Sally could hit between New Orleans and Mobile at hurricane strength – Yale Climate Connections

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Tropical Storm Sally radar
Radar image of Tropical Storm Sally at 2:19 p.m. EDT Saturday, September 12, 2020. (Image credit: Mark Nissenbaum/Florida State University)

Tropical Storm Sally, which formed along the coast of Southwest Florida on Saturday afternoon, is expected to intensify into a hurricane before making landfall between southeast Louisiana and the Florida Panhandle on Tuesday. Sally will be moving very slowly for multiple days up to the time of landfall, increasing chances of dangerous heavy rains in excess of 10 inches along the coast.

At 2 p.m. EDT Saturday, September 12, 2020, Sally was centered near the coast of Southwest Florida, 35 miles south-southeast of Naples. Sally was a minimal tropical storm with 40 mph winds, moving west at 7 mph. The storm was bringing torrential rains and wind gusts of tropical storm force to the Florida Keys. At 12 p.m. EDT, Fowey Rocks in the Florida Keys reported sustained winds of 45 mph, gusting to 47 mph. As of 2 p.m. EDT Saturday, heavy rains of 4 – 8 inches had fallen across much of the Middle and Upper Keys, with over eight inches in Lower Matecumbe Key, the Key West National Weather Service reported. Satellite and radar images showed a steady increase in the organization and intensity of Sally’s heavy thunderstorm activity.

Figure 1
Figure 1. GeoColor visible satellite image of Sally as of 1550Z (11:50 a.m. EDT) Saturday, September 12, 2020. (Image credit: RAMMB/CIRA/Colorado State University)

Forecast for Sally

Sally will move in a general west-northwest motion toward the central Gulf Coast over the next several days. Steering currents will weaken by Sunday and Sally will slow down to a forward speed of about 5 mph, giving it time to gain strength from the very warm waters of the eastern Gulf of Mexico, where sea surface temperatures are around 30°C (86°F). The Gulf waters have largely recovered from the cool wake left by hurricanes Marco and Laura, and Sally should remain just northeast of a cool eddy with low oceanic heat content over the southeast Gulf.

Although wind shear will be moderate over the next several days, at 10-20 knots, Sally will move beneath a broad upper ridge, and the upper winds will support favorable outflow from the storm. The air mass should remain reasonably moist, with a mid-level relative humidity around 60%, so dry air is unlikely to be a major hindrance. By late Monday and into Tuesday, wind shear is expected to tick up a notch, to around 20 knots, which may slow the intensification process.

Figure 2
Figure 2. Sea surface temperatures across the Gulf of Mexico are running about 0.6 degrees Celsius (1°F) above average for this time of year, as depicted in this graphic of anomalies (degrees Celsius) from Saturday, September 12, 2020. (Image credit: tropicaltidbits.com)

How much Sally strengthens will depend in part on how quickly it develops a vertically aligned inner core. A period of rapid intensification cannot be ruled out if the storm organizes quickly enough. The official forecast is for more gradual strengthening, with Sally approaching the central Gulf Coast as a category 1 hurricane. The 06Z Saturday HWRF and HMON intensity models, among the best guidance for intensity, both bring Sally to the coast as a Category 2 storm. The 12Z Saturday HWRF model predicted that Sally would hit southeast Louisiana early Tuesday morning as a high-end category 3 hurricane with 125 mph winds, passing over New Orleans at category 2 strength. The 12Z Saturday HMON model had a category 1 storm with 85 mph winds making landfall near Mobile on Tuesday afternoon. So the uncertainty in landfall intensity is large, with Sally likely to range between a strong tropical storm with 65 mph winds to a high-end category 3 hurricane with 125 mph winds on Tuesday.

The landfall location is also not yet set in stone. There was fairly close agreement between the Friday evening and Saturday morning runs of the GFS, UKMET, and European track models that Sally would approach the coastline between New Orleans and Mobile by Tuesday. However, the center was further south on Saturday afternoon than had been predicted by Saturday morning’s computer models. This implies that the subsequent track may also be further south, bringing the system closer to Louisiana. The European and GFS ensembles from early Saturday morning (06Z) included a number of members with a Louisiana landfall. Because of Sally’s angle of approach, only a slight southward displacement of the track could allow impacts to extend substantially farther west over coastal Louisiana. We can expect track forecasts to shift west as the models incorporate this farther-south location.

One serious concern with Sally is an extended period of torrential rain along the central Gulf Coast, due to the storm’s expected slow motion of less than 5 mph on Monday through Thursday. Models suggest that a pocket of 10-15 inches of rain is likely near the storm’s center, with even higher localized totals possible. A larger corridor of 5-10 inches can be expected near the coast from southeast Louisiana to the Florida Panhandle. After landfall, the system may linger near the coast for another day or more, adding to the rainfall totals.

The type of west-northwest track predicted for Sally is especially favorable for driving storm surge into the coast of southeast Louisiana. The amount of surge will depend on how quickly Sally strengthens, the exact track it takes, and the eventual size of its wind field. Adding to the mix is the unfortunate timing that the new moon arrives on Thursday, September 17, bringing some the highest tides of the year next week – the king tides.

Figure 3
Figure 3. Rainfall forecast for the five days from 8 a.m. EDT Saturday, September 12, 2020, to September 17. Rainfall amounts in excess of 10″ (yellow colors) are predicted along the Gulf Coast near where Sally makes landfall. (Image credit: NOAA/NWS/WPC)

The 2020 parade of record-early named storms continues

Sally’s arrival on September 12 marks the earliest date that any Atlantic season has produced its eighteenth tropical storm, topping the record held by Stan from October 2, 2005. Only three more names remain on the 2020 Atlantic list: Teddy, Vicky, and Wilfred. Invest 95L in the eastern Atlantic is likely to become Tropical Storm Teddy by Sunday; the record for earliest-forming nineteenth storm in the Atlantic is October 4, 2005 (it was unnamed, as it was classified after the season was over).

With the Atlantic hurricane season just two days past the climatological half-way point, we’ve already had 18 named storms, five hurricanes, and one intense hurricane. Only seven Atlantic hurricane seasons since 1851 have had more named storms during an entire season. According to Colorado State University hurricane scientist Phil Klotzbach, the averages for this point in the season are seven named storms, three hurricanes, and one intense hurricane.

Figure 4
Figure 4. GeoColor visible satellite image of Tropical Storm Paulette (upper left) and much smaller Tropical Depression Rene (lower right) at 1750Z (1:50 p.m. EDT) Saturday, September 12, 2020. (Image credit: RAMMB/CIRA/Colorado State University)

Paulette expected to bring hurricane conditions to Bermuda

Resilient for days amid relentless wind shear, Tropical Storm Paulette is now on its way toward more favorable conditions as it approaches Bermuda. A hurricane warning is in effect for the island, as Paulette is predicted to swing over or very close to the island as a Category 1 or 2 hurricane.

Showers and thunderstorm activity (convection) were limited around Paulette on Saturday, but the storm has a strong, well-defined circulation, with midday Saturday top winds of 70 mph. Paulette will be passing over unusually warm waters of 28-29°C (82-84°F), about 0.5-1.0°C above average for mid-September. Wind shear is expected to drop dramatically to less than 10 knots from Sunday into Monday. Despite being engulfed by dry air (mid-level relative humidity around 40%), Paulette may still preserve a pocket of moist air around its core.

Paulette’s approach to Bermuda from the southeast is unusual: most of the many hurricanes that have threatened the island came from the south or southwest. Moreover, Paulette will be making a sharp recurvature toward the northeast in the vicinity of Bermuda. This adds a wrinkle to the track forecast in terms of the island. The most reliable track models are calling for Paulette to make its northeastward bend very close to Bermuda on Monday morning, as reflected in the NHC forecast. Peak winds in Bermuda will hinge on whether the center passes just east or just west of the island; the latter would put Bermuda in the stronger right-hand side of Paulette’s inner core.

Given its extensive experience with intense hurricanes, Bermuda is well equipped to handle Paulette. Rainfall could total 5″ or more on the island, and significant storm surge is possible depending on the exact track of Paulette’s center.

Rene weakens to a tropical depression

To the southeast of Paulette, Tropical Storm Rene was downgraded to depression status on Saturday morning. Choking on dry air, Rene may become a remnant low or could hang on to depression status as it lingers across the subtropical Atlantic south of Paulette for the next several days.

95L in the central Atlantic likely to become TD 20

Satellite imagery showed that the tropical wave located on Saturday afternoon in the eastern Atlantic, a few hundred miles west-southwest of the Cabo Verde Islands, was near tropical depression status.

This system, designated 95L by NHC, is predicted to initially move westward at 15 to 20 mph. Computer model forecasts increasingly have been suggesting that 95L will turn to the northwest well before reaching the Lesser Antilles Islands, and 95L could well turn out to be a “fish” storm – one that will only be of concern to shipping. In a 2 p.m. EDT Saturday Tropical Weather Outlook, NHC gave 95L two-day and five-day odds of development of 90%.

97L near the Cabo Verde Islands may be a short-lived tropical cyclone

A tropical wave that emerged from the coast of Africa on Friday was near the Cabo Verde Islands on Saturday afternoon. This wave, designated 97L by NHC, will have favorable conditions for development through Sunday, with SSTs near 27.5 Celsius (82°F), moderate wind shear of 5 – 15 knots, and a very moist atmosphere.

However, wind shear is predicted to rise to a high 20 – 30 knots on Monday, then increase to a prohibitively high 50 – 70 knots on Tuesday, likely destroying the system. In a 2 p.m. Saturday EDT Tropical Weather Outlook, NHC gave 97L two-day and five-day odds of development of 40% and 60%, respectively. This system will move slowly west-northwest through Sunday.

The most reliable hurricane models, according to their 2019 performance

A Gulf of Mexico disturbance worth watching

NHC was monitoring an area of interest over the north-central Gulf of Mexico producing a few disorganized showers and thunderstorms on Saturday afternoon. Some slow development is possible while this system moves westward and then southwestward over the northern and western Gulf of Mexico through Tuesday. Dry air over the western Gulf of Mexico is likely to inhibit its development. In a 2 p.m. EDT Saturday Tropical Weather Outlook, NHC gave this system two-day and five-day odds of development of 20% and 30%, respectively.

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Posted on September 12, 2020 (4:16pm EDT).

Topics: Weather Extremes

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