Dorian may be a candidate for a similar extreme rainfall to Harvey (2017) and Florence (2018) as blocking high pressure systems either side of Dorian cause a slow turn over Florida that could take up to 3 days to execute. The NOAA/WPC 7-day rainfall forecast already indicates a large swath of 15-20 in. of rain for much of eastern Florida.

Dorian may slow on approach to Puerto Rico today, lose some intensity tonight crossing the island then re-emerge north of Puerto Rico reaching hurricane strength in 2 days. Dorian turns more west-northwest due to steering guidance of a strengthening Bermuda High on the weekend. Tropical cyclone models indicate Dorian could become a major hurricane in the day 4 to 5 period while east of Florida.

The D1, D2 drought areas in southwest/southeast Brazil look to receive some beneficial rainfall during the medium-range period agreed upon by both the GFS and ECM. The wetter trend for these areas which desperately need the rain is a forecast change (from drier outlook late last week).

Two potential tropical cyclones have emerged in the North Atlantic tropics. Tropical Disturbance 98L moves into Florida this weekend and may reorganize moving northeastward off the U.S. East Coast becoming a tropical storm Monday night or Tuesday. In the central North Atlantic tropics Tropical Disturbance 99L may become a tropical storm moving toward the Caribbean Sea early next week.

Neutral ENSO ahead BUT with a Modoki-like character for northern hemisphere autumn. Indian Ocean Dipole forecasts indicate an immensely intense positive phase.

Research has shown that when the far eastern equatorial Pacific is cool and near the Dateline in the tropics surface water is warm there is a tendency in the upper air pattern over the subtropical western North Atlantic for a low pressure trough to form. This phenomenon is known as the tropical upper troposphere trough (TUTT). The TUTT pattern produces upper level shear suppressing tropical activity.

Solar Cycle 24 is ending as a new solar minimum arrives as of late 2018 and 2019. Interestingly, NASA forecasts an unusually lengthy solar minimum extending through 2022 and into 2023. Solar minimum usually lasts less than 2 years as part of the complete 11-year solar cycle.

Emergence of phase_1/phase_2 of the Madden Julian oscillation implies increased vertical motion and lowering upper level wind shear in the tropical North Atlantic (where tropical cyclones develop) and tropical Africa (where tropical waves which become Atlantic tropical cyclones develop). The MJO has anchored over Maritime Continent/West Pacific tropics recently certainly contributing to an increase in tropical cyclone activity. However, today’s ECMWF MJO forecast indicates MJO weakens over Maritime Continent and is likely to regenerate over the tropical North Atlantic/Africa in 2 weeks.

A summary of seasonal forecasts of North Atlantic basin tropical cyclone activity issued initially in April and updated in June and August by the Tropical Meteorology Project at Colorado State University, Tropical Storm Risk of U.K. and Climate Impact Company indicate each revision is slightly more active due to less inhibiting upper shear due to a weakening El Nino (compared to early forecasts) coupled with a warmer than normal North Atlantic basin especially just north of the tropics.