|Above: Infrared satellite image of Tropical Storm Gordon at 1447Z (10:47 am EDT) Monday, September 3, 2018. Image credit: NASA/MSFC Earth Science Branch.|
Tropical Storm Gordon formed on Monday morning in the waters of the Upper Florida Keys, and it will be making a fast trek toward the upper Gulf Coast by late Tuesday. As of 11 am EDT Monday, Gordon was centered just southwest of the Everglades, about 50 miles south-southeast of Marco Island, with top sustained winds of 45 mph. A Hurricane Watch was in effect for the Mississippi and Alabama coastline, and Tropical Storm Warnings were up for the Keys from Craig Key to Ocean Reef and for the far southwest and far southeast coast of Florida, from Bonita Beach to Golden Beach, as well as for the upper Gulf Coast from Morgan City, LA, to the Okaloosa-Walton County Line of the far western Florida Panhandle. A Storm Surge Warning was in effect from Shell Beach, Louisiana, to the Mississippi-Alabama border, with a Storm Surge Watch eastward to Navarre, Florida, and westward to the mouth of the Mississippi River.
Gordon’s formation date of September 3 comes nearly two weeks before the average September 16 date for development of the season’s seventh named storm. This year’s tally of 7 named storms, 2 hurricanes, and 0 intense hurricanes in the Atlantic is thus above average for the number of named storms for this point of the season, which reaches its halfway point around September 10 – 11. However, according to Colorado State University, the Atlantic season has produced only about 53% of the accumulated cyclone energy (ACE) one would expect at this point in the year. This is because the year's named storms have not been especially strong or prolonged.
Gordon benefited on Sunday night from the very warm waters between Florida and Cuba, as well as the usual nighttime maximum in showers and thunderstorms (convection) over developing tropical cyclones. Land interaction with the swamps of South Florida might have actually helped rather than hindered Gordon’s development, as the slight frictional effect may have helped focus the embryonic surface circulation.
Gordon appeared increasingly organized on radar on satellite Monday morning, and Hurricane Hunters that were still investigating the system late Monday morning found flight-level winds of 43 knots (49 mph). The Surface Frequency Microwave Radiometer aboard the flight found top near-surface winds of 36 knots (41 mph) just before 11 am EDT. Overall, the reconnaissance data supports 45-mph surface winds and a central pressure of 1009 mb, as noted in the 11 am EDT advisory from NHC. Gordon’s winds were strongest on the north side of its small circulation. Easterly winds gusted to 43 mph at Fort Lauderdale International Airport at 10 am EDT, but Key West reported east winds of just 3 mph at the same time.
|Figure 1. WU depiction of NWS/NEXRAD radar for southern Florida as of 11:15 am EDT Monday, September 4, 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 west-northwest at about 17 mph on Monday morning, and that general track and speed are expected to persist all the way until landfall on Tuesday evening or early Wednesday, with a slight angle toward the right. Forecast models are 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.
The intensity forecast for Gordon is more challenging. 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 12Z 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. It appears most of Gordon’s circulation will be passing north of the warm and cool eddies we discussed in Sunday’s post, so those should have little major impact on the storm. 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.
The 06Z Monday run of our top intensity model of 2017—the HWRF—called for Gordon to make landfall in Mississippi near 8 pm EDT Tuesday as a borderline Category 1/2 hurricane with winds of 95 mph. Our other top intensity models, the DSHIPS and LGEM models, were less aggressive in their 12Z Monday forecasts, calling for Gordon to make landfall Tuesday evening with top winds of 75 – 80 mph--a low-end Category 1 hurricane. The official NHC forecast as of 11 am EDT Monday calls for Gordon to reach the Mississippi/Alabama coast on Tuesday night with top sustained winds of 70 mph, but the center noted that Gordon could reach Category 1 hurricane strength just before landfall.
Potential storm surges from Gordon, as predicted by NHC, include:
Shell Beach to the Mississippi-Alabama border...3 to 5 ft.
Navarre Florida to the Mississippi-Alabama border...2 to 4 ft.
Shell Beach to the Mouth of Mississippi River...2 to 4 ft.
Mouth of the Mississippi River to the Louisiana-Texas border...1 to 2 ft.
Easterly winds over the northern Gulf of Mexico were already causing a rise in water levels more than one foot above normal along the coasts of Mississippi and Southeast Louisiana at noon EDT Monday, as seen using our wundermap with the Storm Surge layer turned on or NOAA’s Quicklook page for tide levels.
|Figure 2. NHC forecast for Gordon as of 11 am EDT Monday, September 3, 2018. Image credit: NOAA/NWS/NHC.|
Two possible analogues: Tropical Storm Hanna and Hurricane Nate
If Gordon makes landfall on the weaker side, a close analogue storm may be Tropical Storm Hanna of September 14, 2002, which made landfall near the mouth of the Mississippi River in Louisiana and then near the Alabama/Mississippi border with sustained winds of 60 mph. Gulfport, Mississippi reported a storm tide of 5.1 ft, and there were other reports in the 3 - 5 ft range. Minor beach erosion was reported from Dauphin Island, Alabama, to Navarre Beach, Florida, as well as in the Florida counties of Walton, Bay, and Gulf. Some storm tide flooding was reported on Dauphin Island and in Mobile County. Three deaths were attributed to rip currents generated by Hanna on the Florida Panhandle coast. Hanna and its remnants produced heavy rains across much of the southeastern states, with numerous reports of storm total accumulations of between 5 and 10 inches. The highest reported storm total, 15.56 in, was from Donalsonville, Georgia. Note that Gordon’s rainfall impacts will be farther to the west, over Louisiana, and Georgia is not expected to get heavy rains from the system. Hanna caused a total of $20 million in damage; agricultural damage in Georgia, primarily to the cotton and peanut crops, amounted to nearly $19 million of that total.
|Figure 3. GOES-16 view of Hurricane Nate at 22Z (6 pm EDT) Saturday, October 7, 2017. Image credit: NOAA/RAMMB.|
If Gordon makes it to hurricane strength before landfall, a closer analogue for the Gulf Coast might be Hurricane Nate from 2017. Heavy rains from Nate inflicted major damage on Central America (more than $500 million in Costa Rica) before the storm entered the northwest Caribbean and attained hurricane strength, with top winds peaking at 90 mph. Nate was the fastest-moving hurricane on record in the Gulf, with a 12-hour average speed of 29 mph. Wind shear kept Nate from intensifying across the Gulf: it made landfall near the mouth of the Mississippi and again near Biloxi, Mississippi, classified as an 85-mph hurricane. There were no land-based reports of hurricane-force sustained winds, though, and Nate’s rapid motion limited the total rainfall at any one spot. Damage in the U.S. totaled around $22 million.
|Figure 4. MODIS visible satellite image of a newly-emerged tropical wave off the coast of Africa on Monday morning, September 3, 2018. Image credit: NASA Worldview.|
Two more African tropical waves to watch 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 8am EDT Monday, and 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), and the fact that the dry air and dust of the Saharan Air Layer (SAL) lay well to the north. Wind shear over the eastern tropical Atlantic was a moderate 10 – 15 knots, favoring development. Satellite images on Monday morning showed that the wave had a moderate amount of spin and heavy thunderstorm activity.
The wave will likely bring heavy rains of 1 – 3” and gusty winds to the southern Cabo Verde Islands on Tuesday as the system heads west to west-northwest at 10 – 15 mph. Monday morning runs of the European and GFS models showed development of the wave by late this week or early next week, and in their 8am Tropical Weather Outlook, NHC gave the system 2-day and 5-day odds of development of 0% and 30%, 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.
Another tropical wave that is predicted to emerge from the coast of Africa this Friday also has model support for development by next week. It’s that time of the year!
We’ll have an update this evening with more on Gordon as well as Tropical Storm Florence, which will be spinning harmlessly in the Atlantic this week but could pose a longer-term threat, and several systems of interest in the Pacific, including a weakening Typhoon Jebi, heading for a landfall in central Japan on Tuesday local time.
Dr. Jeff Masters wrote the Hanna and tropical-wave sections of this post.