A robust tropical wave – a ripple in east-to-west flowing trade winds at about 10,000 feet resembling an ocean wave – plodding through the central Atlantic this week is poised to develop into a tropical depression or named storm and could strengthen as it approaches the Leeward Islands of the northeastern Caribbean by Thursday or Friday.

It’s still unclear if the system – designated Invest 95L by the National Hurricane Center – will pose a direct threat to the islands, including Puerto Rico and the Virgin Islands, or steer north later this week and weekend, but recent trends favor a recurve just north and east.

Probabilities of Invest 95L or what forms from it passing to within about 100 miles of a given location based on our most reliable global forecast models. Overnight models continue to favor a recurve just north and east of the islands. Until the system forms, however, there’s higher-than-usual uncertainty in the forecast. Interests in the Leeward Islands, Puerto Rico, and the Virgin Islands should continue to monitor the trends.

That said, it’s a little close for comfort and interests in these areas will want to continue to keep close tabs on the forecast.

Intensity models are generally bullish on strengthening beyond mid-week, though guidance has fluctuated wildly and runs the gamut from a tropical depression to a major (Category 3 or stronger) hurricane before the week is out.

Intensity aids for Invest 95L. Reliable guidance runs the gamut from an ill-formed tropical depression to a major (Category 3 or stronger) hurricane by the end of the week. Most guidance, including the better consensus aids, suggest gradual strengthening into a tropical storm or hurricane in the days ahead. (cyclonicwx.com)

While statistical models indicate persistent wind shear could cap strengthening, at times the high-resolution dynamical models like NOAA’s next-generation Hurricane Analysis and Forecast System (HAFS) have suggested the potential of robust or even rapid strengthening as it makes its closest approach to the northeastern Caribbean islands this weekend.

Given the run-to-run inconsistencies of the higher-resolution dynamical models, the upper-end scenarios should be met with a healthy dose of skepticism but they do suggest at least some potential for quicker strengthening if 95L finds its footing.

No concern for the U.S. mainland

As we discussed in this newsletter last Thursday, the odds of a system forming east of the islands and making it clear across the Atlantic to the mainland U.S. in October or November is exceedingly uncommon.

The jet stream finds its way farther south in October, dragging fall fronts into the deep Atlantic that more frequently impede development or form roadblocks, flinging westward-moving systems quickly northeastward and out to sea.

That’ll be the case by the end of the week, with the first major fall front plunging into the southeast U.S. Forecast models are even advertising the possibility that it clears us here in South Florida by late weekend, which would be an earlier-than-average first frontal passage.

Surface map forecast for Sunday morning from the National Weather Service showing the cold front sliding into South Florida. It’s possible the front stalls before clearing South Florida, but it’s something we’ll be watching for this weekend. (National Weather Service)

We know better than to hold our breath for a cold front in early October, but it’s definitely something to look forward to after what’s been a top 10 wettest September on record for most stations in southeast Florida (fourth wettest for the Fort Lauderdale area based on a 105-year period of record).

Ranking for total precipitation observed since Sept. 4 for Broward and Miami-Dade Counties in southeast Florida. Both the Miami and Fort Lauderdale areas observed a top 10 wettest September based on a 100-plus year period of record. The Fort Lauderdale area recorded its fourth wettest September in records going back to 1912. (Southeast Regional Climate Center)

Regardless, the sharp jet stream dip accompanying the strong fall front will steer any system approaching from the east quickly out to sea.

Tallying up the season so far

The Atlantic hurricane season struggled around its traditional September peak, going dormant for almost three weeks through what is historically its busiest stretch. This meant the hurricane season quickly ran a deficit in activity by mid-September.

It’s since done some work to try to overcome that deficit – with Category 4 Gabrielle, Category 5 Humberto, and Category 2 Imelda in quick succession.

Nevertheless, activity is still running about 10% below the 30-year average. Although we’ve recorded two fewer named storms and about one to two fewer hurricanes than average so far, we’re running slightly above average in the number of hurricanes that have reached Category 3, 4, or 5 status (we’ve had two Category 5s and one Category 4 hurricane so far this season).

Typically about 20% of hurricane season activity remains after Oct. 5.

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Tropical system likely to form, could strengthen as it nears eastern Caribbean later this week

Invest 95L could become Jerry before the week is out, passing near or north of the Lesser Antilles

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Michael Lowry