Sunday, February 24, 2013

Record Low Mid-Atmospheric Pressure Predicted for Midwest Blizzard

The maximum intensity for the developing Midwest blizzard is nearly 2 days away, but a major forecast model is predicting mid-atmospheric pressures at least as low as any observed during February in the region since systematic upper-air observations began in 1948. Based on data from this morning, the NOAA/NCEP GFS model is predicting super-low pressures by Tuesday morning near the middle of the atmosphere above the storm. The map above shows the predicted height (meters) of the level at which the pressure is 500 mb, or roughly half the value at the surface. The small circle near the Missouri-Arkansas border is labeled 528, or 5280 meters (approximately 3 miles). The values inside the circle would be even lower. According to historical data from NOAA's Earth System Research Laboratory, the 500 mb height has never been as low as 5280 meters in that region since 1948. Here is a list of the top 10 lowest observations in February at latitude 35 N, longitude 90 W:
Date       Meters
1991,2,15, 5287
1964,2,19, 5314
1978,2,21, 5318
1984,2,28, 5323
1965,2,25, 5334
1970,2, 3, 5345
1971,2, 9, 5357
2010,2,15, 5357
1958,2, 2, 5361
1984,2, 5, 5361 

Looked at another way, the composite forecast from an ensemble of GFS forecast runs made from last night's data indicates that the 500 mb height departure from the long-term average will be as much as 5 standard deviations by Monday night (chart to the right, click to enlarge). For a purely randomly distributed event, a negative departure from average of 4.5 standard deviations would occur only 3 times in a million.

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