The above surface weather map of Asia shows that an area of low pressure has formed offshore to the East of the island of Japan overnight. Tokyo is roughly located at the tip of the red arrow, and the damaged Fukushima nuclear reactor site is roughly located at the tip of the purple arrow.
Winds blow in a counter-clockwise direction around an area of low pressure (in the Northern Hemisphere anyway). This means that a Northwesterly to Northerly wind will be in place over the Tokyo region for the next 12 hours or so (see the latest forecast for Tokyo here). Unfortunately, this could lead to higher levels of radation entering the city from the damaged reactor to the North.
Officials are working diligently to try and contain/stop the leak, but as you can imagine this is a very delicate operation.
Our thoughts & prayers certainly remain with the people suffering in Japan, and with those who are risking their lives to contain the reactor leak.