Orographic Cirrus Preludes the Coming Cool Season

Orographic cirrus is a common phenomenon during the cool season in the lee of the Appalachian Mountains. On Monday evening, an area of orographic standing wave clouds developed in the lee of the southern Appalachians in western North Carolina.

Ryan Ellis from the NWS Raleigh examined more than 40 orographic events in 2009 and 2010 to learn more about anticipating and better forecasting these phenomena which can result in unanticipated areas of dense mid/high level cloudiness which can negatively impact sky and temperature forecasts. Details on this study were outlined in a presentation which can be found here. A preliminary forecasting methodology was developed based on this study and is available as a two page quick reference guide here. Finally, Ryan will present an update on his study, which now includes a dataset of null cases at the Second Eastern Region Virtual Satellite on October 30th (web site).

The animation below shows water vapor imagery on the left and IR satellite imagery on the right from 1701 UTC on 15 October through 0915 UTC on 16 October which highlights an area of thin cirrus that moves southeast across the upper Mississippi Valley and into the Ohio Valley and then increases in areal coverage and opacity as it is enhanced leeward of the Appalachians (note you may need to click on the image to animate in some browsers).

The 12 UTC RAOB on 16 October from KRNK shown below includes many of the environmental conditions associated with Appalachian orographic cirrus outbreaks including:

  • An upper air pattern dominated by a trough near the U.S. East Coast
  • Water vapor imagery indicating upstream moisture moving into the southern Appalachians
  • An inversion or isothermal layer present near the mountain top level between 850 and 750 hPa
  • Winds increasing with height from the inversion base up to the tropopause with at least 30kts of flow at the inversion
  • Winds typically from some component of the west at the top of the inversion that backs with height

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3 Responses to Orographic Cirrus Preludes the Coming Cool Season

  1. Andrew Kimball says:

    I tried opening the referenced presentation and it opens a zip file which has no power point in it. I also tried opening the reference guide, but it said page not found.

    • Jonathan Blaes @ WFO RAH says:

      Hey Andrew,
      Thanks for the catches. The link for the reference guide has been fixed, it was malformed. The presentation link pointed to a pptx file which when viewed on a PC with an old version of power point is often mistaken for a zip file. I loaded a PDF version of the presentation and changed the link.

  2. blsmithwx says:

    The upstream soundings at ILX from the night before (16/00Z) had a lot of similarities to the RNK sounding at 16/12z. http://www.spc.noaa.gov/exper/soundings/12101600_OBS/ILX.gif

    Not sure what the forecast soundings looked like, so but maybe the ILX sounding would have been a good proxy. A lot of times the moisture channel in water vapor imagery does seem to be the biggest clue in the 6-12 hrs leading up to orographic cirrus.

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