Saturday, December 5, 2009

Another Fabled Fifth

12:00 PM Saturday 12-5-09  [updated from 8AM 12/5] At one point this morning, winter weather-related advisories and warnings extended nearly 1,500 miles from southern Louisiana to downeast Maine. Central Maryland and surrounding areas also has its fair share of advisories. It appears this year's "fabled fifth" will turn out to be an over-performer in many categories. 

For this site, the storm is being treated as a training exercise to test the original hypothesis from 11/01 that arrival of significant winter weather would occur in the Mid-Atlantic by 12/5. That long range projection of "significant" implied a snow accumulation of 4 inches or greater, following NWS terminology. The near term call made at 10:00 PM on 12/4 was for 2.4" at BWI airport for 12/5.

THE MAGIC OF COOLING: Despite the lack of a noticeable surface high pressure to our north, the process of dynamical cooling has enabled this storm to generate its own supply of cold air. Evidence can be observed on basic radar loops, for as the coastal low deepens, cold air is being drawn in from the north, resulting an eastward march of the low-level rain/snow line. The simple falling of precipitation through a cool layer creates the additional process of evaporative cooling. As the precip evaporates, it cools the surrounding air, creating a feedback loop which eventually reinforces additional conversion of moisture to snow.

COMMENTS & OBSERVATIONS: All readers are welcome to post their observations and include the reporting location. This gives all of us a good sense of ground truth. Of particular value will be your snow arrival time, full changeover time, the temperature and crystal type.

NOWCASTERS: The tracking links have been relocated for easier access. Anyone is welcome to recommend links to data sources we would find useful in a nowcast event.

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