I've been asked to add this info to our typhoon preparedness sticky. I've written it in extremely simple "typhoons for dummies" style so that you can read it while drunk and still follow along.
[color=blue][b]How to read the Joint Typhoon Warning Center forecasts[/b][/color]
If you’re living in Taiwan, it’s a good idea to access the JTWC website(<--click here) and to learn how to read the typhoon maps there; when storms are approaching, we also regularly put them up in a thread entitled “Typhoon 200x” (that’s the year, silly rabbit!) which is in the Living in Taiwan forum. If you can't open the site or can't see the pics, just post in that thread letting us know; we'll either upload a snapshot copy which you should be able to see, or call you names and make fun of you.
In case you have trouble reading the maps, here’s a little help. First of all, here’s a sample map which you would see posted on ‘mosa, or which you would get to by clicking the link entitled TC Warning Graphic on the JTWC site. The red terms are explanatory and would not normally appear:
The typhoon center is shown with its past history as black circles, taken like snapshots every 6 hours; it’s forecast future position is shown in happy little pink circles, so the direction of its path is from black to pink. There are dates attached to the pink circles showing the present and future forecast positions. 2618Z, as in the present position in the above example, is the 26th, at 18:00 hours (that's 6pm) Zulu time. To get from Zulu time (GMT) to Taipei time, have another beer and add 8 hours; the center dot at that position is therefore where the storm will be centered at 2am on the 27th, our time.
The typhoon itself is much bigger than the center of the happy little circle thingies. You can see the clouds by going to a different link on the JTWC site, entitled Multispectral Satellite Imagery (I can't post the URL here because the link is storm-specific, but I've attached an example below).
We don’t usually post those. It’s also not a discreet thing; expect to have blustery weather with periodic, sudden downpours, perhaps even out of a partly blue sky. I’ve even gotten hit with fat raindrops from a blue, cloudless sky. Or maybe it was bird pee.
Next, you want to look at the wind speed of the storm; the stronger the wind, the more powerful the storm is. This information is present on the maps in three ways. First, there’s a white box like this next to the TC Warning Graphic map. It looks like this:
Look in the first box for the lines saying the date/time, winds so many knots, gusts to so many knots. Knots are gnarly bits in wood, and the more of them there are, the stronger the wind. No, seriously, here’s a chart I got off the internet:
winds < 34 knots (kts)
x 1.157 = <39 mph
x 1.853 = <63 kph
Tropical Storm aka light typhoon
winds 34-63 knots
x 1.157 = 39-74 mph
x 1.853 = 63-119 kph
Typhoon, aka medium typhoon
winds 64-129 knots
x 1.157 = 74-149 mph
x 1.853 = 119-239 kph
winds over 130 knots
x 1.157 = 150+ mph
x 1.853 = 240+ kph
Not all such categorizations agree in terms of when a storm qualifies as a typhoon or super typhoon. This is just for rough reference; the key point is what ‘knots’ are in mph and kph. The CWB (Taiwan’s Central Weather Bureau) uses a different scale:
" qīng dù typhoon" (tropical storm; literally, ‘light degree’):
34 - 63 knots; Force 8 - Force 11
" zhōng dù typhoon" (typhoon; literally, ‘medium degree’):
64-99 knots; Force 12 - Force 15
" qiáng lìe typhoon" (super typhoon; lit. ‘strong degree’)
100 knots or greater; Force 16 or greater
The second and fastest way to check strength is to look at the circle at the center of the storm on the map. They use different centers for different storm strengths.
Hollow circle centers are tropical depressions, not yet a storm. Hollow centers with protruding curved arms are tropical storms (in English) or light typhoons (in Chinese); when the center is solid, that’s a typhoon (in English) or medium typhoon (in Chinese). There’s no change in the graphic for a super typhoon – you have to look at the white wind speed box on the right to read that. Note that as a storm moves, it usually gets stronger, reaches a maximum forecast strength, and then (often around the time it hits Taiwan or Japan) rapidly weakens. So the type of center symbol used will vary along the path. The one that matters is for the time that the storm is forecast to be on top of YOU!
Third, there are concentric circles added around the typhoon center (for present and future dates only) to show "wind radii", as the JTWC terms them.
If there’s no extra circle like this, the wind speed is weak (less than 34 knots (39mph; 63kph). The stronger the storm is, the more circles are added. The outermost radii represents 34 knot winds; the second wind radii represents 50 knot winds; the next radii represents 64 knot winds. If that circle passes over you, it means you forgot to sacrifice the right animal to the gods. Stay indoors and preferably away from windows!
Now, what does all this mean for you?
The typhoon outside the biggest pink circle is just rain on and off, and a little wind. It's ok to go to work, go on short hikes, etc. but bring a strong umbrella, and plan to be safe at home before the storm hits. On your way home, buy food, water, and other things recommended in this thread (Preparing for a Typhoon). Better yet, have all those things in advance!
The typhoon inside the biggest circle but outside the dark red one is stronger rain and wind, and pretty much when that hits where you live, it's time to be indoors, well stocked with candybars and alcohol, and maybe some naughty videos.
The typhoon inside the smaller bright red circle is a big bad wolf. Stay indoors and play naked twister with your roommates. Now, follow the magic line connecting the circles. Does that line go near your home on the map? If so, it's going to be bad. If it goes right on top of your home, you're going to die.
No, just kidding. Follow the precautions, stay inside, and you’ll be fine.
Q1. Why does the forecast keep changing? How do I know if it will really hit us?
A1. Silly rabbit! Weather is variable. Have another look at that typhoon forecast map. Notice that there's a large area which is cross-hatched (shaded), and often roughly conical, as here:
What this area means is that the storm could on average veer right or left within that hatched area. The farther away the storm is, the more room for change there is, so don't make firm plans based on the forecast of a storm still 3-4 days away! You really never know what it will really do until it's almost on top of you.
Q2. Will school and work be cancelled?
A2. That decision is made at the local level by city and county governments, generally around 10 to 11pm the night before the storm hits, and announcements are broadcast in Chinese; they get repeated on ICRT and sometimes on Forumosa in English. Just stay tuned to your local TV and radio news, and check the 'mosa Typhoon 200x thread for updates around that time.