Tropical South Pacific weather resources
Rory Garland, S/Y Streetcar
March 2014
For those cruising the tropical south Pacific and trying to make decisions based on the weather, the
number of different weather and climate resources available can seem daunting. I hope these notes
will help make some sense of it all.
I’ve spent the last 2 years in the region (including two cyclone seasons) and have had the
opportunity to consider most sources of information. In the end, for me, it is the diversity of weather
information that helps make better decisions. I try to make sure I choose and compare information
from across the field of uncertainty. In this way I can get an idea of the likelihood of a forecast being
correct.
It is human nature to want to know exactly what will happen, and dockside chatter is commonly
about people pinning their colours to a particular scenario. But it is worth remembering that nothing
is as certain as it seems when it comes to forecasting the weather. Whilst a single model forecast
could confidently show a feature headed off in a particular direction, it is entirely possible that in
reality the particular direction was determined on a very marginal basis over the other alternatives.
In these notes I describe the sources I use on a regular basis to build a picture of what is happening
and how it might play out. These sources primarily cover my immediate area of interest in the
western tropical south pacific, however many of them have global coverage too.
For convenience the weather source links are all summarised in a table at the end of these notes.
Climatic analysis and forecasts
There are two climatic conditions it is worth tracking regularly:
- Madden Julian Oscillation (MJO); and
- the Southern Oscillation (SO).
These provide good indicators of general conditions in the South Pacific.
Following these two phenomena on regular basis helps to build up a picture of the type of weather
to expect on a weekly, monthly and seasonal basis.
I keep regular watch on the MJO, especially during cyclone season. The location of the MJO ‘pulse’
provides a very good indication of the amount of convection (and likelihood of cyclone formation).
The ‘phase’ diagram is particularly useful for locating the pulse around the globe.
The Australian Bureau of Meteorology (BOM) links below provide good explanations of MJO and SO.
I find the NOAA sites better for keeping tabs on the two conditions.
1. NOAA Global Tropics Hazards and Benefits Outlook
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ghazards/index.php
2. NOAA Madden Julian Oscillation (MJO) status and forecasts
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjo.shtml
3. NOAA MJO weekly discussion
www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjoupdate.pdf
4. BOM (Australia Bureau Of Meteorology) MJO monitoring
http://www.bom.gov.au/climate/mjo/
5. NOAA Southern Oscillation Index (SOI) status and forecasts
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml
6. NOAA SOI weekly discussion
www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-
web.pdf
7. BOM (Australia Bureau Of Meteorology) ENSO Wrap-Up
http://www.bom.gov.au/climate/enso/
Satellite Imagery
I find satellite imagery to be one of the most useful tools for understanding weather in tropical
regions. From a passage making perspective it is especially useful for locating fronts or convergence
features such as the Inter Tropical Convergence Zone (ITCZ) and South Pacific Convergence Zone
(SPCZ).
If I have the internet bandwidth I watch the animations to see what’s developing and where its
headed. If I am at sea or in a remote location I use weatherfax to download individual images from
Honolulu (NWS) 4 times a day.
Infra-Red (IR) images provide good 24hr coverage and show the colder (high) cloud tops as whiter.
The visible image is only useful during daylight hours, but usually gives a clearer picture of what
clouds at different levels are doing. Once you get familiar with interpreting them, I find the visible
images sometimes give a better indication of where low level features might be centred.
1. NOAA Southern Hemisphere Sectors (images and animations)
http://www.goes.noaa.gov/sohemi/
(use the images and loops under the MTSAT East column)
2. Honolulu NWS radiofax charts for the Central, Southeast and North Pacific
http://weather.noaa.gov/fax/hawaii.shtml#SAT
Modelled weather forecasts
I use results from two different global weather models for short and medium range forecasts:
- Global Forecasting System (GFS) model run by the US National Weather Service
- ECMWF model (European model)
The GFS model is the basis for most commonly downloaded GRIB files as well as many of internet
based surface wind forecasts including Passageweather, Windguru and Metvuw. It’s important to
recognise that these are not independent weather forecasts, and therefore comparison does not
provide any indication of confidence or uncertainty. I have heard many people say they’ve compared
two of these internet forecasts and found them to match perfectly so it must be a very confident
forecast… but it’s just a case of them both using the same source data.
What is good on these weather sites is how they display the data. I find some of the images very
useful for visualising conditions. Probably the most beautiful illustration is the “earth wind map”
http://earth.nullschool.net/
ECMWF is a very good performing weather model. In fact, research shows it to be better than GFS in
many circumstances. Unfortunately the Europeans have chosen not to make the model output easily
available in GRIB or any detailed format. So the publically available forecasts are generally limited to
regional synoptics.
I find it a great shame that ECMWF (and the UKmet) choose not to make details of their model
forecasts available. It could provide a real improvement to our weather forecasting, especially in
cyclone prone areas such as the South Pacific.
Another source used by sailors in the region is the Predictwind service. This is a pre-pay online
service which gives access to a range of global and detailed forecasts.
Predictwind uses their own proprietary model to produce two forecasts for users to compare.
The forecasts are outputs from different runs of the same model. The two forecasts differ because
two largely independent sets of input data are used for each model run. The two forecasts therefore
represent differences in the input data rather than differences in the forecasting process.
I have found the Predictwind model to be very good a forecasting detailed local conditions,
especially where interaction with land is likely. I have not had the opportunity to properly evaluate it
for the more extreme tropical events in our region.
Understanding the uncertainty in the model forecasts
In order to get a picture of uncertainty I try to assess the variability between the models and
between successive model runs. The first thing I do is compare 7 days of forecasts between the GFS
and the ECMWF.
Then, if I see a particular event in a forecast (a developing low for example) I will compare its
development and progress as each successive model run becomes available (4 times a day for GFS
and 2 times a day for ECMWF).
Through practice it is possible to build a pretty good picture of how confident the models are.
However it must be remembered they are just computer models and the output is only intended to
help build a picture of what the weather might do. They all need interpretation.
Accessing and viewing weather model forecasts
I find it best to always view my GRIB files on the same package and at the same scale for consistency.
I generally use Viewfax, a freeware grib viewer. It’s very simple.
When I have bandwidth I use Viewfax to specify, download and view the GRIB file. When I am out of
range I specify the GRIB file in an email to Saildocs and then open it in viewfax.
There are many other options, some of which use routeing algorithms. Open CPN is used by many to
display and analyse their GRIB files. Expedition LT is another favourite.
Here are some of the sites I use for viewing various weather model forecasts:
1. ECMWF European model for selected regions
http://www.ecmwf.int/products/forecasts/d/charts/medium/deterministic/msl_uv850_z50
0!Wind%20850%20and%20mslp!0!Australia!pop!od!oper!public_plots!2014010112!!/
(use the ‘step’ drop down menu to choose forecast days, use the ‘forecast base time’ drop
down to compare different model runs)
2. ECMWF and GFS comparison
http://www.wunderground.com/wundermap/?lat=13.60000&lon=-
49.30000&zoom=4&type=hyb&units=english&rad=0&wxsn=0&svr=0&cams=0&sat=0&riv=0
&mm=1&mm.mdl=ECMWF&mm.type=SURPRE&mm.hour=180&mm.opa=100&mm.clk=0&h
ur=0&fire=0&tor=0&ndfd=0&pix=0&dir=0&ads=0&dd=0&tfk=0&ski=0&stormreports=0
(use the menu on the right to scroll to ‘model data’ and use the settings button to change
model)
3. Windguru provides a good way of looking at ‘spot’ forecasts from the GFS model
http://www.windguru.cz/int/index.php?sc=207053
4. Metvuw provides nice clear images showing surface wind and rain. I have never received
confirmation, but I believe that this uses GFS
http://www.metvuw.com/ows/
5. Weather Online Expert Charts provide good comparisons of various different models
http://www.weatheronline.co.uk/cgi-
bin/expertcharts?LANG=en&MENU=0000000000&CONT=sepa&MODELL=gfs&MODELLTYP=
1&BASE=-&VAR=prec&HH=3&ARCHIV=0&PANEL=0&ZOOM=0&PERIOD=
6. Predictwind provides two forecasts based on the same model using different sets of input
data
http://www.predictwind.com/
Other synoptic chart analyses and forecasts
I also like to look at analyses and short term (1 to 3 day) forecast synotpics which have been
produced under the supervision of skilled forecasters.
1. NOAA weatherfax charts can be downloaded using SSB and weatherfax, or
http://weather.noaa.gov/fax/hawaii.shtml
2. New Zealand Metservice forecast synoptics
http://www.metservice.com/maps-radar/maps/southwest-pacific-low-bandwidth
3. Fiji Meteorological Service surface analysis
http://www.met.gov.fj/weather_maps.php
4. Australian Bureau of Meteorology (BOM) weather charts
http://www.bom.gov.au/australia/charts/?ref=ftr
Text discussions and analyses
I find these very useful for low bandwidth downloads. Some of them also provide good
interpretation by experienced weather forecasters. One of the most useful weather discussions for
the region is of course Bob McDavitt’s ‘Weathergram’. Bob produces a new Weathergram every
Sunday night. It should not be missed. He also offers weather services to assist passage making.
1. Fiji Met Service weather bulletin
http://www.met.gov.fj/aifs_prods/10140.txt
2. Weather discussion from Guam for Micronesia
http://www.prh.noaa.gov/data/GUM/AFDPQ
3. Bob McDavitt’s regular Sunday night Weathergram
http://weathergram.blogspot.com/
Additional cyclone monitoring
All the official local and regional sources should be monitored for advisories. I also track several
other sources of forecast and information to make sure I am fully in the picture and able to make
decisions I am happy with. These include:
1. Joint Typhoon Warning Centre for various text and graphic products on current threats
http://www.usno.navy.mil/JTWC/
2. Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin
http://tropic.ssec.wisc.edu/
(there is some really useful pages which help you to understand the steering forces which a
cyclone is subject to)
Swell forecasts
Many anchorages or passes are affected by swell conditions, particularly if there has been a big
storm in temperate latitudes. There are plenty of good surfer web sites. Look at the wave period and
wave height forecasts. Wave period over about 12 seconds starts to produce long swell conditions
which can build up in shallower water.
1. Magic Seaweed is one of many good sites
http://magicseaweed.com/Pacific-Ocean-Islands-Surf-Chart/58/?chartType=PERPW
2. Windguru gives swell and wave forecast data in easy to read tabular form
http://www.windguru.cz/int/index.php?sc=207053
Summary of links for weather and climate in the western South Pacific
View this table online
Climate analyses and forecasts
NOAA Global Tropical Hazards BOM MJO monitoring
NOAA MJO status and forecasts BOM ENSO Wrap-up
NOAA MJO weekly discussion
NOAA SOI status and forecasts
NOAA SOI weekly discussion
Satellite Imagery
NOAA MTSAT animations NOAA Honolulu Weatherfax charts
Short and medium term model forecasts
ECMWF surface pressure forecasts Windguru (GFS model)
Wunderground ECMWF and GFS comparison Metvuw (GFS model?)
Weather Online expert charts (GFS, ECMWF etc) Predictwind
Synoptic analyses and forecasts
NOAA Honolulu Weatherfax charts New Zealand Met Service
Fiji Met surface analysis Australia BOM
Text weather discussions
Fiji Met bulletin NOAA Guam (Micronesia only)
Bob McDavitt weekly Weathergram
Additional cyclone monitoring
Joint Typhoon Warning Centre CIMSS (University of Wisconsin)
Swell conditions and forecasts
Windguru Magicseaweed
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