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Invest 97L off Southeast U.S. Coast; Tropical Storm Ida Churns the Central Atlantic

By: Bob Henson and Jeff Masters 3:43 PM GMT on September 22, 2015

An area of disturbed weather off the coast of North Carolina (Invest 97L) 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. The disturbance is under high wind shear of 20 - 30 knots, and there is plenty of dry air around it, which is inhibiting development. Satellite loops on Tuesday morning showed that 97L had some rotation but no surface circulation, and a modest area of heavy thunderstorms that were increasing in intensity and areal coverage. Long-range radar on Tuesday 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 Tuesday and Wednesday, and to the coast of South Carolina on Wednesday and Thursday. An upper-level trough of low pressure along the U.S. East Coast will bring high wind shear over 97L for the remainder of the week, which will make development difficult. None of our three reliable models for predicting tropical cyclone genesis show development of 97L into a tropical or subtropical cyclone. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 10% and 20%, respectively.

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

Tropical Storm Ida no threat to land
Tropical Storm Ida is essentially stalled out over the Central Atlantic, well away from any land areas. Satellite images on Tuesday 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.

Figure 2. Latest satellite image of Tropical Storm Ida.

Gulf of Mexico development 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 the long-range forecasts from the GFS and European models are showing an area of low pressure capable of becoming a tropical or subtropical depression forming near Mexico's Yucatan Peninsula on Monday. Wind shear is likely to be moderate to strong over the region, limiting the potential for this system 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 the middle of next week, but a strong trough of low pressure over the Western Gulf of Mexico will likely bring high wind shear to the Gulf, limiting the odds of a this system becoming a tropical or subtropical storm that will hit the U.S. next week.

Tropical Storm Malia moving through Central Pacific
Tropical Storm Malia formed on Monday and moved though the Papahanaumokuakea Marine National Monument, several hundred miles west of the Hawaiian Islands. Malia is headed north-northwestwards at 8 mph and will likely become an extratropical storm on Wednesday, between Hawaii and Alaska's Aleutian islands. Malia was the record 6th named storm to form in 2015 in the North Central Pacific (between 140°W and the Date Line.) According to wunderblogger Dr. Phil Klotzbach, prior to 2015, the previous record for named storms in the North Central Pacific for an entire season was four set in 1982. The other named storms that formed in the North Central Pacific in 2015 were Halola, Ela, Iune, Kilo and Loke. This year's record activity has been due to unusually low wind shear and record-warm ocean temperatures caused by the strong El Niño event underway.

TD 16E’s rains less than expected in Southwest
After moving into northwest Mexico on Monday, Tropical Depression 16E quickly dissipated, with no discernable surface circulation detectable in the depression’s remnants over Arizona by early Tuesday morning. The remnants are now being swept northeastward in fast upper-level flow between a ridge over Texas and a persistent upper low over southern California. Between the system’s rapid motion and disorganized state, plus drier air infiltrating at upper levels, the rainfall associated with TD 16E’s remnants was less intense and widespread than earlier expected. Reports from CoCoRaHS observers on Tuesday morning for the preceding 24 hours showed most of the Phoenix area getting little more than sprinkles, with about 0.45” recorded on the eastern fringes of the metro area. Reports in the Tuscon area ranged from about 0.5” to more than 1.5”, with around 3” recorded well south of town in Santa Cruz County. Nogales, AZ, picked up 1.99”. Further west, the upper-level low over southern California was another underperformer in terms of rainfall. The San Diego area received just trace amounts, and the Los Angeles metroplex stayed high and dry.

Figure 3. Washes quickly filled up in Tuscon, AZ, as rains associated with the remnants of TD 16E reached town on Monday, September 21, 2015. Image credit: wunderphotographer pjkace70.

Figure 4. Heavy rains sweep across the countryside southwest of Phoenix near Gila Bend, AZ, on Monday, September 21, 2015. Image credit: wunderphotographer Milker13.

Another Pacific typhoon may develop this week
Tropical Depression 21W, located more than 300 miles west of the Marianas Islands in the open Northwest Pacific, appears destined to become the region’s 21st named storm and 13th typhoon of 2015. Moderate wind shear has kept 21W somewhat disorganized, despite warm sea-surface temperatures: convection is focused to the west of the ill-defined center of circulation. TD 21W should move into a more favorable spot on the southwest corner of an upper-level ridge by midweek. The Joint Typhoon Warning Center projects that TD 21W will be a recurving Category 4 typhoon by Sunday. It is too early to know whether the expected recurvature will be sharp and soon enough to spare Japan or neighboring areas from any typhoon-related impacts. This year has been exceptionally busy in the Northwest Pacific: as of mid-August, the region had broken its record for the greatest amount of accumulated cyclone energy for any year since 1950.

Wunderblogger Steve Gregory has an update on El Niño in his Monday afternoon post. Later today, we’ll have a post on this week’s climate-related events in Washington, D.C., and New York and how carbon dioxide emissions have evolved over the last four-plus decades.

Bob Henson and Jeff Masters

Figure 5. Latest satellite image of Tropical Depression 21W.


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.