Is it possible if I have, say, two small moons orbiting the planet that individually don't affect tides too strongly, but when their orbits get close to each other every few years they cause such a storm?

On my planet there is a race of sentient Trees, which are very tall for their width. They have enormous leaves but these leaves stem directly off the central trunk, with no branches. Normally they handle wind and rain relatively well. Partly that is because they grow in sheltered valleys where the wind is not as strong.

But it is important to the plot that every few years (not sooner) a very large storm comes whose winds are too strong for the leaves to handle as they are. So the tree shuts its leaves, a bit like an umbrella closing upside down; the leaves bend upward and overlap each other until they are relatively flush against the trunk, which can then bend as needed to avoid breaking.

I'm just trying to think what could cause that to happen. I suppose if it had to happen every year in order to be plausible I could work with it, but it would lessen the impact a bit since it would be commonplace.

What if you had a volcanic eruption on the sea floor that heated the ocean water enough to cause a storm to form? You couldn't blame that on the moons, though.

For example a comet travels through the system making a circuit every three years. Debris from its tail bombards sections of the planet causing intense dust storms that block the sun causing air temperatures to change abrutly. This spawns a wind storm with hurricane force winds.

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But it is important to the plot that every few years (not sooner) a very large storm comes whose winds are too strong for the leaves to handle as they are. So the tree shuts its leaves, a bit like an umbrella closing upside down; the leaves bend upward and overlap each other until they are relatively flush against the trunk, which can then bend as needed to avoid breaking.

I'm just trying to think what could cause that to happen. I suppose if it had to happen every year in order to be plausible I could work with it, but it would lessen the impact a bit since it would be commonplace.

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This behavior from the sentient trees seems to me something that would be evolutionarily derived, and that means the triggering event has to happen with enough frequency and over a long enough time frame to lead to it. That points to regularity.

I'm afraid I don't agree with the idea that if it were regular or commonplace, as you say, that that would lessen its impact any. Hurricane and monsoon seasons are quite regular, and that doesn't lessen their impact a whit. You are bound to lose some trees with each season of storms. Thus evolution encourages the propagation of trees that can withstand storms.

Of course, you could also flip it around and have your trees take advantage of storms, as nature is wont to do, and turn a potential enemy into an ally by dropping storm-blown seeds, which then settle in valleys when the winds weaken and drop them.

http://www.extremescience.com/weatherport.htm

So you need a nonseasonal, but fairly regularly occurring storm to wreak some pretty good havoc, you have two moons which, combined, could have an abnormally large tidal effect.

Try this on for size--hurricanes form strictly over water, since land does not have enough moisture or heat energy to feed one. Suppose your double moon tide swells into an area of your planet that is just barely above sea level--something like Holland without the dikes. It would have to be somewhere equatorial in order to provide the amount of heat energy required. Now this tidal effect is generally nothing to worry about, but every once in a while the tidal effect coincides with the storm season and a hurricane comes farther inland (over the warm tidal sea) than is usually possible. This effect is fairly predictable (as predictable as hurricane occurence and the occurence of the tidal effect can be), but not seasonal.

You devise a complex tidal system, as described above, with at least two moons affecting the tides. Usually the moons pulls against each other, which makes your sea level pretty stable. A really high tide comes when the moons align and pull together. This is a rare but regular and predictable occurance, perhaps once every eight months Earth time.

On one side of the planet you have a coastal desert that is just inches above sea level. This desert builds up a huge amount of heat energy every day. Normally it dissipate the energy via radiation every night.

But when the extra high tide happens to hit the desert during the hottest part of the day (perhaps 1/3 of all high tides) the desert floods. When inch-deep water covers thousands of square miles of burning desert you get a lot of evaporation. Sea level drops a bit, and the atmospheric water content surges. Equilibrum will be restored when it cools and condenses somewhere, releasing a lot of energy and rainwater.

Survivor said look into El Niño. I agree.

And I think I'll still have two small moons orbiting the planet, but I won't blame the weather on them

El Nino is caused by the bathythermic configuration of the deep Pacific. Basically, there is a deep but placid area where heat energy collects over a span of time (the actual El Nino cycle is 7-10 years, but with different large placid oceans it could be anything). When sufficient heat energy is collected, a local current (also called El Nino, or rather the cycle takes its name from the current, or perhaps both...never mind) is dramatically affected and causes a powerful alteration in the weather along the South American coast.

El Nino and similar phenomena are short term climactic change. Changes that effect global circulation systems (ocean or atmosphere) have time spans ranging in hundreds to thousands of years (See the November, 2004 issue of Scientific American for an excellent discussion.)

In fact, if the trees have evolved around this phenomenon, then it should be long term episodic—there are two seasons every year, the Season of Tranquility and the Season of Winds.

I'm not sure how evolutionary this needs to be. I was thinking it might also be connected to the sentience of the Tree. In the story, the Tree is also able to open archways into its hollow trunk from the base of each leaf, to facilitate the movement of the humans getting up and down it. That was something born of courtesy to humans, who have only been on the planet a couple of hundred years. The theory I've been going on is that the Tree has the ability to manipulate its physical form somewhat, though not to the extent of say, uprooting and walking. For instance, Trees that may grow in other places in the jungle may not be as affected by the storms, and not need to move their leaves the same way. Similarly, Trees without human villages don't automatically grow arches into their trunks. Although, Trees who can use human and animal waste as fertilizer grow taller and stronger, so it is in their favor to have humans living in them and to shelter said humans.

I assumed the reason you wanted a recurring storm was to make the trees ability to close their leaves evolutionarily plausible. But if your trees are so adaptable, then they could plausibly close up the leaves for the first storm that they ever encouter.