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Heavy Rains Ahead for Gulf and Mid-Atlantic; Niala, Dujuan Prowl the Pacific

By: Jeff Masters and Bob Henson 4:17 PM GMT on September 25, 2015

A trough of low pressure is producing a large area of disorganized showers and thunderstorms over the northwestern Caribbean Sea and portions of Central America, and will move northwest across Mexico's Yucatan Peninsula over the next few days. By Sunday night or Monday morning, this activity will push into the Gulf of Mexico, where development into a tropical or subtropical depression could occur. However, 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, and our top three models for predicting tropical cyclone genesis are showing little development of this system once it reaches the Gulf. The system will get pulled northwards to affect the U.S. coast from Louisiana to the Florida Panhandle by Tuesday, bringing heavy rains of 2 - 5". In their 8 am EDT Friday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 0% and 20%, respectively.

Little change to Ida
There is not much new to say about Tropical Depression Ida, which is wandering slowly over the Central Atlantic, well away from any land areas. Satellite images on Friday morning showed Ida's center of circulation was fully exposed to view by high wind shear, and all of Ida's heavy thunderstorms were limited to the east 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 Friday morning runs of the GFS model.

Figure 1. Latest satellite image of Niala.

Tropical Storm Niala forms near Hawaii
Yet another tropical storm has formed in the Central Pacific near Hawaii, where Tropical Storm Niala was named on Friday morning. Niala was headed northwest at 6 mph towards Hawaii at 11 am EDT Friday, and is expected to take a more westerly path over the weekend, passing about 200 miles south of the Big Island on Saturday afternoon. With tropical storm-force winds expected to extend out up to 90 mph from the center at that time, a Tropical Storm Watch may be hoisted for The Big Island on Friday evening. The 11 am EDT Wind Probability Forecast gave a 21% for South Point on the Big Island to receive tropical storm-force winds of 39+ mph from Niala. The bigger threat from the storm will be heavy rain, which could bring dangerous flash floods to the Big Island on Saturday.

Niala is the record 7th 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 Malia, 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.

Figure 2. MODIS image of Typhoon Dujuan as seen from NASA's Terra satellite on Friday, September 25, 2015 at 02:15 UTC. Image credit: NASA.

Typhoon Dujuan a threat to Japan, China, and Taiwan
Typhoon Dujuan, a Category 1 storm with 90 mph winds located about 510 miles south-southeast of Okinawa in Japan's Ryukyu Islands at 8 am EDT Friday, is steadily intensifying as it heads northwest at 9 mph towards China. Wind shear is in the moderate range, 10 - 20 knots, and ocean temperatures are a very warm 30°C (86°F), conditions which favor intensification. Satellite loops on Friday morning showed a large, well-organized storm with a 28-mile wide eye and a strong upper-level outflow channel to the south. The Joint Typhoon Warning Center (JTWC) projects that Dujuan will intensify to Category 4 strength by Sunday, and the storm is expected to pass between Okinawa and Miyakojimi island in Japan's Ryukyu Islands near 00 UTC Monday. 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. The 00Z Friday run of the European model showed a path very close to Taiwan and into mainland China, while the 00Z and 06Z Friday runs of the GFS model had Dujaun missing China entirely, and instead curving to the north and affecting Japan.

Figure 3. Latest satellite image of Invest 93E (left side of image) and 92E (right side of image, close to the coast of Mexico/Central America.)

Tropical disturbances 92E and 93E a heavy rain threat to Mexico and Central America
Our three top models for predicting tropical cyclone genesis predict that an area of disturbed weather about 300 miles south of Acapulco, Mexico (Invest 93E) will develop into a tropical depression on Sunday or Monday. This system is expected to move northwards and be very near the coast close to Acapulco on Monday through Thursday, potentially bringing an extended period of dangerous flooding rains. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 30% and 80%, respectively.

Another area of disturbed weather in the Eastern Pacific along the coast near the Mexico/Guatemala border (Invest 92E) is drifting slowly to the northwest, and will bring heavy rains to El Salvador, southern Guatemala, and southeastern Mexico over the next few days. None of our reliable models for forecasting tropical cyclone genesis develop 92E into a tropical depression. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 20%.

Apparent tornado near Charleston; heavy rain, coastal flooding on tap for mid-Atlantic
Coastal flooding and heavy rains will be a concern throughout the upcoming weekend over parts of the mid-Atlantic coast, as a long fetch of strong wind gets funneled between high pressure building over the Northeast and a persistent zone of low pressure hugging the Southeast coast. A weak upper-level low drifting across Georgia on Friday will provide support for occasional strong thunderstorms, especially near the coast. One intense cell tracking north-northwest near Charleston, SC, apparently spawned a long-lived tornado early Friday that struck the communities of Johns Island and West Ashley, just a few miles southwest of Charleston. Numerous trees are down, and at least a dozen homes appear to have been damaged, according to the Associated Press. The National Weather Service in Charleston is planning a damage survey for Friday. In two METAR reports, the NWS office indicated a lightning-illuminated tornado just southwest of the office between 12:59 and 1:37 a.m. EDT Friday morning. During the preceding half hour, a significant debris signature was apparent on dual-polarization NWS/NEXRAD radar, as posted in @USTornadoes. The NOAA Storm Prediction Center was projecting a marginal risk of severe weather across far eastern North Carolina on Friday, with a small chance of a brief tornado.

Figure 4. Radar imagery from the NWS NEXRAD radar near Charleston, SC, at 0450 GMT (12:50 am EDT) on Friday, September 25, 2015. A diffuse hook echo is evident in the precipitation signature (left), in the same location as a strong contrast in inbound and outbound winds, shown by closely packed green and red echoes (right). These radar indications of a possible tornado are consistent with reports of tornado damage early Friday morning from just southwest of Charleston. Image credit: NWS/Charleston.

Figure 5. Predicted 72-hour rainfall amounts from 1200 GMT (8:00 am EDT) Friday, September 25, through Monday, September 28. Image credit: NOAA Weather Prediction Center.

This weekend’s multiday spell of wet weather should bring some welcome relief from gradually intensifying drought conditions across parts of the Carolinas and Virginia. However, it will also torpedo weekend recreation for thousands of people. Given the dismal forecast, organizers of the 42nd annual Neptune Festival in Virginia Beach, VA, have cancelled many of this weekend’s events, with other activities moved indoors.

Persistent easterly winds, perhaps as strong as 30-40 mph by Saturday evening into Sunday (see Figure 6 below), will lead to strong rip currents and mostly minor but persistent coastal flooding from New Jersey to the Carolinas. The approaching full moon (see below) will add to the problem by boosting peak tidal levels. Water could be 2 to 3 feet above normal in spots, and 6’ – 9’ surf will be widespread. This prolonged event could produce a substantial amount of beach erosion, especially along the Outer Banks of North Carolina. The islands have long struggled against erosion events that scrub sand from heavily touristed beaches, and long-term sea level rise is only adding to the problem. Barrier islands are naturally fluid structures--they build sand on one side as it disappears on the other--so any effort to preserve specific structures and beaches along barrier islands can be a never-ending battle. Structures such as groins, walls that extend perpendicular to the coast, can help retain sand upstream of prevailing longshore flow, but they typically exacerbate erosion downstream. in 2011, North Carolina ended a decades-long ban on terminal groins, which are placed next to inlets. At least four terminal groins are now being installed or planned along the Outer Banks.

Figure 7. Wundermap depiction of forecast surface winds at 0000 GMT Sunday (8:00 pm EDT Saturday) shows a long fetch of sustained 30-40 mph winds pushing into the mid-Atlantic coast.

September heat still at record pace in Colorado, NYC, New England
It may be autumn, but the atmosphere hasn’t gotten the memo across large parts of the lower 48 U.S. states. The rest of this month will skew warm and dry as a sprawling upper-level ridge predominates, which raises the odds of a record-warm September for the nation as a whole and for a number of U.S. cities. The table below has been updated from a post last week; it shows how warm the remainder of the month will need to be in various cities in order to set a September record. If the NWS forecasts from Thursday are reasonably correct, then much of New England--including Portland, Maine, and Burlington, Vermont--will end up with a record-warm September. New York City’s Central Park also has a good shot. Some intrusions of cool air into the Midwest have tamped down the odds of record monthly warmth in places like St. Louis and Minneapolis. However, the entire Front Range of the Rockies from Pueblo, Colorado, to Cheyenne, Wyoming, is right on track for its warmest September in records dating back more than a century.

Moon mania! Sunday night to bring a rare combo of supermoon and total lunar eclipse
Between 10:11 and 11:23 pm EDT on Sunday night, an unusually large “supermoon” will be completely eclipsed by Earth’s shadow, leading to a rare treat for astronomical buffs. Partial or total eclipses occur only when the Sun, Earth and moon are lined up, so by definition they only occur when the moon is full. Because the moon’s orbit around Earth is not perfectly symmetric, the moon is sometimes closer to us than at other times. Supermoons can appear as much as 14% larger in diameter than usual. There are typically two to five partial or total lunar eclipses per year, but getting a total lunar eclipse (also called a “blood moon,” referring to the common reddish hue) during a supermoon is a once-in-several-decades event. According to space.com, the last one was in 1982 and the next one won’t occur until 2033. Sunday night will also mark the fourth consecutive total eclipse without any partial eclipses, an unusual event called a tetrad.

Figure 8. The total lunar eclipse of October 8, 2014, as photographed in California by Alfredo Garcia, Jr. Image credit: Wikimedia Commons/Tomruen.

Sunday’s lunar eclipse is timed especially well for the Americas, with totality falling between 7 pm and midnight across the contiguous U.S. Evening skies will be fair or clear over much of the nation, apart from the stubborn Southeast storm and another cloud-bearing system over the north central states. Space.com is offering “full ‘blood moon’ coverage” this weekend. Have a great one!

Jeff Masters and Bob Henson

Hurricane Tornado Flood

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