February 19-22, 2019 | Commander-in-Chief Winter Storm
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I think we have differing definitions of CAD. It sounds like you are describing cold air drainage, which is a local phenomena where colder and denser air settles into valleys when winds are light. Cold air drainage amplifies the diurnal oscillation of temperatures in valleys/hollows relative to nearby peaks/ridges. Alternatively, cold air damming refers to how synoptic scale atmospheric currents (e.g. low level jets) flow over and around mountain barriers. Because warm and moist air is less dense than cold and dry air the two layers stay stratified unless winds are either strong enough to instigate mixing or winds are from the correct direction (i.e. parallel rather than perpendicular to the valley); threshold values of the Froude number determine the magnitude of 'damming'. Cold air damming is particularly prevalent in the ridge and valley province to the east of the Allegheny Front (a continuous ridgeline >800 m elevation that extends across several states), meaning the most frequent and profound CAD examples are in the ridge and valley province of southeastern PA, western MD and VA. Easterly flow ahead of traditional cyclones is 'dammed' by the mountains and instead funnels the cold air from northeast to southwest as southeasterly flow aloft transports moisture overhead. Since precipitation type is largely a factor of temperature profiles in the lowest km, CAD strongly influences regional p-types.
What most folks don't understand about CAD is that there's an equal and opposite process on the west side of the Allegheny Front that isn't formally named, because it often results in benign weather (i.e. cold rain). I refer to this process as warm air ramming since it 'rams' warm/moist air northeast along the west side of the Allegheny Front meaning if the Gulf is open for business then warm air will push the cold air out of the way just in time for precipitation to fall as rain (most of the time). Today is a great example of damming/ramming phenomena and I'll use Morgantown, WV (latitude 39.65ºN, elevation 1227ft) and Martinsburg, WV (latitude 39.4ºN, elevation 538ft) as examples. Morning observations in Morgantown indicate light easterly winds overnight transitioned to 10-20 mph southeasterly winds with gusts of 25-35 mph and the temperature rose from this morning's low of 36F to 49F at noon as winds veered and strengthened. However, observations in Martinsburg indicate the temperature has been remarkably steady between 31F and 33F for the past 30 hours with northeasterly winds that transitioned to light and variable winds due to 'damming'. Thus, even though Martinsburg is farther south and ~700 ft lower in elevation it's still 16F cooler than Morgantown despite being less than 130 miles away.
That's just a really long-winded way of saying this type of storm is no bueno for MBY and I'll continue to be very concerned about 850 mb temperature despite most folks complacency on the matter. I don't think it's a coincidence that the complacent folks also happen to live east of the Allegheny Front.
Great points covered. Please take time to read!
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