Another tornado outbreak

Yesterday, atmospheric conditions were particularly favorable for large, destructive tornadoes across the Central Plains.  In brief, the system that brought thunderstorms to San Francisco on Thursday moved across the Rockies and into the Plains yesterday, where it encountered a warm, humid air mass.  This system had a strong jet stream aloft and good directional shear - that is, winds from different directions at different heights in the atmosphere.  In this case, south and southeasterly winds at the surface brought in warm, moist air; these winds veered to the southwest and west higher aloft, and brought in much drier air.  This change in wind direction with height is a critical ingredient in supercell thunderstorms and tornadoes.  The thunderstorms with this system formed along a dryline - similar to a front, but very shallow and separates warm, humid air to the east from dry, hot air to the west.  Drylines are focal points for convergence and rising motion, making them a trigger for cloud and thunderstorm development.  The top picture is a radar image of one of the tornadic supercell thunderstorms, this one over south central Kansas.  The radar itself is located in the black dot at the top-right of the image, in Wichita (KICT).  Green indicates light precipitation; red indicates heavy (and probably large hail).  Notice the hook-like appendage at the southwest corner of the storm - the tornado is located there, south of the heavy rain and hail.  The second figure shows radar velocities at the same time.  Doppler radars have the ability to detect whether cloud/rain drops are moving toward or away from the radar.  Green/blue areas indicate motion toward the radar (toward Wichita) and red is motion away.  The figure shows a textbook velocity "couplet" with this storm - the dark green and blue area next to the red/white/purple area.  This indicated rotation and likely a tornado.  Here's a link to video of some of the tornadoes.

Several tornadoes occurred across Oklahoma, Kansas, and Nebraska.  Some of the tornadoes were quite large and long tracked; fortunately, this part of the country is sparsely populated, so injuries and fatalities were relatively low considering the number and size of these storms.  But as important was excellent forecasting by the National Weather Service (NWS) and the Storm Prediction Center (SPC).  On Friday, 24 hours before the event, SPC issued a High Risk severe weather outlook for the central Plains.  High Risks are rarely issued, and are almost never issued a day in advance.  This gave the region a good heads-up that tornadoes were likely.  Then, on Saturday during the event, the local NWS offices in the region did a great job in issuing tornado warnings.  The map on the right shows the observed tornadoes (dots) and the tornado warnings issued by NWS (red).  Most all of the tornadoes had warnings associated with them, with only a few false alarms (when warnings were issued and no tornado was reported).  Hats off to those forecasters.

Unfortunately there were five fatalities, all in Woodward, Oklahoma.  A tornado struck the town in the middle of the night and the tornado sirens weren't working - they were damaged in a thunderstorm earlier that same day.  The situation could have been much worse though, because larger population centers, like Wichita, Kansas, were also hit, but without fatalities. 

We have very benign weather for the next week in San Francisco.  There will be occasional fog especially near the coast, but otherwise mostly sunny all week with highs in the 60s and by Friday/Saturday into the 70s.