Bob Blog 12 May

WEATHERGRAM

YOTREPS

Compiled Sun 12 May 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.

 

We can now say that the 2018/2019 Cyclone season is nearly over.

There has been one last “cyclone” named today as ANN located west of New Caledonia and going west. It seems to have managed to make the threshold for being a tropical cyclone, in the lowest category 1, but is expected to weaken back to a tropical depression by Tuesday.

 

During the past week there has been the passage of an active phase of MJO or Madden Julian Oscillation across the South Pacific. It takes 10 to 20 days for the active part of an MJO event to travel across the Pacific (a burst of extra convection that can trigger cyclones). Since this MJO event is now expected to move off, we are likely to have a more subdued South Pacific Convergence zone over the next few weeks.

 

Another parameter that helps us watch pattern sin the tropics is Outgoing longwave radiation or OLR. When the tropical convergence zones are active their cloud blocks radiation from escaping to outer space, and OLR is low, coloured blue (for bubbly). When skies are clear radiation can escape to outer space and OLR is high, coloured orange or (mellow) yellow.

 

A constructed analogue forecast of future values of the OLR show that we are expected to have a phase of high OLR (orange values) over the next few weeks, and that means a subdued South Pacific convergence zone.

See diagram at www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/forca.shtml

 

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THE TROPICS

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

TC ANN was named today and is expected to weaken as it moves of to the west.

No other cyclones around.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to weaken this week in the Coral Sea, and to be most active this week between Tokelau (north of Samoa) and Southern Cooks.

 

Subtropical ridge (STR)

New HIGH is expected to travel east along 30S across Tasman Sea on Mon and Tuesday and fade on Wed.

Next HIGH is expected to travel east along 30S across Tasman Sea from Thursday, crossing central NZ on Sat/Sun 18/19 May, followed by active trough on Sun/Mon 19/20 May.

 

Australia/Tasman Sea / New Zealand

Looks OK for departure from NZ to Tonga/Fiji/Noumea on Tuesday/Wednesday 14/15 May.

A Monday 13 departure may have a bumpy start in left over northerly swell.

After Wed, stay put or may encounter some NE winds.

 

Fast moving active FRONT is expected over Tasmania on Monday and NZ on Tuesday/Wednesday followed by large swells around NZ on Thu to Saturday.

 

As that front crosses the south Tasman Sea on Tuesday, there is a brief twist to southerly in the winds between Queensland and Noumea—offering possibilities of sailing to Noumea, then again a Fri/Sat departure is looking possibly better than a Tuesday/Wednesday/Thursday departure.

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