There were multiple rounds of orographic cirrus cloudiness east of the Blue Ridge on Sunday. The image to the right is a visible satellite image from 23 December. An animation of the visible satellite imagery from 1301 to 2145 UTC on Sunday 23 November shows the multiple rounds of high/mid-level cloudiness that was triggered or enhanced by standing waves downstream of the Appalachians.
Orographic cirrus is a somewhat common phenomenon during the cool season in the lee of the Appalachian Mountains. The environment conducive to the development of this feature is fairly well known and its potential can be anticipated through pattern recognition. A previous blog post highlighted another orographic cirrus event and an orographic cirrus forecasting guide has been developed. The climatological and environmental conditions favorable for these situations are noted below:
• It is typically a cool season occurrence when a deep layer northwest flow is established
• They often develop during the evening and overnight and persist into the late morning and early afternoon hours.
• The atmospheric thermal profiles include an inversion or isothermal layer near the mountain top level between 850 and 750 hPa
• The atmosphere wind profiles include winds of 35 kts or more that are unidirectional and increase with height
• The presence of some mid or high level moisture is required for the vertically propagating wave to act upon and generate cloudiness
The cloud cover had an impact on the 17 UTC surface temperatures across western North Carolina and Virgina, especially the RTMA analyzed surface temperatures in the upper 30s (shown in purple) across northwestern NC, just east of the higher elevations. Another minimum with temperatures in the lower 40s (shown in light blue) is located across western VA, just northwest of the large area of temperatures in the lower 50s.
The cloud cover as depicted in the IR satellite imagery is superimposed on the RTMA temperature analysis in the image to the right from 17 UTC. Other locations where the cloudiness impacted temperatures can be seen in south-central NC near Fayetteville and in far northwestern SC. Orographic cirrus events pose a forecast problem in that they can produce large areas of opaque high-level cloudiness in an environment that forecasters might assume would produce clear skies and fair conditions. In some cases, as shown above, the cloudiness can have localized impact on temperatures of 5 to 10 degrees F.
Finally, a couple of forecast images from the 12 UTC, 23 December NAM12 valid at 15 UTC are shown below and to the right. The first is a cross section extending northwest to southeast across WV and VA that is shaded with RH, shows omega in white contours (solid is upward motion) and theta in the tan contours. The cross section is consistent with the conceptual model of the typical standing wave environment with a large area of upward motion shown thorough a deep layer that results from the vertically propagating wave.