First Orographic Cirrus Event of the Cool Season

20131028.1315z.irDuring the morning hours of Sunday 27 October 2013 a brief orographic cirrus event developed across Virginia. Forecasters at the NWS Raleigh, NC anticipated the potential for the event but were unsure of whether the mountain enhanced clouds would extend into northern North Carolina. The IR satellite image from 1315 UTC on 27 October shown to the right provides a view of the small cirrus shield across the region near its peak in coverage and intensity. An animation of the evolution of the event can be accessed from the UCAR weather website at this link.

This event fit the conceptual model for orographic cirrus events initiated in the southern Appalachians. First, orographic cirrus events most frequently occur in the cool season, last between 4 and 8 hours, typically begin during the evening or early morning hours and dissipate during the morning or early afternoon hours. The typical synoptic pattern is given by an upper level trough over the eastern U.S. with an enhanced northwest or westerly flow aloft across the Mid-Atlantic.  The environmental profile near and east of the southern Appalachians features an inversion near and above mountain top level  between 850 and 700 hPa, unidirectional winds at and above mountain top of 30 kts or more that increase with height, and some enhanced moisture (not necessarily saturated) at mid and upper levels. The observed RAOB at Blacksburg, VA (KRNK) from 1200 UTC on 27 October is shown below.

131027.12z.raob.rnk.final

An investigation into these event led by Ryan Ellis at the NWS Raleigh, which was to be shared at the NWA annual convention in October, can be viewed by expanding the image below.

RAH.Ellis.poster.orographic.cirrus.final

 

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