More Midwest/East tornadoes

Friday, March 2, 2012 will long be remembered in the Ohio Valley for its outbreak of severe thunderstorms and tornadoes.  Dozens of tornadoes were reported across a large area from Indiana southward to Alabama, some large, and resulting in about 40 fatalities.  A particularly strong, long-tracked tornado carved a long path across southern Indiana into far northern Kentucky, narrowly missing Cincinnati.  The image on the right is a compilation of large hail, damaging wind, and tornado reports from March 2.

This event was a classic setup for severe weather.  A low-pressure system rapidly developed over Missouri and accelerated northeastward toward Michigan.  Ahead of this system, warm, moist air from the south surged northward.  And, a strong jet stream aloft resulted in strong vertical shear - that is, winds changing in speed and direction with height above the ground.  This is an essential ingredient for tornadoes.  And, much colder air rushed in behind this system.  Heavy, wet snow blanketed the region from Chicago and Milwaukee and eastward across Michigan.  I'll talk more details about the meteorological factors behind severe thunderstorms as the spring continues.

Conversely, we have had continued benign weather here in the Bay Area.  Temperatures were well above normal this weekend, with highs in the 70s and even a few 80s well inland.  This will quickly change Monday and Tuesday as a cold front moves through.  Precipitation will be light, but it will become quite windy, especially on Tuesday.  Thursday, Friday, and possibly Saturday look nice though.  After that, indications are still there that the middle of March could be wetter than normal across northern and central California.  This is still a long way out in terms of reliable forecasting, but it would be welcome precipitation after a very dry winter.