Why no frost?

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It was quite cold this morning, with temperatures dropping to the low 20s and even teens in some western Washington locations. You don't want to know how cold it is in eastern Washington! Tonight will be even colder.

Several people have asked...why was there little or no frost this morning, even though we had the coolest temps of the year so far? The answer: low humidity.

Our dew points have been very, very low. (Remember what dew point is? The temperature to which you have to cool the air down to get saturation. Below 32F we really should call it "frost point" but we never do). The dewpoints this morning were in the single digits and even below 0F for most of the region. So you would have to cool the air down that low to get frost...and we didn't. So the temperature remained well above the dewpoint and we didn't see any frost or fog. Here are the current observations....same story tonight...LOW dewpoints

That is good thing. We had lots of frost on Saturday morning and slippery roads and accidents were everywhere. I almost spun out myself. One woman was killed. Never forget, roadway icing is the number one weather killer in our area. More than floods, more than windstorms. I put a whole section on this in my book for a reason. I have also created a web page just on this topic (sponsored by Washington DOT):


Most of the time such accidents can be avoided by driving slower. And make sure you have decent tires and get Vehicle Stability Control on your new or used car if you can afford it. Remember: the road temps on clear nights can be colder by 2-5F than what you see on your car thermometer, the official weather reports (which are for locations about 6 feet above the surface), and certainly the often inaccurate bank thermometer. And bridges ice up soon because they don't get the heat conducted from the soil.

Air quality is still quite good, but it may decline in time. A good place to check on it is the web site of the Puget Sound Clean Air Agency (http://www.pscleanair.org/)

Finally, some of your have noted that PROBCAST is going 7F for Seattle tomorrow. Probcast is wrong, trust me. And I know why...we are using an early version of our current computer models to drive it and it is not dealing with this cold period correctly. As soon as I can secure funding for new computers I will upgrade the whole system--so don't trust the low temperatures. Probcast is an experimental system.

Finally, what about snow? Well, to get snow in our area you need cold and wet and it is very hard to get both together. Well, we have cold now...all we need is wet. The big snow threats are moving into and out of cold periods. We got in without much, but what about the exit period? Thursday could have some scattered snow showers, but the real threat is over the weekend as we transition to a warmer, wetter regime...more on that tomorrow.
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