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Living in southeast Washington, ForeFlight often paints large swaths of mountainous terrain to the east as IFR “mountain obscuration”. I live at the foot hills, and often as I look that way, I see only a few spotty clouds here and there. I find this class of IFR very vague/confusing.

Can anyone shed any light on what rubric they use to declare this? Is it simply saying “hey VFR pilots, you shouldn’t go in a cloud in the first place, but the clouds in this region might have solid cores because the highest elevation in this quartile is at or above the broken cloud floor”? Or is it more specific than that?

I’m not looking to bend the rules or push the envelope here. My goal is to be old rather than bold. It’s more that the vagueness of the declaration bothers me; I want to understand at a more fundamental level what the envelope of weather is that fits inside “mountain obscuration” really is.

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(Source) From the "The United States of America Aeronautical Information Publication (AIP) is published by the authority of the Federal Aviation Administration."

7.1.4 Understand Mountain Obscuration. The term Mountain Obscuration (MTOS) is used to describe a visibility condition that is distinguished from IFR because ceilings, by definition, are described as “above ground level” (AGL). In mountainous terrain clouds can form at altitudes significantly higher than the weather reporting station and at the same time nearby mountaintops may be obscured by low visibility. In these areas the ground level can also vary greatly over a small area. Beware if operating VFR-on-top. You could be operating closer to the terrain than you think because the tops of mountains are hidden in a cloud deck below. MTOS areas are identified daily on The Aviation Weather Center located at: http://www.aviationweather.gov.

(emphasis is mine)

Source for para. 7.1.4

Also, from FAA Advisory Circular AC-00-6B, Aviation Weather, paragraph 16.2.2:

16.2.2 Mountain Obscuration. A mountain obscuration is a condition in which mountains or mountain ridges are obscured due to clouds, precipitation, smoke, or other obscurations. Flight can be especially hazardous over mountain routes when the mountains are obscured. The large elevation variations around mountains can cause surface weather observations to mislead. For example, a weather station located in a valley could report a visual flight rules (VFR) cloud ceiling, while a hiker in the mountains sees fog.

(emphasis is mine)

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  • $\begingroup$ This helps some. Thanks. It’s still a less measurable/quantifiable IFR condition than others, but I think I get a little better the basic jst: ridges peaks and increased variation from what little can be measured. $\endgroup$ Commented Nov 5, 2022 at 18:55
  • $\begingroup$ @TravisGriggs The part about ceilings being reported as AGL is the key. In flat terrain, you can assume the ceiling is valid over a wide area; in the mountains, it’s only valid at that point, and the ceiling may be zero or negative a mile away. So, covert the ceiling to MSL and compare to the altitude of nearby terrain. $\endgroup$
    – StephenS
    Commented Nov 19, 2022 at 20:15

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