On 10 April 2012, an orographic cirrus event manifested itself over central and eastern North Carolina. Climatologically this was a late season event as April is typically the last month we see a deep northwest flow aloft to support the development of orographic cirrus east of the Appalachian Mountains. Looking at the 1200 UTC sounding from GSO, the classic signals for orographic cirrus were present, including an inversion near or just above mountain top level, increasing northwesterly winds through and above the inversion, and a mostly unidirectional and slightly backing wind profile above the inversion. While there was a moisture plume moving into the area as seen on water vapor imagery, it was not incredibly pronounced and produced a minimal amount of cirrus across the Tennessee Valley, southern Appalachians, and the Carolinas. At around 0800 UTC, an enhanced area of moisture moved east of the Appalachians and initiated the development of the standing wave. The cirrus spread east across central and eastern portions of North Carolina before slowly dissipating from west to east after 1400 UTC. Another smaller area of orrographic cirrus developed in northwestern North Carolina, much closer to the mountains, between 1630 and 1900 UTC.
This event was interesting in that it occurred during a Red Flag Warning that was issued for nearly the entire state. Once the cirrus was in place, it had a noteworthy impact on surface conditions. The diminished insolation was responsible for slightly cooler temperatures and reduced mixing. This resulted in lower wind gusts, and a delay in reaching the 30 knot wind gusts needed to reach red flag criteria. The decreased mixing also kept relative humidity values elevated during the late morning and early afternoon. Since the cloud shield was elongated from west to east and the surface wind flow was primarily from the southwest, drier and warmer air eventually advected into the region below the cloud shield. Eventually, red flag criteria was likely achieved late in the afternoon across central and eastern North Carolina as warmer and drier air undercut the cloud cover and as the cirrus moved off the coast resulting in increased insolation and mixing. The red flag warning worked out for much of the state, but it was certainly harder to achieve red flag conditions under the cirrus, than it was to the west where insolation was not hindered.
WFO RAH developed an Orographic Cirrus Forecasting Guide (
) which provides a conceptual model along with tips to analyze and forecast these events.
Included below are the 1200 UTC GSO sounding from the University of Wyoming, a photograph of the cirrus over the Raleigh WFO taken by Jonathan Blaes, IR images from the beginning and peak of the event, IR image combined with surface relative humidity values from the RTMA, and also a GOES-R proving ground cloud type image from the folks at SSEC in Madison, WI and sent to us via Larry Lee, Science Operations Officer at WFO Greer, SC. It should be noted that the GOES-R cloud type photo is labeled MODIS which is incorrect and leftover from a previous experiment.