17 June 2023

What month is it? Next storm likely to form from African wave

A tropical wave that left the African coast a couple days ago is strongly favored to become the next named storm, Bret, in the coming days.  This would not be unusual at all in August or September, but it's mid-June.  It could even become the season's first hurricane next week.

A key player in this accelerated "Cabo Verde" season is the Azores High, a quasi-permanent feature of the subtropical North Atlantic Ocean. It gets its name because it's typically centered near the Azores Islands (about 1000 miles west of Portugal)... though it does have some variability in its position and strength.  However, if we look at the average surface pressure over the past couple of weeks and compare that to its normal position and intensity, we clearly see that it is significantly weaker and displaced to the southwest.

Average sea level pressure from June 1 - June 14 during 2023 (top) and during a climatological period of 1991-2020 (bottom).  Pressure values (in millibars) are plotted on the same scale for easy comparison.

This huge difference is allowing the easterly winds that blow across Africa and across the tropical ocean to be weaker, resulting in less Saharan dust transport from the continent (those dust plumes are good at intercepting the sun's energy before it reaches the ocean) AND in reducing the evaporative cooling of the water.  The combined effect has been to create alarming and record-smashing sea surface temperature anomalies in the tropical east Atlantic.  This has turned that part of the basin from what would typically be too cool for tropical cyclone development to plenty warm for development.  We're seeing a 2-month boost to the Cabo Verde season as a result.

Daily SST anomaly relative to a 1991-2020 climatology.  The warm anomalies in the eastern tropical Atlantic are accelerating the onset of the Cabo Verde season.  The approximate location of Invest 92L on Saturday morning is circled in white.  Available at  https://bmcnoldy.earth.miami.edu/tropics/sectors/

Back to the feature of interest, which is tagged as Invest 92L.  With this unusually-favorable environment to work with, the National Hurricane Center is giving it a 40% chance of development within the next two days and a 70% chance within the next seven days. The area of potential formation within the next seven days is illustrated by the red shape on this map.  This is not a track forecast or a "cone of uncertainty", just an envelope of potential formation as the disturbance plods along.

The outlook from some of the earliest model guidance available for Invest 92L is very bullish, with two reliable hurricane models, HMON and HWRF, bringing this to nearly a Category 3-4 hurricane in a week.  Most models are not that high, but still a hurricane appears likely as it nears the Leeward Islands next weekend.  This is definitely something to watch closely. 

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