Favorable winds over Japan carrying radioactivity out to sea
If there is going to be a major nuclear disaster with massive release of radioactivity into the atmosphere from Japan's stricken Fukushima nuclear plant, today would be the best day meteorologically for this to occur. The low pressure system that brought rain and several inches of snow to Japan yesterday has moved northeastwards out to sea, and high pressure is building in. The clockwise flow of air around the high pressure system approaching Japan from the southwest is driving strong northwesterly winds of 10 - 20 mph over the region. These winds will continue through Thursday, and will take radiation particles emitted by the stricken reactors immediately out to sea, without lingering over Japan. Since high pressure systems are regions of sinking air, the radiation will stay close to the ocean surface as the air spirals clockwise over the Pacific. The contaminated air will remain over the ocean for at least five days, which is plenty of time for the radiation to settle out to the surface.
Figure 1. Surface weather map for 8am EDT today, taken from the 6-hour forecast from this morning's 6 UTC run of the GFS model. A high pressure system to the southwest of Japan, in combination with a low pressure system to the northeast are driving strong northwesterly surface winds over the country. Image is from our wundermap with the "Model" layer turned on. The lines are sea-level pressure (blue contours, 4 mb interval) and 1000 to 500 mb thickness (yellow contours, 60 m interval). Thickness is a measure of the temperature of the lower atmosphere, and a thickness of 5400 meters is usually close to where the dividing line between rain and snow occurs.
Thursday night and Friday morning (U.S. time), the high pressure system moves over Japan, allowing winds to weaken and potentially grow calm, increasing the danger of radioactivity building up over regions near and to the north of the nuclear plant. On Friday, the high departs and a moist southwesterly flow of air will affect Japan. These southwesterly winds will blow most of the radiation out to sea, away from Tokyo. Southwesterly winds will continue through Sunday, when the next major low pressure system is expected to bring heavy precipitation to the country. Beginning Thursday night, the sinking airmass over Japan will be replaced a large-scale area of rising air, and any radiation emitted late Thursday through Friday will be carried aloft towards Alaska and eastern Russia by this southwesterly flow of rising air.
Ground-level releases of radioactivity are typically not able to be transported long distances in significant quantities, since most of the material settles to the ground a few kilometers from the source. If there is a major explosion with hot gases that shoots radioactivity several hundred meters high, that would increase the chances for long range transport, since now the ground is farther away, and the particles that start settling out will stay in the air longer before encountering the ground. Additionally, winds are stronger away from ground, due to reduced friction and presence of the jet stream aloft. These stronger winds will transport radioactivity greater distances. I've made trajectory plots for the next three days assuming two possible release altitudes--a surface-based release near 10 meters, which should be the predominant altitude in the current situation, and a higher release altitude of 300 meters, which might occur from an explosion and fire from a Chernobyl-style incident. Given that the radioactivity has to travel 3000 miles to reach Anchorage, Alaska, and 5000 miles to reach California, a very large amount of dilution will occur, along with potential loss due to rain-out. Any radiation at current levels of emission that might reach these places may not even be detectable, much less be a threat to human health. A Chernobyl-level disaster in Japan would certainly be able to produce detectable levels of radiation over North America, but I strongly doubt it would be a significant concern for human health. The Chernobyl disaster only caused dangerous human health impacts within a few hundred miles of the disaster site, and the distance from Japan to North America is ten times farther than that.
Figure 2. Five-day forecast movement of plumes of radioactive air emitted at 10 meters altitude (red line) and 300 meters (blue line) at 18 UTC (2pm EDT) Wednesday, March 16, 2011 from the Fukushima Daiichi nuclear plant. The plumes spiral clockwise around the high pressure system to the southwest of Japan and stay near the surface. Images created using NOAA's HYSPLIT trajectory model.
Figure 3. Five-day forecast movement of plumes of radioactive air emitted at 10 meters altitude (red line) and 300 meters (blue line) at 18 UTC (2pm EDT) Thursday, March 17, 2011 from the Fukushima Daiichi nuclear plant. The plumes initially spiral clockwise around the high pressure system to the southwest of Japan and stay near the surface. By Saturday, though, the plumes get caught in a southwesterly flow of air in advance of an approaching low pressure system. Ascending air lifts the plumes to high altitudes, where winds are stronger and rapid long-range transport occurs. Images created using NOAA's HYSPLIT trajectory model.
Figure 4. Five-day forecast movement of plumes of radioactive air emitted at 10 meters altitude (red line) and 300 meters (blue line) at 18 UTC (2pm EDT) Friday, March 18, 2011 from the Fukushima Daiichi nuclear plant. The plumes get caught in a southwesterly flow of air in advance of an approaching low pressure system. The plume emitted near the surface (red line) stays trapped near the surface, but the plume emitted at 300 meters is lifted to 3.5 km altitude by the rising air associated with the approaching low pressure system. Images created using NOAA's HYSPLIT trajectory model.
Seven-day weather forecast for Sendai near the Fukushima nuclear plant
The Austrian Weather Service is running trajectory models for Japan.
Current radar loops from the Japan Meteorological Agency
Rare subtropical cyclone forms near Brazil
An unusual low pressure system that came close to becoming a tropical storm is in the South Atlantic, a few hundred miles east of the coast of Brazil. The Brazilian Navy Hydrographic Center has officially named the system Subtropical Storm "Arani", but I'm not sure the low would have been named by NHC, since Arani has somewhat of a loose circulation and limited heavy thunderstorm activity. The storm is expected to move slowly eastward out to sea, and does not pose a threat to South America. The latest run of the GFDL model shows little development of Arani, and the storm is now encountering a frontal system, which is bringing 20 - 30 knots of wind shear. It is unlikely that Arani will become a tropical storm. Some runs of the GFDL last weekend were predicting Arani would intensify into a Category 3 hurricane; that's the first time I've even seen such a prediction for a South Atlantic storm. The metsul.com blog has more info on Arani, for those of you who read Portugese.
Figure 5. During the daytime on Tuesday 15 March 2011 at 1820 UTC the TRMM satellite flew over a rare cyclone labeled Arani in the South Atlantic. Arani had the appearance of a tropical cyclone but has been classified as a subtropical cyclone. NOAA's Satellite and Information Service classified Arani as a T1 on the Dvorak intensity scale which would indicate an estimated wind speed of about 29 kt (~33 mph). TRMM's Microwave Imager (TMI) and Precipitation Radar (PR) data were used in the image above to show rainfall near Arani. Image credit: NASA.