Bob Blog

WEATHERGRAM

YOTREPS

Issued 23 November 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

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin.

It has been negative since July and dived below -10 (Australian units) for much of September, slightly relaxed for a while in October, and dived below -10 early in November, but is now relaxing a little .

image

SOI as shown at http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi, showing that we are now having an at-least-weak El Nino.

TROPICAL TOPICS

Tropical cyclone activity has now reduced to one possible area in the Indian Ocean, and an “area of interest” to the NW of Samoa.

The “area of interest” looks like this on Google Earth at present:

image

The latest tropical disturbance summary at http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/ab/abpwweb.txt has this to say about this system:

ABPW10 PGTW 230600

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//

SUBJ/SIGNIFICANT TROPICAL WEATHER ADVISORY FOR THE WESTERN AND

/SOUTH PACIFIC OCEANS/230600Z-240600ZNOV2014//

RMKS/

1. WESTERN NORTH PACIFIC AREA (180 TO MALAY PENINSULA):

A. TROPICAL CYCLONE SUMMARY: NONE.

B. TROPICAL DISTURBANCE SUMMARY: NONE.

2. SOUTH PACIFIC AREA (WEST COAST OF SOUTH AMERICA TO 135 EAST):

A. TROPICAL CYCLONE SUMMARY: NONE.

B. TROPICAL DISTURBANCE SUMMARY:

(1) THE AREA OF CONVECTION PREVIOUSLY LOCATED NEAR 8.2S

177.1W, IS NOW LOCATED NEAR 11.5S 174.4W, APPROXIMATELY 204 NM NORTH

OF PAGO PAGO, AMERICAN SAMOA. ANIMATED MULTISPECTRAL SATELLITE

IMAGERY DEPICTS UNORGANIZED FLARING CONVECTION ASSOCIATED WITH AN

ELOGATED LOW-LEVEL CIRCULATION CENTER. A 230354Z NOAA-19 MICROWAVE

IMAGE REVEALS VERY FRAGMENTED CONVECTIVE BANDING ON THE EASTERN SIDE

BROADLY WRAPPING INTO THE CENTER. ALSO, A CIMMS 230300Z 850MB

RELATIVE VORTICITY PRODUCT SHOWS AN ELONGATED VORTICITY SIGNATURE.

UPPER-LEVEL ANALYSIS INDICATES THE DISTURBANCE IS LOCATED NORTH OF

THE RIDGE AXIS IN A FAVORABLE ENVIRONMENT WITH LOW TO MODERATE (05

TO 15 KNOT) VERTICAL WIND SHEAR AND EXCELLENT POLEWARD OUTFLOW.

ADDITIONALLY, SEA SURFACE TEMPERATURES WITH A RANGE OF 26 TO 28

DEGRESS CELCIUS IN THE AREA ARE FAVORABLE FOR DEVELOPMENT. MAXIMUM

SUSTAINED SURFACE WINDS ARE ESTIMATED AT 20 TO 25 KNOTS. MINIMUM SEA

LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1004 MB. THE POTENTIAL FOR

THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24

HOURS REMAINS LOW.

(2) NO OTHER SUSPECT AREAS.//

It is judged as having a low chance of any further deepening. The GFS models and their GRIB files are picking that this system may deepen into a depression on Mon/Tuesday and bring clockwise gale winds between the Niuas and Niue, then Southern Cooks on Wednesday/Thursday- but the more reliable ECMWF model suggests this isn’t likely.

Weekly rain maps over the past fortnight show an increase in convection over the past week in the Indian Ocean.

image

image

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

This increase in convection in the Indian Ocean is possibly part of a MJO cycle that will gradually make its way into the Pacific Ocean over the next few weeks. For the next week or so it increases the risk of a tropical cyclone formation in the area to NW of Aussie.

image

For the following weeks this increased risk spreads east

image

This is shown at http://www.meteo.nc/cyclone/coin-des-experts

WEATHER ZONES

image

Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS/WW3/NOGAPS models) showing isobars, Sig wave height green lines, swell direction arrows SPCZ and STR.

SPCZ=South Pacific Convergence zone.

SPCZ has strengthened in the past week and is expected to drift south onto northern Tonga and SE onto Niue, then later onto southern cooks. The GFS model is picking that a depression may form on the SPCZ and move along it to the SE, but other models only have a weak feature. The GFS model also has a tropical low forming SE of French Polynesia. These may not actually happen but if you are sailing in these areas this week then brace for strong winds anyway.

STR= Sub-tropical Ridge

The STR is strong and well defined and mostly at its normal latitude for the time of year, but north of normal in the Tasman Sea/ NZ area.

HIGH at 30S between New Caledonia and New Zealand is expected to stay put until Wednesday and then slowly fade away as it travels east from Thursday to Saturday.

NEXT High cell is expected to travel east along 45S from Saturday 29 Nov , reaching North island around Wed 3 December – followed by a northerly flow -that is GOOD for arriving in NZ – late next week (Thursday to Saturday 4-6 Dec).

Between the tropics and NZ

At north Minerva:

Trade winds until Thursday, light and variable on Fri 28 to Mon 1 Dec, then more trade winds. Good idea to depart around Thursday so as to catch a northerly flow when approaching NZ – voyage will encounter a period of southerly winds around Sun 30 Nov/Mon 1 Dec and requires waypoints.

Over northern NZ:

Westerly flow on Monday/Tuesday, then an active trough on Wednesday followed by a SW/S flow on Thursday 27 to Tues 2 Dec. NOT the best week (after Tuesday) for arriving in NZ.

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

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

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