Originally Posted by KD on Tropic
Am off to Thailand
to sail around the East coast
riviera islands of Koh Chang, Koh Mak, Koh Kood etc. in a weeks time.... never a bad place to sail due to lots of protected areas should the weather turn.... so as a safe haven to learn in... it is perfect.... and this where I would like to get the practical side of "Understanding the Weather class 101" done.
For Ko Mak, Ko Chang etc, I think you need Weather in Southeast Asia
115, not Weather on Temperate Continental Coasts 101.
Here's my quick take on Weather 115 (if I had more time I would have written more concisely):
That's because the weather at latitude 12 degrees N has a different mix of pattern and chaos, requiring different rules of thumb, than the weather in a temperate clime such as the Melbourne VIC near latitude 38 South or the Melbourne FL near latitude 29 North (by which I mean I don't know which Melbourne is your home).
1. The weather in the Gulf of Thailand
is part of the Asian-Australian Monsoon system, a seasonal weather system with two monsoon seasons (named after the prevailing wind) and two intermonsoon seasons.
Every December thru early March in SE Asia
is the NE monsoon in the N hemisphere (and in N Australia
, the NW monsoon). So you can expect Northeast winds around Ko Chang et al.
The strength of each monsoon is both forecast
and monitored. The Asean Specialised Meteorological Centre does some of that reporting and forecasting. You can find the current forecast at: Regional Climate – Seasonal Outlook |
You'll note that Ko Chang et al are in the zone predicted to enjoy (if that is the word) below normal seasonal rainfall.
One of the very best sites observing and reporting on the Asian-Australian monsoon system is at U of Hawai'i: Monsoon Monitoring
At that U Hawai'i website, charts
A and B related to the NE monsoon experienced at Asian end of the Asian-Aus monsoon system. And chart C relates to the NW Monsoon experienced in N Australia
(and the parts
of the Maritime Continent (aka the Great Archipelago aka Maritime SE Asia) S of the Equator. You'll see that so far in January 2016, the NE monsoon in the Indian subcontinent and in SE Asia is weaker than average. But the NW monsoon in N Australia is stronger than average (big mobs of rain in N Queensland
and the NT).
2. Another pattern is imposed on the cycle of NE Monsoon - Intermonsoon - SW Monsoon - Intermonsoon. That other pattern is the Madden-Julian Oscillation (MJO). You can read more at Wikipedia or other info sources. But here's a quick run-down:
- If you look at the red data curve on chart C from that U of Hawai'i website. you'll note that the strength of the current monsoon cycles up and down about every 40 - 50 days. That's the same pattern observed by Roland Madden and Paul Julian in 1971 when they were working at the National Center for Atmospheric Research
in Boulder, Colorado. Madden and Julian traced it to a pulse of convection that starts at about the East African/Indian Ocean coast and proceeds eastward at about 10 knots, crossing the Indian Ocean
, then thru the Maritime Continent, then across the Pacific Ocean
(where it can sometimes speed up to 30 knots as it loses its rain into the Pac, stays at a stable speed other times and produces other effects, and at still other times dies away to nothing in the Pac O), and sometimes into the Atlantic. O (where it is associated with Atlantic hurricanes and the West African monsoon).
- That pulse of convection is a double pronged thing, usually a dry prong of uplift that delivers sunny and dry weather followed by a wet prong of uplift that delivers squally and wet weather.
- The pulse of convection is generally strictly tropical, directly affecting latitudes 10 N to 10 S. But the pulse can reach intro sub-tropical zones (e.g. the Pineapple Express in Hawai'i).
- To date no one has mastered forecast of the Madden-Julian Oscillation (hence it's other name, the maddening Julian Oscillation). Nine of the richer economies on the planet run 15 supercomputers, each doing their best to come up with a prediction of the MJO. The US Climate Prediction Centre refuses to call a winner and instead runs a website which shows you each of the 15 predictions for the two weeks. You can find those predictions at the CPC's CLIVAR program website: CPC - Climate Weather Linkage: Madden - Julian Oscillation
You'll see a table with 15 cells, each labelled with abbreviation of a prediction centre (BOMM = Australia's BOM, E for Europe
, J for Nippon, T for Taiwan
, N for the US, CM for Canada
Met, UK for Britain, CPTC for Brazil). Rest your pointer over one of the cells (e.g. the TCWB, which is one of my favourites for its accuracy in the NE monsoon) and you'll see a red curve, dotted with past dates, leading up to a green line that is the prediction for the 14 days since the model was last run. The deal is to look at which zone (Indian O, Maritime Continent, etc) has a peak (where the curve is far from the central circle. So if TCWB etc show a green peak in zones 3 or 4 (E Indian O and W Maritime Continent) when you expect to be in Ko Chang, then expect to see squalls, rain, etc.
3. GRIBs are of course useful. If you use software
such as zygrib, download the latest GRIBs for your cruising area of Ko Chang, Ko Mak et al. Make sure to configure zygrib or the GRIB viewer of your choice to see CAPE (Convective Available Potential Energy) and CIN (Convective INhibition) forecasts. If CAPE is >1.5 kJ/kg, expect updrafts (as explained above) with squalls, rain etc. If CIN is > 200 J/kg, a capping layer may counter the updraft effect. If CIN is < 50 J/kg and CAPE is > 1.5 kJ/kg, then reef early and set your anchor
4. At latitude 12 degrees N (i.e. Ko Chang et al) don't waste your time looking at sea level pressure synoptic charts
. Synoptic charts and the associated rules of thumb (Buys Ballot and so on about Lows on your Left) that are fine in temperate latitudes are not worth much in the tropics.
Instead of synoptic charts (with isobars of sea level atmospheric pressure), in the tropics streamline charts are the more useful tool. The US Navy
Fleet Numeric Ops Center has a neat website that delivers great forecasts into the near future. But the easy choice for the Gulf of Thailand (remember to pronounce it ThaiLand, with the capital letters being point of pronunciation emphasis), is the daily Regional Weather Chart issued each afternoon from the Singapore
Agency. See: Singapore Weather Information Portal - Regional Weather Chart
The lines are surface streamlines (i.e. the direction of wind
at 10 metres above sea level, which will be close to the metacentre of your sailplan). At selected points you'll see a wind
barb (the usual deal, a half fleche for 5 knots, a full fleche for 10 knots), the expected temperature in Celsius, and an icon for expected weather conditions (a black dot for overcast, a white dot for clear sky, etc).
Of course, around Ko Chang, Ko Mak etc, you'll have other local conditions (island effect wind conditions, sea breeze etc) so don't expect any general prediction to be accurate at micro levels.