Bob Blog 15 March 2020

WEATHERGRAM

YOTREPS

Compiled Sun 15 March 2020

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.

EQUINOX

The sun is directly over the equator on Friday 20 March at 03:49UTC (in NZ that’s Friday afternoon at 4:49 NZDT) and that’s the equinox. Australia, New Zealand and Samoa switch from Daylight or Summer-time to standard time on 5 April, the first Sunday in April.

The equinox is when the latitude of the overhead sun (or declination) becomes zero.

You can find your solar altitude (local sun angle at solar noon) by computing (for the southern hemisphere) 90-L-D, where L is our latitude in degrees South, and D is the declination

SO at the equinox, latitude L of Auckland is 37 degrees so solar angle/altitude is 90-L-D = 90-37-0 = 53 degrees.

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There is a period in early March when the overhead sun is just south of the equator, and on 8 March is was directly above 5S.

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It so happens that this direct sunlight helps to form a “mirror ITCZ convergence zone” with extra convection mainly along around 5S from around 100W to 85W. This often happens in March, and once formed the zone lasts a few weeks even though the overhead sun continues northward. This has happened this March as seen on today’s windy rain accumulation map for the next 5 days:

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This affects the yachts sailing between Galapagos and Marquesas and probably the best strategy to avoid these squalls is to stay north of 5S as far as is desirable.

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The Tropics

The latest cyclone activity report is at tropic.ssec.wisc.edu and Tropical Cyclone Potential is from http://www.ssd.noaa.gov/PS/TROP/TCFP/index.html

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Th e MJO is spreading into the western South Pacific helping TC GRETEL which is expected to brush past northern NZ on local Tuesday/Saint Pats day. There is also TC HEROLD just east of Madagascar and travelling off to the Southeast.

WATHER ZONES

Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta) STR, and SPCZ. Pink area = lightning likely (high CAPE)

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SPCZ=South Pacific Convergence zone.

The SPCZ is expected to become less active in the Coral Sea as GRETEL travels off to the southeast, and it should stay much the same from Solomon Islands to Vanuatu to Fiji to Tonga to southern Cooks.

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Accumulated rainfall for next week from windyty.com (isobars are for Sunday night).

Subtropical ridge (STR)

HIGH over Tasmania tonight is expected to travel NE across the Tasman Sea this week and reach northern NZ by Friday local.

Tropical Cyclone GRETEL down to 976hPa just south of New Caledonia tonight is expected to travel Southeast and brush past north and far east of North Island on Monday night /Tuesday local.

Tasman Sea /NZ/Aus

This cyclone is expected to give 3 to 5m swells in East Tasman Sea on Monday night/Tuesday with swells to 8-9m near the cyclone.

After the SE winds that follow that cyclone move off on Wednesday, a High is expected with a zone of light winds. This should be followed by an increasing westerly floe south of 35S.

As for sailing east across the Tasman: it may be possible south of 35S after swells ease with Tuesday or Wednesday departure.

As for New Caledonia to Australia: swells are over 3metres until Friday

From Panama to Galapagos/Marquesas:

North to northeast winds to 5N this week, but easing to light and variable next week, then a good current and Easterly winds to Galapagos.

To get to Marquesas this week there is a change: NORTH of Galapagos is now the way to go with stronger current and better wind. That mirror ITCZ along 5S 10 is also worth avoiding. When west of 95W aim for 5S 105W.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

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Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

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