Bob Blog 9 May 2021

Bob McDavitt’s ideas for sailing around the South Pacific.

Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.

Compiled Sunday 9 May

Regarding the LUCKGRIB app I mentioned last week—The only supported ways to run the app are by downloading it to your Mac, iPhone or iPad via the Apple App store. Anybody downloading the app via that link for Android should first do a backup, as there is a great chance that the download may corrupt your device…

Regarding the weather links I mentioned a few weeks back, the ones for the Fiji and New Zealand weather maps are crook and linking to old data at present. This is being worked on and hopefully can be fixed. I’ll keep you posted.

The Tropics

The 2020-2021 Cyclone Season (from Wikipedia)


On December 8, a disturbance formed near Fiji, starting the 2020–21 South Pacific Ocean cyclone season, it gradually intensified into a depression and attained a tropical storm status according to JTWC. It reached at the maximum 10 minutes sustained wind speed of 55 km/h (35 mph) and minimum pressure of 1000 mb (29.53 inHg). Another disturbance formed near the existing 01F and rapidly intensified into a depression. It hampered the system intensification due to a brief interaction with Tropical Depression 01F. Following the same day, another disturbance formed and intensified into a depression the next day. 01F became a remnant low and was absorbed by Tropical Depression 02F. On December 13, 15:00 UTC, 02F intensified into a Category 1 tropical cyclone, giving it the name Yasa. It quickly strengthened into a Category 5 tropical cyclone before impacting Fiji. 03F intensified into Category 1 tropical cyclone Zazu, strengthening to Category 2. Yasa became the most intense tropical cyclone of 2020, surpassing Goni with a minimum barometric pressure of 899 mb (26.55 inHg) and a maximum wind speed of 250 km/h (155 mph). It was also the fourth most intense South Pacific tropical cyclone in terms of pressure after Winston, Zoe, and Pam. Yasa caused heavy damage in and four deaths in Fiji. It then became extratropical on December 20.

After all that, the basin remained quiet for a while until activity picked up in late January. Four depressions formed within a few days of each other, with two becoming named, Ana and Bina. Ana became a Category 3 tropical cyclone struck Fiji like Yasa did and caused damage in the island nation. Bina was a short-lived storm that affected Fiji and Vanuatu. Then, on February 1, Lucas entered the basin form the Australian region. It peaked as a Category 2 tropical cyclone and affected the Loyalty Islands. Afterward, another depression formed.

Another pause of inactivity ensued again, with only a small disturbance forming, in late February. However, in early March, Niran entered the basin from the Australian region as a Category 5 severe tropical cyclone. It passed close to New Caledonia before becoming extratropical north of New Zealand. The basin became quiet once again until a disturbance formed on April 9, strengthening into a depression later.

It is interesting to compare these cyclone clusters with the passing MJO pulses, shown here as blue zone sin the OLR anomolies, from


The latest cyclone activity report is at and Tropical Cyclone Potential is from

There are some weak zones of potential development east of Philippines and west of Mexico.



Weather Zones Mid-week GFS model showing isobars, winds, waves(magenta), Rain (Blue),

STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) and CAPE9 (in pink)


Predict wind CAPE mid-week as seen by EC and GFS


SPCZ=South Pacific Convergence zone.

The SPCZ is expected to be weak and mainly from northern Vanuatu to Samoa and then southeast towards between Raro and Tahiti.

A low is expected to form on the SPCZ southeast of Raro around mid-week and deepen as it moves off to the south.


Rain Accumulation next five days from

Subtropical ridge (STR)

HIGH 1030+ east of NZ is travelling east along 35S.

HIGH 1030+ over Tasmania on Tuesday is expected to travel east along 45S and across South Island on Thursday and then fade.

Aussie/Tasman/NZ troughs

Trough over Tasmania tonight reaching South Island by Tuesday and North Island on Wednesday followed by a one-day southerly, possibly forming a Low east of North Island on Thursday.

Low is expected to form off NSW coast on Friday and go SE onto Southland by Saturday with a trough over rest of Tasman/NZ. This trough may offer a chance for sailing from Queensland to New Caledonia with a mid-week departure.

Panama to Galapagos/Marquesas

Light winds for Panama. SW/S winds from 5North to Galapagos. Currents help make the trip to Marquesas via north of Galapagos better this week.


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