Coldest Temps

The answer to my question in the last blog: the coldest temperatures are generally in the hour AFTER sunrise--so this time of the year around 8 AM. Temperature is controlled mainly by two factors: solar radiation during the day that warms, and infrared radiation emitted by the surface that cools (day and night). When the sun sets there is only infrared cooling and thus fairly rapid cooling sets in, cooling continues albeit at a lesser pace all night, when the sun just rises its feeble warmth is still dwarfed by the infrared cooling, so net cooling occurs and temp can still fall. Not until about an hour after sunrise is the sun's warmth large enough to balance the infrared cooling and temperature starts to rise.
I have attached the plot of temperature on the roof of the atmospheric sciences bldg at the UW that illustrates many of these points...the plot on the bottom is the solar radiation and air temperature is the third plot from the top.
Weatherwise, a weak front will approach western Washington late tomorrow afternoon, and showers should be felt tomorrow evening. The mountains will get some light snow--but not enough to really help. A convergence zone should form Saturday am and with cold air moving in behind the front, there could be some wet snowflakes in the CZ, particularly above 300 ft. More on this tomorrow.