Unprecedented warmth in the Midwest

The Midwest/Great Lakes region continues to bask in historic warmth for the middle of March.  Temperatures have been in the 70s and 80s across a wide area from Missouri to Minnesota and east to the Mid-Atlantic.  The culprit is a highly amplified upper-level pattern over the country.  A strong ridge has persisted and strengthened from the Southeast northward to Ontario, with a deep trough over the Rockies.  That trough brought us all the rain last week and snow into Arizona and northern Mexico.  Also of note is the wind flow from Texas due north to central Canada.

Just how warm has it been?  Numerous locations are on track for their warmest March on record.  The graph on the right is from the Chicago National Weather Service office, showing the average March temperature and the previous 4 warmest Marches on record.  2012 is on pace to blow away the old record.  Tomorrow's forecast high for Chicago is 85º - this is warmer than Chicago's average high in the middle of July.  This is indeed a historic event, and as such, it's difficult to predict what's coming next.  Much of weather forecasting is based on a phenomenology of what's happened in the past, but we don't really have much to compare these warm temperatures to.  While good for outdoor spring/summer activities, this unusual warmth could have implications on things like crops and lake water levels.

As for California, last week's series of systems brought us much needed rain, with over 10 inches falling in some of the higher elevations of the Bay Area.  For San Francisco, we're up to about 50% of normal for rainy season precipitation.  And, the Sierra received several feet of snow.