Posted on 29 September 2020 by Guest Author

This is a re-post from Yale Climate Connections by Jeff Masters

The enjoyable lull in Atlantic tropical cyclone activity over the past week may come to an end this weekend, when a large low-pressure system that is expected to develop in the western Caribbean has a 50% chance of spawning a tropical depression, according to the National Hurricane Center.

A large, complex area of low pressure called the Central American Gyre (CAG) will develop in the western Caribbean late this week, generating heavy rain as it moves slowly west-northwest. CAGs tend to develop near the beginning and end of the Atlantic hurricane season, and they can sprawl over hundreds of miles. They are notoriously long-lasting and slow-moving, making them prodigious rain producers. For more background, Phillipe Papin is an expert on CAGs (click here to animate his tweet, shown right), and he maintains an excellent CAG forecast page.

As smaller-scale vortexes spin around the CAG, one or more of them can consolidate into a tropical cyclone and eventually break away from the gyre. That outcome can happen in either the Atlantic or the Pacific. For example, on May 31, the first named storm of the Eastern Pacific hurricane season, Tropical Storm Amanda, developed from a CAG, and it made landfall on the south coast of Guatemala later that day. In October 2018, after a week of gestation, a disturbance on the north end of a CAG became a tropical depression in the western Caribbean. Three days later, that depression had detached from the gyre and barreled into the Florida Panhandle as mighty category 5 Hurricane Michael.

The exact location and timing of any tropical cyclones that may develop from this week’s CAG cannot yet be predicted with much probability of success. If the CAG does manage to spawn a hurricane in the coming week, the most likely location for a storm would be in the waters of the extreme northwest Caribbean, near western Cuba and the northeast Yucatan Peninsula. It is more likely that a tropical depression or weak tropical storm would form farther to the south, as predicted by the 6Z Tuesday run of the GFS Ensemble Forecast System (Figure 1).

Figure 1. Predictions for 6Z (2 a.m. EDT) Saturday, October 3, from the 6Z Tuesday, September 29, run of the recently updated and expanded GFS ensemble forecast system (GEFS), of the locations of surface low- and high-pressure systems. (Image credit: Tropical Tidbits)

In that figure, predicted locations of centers of low pressure are shown as orange numbers in millibars, with the leading “10” or “9” omitted, depending on whether the low’s central pressure were above or below 1000 mb. For example, a 999-mb low pressure center will be displayed as “99”, and a 1000-mb low pressure system will be displayed as “00”. In blue numbers, with the leading “10” left off, are the predicted locations of centers of high pressure. For example, a 1020-mb high pressure system will be coded as “20”. Each of the 31 forecasts from the individual member forecasts generated a different location and central pressure for major high- or low-pressure systems. The color-coding is a measure (in standard deviations) of ensemble spread – the difference in pressure between the ensemble mean and the individual member. Six ensemble members predicted a hurricane-strength system with a pressure of 985 mb or lower in the western Caribbean on Saturday. These were color-coded orange, showing a high spread from the ensemble mean, since their central pressure differed greatly from the mean predicted pressure.

Figure 2. Departure of sea surface temperature (SST) from average on September 29. SSTs were approximately 1.0 degrees Celsius (1.8°F) above average in the Caribbean. (Image credit: Tropical Tidbits)

Even busy hurricane seasons have extended lulls

Even the busiest hurricane seasons have lulls. For example, during the record-busy 2005 Atlantic hurricane season, Irene was named on August 7, but the next storm, Jose, didn’t get named until August 22.

Lulls like these are usually caused by sinking air over the tropics, leading to drying, high pressure, and reduced odds of tropical storm formation. Sinking air is caused by an unfavorable state of the Madden Julian Oscillation (MJO), or by the suppressed phase of an atmospheric disturbance called a convectively coupled Kelvin wave (CCKW). The MJO is a pattern of increased thunderstorm activity near the equator that moves around the globe in 30 to 60 days; odds of tropical cyclone formation increase when the MJO is strong and located in the proper location, but typically decrease for ocean basins not in the active portion of the MJO. Similarly, a CCKW is a large but subtle atmospheric impulse, centered on the equator, that rolls eastward at 30-40 mph, with showers and thunderstorms along its forward flank. On either side of this center of action, sinking air, high pressure, and reduced odds of tropical storm formation typically occur.

Passage of the suppressed phase of a CCKW and an unfavorable MJO have been acting to dampen Atlantic tropical cyclone activity since last week, but that situation will change by mid-October. With ocean temperatures still much above average (Figure 2) and a season that has a proven track record for spitting out large numbers of named storms, we likely will see at least three named storms form in October. As discussed above, there is even a chance of one spinning up this weekend in the western Caribbean, despite the relatively unfavorable MJO.

Figure 3. In this velocity potential (VP) anomaly plot* from September 29, the MJO brings positive 200 mb VP anomalies (yellow colors) and reduced odds of tropical cyclone formation to the Caribbean (top). However, by mid-October, this pattern is predicted to shift, with negative 200 mb VP anomalies (blue colors) over the Caribbean. (Image credit: Michael Ventrice).

*The amount of rising or sinking air can be inferred from the 200 mb velocity potential (VP) departure from average (also called the anomaly). Negative 200 mb VP anomalies mean that upper-level winds at the 200 mb level are diverging, causing rising air from below to replace the air diverging away at high altitudes. This rising air helps nurture thunderstorm updrafts, and favors low pressure and increased chances of tropical cyclone formation. Conversely, positive 200 mb VP anomalies imply converging air aloft, where sinking air, high pressure, and dry conditions will be unfavorable for tropical cyclone formation. In this plot, negative 200 mb VP anomalies (divergence) are cool-colored contours (the scale shows the departure from average in standard deviations); positive 200 mb VP anomalies (convergence) are warm-colored contours.