Melissa likely to rapidly intensify this weekend

Almost nothing has changed since my previous post on Tuesday, but I'll provide the few updates there are regarding Melissa. Melissa is still a tropical storm, and is still in the central Caribbean and is barely moving.  The extremely warm and deep waters of the Caribbean near Jamaica will provide an endless fuel source with no opportunity to upwell cooler water. The vertical wind shear which has been inhibiting intensification so far is quickly decreasing. All signs point to rapid -- or "explosive" -- intensification very soon.

On Friday afternoon, Tropical Storm Melissa is intensifying and is forecast to become the season's fifth hurricane shortly.  It has barely moved in the past couple of days, and is only forecast to drift to the west toward Jamaica over the next 3-4 days. This will undoubtedly result in a significant disaster in Jamaica, but also further east in Haiti and perhaps even the Dominican Republic.  A Category 4 or 5 hurricane landfall is bad enough, but to have it last for a few days is much worse (remember Dorian over the western Bahamas in 2019?), and the rainfall and resulting flash floods and mudslides in mountainous areas will be devastating. I have long updating radar loops from Jamaica available at https://bmcnoldy.earth.miami.edu/tropics/radar/.

The track forecast takes Melissa over Jamaica on Tuesday, then eastern Cuba and the Bahamas on Wednesday.  The multi-day stall will be definitively ended as a deep trough sweeps in across the eastern U.S. and picks the storm up and accelerates it off to the northeast. As of now, hurricane watches are in effect for Jamaica and Haiti's Tiburon Peninsula.

Assuming Melissa becomes a major hurricane (Category 3+), it would not be out of place at all historically. Below is a map showing tracks of the twenty major hurricanes that have been in the Caribbean Sea during October, going back to 1920.  The most recent were Delta (2020), Matthew (2016), and Sandy (2012).  There have been five Category 5 October Caribbean hurricanes: Matthew (2016), Wilma (2005), Mitch (1998), Hattie (1961), Unnamed (1924). They all made landfall eventually of course because it's virtually impossible to escape the Caribbean without hitting land.

The season's Accumulated Cyclone Energy (ACE) is at 90% of average for the date, but that's about to change quite a bit as Melissa intensifies. Based on NHC's current intensity forecast, 2025's ACE will cross above the average line on Monday.

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Melissa forms in Caribbean, forecast to intensify and stall over exceptionally warm water

The easterly wave that I mentioned in the posts on October 13 and again on October 15 finally made it into the Caribbean on Sunday and then was upgraded to Tropical Storm Melissa on Tuesday morning. It is located in the central Caribbean Sea, south of Hispaniola and just north of Venezuela.

Melissa is the 13th named storm of the season, and formed just four days ahead of the climatological date of 13th named storm formation.

The storm is forecast to gradually strengthen in the coming days, but also slow down to a crawl as it loses steering currents. And any time there is a forecast involving weak steering currents, there is an exceptional amount of uncertainty. Of course, the "cone of uncertainty" can't actually capture that because it's the same size for all storms all year long -- so on the map below, you need to mentally inflate the later days of the cone in this case. [see "Cone of Uncertainty" Update & Refresher]

If we take a look at some ensembles, we get a sense of the range of possibilities that helped to inform the NHC forecast shown above.  Some members track it across the Caribbean toward Central America, while others show a stall followed by a sharp turn to the north. Several others linger somewhere in between the two.  Any track forecast more than a few days out has very low confidence.  These ensemble members' tracks are shown out to one week (October 28)... American model on the left, European in the middle, and Google Deepmind on the right.

NHC is leaning slightly toward the north turn scenario, and has issued a tropical storm watch for Jamaica and a hurricane watch for Haiti's Tiburon Peninsula. Certainly anyone in the western Caribbean should be paying close attention to this... and if the lingering lasts longer and it drifts further west then a turn to the north eventually happens, it's not out of the realm of possibility that Florida will need to be on alert -- but not yet.

Quite a number of the ensemble members indicate Melissa becoming a strong hurricane in the coming week, while others keep it very weak -- again, huge uncertainty. One element that looks much more certain is flooding rainfall across the Greater Antilles, particularly Haiti and the Dominican Republic.

The ocean heat content is exceptionally high for this time of year across much of the Caribbean, so even a stalled storm will not be able to upwell cooler water from below. The Caribbean is famously an endless fuel source for hurricanes, and these huge anomalies only boost that further.

https://bmcnoldy.earth.miami.edu/tropics/sectors/

If the forecast scenario of an "M" storm in October cruising into the Caribbean, stalling, and turning north as a major hurricane seems familiar to you, it should: this outcome would be remarkably similar to what we saw with Matthew in 2016:

Track of Hurricane Matthew (2016)

... and also Hurricane Hazel in October 1954:

Track of Hurricane Hazel (1954)

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Lorenzo dissipates, and eyes still on Caribbean early next week

After just three lackluster days, Tropical Storm Lorenzo dissipated on Wednesday afternoon in the central Atlantic... far from any land.

The African easterly wave I referenced in Monday's post is still showing up in the model guidance, though taking a bit longer to develop.  The ensembles generally suggest the wave reaching the Lesser Antilles on Monday then perhaps a chance of intensification in the Caribbean.  

The three maps below show track forecasts of the potential next system from the European (top), American (middle), and Google Deepmind (bottom) ensembles... all of them end one week from now. The background shading is the sea surface temperature.  You can easily see varying degrees of bullishness on developing this wave, and within the Caribbean, the intensity is all over the place -- but the potential for a hurricane in the eastern Caribbean mid-week is there. The next name on the list is Melissa.

Now that there's a lull in the activity, I'll share some preliminary season-to-date materials.

First is the Accumulated Cyclone Energy, which I share frequently throughout the season.  That's at 91% of average for the date and will drop to 88% of average by next Wednesday if nothing else develops by then. The ACE was only above-average for a couple weeks at the end of August.

Next is a map of all 12 tropical cyclones so far this season, with their peak intensity, minimum pressure, and ACE contribution listed on the right. It is really striking how the storms have so far stayed out over the ocean for the most part. Of course, that may not hold true for the rest of the season.

Finally, some verification statistics for track and intensity forecasts made by the National Hurricane Center -- these are averaged over all twelve storms as well. The track errors have been right around average for all lead times, from 1 to 5 days. The 4-day (96-hour) track errors are just slightly above average.  Intensity errors, on the other hand, have been decidedly above average this season so far.

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