Bob Henson ·

Above: Visible GOES-16 satellite image of Tropical Storm Gordon at 2033Z (4:33 pm EDT) Monday, September 3, 2018. Image credit: tropicaltidbits.com.

Tropical Storm Gordon continued to organize on Monday afternoon just off the southwest coast of Florida, and it is predicted to reach hurricane strength by Tuesday before it slams into the central Gulf Coast. As of 5 pm EDT Monday, Gordon was centered just west of the Everglades, about 50 miles west-southwest of Fort Myers, with top sustained winds of 50 mph. A Hurricane Warning was in effect for the Mississippi and Alabama coastline, and Tropical Storm Warnings were up for Chokoloskee to Bonita Beach on Florida’s southwest coast, as well as along the Louisiana coast from Morgan City, LA, eastward and along the Florida coast from the Okaloosa-Walton County Line westward. A Storm Surge Warning was in effect from Shell Beach, LA, to Dauphin Island, AL, with a Storm Surge Watch eastward to Navarre, Florida, and westward to the mouth of the Mississippi River.

Gordon was a very small storm on Monday afternoon, with tropical-storm-force winds extending just 45 miles from its center on Monday afternoon, mainly on the north side. Smaller tropical cyclones can undergo more rapid swings in intensity. At one point on Monday, Gordon’s showers and thunderstorms (convection) tightened around a tiny proto-eye, which dissolved just hours later. Radar images suggested that Gordon was making another attempt to form an eye, this one somewhat larger, late Monday afternoon. Outflow at upper levels was distinctly improving around Gordon late Monday, helping to ventilate the storm. Once its inner core consolidates around an eyewall, Gordon will be in a very favorable position to intensify through Tuesday. Gordon’s central pressure was estimated at 1006 mb, on the high side for a tropical storm—another sign of Gordon's very compact structure.

Figure 1. WU depiction of NWS/NEXRAD regional radar for Tropical Storm Gordon as of 6:48 pm EDT Monday, September 2, 2018.

Forecast for Gordon

Gordon’s track forecast is quite straightforward, as steering currents are predicted to change little between now and Wednesday. Gordon was moving just north of due west at about 15 mph on Monday evening, and that rough speed is expected to persist all the way until landfall on Tuesday evening or early Wednesday, with an angling toward the right over time. Forecast models from 12Z Monday morning were tightly clustered around a landfall between far southeast Louisiana and southern Alabama, with Gordon continuing north-northwest across the lower Mississippi Valley, perhaps slowing or stalling by Thursday somewhere near the Arklatex region.

Because Gordon has developed a closed circulation more quickly than expected, it will be able to make the most of the favorable conditions across the eastern Gulf. The 18Z Monday run of the SHIPS statistical model suggests that a moist atmosphere (mid-level relative humidity around 65%), light to moderate wind shear (10 – 15 knots), and warm sea surface temperatures (SSTs) around 30°C (86°F) will prevail throughout Gordon’s trek over the Gulf. We can thus expect steady strengthening, and it appears that Gordon will most likely landfall as a strong tropical storm or a Category 1 hurricane. There is the possibility that Gordon could strengthen more rapidly, but predicting rapid intensification remains one of the toughest challenges that face hurricane forecasters.

The rapid intensification index in the SHIPS model gives a 13% chance of Gordon gaining 45 knots of strength in 36 hours, which would bring it well into the Category 2 range. The 12Z runs of our top intensity models, the HWRF, DSHIPS,and LGEM models, called for Gordon to make landfall Tuesday evening with top winds of 75 – 80 mph--a low-end Category 1 hurricane. However, Gordon's small size makes it more difficult for such models to correctly capture the storm's evolution. The 18Z HWRF run was considerably weaker, bringing Gordon onshore as a tropical storm.

The official NHC forecast as of 5 pm EDT Monday calls for Gordon to reach the Mississippi/Alabama coast on Tuesday night at minimal hurricane strength (75 mph). Given the favorable conditions all the way to landfall, it's not out of the question that Gordon could strengthen into the upper Cat 1 or lower Cat 2 range, though minimal hurricane or top-end tropical storm strength is the more likely outcome.

Figure 2. Probability of experiencing tropical-storm-force winds from Hurricane Gordon. Image credit: NOAA/NWS/NHC.

Impacts from Gordon

There is still some uncertainty around Gordon’s wind impacts, but given the storm’s small size and the short amount of time before landfall, any areas of hurricane-force sustained winds are likely to be confined to within roughly 30-50 miles to the east of where Gordon’s center comes ashore. Tropical-storm-force winds will be more widespread. These are capable of bringing down trees and power lines, and several deaths in recent hurricanes and tropical storms have occurred from trees falling atop homes and cars.

There’s more confidence that Gordon will bring heavy rain to a broad area around its center. Gordon’s steady motion will help limit the rainfall totals near the coast, but widespread totals in the 5” - 8" range can be expected within a 100-mile swath along Gordon’s path, as well as over parts of South Florida. If Gordon stalls in the Arklatex region later this week, as suggested by models, even heavier rains could develop there. Gordon’s moisture will also help fuel rains along a frontal system in the Midwest, where some areas were hard hit by August flooding.

Potential inundation above ground level from Gordon’s storm surge, as predicted by NHC, could reach:

1 to 2 feet: Louisiana-Texas border to the mouth of the Mississippi River

2 to 4 feet: Mouth of the Mississippi River to Shell Beach, LA

3 to 5 feet: Shell Beach, LA, to Dauphin Island, AL

2 to 4 feet: Dauphin Island, AL, to Navarre, FL, including Mobile Bay

Easterly winds over the northern Gulf of Mexico were already causing a rise in water levels about 1.4 feet above normal at Shell Beach, LA, at 5 pm EDT Monday, as seen using our wundermap with the Storm Surge layer turned on or NOAA’s Quicklook page for tide levels.

Figure 3. 5-day rainfall totals from 8 pm EDT Monday, September 3, through Friday, September 8, are expected to top 5” along a swath from the central Gulf Coast to eastern Oklahoma. Image credit: NOAA/NWS/WPC.

Figure 4. Tropical Storm Florence as seen on Monday afternoon, September 3, 2018. Image credit: NASA Worldview.

Tropical Storm Florence headed into the open Central Atlantic

Tropical Storm Florence was headed west at 15 mph into the Central Atlantic on Monday afternoon, and it poses no threat to any land areas this week. Conditions were marginal for development on Monday afternoon, with sea surface temperatures (SSTs) a relatively cool 26.5°C (80°F) and the storm in a region where the dry air of the Saharan Air Layer (SAL) was creating a mid-level relative humidity of 50 – 55%. Moderate wind shear of 10 – 20 knots was keeping intensification slow, and Florence had top sustained winds of 70 mph at 5 pm EDT Monday. Satellite images showed a well-organized system, but no inner core (eyewall).

The intensity forecast for Florence shows conflicting influences. The atmosphere surrounding Florence will remain dry this week, with a mid-level relative humidity of 50 – 55%. With the storm expected to experience moderately high wind shear of 15 – 20 knots during much of this period, slow weakening is indicated. However, Florence will experience slowly increasing SSTs this week, traveling over 29°C (84°F) waters by Saturday. At the same time, wind shear is expected to fall on Friday and Saturday. This will tend to favor strengthening. Overall, the NHC forecast of little strengthening this week appears to be the best bet.

Steering currents favor a west-northwesterly to northwesterly track for the rest of the week, which will take Florence through the Central Atlantic, far from land. This weekend, though, Florence will have a major choice to make. If the storm is relatively strong—at Category 1 hurricane strength or stronger--Florence will most likely recurve to the north and northeast, out sea, in response to a strong upper-level trough of low pressure passing to the north. This is the solution of the 12Z Monday GFS model. However, if Florence fails to intensify much, the storm will tend to be steered by the atmospheric flow at lower levels of the atmosphere, which favors a more west-northwesterly track. In this scenario, endorsed by the 12Z Monday runs of the European and UKMET models, Florence would avoid recurvature and get trapped to the south of a strong ridge of high pressure. In this case, Florence would continue moving to the west-northwest, perhaps passing close to Bermuda on Monday, September 10--and potentially posing a long-range threat to the U.S. East Coast later next week. This is the solution favored by the 12Z Monday European and UKMET models. The steering currents a week or more in advance are impossible to accurately predict, though, and will likely depend upon the very uncertain interaction of Typhoon Jebi in the Western Pacific with the large-scale atmospheric circulation later this week. So, stay tuned, and root for Florence to grow strong now, increasing its odds of recurvature.

Figure 5. MODIS visible satellite image of tropical wave 92L off the coast of Africa on Monday afternoon, September 3, 2018. Image credit: NASA Worldview.

92L in Eastern Atlantic may develop late this week

A strong tropical wave that emerged from the coast of Africa on Monday morning was located about 400 miles southeast of the Cabo Verde Islands at 2 pm EDT Monday, and was designated 92L by NHC on Monday afternoon. This system may be a long-range threat to the Caribbean next week. Favoring development of the wave were warm sea surface temperatures (SSTs) near 28°C (82°F), a moist atmosphere, and moderate wind shear of 10 – 20 knots. Satellite images on Monday afternoon showed that the wave had a moderate amount of spin and heavy thunderstorm activity, but was poorly organized.

The 18Z Monday run of the SHIPS model predicted that SSTs would cool and the atmosphere would grow drier through Thursday, limiting the potential for 92L to develop in the short term. However, SSTs will begin to warm on Friday and wind shear is predicted to be moderate, which may allow the system to organize into a tropical depression late this week. The 12Z Monday runs of our top three models for predicting tropical cyclone development—the European, GFS and UKMET models—all showed development of the wave by late this week or early next week. In their 2pm Tropical Weather Outlook, NHC gave the system 2-day and 5-day odds of development of 10% and 40%, respectively. Steering currents favor a west to west-northwesterly track over the next week, and the wave could arrive in the Lesser Antilles Islands as early as Wednesday, September 12. Keep in mind, though, that steering currents so far into the future are very uncertain. The next name on the Atlantic list of storms is Helene.

Another tropical wave that is predicted to emerge from the coast of Africa this Thursday also has model support for development by next week.

Norman and Olivia: Busy times continue in the Central/Eastern Pacific

Two named storms are prowling the Central and Eastern Pacific, just days after the region saw its most active month of tropical cyclones on record. Fortunately, neither of these systems are likely to threaten land. Hurricane Norman—a former Category 4 storm still packing 105-mph winds on Monday afternoon—will continue west through Wednesday; it will then turn northwest, arcing away from Hawaiian Islands into the open Central Pacific.

Meanwhile, Tropical Storm Olivia is on track to become the region’s next hurricane. Located about 555 miles west-southwest of Cabo San Lucas, Mexico, on Monday afternoon, Olivia is heading west and poses no immediate threat. By later this week, Olivia will be passing over cooler waters and will most likely follow in the watery footsteps of Norman, curving well northeast of Hawaii as it weakens.

Figure 6. Infrared satellite image of Typhoon Jebi as of 2130Z (5:30 pm EDT) Monday, September 3, 2018. Image credit: RAMMB/CIRA/CSU.

Jebi weakens on approach to Japan

Typhoon Jebi—a former Category 5 super typhoon—will strike Japan on Tuesday local time as a shadow of its former self, but significant impacts are still possible, including heavy rains and very strong wind gusts as far east as Tokyo. Jebi’s top sustained winds were down to 90 mph as of 2 pm EDT Monday. The typhoon is predicted to strike central Japan near Osaka and Kyoto around midday Tuesday local time as a Category 1 typhoon.

Since Japan was preparing for a stronger landfall, the nation should be well prepared for Jebi. Widespread 3” – 6” rains can be expected, with higher totals along south-facing slopes east of Jebi’s center. Because Jebi’s large and strong pre-existing circulation will take time to spin down, some of the most powerful winds will occur above the surface: hurricane-force sustained winds possible across the higher elevations of central and eastern Honshu. The Japan Times is live-blogging the storm’s arrival.

Dr. Jeff Masters co-wrote this post.