropical Storm Dujuan at the right side of the image. The equinox marks the day that every place on Earth receives exactly twelve hours of daylight. From now until the spring equinox, the North Pole will be in 24-hour night, and the South Pole will have 24-hour daylight. Image credit: Dan Lindsey, NOAA/CIRA.

Tropical disturbance south of Acapulco, Mexico a threat to the coast
Our three top models for predicting tropical cyclone genesis predict that an area of disturbed weather about 300 miles south of Acapulco, Mexico will develop into a tropical depression on Monday. This system is expected to move northwards and be very near the coast close to Acapulco on Tuesday, potentially bringing dangerous flooding rains. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 20% and 80%, respectively.

Jeff Masters and Bob Henson