Monday, October 27, 2014

Orographic Convection

Looking southeast at orographic convection over the central Wasatch @ 7:45 AM this morning
There's a lot to be learned by observing the weather during periods that are relatively benign.  Big storms are great, but you can't always peer into them and see the underlying processes that are contributing to precipitation generation.   Weak events, however, enable you to see some of the phenomena that ultimately contribute to big events.

Early this morning provided such an example.  We had weakly unstable northwesterly flow impinging on the central Wasatch.  This is led to the development of orographic convection over the central Wasatch and adjoining Traverse Range (see photo above).  Orographic convection features strong updrafts that are initiated as unstable or potentially unstable flow is forced over a mountain range.  Typically the intensity, depth, frequency, and or spatial coverage of the updrafts is greater over the mountains than the upstream lowlands, resulting in more widespread and deeper clouds.  

In some instances, such convection produces or enhances precipitation rates over the mountains, sometimes resulting in heavy snowfall or rainfall.  Today, however, the airmass is relatively dry, so for the most part we are seeing non-precipitating cumulus clouds developing and persisting over the mountains, with just a few showers at times.

Here are a couple of views, the first looking southward from near Red Butte Canyon.

And a second looking eastward from the western Salt Lake Valley.

Note in particular the greater depth and persistence of cumulus clouds over the Wasatch Mountains compared to the upstream lowlands.

This provides a weak example of what often happens in unstable, post-frontal, northwesterly flow over northern Utah.  Lifting by the mountains helps to initiate or intensify orographic convection, enabling storms to rage on.

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