Potential Gulf of Mexico Development Next Week; Japan Watches TS Dujuan

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 Monday. 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 Florida by next Wednesday, but forecasts this far into the future are low-confidence. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 0% and 20%, respectively.


Figure 1. Latest long-range radar image from the Cape Hatteras, North Carolina radar.

High seas, strong winds for Southeast U.S. coast
An area of disturbed weather off the coast of North Carolina, which was being tracked by NHC on Tuesday as Invest 97L, is no longer organized enough to be considered an area of interest, but is bringing heavy rains to the waters just offshore from the Outer Banks of North Carolina, where a High Surf Advisory for waves of 6 - 9 feet is posted until Thursday morning. The disturbance is under high wind shear of 30 - 50 knots, and there is plenty of dry air around it, which is preventing development. Long-range radar on Wednesday morning from the Cape Hatteras, North Carolina radar showed no spiral banding or signs of organization to 97L's precipitation echoes. 97L will move slowly west-southwest the next few days, bringing strong winds and occasional heavy rains to the coast of North Carolina on Wednesday, and to the coast of South Carolina on Thursday. None of our three reliable models for predicting tropical cyclone genesis show development of 97L into a tropical or subtropical cyclone.


Figure 2. Latest satellite image of Tropical Storm Ida.

Tropical Storm Ida no threat to land
Tropical Storm Ida continues to wander aimlessly over the Central Atlantic, well away from any land areas. Satellite images on Wednesday morning showed that Ida continued to struggle against moderate wind shear, with the center of circulation partially exposed to view, and all of Ida's heavy thunderstorms limited to the southeast side of the center. Ida will meander slowly today through Thursday, but by Friday, a trough of low pressure passing to its north will likely pull Ida to the north. It appears unlikely that Ida will pose a long-range risk to North America.

Tropical Storm Dujuan a threat to Japan
Tropical Storm Dujuan, located about 700 miles southeast of Okinawa in Japan's Ryukyu Islands at 8 am EDT Wednesday, appears destined to become a major typhoon that will threaten Japan early next week. High wind shear of 20 - 25 knots has kept Dujuan somewhat disorganized, despite warm sea-surface temperatures. However, the storm should move into the southwest corner of an upper-level ridge by Thursday, allowing wind shear to drop and a strong upper-level outflow channel to develop, promoting possible rapid intensification. The Joint Typhoon Warning Center (JTWC) projects that Dujuan will intensify to Category 3 strength and pass near Okinawa island near 00 UTC Monday. However, this forecast is much lower confidence than usual, given a complex upper-level steering pattern that will develop this weekend. We have models showing a weak Dujuan passing well south of Okinawa, and models showing a stronger typhoon recurving and missing the Ryukyu Islands entirely. JTWC is taking a middle approach, and it remains to be seen if this will be correct.

It's been a remarkably active typhoon season in the Western Pacific; Dujuan is the 21st named storm so far in 2015. According to statistics from Digital Typhoon, only six other typhoon seasons since 1951 have had more than 21 named storms by this point in the season. An average season has 26 - 27 named storm during the entire year.

Fall is here!
The September equinox (also called the southward or autumnal equinox in the Northern Hemisphere) arrived on Wednesday morning at 4:21 am EDT. This is the latest September equinox since 2011. Very rarely, it can occur on September 24: the last time this happened was in 1931, and the next time will be in 2303. Happy autumn--or happy spring, if you're south of the equator!

Bob Henson and Jeff Masters