10:00 AM Thursday, May 29th - **The initial threat of the 2025 Atlantic Basin tropical season likely comes in about ten days**

The MJO appears to be headed to “Phase 8” in about ten days or so as depicted here by the European computer forecast model (indicated with arrow) and this is often a favorable location for tropical activity to take place somewhere in the Atlantic Basin. Map courtesy ECMWF, NOAA

Overview

The 2025 Atlantic Basin tropical season officially gets underway on Sunday, June 1st, and there are signs that the initial threat could come as early as ten days or so from now (i.e., June 8, 9, 10th time frame). A teleconnection index known as the Madden-Julian Oscillation or MJO appears to be foreshadowing the potential of tropical troubles in this time frame as it transitions into a “phase” that is generally quite favorable for activity in the Atlantic Basin. In addition, a second favorable sign may be the evolution of the upper-level pattern of highs and lows across North America which may feature strong ridging over southeastern Canada in about ten days or so. A strong ridge in this part of North America is often favorable this time of year for an “undercutting” tropical system in the southwestern Atlantic, Gulf of America, or Caribbean Sea where sea surface temperatures are generally warmer-than-normal.

While the tropical season will be very young, the sea surface temperatures in the Gulf of America and southwestern Atlantic (circled region) are generally warmer-than-normal and likely can support the development of a tropical system during the early or middle parts of June. Map courtesy NOAA, tropicaltdbits.com

Madden-Julian Oscillation or MJO

The official beginning of the 2025 Atlantic Basin tropical season comes this Sunday, June 1st, and there are signs that point to an initial threat in about ten days from now (i.e., around the 8th, 9th or 10th of June). One of the signs comes from a teleconnection index known as the Madden-Julian Oscillation or MJO which tracks a tropical disturbance that propagates around the global tropics. Unlike El Nino or La Nina which are stationary features in the equatorial Pacific Ocean, the MJO is an eastward moving disturbance of clouds, rainfall, winds and pressures that traverses the planet in the tropics and returns to its initial starting point in 30 to 60 days, on average. The MJO was first discovered in the early 1970s by Dr. Roland Madden and Dr. Paul Julian when they were studying tropical wind and pressure patterns. 

Some computer forecast models including the GFS (not shown) and Euro (shown here) are sending signals for low pressure development in about ten days or so somewhere in the SW Atlantic, Gulf of America or Caribbean Sea. Map courtesy ECMWF, Weather Bell Analytics (Meteorologist Joe Bastardi, X)

Research and empirical observations have found that the location or “phase” of the MJO is linked with certain temperature and precipitation patterns around the world. The MJO phase diagram illustrates the progression of the MJO index through different phases, which generally coincide with locations along the equator around the globe. When the index is within the center circle, the MJO is considered weak, meaning it is difficult to discern. Outside of this circle, the index is stronger and will usually move in a counter-clockwise direction as the MJO moves from west-to-east. The very latest phase diagram produced by the European computer forecast model propagates the MJO into Phase 8 in about ten days or so and this is often an active location for tropical activity to take place in the Atlantic Basin during this time of year.

A strong ridge of high pressure (shown in red/orange) may build into the southeastern part of Canada in about ten days or so which can often lead to tropical activity this time of year across the southwest Atlantic, Gulf of America, or Caribbean Sea. Map courtesy NOAA, tropicaltidbits.com

Upper-level signals

The 500 millibar height field across North America is another important factor that I focus in on during the tropical season in order to determine the potential for tropical activity in the Atlantic Basin. If, for example, a strong ridge of high pressure builds aloft over southeastern Canada or the northeastern US during the tropical season, the atmosphere may respond with low-level “convergence” across the southwestern Atlantic, Gulf of America, or Caribbean Sea which, in turn, can lead to the development of a tropical wave that will have the ability to move from east-to-west and, in effect, “undercut” the ridge of high pressure to the north. Indeed, some computer forecast models point to the formation of a strong ridge of high pressure across southeastern Canada in about ten days or so (example shown comes from the 00Z GFS) at the same time the MJO appears to be headed into a more favorable location or phase (8) for tropical activity in the Atlantic Basin.

The bottom line...we’ll have to closely monitor the SW Atlantic/Gulf of America/Caribbean Sea region in coming days for possible tropical activity during the time period of around June 8th, 9th or 10th.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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