Today was a spectacular example of how the atmosphere can go unstable over a broad region, bubbling up into convective clouds and showers as the earth warms. As I mentioned in an earlier blog, when there is a large difference in temperature in the vertical the atmosphere can start to convect...just like in your hot cereal saucepan. The action was particularly clear over eastern Washington today.
Let's look at the satellite imagery to see this happen from space. At 8:45 AM, western Washington and the coast experienced fog and low clouds, while most of eastern Washington was clear. With a cool surface and lower atmosphere, there was no convection.
By noon, as the ground was warming rapidly, lots of convective clouds had formed, aligned in what is known as "streets."
By 2:30 PM, the transformation was complete, east of the Cascades was a vast ocean of convection, and some convection had built up away from the water over western Washington (cool water is bad for the convective business since it works against getting a large change of temperature with height).
4 PM is pretty similar.
And at 7 PM, as the ground began to cool, the convection started to die.
Over eastern Washington the convection did not lay down much rain, but some of the western Washington showers provided a few downpours. Most of this convection was fairly shallow (less than 10,000 ft high), but could produce some modest bumps coming into or out of a few airports. The pilot report (PIREP) at 10:45 AM at Fairchild AFB outside of Spokane confirmed this.
SKA UA /OV GEG235020/TM 1745/FLUNKN/TP DH8D/TB LGT BLO 060
Finally, it would like to see a wonderful video of the atmosphere going from stable (low clouds and fog) to unstable (convective clouds and showers), check out this video from the top of my building today (click on image or link):
http://www.atmos.washington.edu/~ovens/loops/wxloop.cgi?/home/disk/data/images/webcam2/20130416+2013041623/23h/10m+-pat+YYYYMMDDHHNNSS.jpg
Convection is a sign of spring around here, just like hay fever and mowing your lawn.
For non-pilots, that pilot report translates as:
ReplyDeleteUA = Pilot Report
OV = Location, 20 miles southwest (235 radial) of GEG (navigational fix near Spokane International Airport)
TM = Time, 1745 Zulu or 10:45am PST
TP = Type of aircraft, a deHavilland Dash-8
TB = Light Turbulence Below 6000 feet
I hope I got all that right... :)
Cliff, thanks for all the interpretations - it is helping me understand the weather here :-)
ReplyDeleteThe effect of the heating differential is quite striking: I missed the thunderstorms Saturday, as I was in Port Townsend, getting the sailboat ready. I left at 5:30Sat PM with a strong northerly wind, anchored at Port Ludlow after dark. On Sunday, I sailed to Everett under mostly sunny skies, but with cumulus congestus all around the Sound. But for me, on the Sound, the winds were surprisingly light much of that afternoon.
ReplyDeleteAnsel