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Bob McDavitt's ideas for sailing weather around the South pacific

17 May 2020

Bob Blog 17 May

WEATHERGRAM

YOTREPS

Compiled Sun 17 May 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.

 

Jetstreams

If you are having problems with working out where and when the systems on the (surface-based) weather map are going next, then it is worthwhile to check out the jetstream.

Jetstreams are rivers of wind found in the upper atmosphere where jets fly. They snake eastwards around the earth faster the earth’s normal eastward rotation, and often look like large waves marking passing troughs, and sometimes can have small buckles that play with the local weather below.

On average, there are TWO mains jetstreams affecting NZ weather-- the polar jet that is sometimes south of us and sometimes on top of us, especially when we have passing troughs. The other is a subtropical jet, sometimes just to north of us, but sometimes on top of us, especially when anything come to NZ from the tropics.

 

Jetstreams can tell us about development (invigoration) and decay (weakening) of surface weather.

In the southern hemisphere, expect development at the surface slightly north of a jetstream- entrance and south of jetstream-exit. Expect decay at the surface slightly south of a jetstream-entrance and north of a jetstream-exit.

 

As for a steering field for surface features, well, there are stagnant areas in the jetstream which indicate where surface weather may be stalling. However, for a better surface-steering indicator, go to windy,com and dial that height/altitude bar to around 850 maybe 800 or 700hpa.

 

The Tropics

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

Today’s map shows TC AMPHAN in Indian Ocean heading north towards Bangladesh.

Also, a weakfish tropical depression of eastern USA.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ stretches from Solomon island to north of Fiji to norther Tonga for much of this week.

 

A LOW is predicted to form south of Tonga on Thursday and then go south, accentuating a squash zone of strong winds and swell between Fiji and NZ from Tuesday (between High to south and trough to NE).

 

Subtropical ridge (STR)

HIGH 1030hPa is expected to enter Tasman sea tonight and cross central NZ mid-week then go off east along about 40 to 35 S.

Next HIGH is expected to enter South Tasman Sea this weekend.

 

Tasman Sea /NZ/Aus

Trough over South Island Mon, North Island Tues.

Squash zone is expected between NZ and Fiji from Tuesday, travelling southeast on Thursday and south from Friday.

Low expected to form off Queensland this weekend and bring wind and rain to North island next week.

 

From Panama:

Not good this week, and maybe for the coming months. The SW winds arrive in Panama this week, and the ITCZ visits at times. I suppose the best way to tackle these SW winds is to go SSE then west

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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

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