After popping into work for a couple hours yesterday afternoon, I took a quick look at the visible satellite and one thing clearly stood out. Why is there an anomalous clear patch in the middle of a state-wide deck of clouds?
The answer lies in something that most people don’t think about in terms of it’s effect on weather: topography.
The Kilbuck Mountains are part of the reason for the clearing. While not terribly vertically imposing at 2,238 ft. above sea level it goes to show that you don’t need much terrain to have sensible changes in the weather.
The Kilbucks lie in the western portion of the image. Anchorage is off the map to the east.
So why is the clearing happening? Here is a model derived cross section that is packed with information. The cross section looks at a vertical slice of the atmosphere, in the case from Bethel to Dillingham.
What is in this image? (for the non-meteorological types) First the vertical axis looks from the surface all the way up into the stratosphere, measured by pressure in millibars logarithmically. The green lines and color fill denote vertical motion. Warm colors denote upward motion, cold colors denote downward motion. Orange lines are equivalent potential temperature, think of it as a combination of temperature and moisture, used to diagnose stability or resistance to vertical displacement. Wind barbs are pretty self explanatory, plotted in polar coordinates.
The first thing that sticks out is the strong unidirectional northwest flow throughout the column. The packed isentropes (equivalent potential temperature) near the surface denotes strong moist static stability. Any air parcel in this region will resist vertical motion and want to return to it’s original elevation when displaced. When the northwest flow encounters the mountain barrier the air is mechanically forced up the slope, condensing out moisture and cooling along the way. Once the parcel reaches the top of the barrier it is going to want to return to it’s original vertical position. As the parcel descends down the lee slopes, compression and adiabatic (no outside heat introduced) warming dry and heat the parcel as it accelerates. These are the physical processes that lead to the clearing in the midst of a very large stratus deck. Another feature to note in the visible satellite are the trapped waves the intermittently show up in the clearing. These come about from the air parcels oscillating between the ground and inversion above as they try to reach equilibrium.
Upstream of the barrier. Afternoon sounding from Bethel (PABE)
Features to look for: unidirectional northwest flow through the column, moist low levels, and surface based temperature inversion reaching the top of the flow obstruction. (Maybe a bit of wet-bulbing in there)
Downstream afternoon sounding from King Salmon (PAKN) on the lee of the barrier.
Notice the warming and well mixed nature of the near surface air, keeping in mind that King Salmon is a good 60+ miles removed from the Kilbucks.
What does all this mean for an operational setting? The warming is going to increase temperatures as well as the lack of cloud cover promoting more shortwave radiation reaching the surface to further enhance warming.
The two highest temperatures for the whole state of Alaska yesterday were 74 at Manokotak and 73 degrees at Dillingham which is pretty damn impressive considering their coastal locations right along Bristol Bay.