Bob Blog

WEATHERGRAM

YOTREPS

Issued 26 October 2014

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

Coming Cyclone Season

NIWA has issued a Tropical Cyclone outlook for the coming South Pacific cyclone season.

For the details go to http://www.scoop.co.nz/stories/SC1410/S00048/southwest-pacific-tropical-cyclone-outlook.htm

In summary NIWA expect the coming season to bring a near average number of cyclones (8 to 12 in all), with the normal slow start, and February/March peak.

VANUATU and NEW CALEDONIA are expected to have lower than normal risk.

A zone of slightly higher than normal risk is expected to stretch from TUVALU across TOKELAU/SAMOA, to between NIUE and SOUTHERN COOKS. These are expected to be only small variations from normal and so vigilance is recommended.

Madden Julian Oscillation MJO

During the warmer months of the year we can anticipate phases of extra convection in the South Pacific by watching the MJO. This index is NOT having much impact on tropical convection during the coming week. My future blogs will give more info about it.

TROPICAL TOPICS

Tropical cyclone activity at present includes ANA which has maintained its integrity since re-curving wets of Hawaii early last week and is now unraveling but its components may well reach the Victoria coast of CANADA this week. There is also a feature currently name 04A FOUR in the Arabian Sea.

image

image

Get updates for these features from http://www.tropicaltidbits.com/storminfo

Weekly rain maps show that there was heavier rain in the past week over Solomons/ northern Coral Sea than in the previous week.

image

image

Weekly rain signatures for past two weeks, as seen at http://trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

WEATHER ZONES

image

Weather Zones (see text) as expected mid-week at 0000UTC on Thursday (GFS model) showing isobars, Sig wave height (coloured at sea), swell direction arrows, SPCZ and STR. (Display constructed with http://www.zygrib.org)

SPCZ=South Pacific Convergence zone.

SPCZ strengthened in the past week but is expected to weaken and become more spread out this week, mostly drifting north.

STR= Sub-tropical Ridge

High between Fiji and NZ today is expected to travel steadily eastwards along 25 to 30S to 150W by Thursday and then to the southeast.

Another HIGH is expected to move form Aussie bight into Tasman Sea on Wed/Thu and then eastwards along 30S on Fri/sat/sun. This makes for a strong STR all week.

Another HIGH is in the south Tasman Sea tonight and expected to fade as it crosses southern/central NZ on Monday. This high is just a break between the troughs in the Southern Ocean and for the rest of the week the strong disturbed westerly winds that are typical of spring dominate the Tasman Sea/NZ pattern.

The ECMWF model (tinyurl.com/ecaus) shows that the STR is likely to spread southwards on to NZ early next week (4 Nov).

image

ECMWF isobar and wind forecast for Tuesday 4 Nov (tinyurl.com/ecaus)

Between the tropics and NZ

Vessels in the All Points Rally (http://www.islandcruising.co.nz/?page_id=1687) are looking for a good weather pattern for sailing to Opua, NZ from Tonga, Fiji, or Vanuatu. This rally is free of charge (thanks to the sponsors) and helps participants with planning, weather info, clearance, and seminars on how to enjoy NZ. I shall do my bit here by commenting on “windows of weather” to get to NZ.

The first factor to consider a departure date is the local weather: the SPCZ does have some convective cloud over northern Vanuatu and parts of Fiji at present but is not very intense and seems to be likely to drift north and spread out. It isn’t really strong enough to delay departure this week.

The second restraint on a weather window is to pick an ETA over northern NZ that does NOT have strong adverse winds. Northern NZ is expected to have strong winds from Monday to Thursday. Then these winds should ease over the weekend 1-2 Nov and be light and variable early next week 3-4 Nov. Arranging your trip to reach NZ on 3-4 and maybe 5 Nov looks OK at this stage. Beyond that is too far away to tell at this stage.

The third factor is to avoid gales and heavy swells during the trip. Sadly a burst of 3 to 4 metre swell from the southern Ocean moving as far north as 23S by Friday in area north of NZ. This is in the STR and winds will be light: no whitecaps, and the swells will have long periods of 11 to 12 seconds, so it is likely to be a gentle encounter. However it may require a watchful hand on the tiller, and may be uncomfortable. Anyone wish in avoid sailing in these condition should delay getting to 23S until after Friday.

Those who intend to go to North Minerva and wait there for a good sailing window are likely to have an OK wait until Thursday this week , then those 3 metre swells are expected to briefly reach the area (outside the reef) on local Friday night (in time for Halloween) easing again on Saturday.

MINERVA REEF

UTC                     | hPa |Dir    kt| m | %

27-Oct-14 0000|1017|088|13|2.1|19

27-Oct-14 0300|1017|093|13|2.0|12

27-Oct-14 0600|1016|092|14|2.0|5

27-Oct-14 0900|1017|087|15|1.9|4

27-Oct-14 1200|1017|085|14|1.9|3

27-Oct-14 1500|1017|087|13|1.8|2

27-Oct-14 1800|1018|092|13|1.7|1

28-Oct-14 0000|1018|098|14|1.6|0

28-Oct-14 0600|1018|108|15|1.6|1

28-Oct-14 1200|1018|102|15|1.6|1

28-Oct-14 1800|1017|104|15|1.5|0

29-Oct-14 0000|1017|093|13|1.5|0

29-Oct-14 0600|1016|087|14|1.4|1

29-Oct-14 1200|1017|056|11|1.4|2

29-Oct-14 1800|1016|036|06|1.3|3

30-Oct-14 0000|1015|092|01|1.3|2

30-Oct-14 0600|1016|150|06|1.2|1

30-Oct-14 1200|1016|118|05|1.2|0

30-Oct-14 1800|1017|124|06|1.3|28

31-Oct-14 0000|1017|132|08|1.6|56

31-Oct-14 0600|1017|130|11|2.5|73

31-Oct-14 1200|1018|103|10|3.0|89

31-Oct-14 1800|1018|114|10|3.0|83

01-Nov-14 0000|1019|121|11|2.8|77

01-Nov-14 0600|1019|119|16|2.5|79

01-Nov-14 1200|1020|107|15|2.4|80

01-Nov-14 1800|1020|114|15|2.2|83

02-Nov-14 0000|1020|114|16|2.3|87

Spot read out at Minerva; hPa is air pressure, Dir kt is wind direction true and speed in knots, m is wave height in significant metres, % is cloud cover

SO the decision about when to depart depends on these three factors as well as speed of vessel and crew expectations, and the answer will be different for different vessels. Normally at this time of the year there can be paralysis by over-analysis, especially at the Minerva yacht club. The good news is that there are NO gales expected to south of 30S after Thursday- just that period of enlightened swells to factor in. Next week could well be different.

 

See my yotpak at boatbooks.co.nz/weather.html for terms used.

Weathergram text only (and translator) is at weathergram.blogspot.co.nz

Weathergram with graphics is at metbob.wordpress.com, click FOLLOW at bottom right to subscribe.

My website is at metbob.com – Feedback to bob@metbob.com

To unsubscribe, send a reply email saying LEAVE.