During the last two days we have had something we haven't seen in a while. No, I am not talking about highs near 60F...although that is true. What we have had is a very large diurnal (daily) range of temperature. Look at the plot above at Sea Tac, today the high was 59 with a low of 39F, a 20F temperature range....far greater than we have had since December. Many locations got colder than that and frost graced many a car this am.
Why so large a range? Most important have been clear skies and our strengthening sun. You feel the warmth? Remember the sun right now is as strong as in mid October, with much longer days than two months ago. And the clear skies associated with high pressure aloft allow maximum heating during the day and maximum cooling at night as the earth radiates infrared energy to space.
But why no clouds and fog...which often occur in midwinter? The secret is that we have a modest east-west pressure difference (higher pressure to the east), which is producing easterly flow across and down the western Cascade slopes. Such downslope air produces drying and warming. Check out the latest profiler data from north Seattle...you can see the easterly flow.
But too much easterly flow would cause warming due to compression and keep our temperatures up at night. But fortunately for diurnal temperature range lovers the easterly flow is strong enough to keep us clear, but not strong enough to cause too much warming.
These conditions will continue through the weekend...warm enough for tee shirts during the day, but cold enough for frost in the morning at many locations.
Today's Contest: WHO HAS THE LARGEST DIURNAL RANGE?
PSS: There is an article in the Seattle Times today (Saturday) on the early spring