Fiona Forms in Tropical Atlantic; Tens of Thousands Flee California Wildfire

A tropical depression in the remote eastern tropical Atlantic became Tropical Storm Fiona on Wednesday afternoon. At 11:00 am EDT Thursday, Fiona was located at 16.4°N, 40.5°W, more than 1000 miles west of the Cape Verde Islands, with top sustained winds of 45 mph. Fiona is a small storm, with tropical-storm force winds extending just 35 miles from its center. On Wednesday evening, Fiona was almost devoid of deep convection (showers and thunderstorms), but tropical cyclones typically experience an uptick in convective activity overnight--a phenomenon called the nocturnal convective maximum. This helped Fiona to develop a compact core of deep convection early Thursday, although some decrease was already evident by late morning.


Figure 1. Latest visible satellite image of Tropical Storm Fiona.

Outlook for Fiona
There are both pros and cons in Fiona’s future as the storm heads west-northwest at a leisurely 8 mph, far from any land areas. Sea-surface temperatures beneath Fiona are around 27°C (81°F) and vertical wind shear is fairly light, less than 10 knots. Wind shear should remain light to moderate for the next day or two, and Fiona’s path will take it over increasingly warmer water of 28-29°C associated with a tongue of unusually warm SSTs (1-2°C above average) extending across the subtropical North Atlantic. While these factors favor development, Fiona is also half-surrounded by a large mass of dry air from the Sahara Desert lying to its north (see Figure 3 below). The storm is likely to ingest some of this dry air, weakening its ability to consolidate convection. Wind shear may also become more of a problem in 2 or 3 days.

The track forecast for Fiona is relatively straightforward, as the storm heads toward a weakness in the upper-level ridge sprawling across the subtropical North Atlantic. Both the GFS and ECMWF--our best models for predicting tropical cyclone tracks--keep Fiona on a west-northwest to northwest heading throughout the next few days, bringing it to around 50-55°W by early next week. The main variable in Fiona’s track is its intensity. Should Fiona remain weak, it is more likely to be steered by low-level winds and continue moving on its present track. The more Fiona manages to intensify, the more likely it would be to arc toward the north, as upper-level winds become more important in steering the storm. The 00Z and 06Z Thursday runs of the GFDL model brings Fiona to hurricane strength this weekend on a north-northwest track east of 50°W. However, other dynamical models do not strengthen Fiona significantly in the next five days, including the most recent operational runs of the GFS (06Z Thursday) and ECMWF (00Z Thursday). Several members of the 00Z Thursday ECMWF ensemble weaken Fiona below the tropical storm threshold (40 mph sustained winds) by the weekend. We have plenty of time to monitor Fiona as the storm gradually moves toward warmer waters. Climatology strongly favors the idea that Fiona will eventually recurve well east of North America, as evident in Figure 2 below.


Figure 2. The tracks and intensities of all tropical storms and hurricanes in August (1851 - 2015) that passed within 2° latitude and longitude of Fiona’s position as of Thursday morning, August 18, 2016.


Figure 3. A large mass of dry air from the Sahara Desert (yellows and reds) was encircling Tropical Storm Fiona, located near 40°W, as of 1500Z (11:00 am EDT) Thursday, August 18, 2016. Another tropical wave was located to the south of the Saharan air layer, centered around 30°W. Image credit: University of Wisconsin-Madison/CIMMS/NOAA

The African wave train continues
As we dive into the heart of the Cape Verde season, tropical waves continue to shuttle off the coast of Africa into the tropical Atlantic. One sizable wave, with an increasing amount of convection on Thursday morning, is located around 30°W, while another wave now over far West Africa will enter the tropical Atlantic during the weekend. In its Thursday morning tropical weather outlook, NHC assigned 10% odds of this second wave developing by next Tuesday, August 23. Longer-range models have been vacillating on the future of these two waves, but they both bear watching, as one or both of them will likely be moving at fairly low latitudes across the tropical Atlantic toward the Caribbean over the next week or so.


Figure 3. Infrared image of Invest 97E off the southwest coast of Mexico as of 1530Z (11:30 am EDT) Thursday, August 18, 2016. Image credit: NOAA/NESDIS.

97E slowly organizing
In the Eastern Pacific, Invest 97E may become a tropical depression or tropical storm by this weekend. Now with sustained winds of around 30 mph, 97E was located about 350 miles west-southwest of Manzanillo, Mexico, moving toward the northwest. Waters along 97E’s path are quite warm, around 29°C (84°F), and wind shear is expected to remain light to moderate (10-20 knots) for the next several days. Although it is not yet well organized, 97E experienced a healthy burst of convection on Wednesday night. Most computer models are keeping 97E below the tropical storm threshold for the next 3-5 days, but given the favorable factors above, it would not be a shock to see 97E become Tropical Storm Kay by early next week. In its Thursday morning outlook, NHC gave 97E a 70% chance of development to at least depression status by Saturday. 97E could eventually take a path toward Baja California, especially if it intensifies.


Figure 4. An infrared image of Tropical Depression Dianmu heading into northern Vietnam as of 1530Z (11:30 am EDT) Thursday, August 18, 2016. Dianmu's showers and thunderstorms span more than 600 miles from east to west. Image credit: RAMMB/Colorado State University

Elsewhere in the tropics
An active monsoon trough in the Northwest Pacific is keeping the region hopping with tropical cyclones. The biggest concern at present is Tropical Depression Dianmu, which is bringing heavy rains to northern Vietnam and Laos. Dianmu’s large envelope of convection could trigger flash floods and landslides as it slogs into the region’s mountainous terrain on Thursday and Friday. Rainfall amounts of 8” - 12” will be widespread, with localized totals on the order of 20”, according to Vietnam’s National Center for Hydro-Meteorological Forecasting.

Meanwhile, Tropical Storm 12 will swing eastward several hundred miles south of Japan over the next couple of days as it remains well below typhoon strength, perhaps intensifying early next week. At the same time, Tropical Depression Ten will be arcing north toward Japan by the weekend, perhaps interacting with TS 12 as it does so.


Figure 5. Embers from the Blue Cut fire smolder along Lytle Creek Road near Keenbrook, CA, on Wednesday, August 17, 2016. Image credit: AP Photo/Noah Berger.

A high-impact wildfire continues to rage east of Los Angeles
More than 82,000 Southern Californians were under mandatory evacuation orders on Thursday as the Blue Cut fire continued burning just north of of the city of San Bernardino, about 60 miles east of Los Angeles. The fire, which now spans more than 31,000 acres, exploded into a huge inferno in a matter of hours on Tuesday afternoon. It’s not yet clear how many structures have already been affected, but officials and locals are bracing themselves for an “immense tally of devastation,” according to the Los Angeles Times. The evacuation warnings include an estimated 34,500 homes, according to InciWeb.

The Blue Cut fire was fed by hot, dry southwest winds heading into the Cajon Pass, which lies between the San Gabriel and San Bernardino mountain ranges. The rugged topography and tinder-dry vegetation has led to challenging conditions for more than 1500 firefighters. Near the center of the fire, Interstate 15, which connects Los Angeles and Las Vegas via the Cajon Pass, was completely shut down on Tuesday, and the southbound lanes remained closed on Thursday morning. As of Thursday morning, the blaze was only 4% contained. A red flag warning remained in effect on Thursday and could be extended into Friday, with relative humidities expected to fall below 10 percent and winds gusting as high as 30-40 mph. The weather will improve only slightly over the next couple of days, with somewhat lower temperatures and higher humidities, but gusty winds are expected to continue.

Largely bypassed by the El Niño rains of 2015-16, Southern California will be extremely vulnerable to major wildland fire over the next few weeks until winter rains (hopefully) arrive. Five years of drought have left a landscape packed with dead or dying trees and brush, and September and October often bring some of the hottest, driest weather of the year. Other parts of the U.S. West will need to stay on alert as well. For Thursday, the NOAA Storm Prediction Center outlook warned of critical fire weather conditions over the Okanogan Valley of eastern Washington, with several other regions of elevated risk (including interior parts of Southern California east of Los Angeles and San Diego).

Bob Henson


Figure 6. The burned-out hulk of a 1960s Ford Falcon is seen on Wednesday, August 17, 2016, after flames from the Blue Cut wildfire overnight swept through a rural area near Phelan, CA. Image credit: AP Photo/Christine Armario.