Hurricane Season Has Begun

by

Weather

Two seasons start today. One is meteorological summer, the three warmest months of the year, June, July, and August. The other is the Atlantic Hurricane Season, which ends on November 30.

We've already hit the ground running. We had two suspect areas in May, one responsible for horrible weather across the state on Saturday and another over a week ago that didn't amount to anything.

One more suspect area formed yesterday in the Gulf of Mexico. Here's an extreme close-up of satellite imagery:

It's looking and acting like a duck, so I'm listening for quacking. It's in an area of semi-favorable conditions with low vertical shear. Sea surface temperatures are less than 80°F, a typical ingredient for tropical cyclone formation, but they can form over cooler waters.

NHC is taking this seriously enough to have scheduled the Hurricane Hunters to fly into it this afternoon. Depending on what they find, it could be declared a tropical depression or storm later today.

Whether it becomes a tropical cyclone or not, it is unlikely to last long. It's also unlikely to have any direct impact on South Carolina. Computer models forecast it to drift southward over the next day or two, taking it over warmer waters but into an area of more shear. After that, it likely turns eastward, moves across Florida, and into the Atlantic. If it survives through the weekend, it will probably find more favorable conditions over the Atlantic. However, it would continue eastward and away from South Carolina.

That is the only feature of interest in the tropics as we begin the hurricane season. Tropical waves are crossing the deep tropics already; the lead car on the African wave train usually departs Dakar Station in mid-May.

One tropical wave is along 64° west longitude, moving through the Lesser Antilles. Another is over the Atlantic near 50° west. A third is southwest of Cabo Verde along 28° west. All three are drifting steadily westward and are disorganized, and none will develop further because upper-level winds are hostile.

Now that we addressed immediate concerns, I want to tell you about the seasonal activity forecasts for this year. Today, several private and public entities issue seasonal tropical cyclone forecasts for the Atlantic Basin. The best known are from the seasonal forecasting team at Colorado State University (CSU) and the National Oceanic and Atmospheric Administration (NOAA). Others include the European Center for Medium-Range Forecasts (ECMWF, creator of 'the Euro' model), and private-sector weather forecasting companies like WeatherBELL.

Here's the range of forecasts for the number of hurricanes predicted to form this season in the Atlantic:

You can see that the forecasts are all clustered around the average number of hurricanes that we see in the Atlantic, which is 7.2, so most hurricane forecasters call for this to be a near-average season.

The CSU team updated their forecast for this season today and has increased their numbers a bit from their original forecast released in April.

The official NOAA forecast is close to CSU's.

While there is some variance between all the seasonal forecasts for the Atlantic hurricane season, the number of forecasts for an active season is roughly the same as those for a below-average season.

These seasonal outlooks consider many factors, including the state of ENSO, which stands for El Niño - Southern Oscillation. It's an all-encompassing term to describe the cycle of changing water temperatures in the equatorial Central and East Pacific and its effects on the atmosphere. During El Niño, water temperatures are warmer-than-usual in this area, and with La Niña, the water is cooler-than-usual. There is also a neutral phase (some scientists jokingly call it 'La Nada') with near-average water temperatures.

After nearly three years of La Niña in the East Pacific (one of the longest La Niña periods on record), El Niño is now coming on strong, like a rubber band snapping.

La Niña periods tend to produce more favorable conditions over the Atlantic for tropical cyclones development, partly explaining why some recent hurricane seasons have been so active. The opposite is true during El Niño, when upper-level winds are less favorable over the tropical Atlantic due to dominant westerly winds aloft. Those westerly winds are roughly the opposite direction as the low-level trade winds, creating vertical shear that tears tropical cyclones apart.

Another important factor for seasonal forecasters is water temperatures across the Atlantic Ocean. The Atlantic Ocean experiences a long-term cycle of warmer-than-average and cooler-than-average waters called the Atlantic Multidecadal Oscillation (AMO). The Atlantic has been in a warm phase of the AMO since the mid-1990s. The warm phase of the AMO, as you might expect due to the warmer-than-average waters, tends to result in more active hurricane seasons. It's another explanation for the busy hurricane seasons of recent years and a reason why we have had several busy hurricane seasons over the last 25 years or so.

Each phase of the AMO typically lasts about 30 years, so we should see the AMO switch to its cool phase in the next 5-8 years. When it does, we should typically see less active hurricane seasons for a few decades.

Seasonal hurricane forecasters consider other factors, too. Rainfall over the Sahel region of Africa correlates to hurricane activity in the Atlantic; wetter periods in the Sahel typically mean more active hurricane seasons. They also consider computer model output and other atmospheric oscillations like the Quasibiennial Oscillation. However, the state of ENSO and the AMO are the two most critical factors in their forecast.

With ENSO and AMO at odds with one another, the seasonal forecast for the Atlantic is more uncertain than usual. It's hard to tell whether ENSO or the AMO will be more dominant.

I've been mulling it over for months, and I expect a near-average season, perhaps slightly above average, in terms of the number of named storms. Those warm waters should result in at least an average normal of tropical storms and a near-average number of hurricanes. However, I think the number of major hurricanes will be less than average. The El Niño-induced vertical wind shear should keep most storms that form from becoming so intense.

These forecasts all take into account that we've already had one storm that you might not have heard about. Back in January, a subtropical storm formed over the Gulf Stream east of Virginia and ended up in Canada. It didn't get a name because it was considered non-tropical at the time but determined to be a subtropical storm well after it occurred during a reanalysis in May.

What does this mean for you? Well, the message is the same as for the start of every hurricane season: Be ready! We have had tropical cyclones impact the state in back-to-back years and even a few seasons with multiple impacts. Even if there was only one hurricane this year, if it hits South Carolina as a hurricane, it's a bad year for us. As many meteorologists and emergency managers say, it only takes one.

So, please have your hurricane (and other disaster) preparations in place. The preparedness experts at SCEMD have hurricane.sc refreshed for 2023, and it's a one-stop-shop of resources.

Finally, as a programming note, this serves as your message for the week since our weather looks mostly tranquil (and mostly cooler-than-average) over the next week. It looks as though this weekend will be mostly rain-free aside from stray showers Saturday night and Sunday. A cold front could bring much of the state a few showers and maybe a thunderstorm on Tuesday. Also, be mindful that the current weather pattern is favorable for cold fronts to move in and become stationary near our coast, which is one way we get 'homegrown' tropical systems. So, be on the lookout for a special alert. Unless one of those becomes necessary, see y'all next Friday!