Bob Blog 24 March

WEATHERGRAM

YOTREPS

Compiled Sun 24 March 2019

 

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.

 

An addendum regarding that DECLINATION map which appeared in last week’s blog , is that 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.

(In Northern Hemisphere  it is 90-L+D, where L is Latitude in degrees North).

SO

At the equinox, latitude L of Auckland is say 37 degrees so 90-L-D is 90-37-0 = 53 degrees.

At winter solstice D is +24.5 so 90-L-D is 90-37-24.5= 28.5 degrees

At summer solstice D is -24.5 so 90-L-D is 90-37+24.5= 77.5 degrees

 

MJO What is it and what might it do?

MJO stands for Madden Julian Oscillation, and can be thought of as being a wave of extra convection or shower activity that travels eastwards around the planet, in the tropics once every 30-60 days taking around a week to pass by.

It was discovered in 1971 by Roland Madden and Paul Julian of NCAR (US national centre for Atmospheric research), and as it passes it triggers extra cloud and rain. This wet phase is followed by a dry phase.

The normal diagram used for following MJO is a phase diagram as found at

www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjo.shtml#forecast

 

In this diagram, the “ Maritime Continent “ is another name for Indonesia/Papua New Guinea. It shows the expected position and strength of the wet phase of the MJO travelling westwards (right to left in this diagram) next few weeks, but one with little strength.

Another diagram we use is the OLR diagram. OLR= Outgoing radiation and is lowest (blue) when blocked by cloud and rain, and highest (yellow/orange) in reasonably clear skies. As a mnemonic, think Blue for “bubbly” and Yellow for “mellow” This diagram shows the wet phase of the MJO having an impact on the western Pacific during the coming week, and then fading as it gets further east.

 

Meteo France have used this data to produce a probability diagram of Cyclone formation during the next three weeks, as seen at www.meteo.nc/nouvelle-caledonie/cyclone/coin-des-experts

These maps reach their strongest probability of a cyclone occurring in the region of TC VERONICA, and second highest but looking rather weak, chance is between New Caledonia and Fiji

 

THE TROPICS

Latest cyclone activity as at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential as seen at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

 

TC JOANINHA is over the open sea in South Indian Ocean. VERONICA is skirting NW Australia, and TREVOR is going inland and has just been downgraded.

The MJO (Madden Julian Oscillation) wave of increased activity is travelling east into the Western Pacific this week, and it is weakening. There is likely to be increased activity between Solomon Islands and Vanuatu/Fiji.

Rain in the past week was strongest near the cyclones. It has weakened a little along the Pacific ITCZ.

See trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

The “mirror CZ” continues just south of the equator in the eastern Pacific, affecting those sailing between Galapagos and Marquesas.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is active over Papua New Guinea and from Solomon Islands to Vanuatu/Fiji this week. A low may form over the Fiji/Tongan area by the end of the week, travelling to the southeast

 

Subtropical ridge (STR)

HIGH to east of NZ tonight is expected to travel slowly east, lingering near 40S 165W. Expect enhanced trade winds on its northern side, between 20 and 30 South.

Next HIGH from Australian Bight is expected to travel east across Bass Strait around Wednesday and around the South Island on Thursday, then off to the east along 45 to 40South.

 

Australia/Tasman Sea / New Zealand

Deep Low is expected to travel southeast across South Tasman Sea from Monday to Wednesday and  associated front should cross New Zealand on Thursday.

A small but intense low may deepen off Sydney on Friday, and then widen outwards and travel south/southeast late this week.

 

Panama to Marquesas

The northerly winds are still over 20 knots at times around Panama, but for a lull on local Monday, and then relaxing from local Thursday.

There is a good tail current to north end of Galapagos, then go to 6S 100W, to avoid the “mirror convergence zone” along around 5 South,

 

Port Vallarta to Marquesas

There is a Tejuantepecer blast forecast to peak from Monday to Wednesday, (for background see en.wikipedia.org/wiki/Tehuantepecer)

However, winds around Puerto Vallarta are likely to be OK for departure anytime this week.

Weak ITCZ likely between 6N and 3N, and then a “mirror CZ” near 3 to 5S

 

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

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

Or Facebook at /www.facebook.com/metbobnz/

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

>>>>>>>>>>>>>>>>>>>>>>>