Jeff Masters Weather Blog

Late-week western Caribbean disturbance may develop » Yale Climate Connections


The western Caribbean is the potential breeding ground for a tropical cyclone this week, with Central America most at risk of seeing impacts by the end of the week — on the heels of another system, Tropical Storm Pilar, that will drench the Pacific side of Central America.

Conditions in the western Caribbean will be favorable for tropical cyclone development this week, with record-warm waters of about 30 degrees Celsius (86°F), plenty of atmospheric moisture, and light upper-level winds bringing low wind shear. There is strong model support for something to develop by late in the week.

The most recent set of model runs predicted that a ridge of high pressure to the north of the Caribbean would keep anything that develops on a mostly westward trajectory, giving Nicaragua and surrounding portions of Central America the potential to see a landfalling system from the Caribbean on Friday or Saturday. By early next week, a trough of low pressure passing to the north might be capable of pulling a potential Caribbean system northward into the Gulf of Mexico or Cuba, though this has looked increasingly unlikely in recent model runs. In their 8 a.m. EDT Monday Tropical Weather Outlook, the National Hurricane Center gave a future western Caribbean disturbance two-day and seven-day odds of development of 10% and 50%, respectively. The next name on the Atlantic list of storms is Vince.

As discussed by Michael Lowry in his Monday Substack post, “the western Caribbean is the place to look this time of year. Roughly a third of all hurricanes that form this late in the season do so in the western Caribbean. Moreover, 9 of the 11 (82%) Category 3 or stronger hurricanes recorded this late or later in the hurricane season have formed in the western Caribbean.” However, it is very unusual to see November hurricanes developing during an El Niño year, as documented by hurricane scientist Andy Hazelton (see Tweet below).

Disturbance brings heavy rains to Puerto Rico

Tropical disturbance 96L formed on Friday just north of Puerto Rico and was located a few hundred miles east of the northwestern Bahamas on Monday morning. Satellite images early Monday afternoon showed that 96L had a robust circulation but only a limited amount of heavy thunderstorms. Wind shear is expected to increase from 20-25 knots on Monday to 40-45 knots on Tuesday, which should put an end to any chances of development. In their 8 a.m. EDT Monday Tropical Weather Outlook, the National Hurricane Center gave 96L two-day and seven-day odds of development of 20%.

On Friday through Sunday, unsettled weather in association with 96L brought heavy rains of five to seven inches to portions of Puerto Rico and the Virgin Islands, causing damaging flooding. In the capital of San Juan, flood damages were estimated in the millions of dollars. Heavy rains are expected to continue on Monday over eastern Puerto Rico and the Virgin Islands, where flood advisories were in effect.

Death toll in Acapulco rising from Hurricane Otis

The death toll from the catastrophic impact of Hurricane Otis in Mexico has risen to 48 with six missing, according to the latest statement from the Mexican government on Sunday. Otis made landfall in Acapulco on Wednesday, October 25, as a Category 5 hurricane with 165 mph winds, making it the strongest Pacific hurricane on record to hit Mexico. Satellite mapping (see Tweet below) has revealed that damage from Otis was heaviest in the high-elevation areas at the edges of the city, where high winds and landslides from heavy rains falling on the steep slopes combined to cause massive destruction. Few portions of the city escaped damage.

According to media reports and eyewitness accounts, the people of Acapulco are suffering greatly. Water, food, and gas are in short supply, and electricity, communications are mostly unavailable. Some bus service has been restored, though, and the airport is partially open. As of Monday, power had been restored to 58% of the city, and the president of Mexico said power would be fully restored to the city on Tuesday.

Heavy rains from Pilar to hit several countries in Central America

Tropical Storm Pilar may never come ashore in Central America, but it still poses a risk of flooding rains to several nations. As of 11 a.m. EDT Monday, Pilar was located about 225 miles southwest of San Salvador, El Salvador, with top sustained winds of 50 mph. Tropical Storm Watches were in effect for the Pacific coastal areas of El Salvador, Honduras, and Nicaragua north of Puerto Sandino. Strong showers and thunderstorms, or convection, were already pushing onshore on the periphery of Pilar, which had a consolidating convective core as well as extensive thunderstorm activity well to its southeast.

satellite image of Central America showing Pilar over the waters to the west
Figure 1. Combined visible/infrared satellite image of Tropical Storm Pilar, including showers and thunderstorms well southeast of its center, at 11 a.m. EDT Monday, October 30. (Image credit: NOAA)

Pilar was headed east-northeast toward Central America at about 6 mph on Monday morning, being pulled by an upper-level low in the Caribbean. The upper low will shift into the Gulf of Mexico, to be replaced by a strong upper high. This change in steering currents appears destined to prompt a 180-degree turn in Pilar’s motion, sending it back toward the open Pacific after midweek. The open question is how close Pilar will get to El Salvador, Honduras, and Nicaragua before it reverses course and how long it will remain close enough to bring rains onshore. Virtually all of the ensemble members of Sunday night’s European and GFS runs bring Pilar within about 100 miles of landfall before it turns away from the coast.

maps of forecast tracks showing Pilar moving toward Central America and then reversing course
Figure 2. Track forecasts out to five days for Pilar from the 06Z Monday run of the European ensemble model (left) and the GFS ensemble model (right). Individual forecasts of the 51 Euro and 31 GFS ensemble members are the lines color-coded by the wind speed in knots they predict; red colors correspond to a Category 1 hurricane. The time in hours from the model initialization time is in gray text. Long-range models such as the European and GFS models are less reliable guides to storm intensity than higher-resolution, shorter-range models such as HWRF, HMON, and HAFS. (Image credit: weathernerds.org)

Tropical-storm-force winds only extend about 70 miles from Pilar, and that radius is not expected to change dramatically, so Central America is unlikely to experience major wind damage from Pilar. However, as the circulation expands over time, persistent heavy rain bands pivoting around Pilar could push well inland, especially on Tuesday and Wednesday, exacerbated by upslope flow into rugged terrain. The heaviest rains — locally five to 10 inches, with higher amounts possible, perhaps triggering flash floods and mudslides — will most likely be in the onshore flow near and just east of Pilar’s center, along the coastlines and the Pacific-facing slopes of eastern El Salvador, southern Honduras, and western Nicaragua.

Pilar is predicted by the National Hurricane Center to intensify to near hurricane strength by Tuesday night, as the storm draws on unusually warm sea surface temperatures (28-29 degrees Celsius or 82-84 degrees Fahrenheit). Working against Pilar will be moderate to strong wind shear of 10-25 knots. As Pilar nears the coast, some dry air flowing offshore around Pilar’s north side may also infiltrate what is otherwise a moist environment (midlevel relative humidity of 70-80%). Pilar should only gradually weaken late this week as it pulls away from the coast, eventually moving into a drier environment by the weekend.

Storm Ciarán aims at Europe

An unusually powerful Atlantic storm will pass just south of Ireland on Wednesday and is expected to blast the UK and France with destructive winds on Thursday, November 2. This storm, named Ciarán by the UK Met Office, is predicted to have one of the lowest pressures ever observed in the UK in November. Wind gusts up to 129 km/h (80 mph) are forecast for portions of the UK and France, and heavy rain will fall over much of western Europe.

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