Subtle Terrain Effects Across Eastern South Dakota


The morning satellite imagery shows the subtle effects of terrain on sensible weather.  The Coteau des Prairies in Eastern SD (sometimes referred to as the Sisseton Hills) rises above the local terrain by 600-800 feet.  A good colored relief map clearly shows this feature:

Strong nearly unidirectional NW flow was present over the entire region, and the synoptic flow pattern was not conducive to any significant regions of forced ascent.  Upper level heights were rising with strong subsidence owing to the effects of differential anticyclonic vorticity advection aloft.  However, a weak mid-level trough axis was present owing to weak thermal advection as the powerful low continued to “wrap” warm and moist air around the large upper trough.

700 hpa trough axis noted on both the 12 GFS and NAM.  Also note the mass convergence at this level–a response to the forced ascent, not the other way around.

Note the region of weak warm thermal advection.

And weak resulting synoptic ascent:

The 0Z sounding from Aberdeen depicts the moist inversion sandwiched in between two dry layers, one aloft and one near the surface.

The subtle effects of terrain across the region had very visible and profound weather effects.  Stable low level NW flow and subsequent downsloping resulted in drying of the low level air mass over Aberdeen with subtle upslope effects clearly visible over the higher terrain.  Let’s look a little closer.

Radar depicts the showers over the area at 21Z.  Note the radar “hole” around Aberdeen (red) and the regions of enhanced returns over the higher terrain (green).

The observations at Watertown (KATY) and Aberdeen (KABR) also show the effects of terrain enhancement.  Note the dewpoint depression at Aberdeen hovers around 4-5 degrees C while Watertown hovers around saturation to 1-2 degrees C.  Ignore the -SN reports from Aberdeen–seems to be a sensor error (also note it reports a constant 9SM visibility, a good indicator something is not right).

Aberdeen:

Watertown:

The visible satellite image the day after clearly shows the terrain enhanced snowfall over the higher terrain in Eastern SD (red).  Also note the west slope favored snowfall across the Bighorns, Black Hills, and Laramies (green).  Also worth noting here is the lack of snow cover over the whole of the Coteau des Prairies region.  This suggests the air mass precipitated out before before reaching farther south and east.

This shows up in even more spectacular fashion on the MODIS multi-spectral image.

This is a clear example  the significant effects even “subtle” terrain can have on weather.  In this case, strong and stable low level flow won out over weak mid level ascent.  If precipitation were heavier, it is unlikely terrain would have played nearly as significant of a role.

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About jason ahsenmacher

I am a meteorologist, and I like baroclinic instability. View all posts by jason ahsenmacher

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