Why no frost?

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):

http://www.atmos.washington.edu/~cliff/Roadway3.html

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.

Snowy Sequim

A few cam shots around Sequim below (Sequim Lavender farm and Pt Townsend Airport) . Looks like they received around 1/2-1 inch of snow last night.


Temps are relatively warm today due to the clouds associated with the weather disturbance that moved through last night and this morning...but clearing will spread over the region today and the real cold is expected tomorrow morning. I spent some time yesterday preparing my home and plants...mulching them and moving some into the garage....they are ready!

Strong NE winds are coming out of the Fraser...some gusts to 30-40 mph in exposed locations last night (a few even higher). Look at the wind graphic below. On the figure there is both temp (upper left) and dewpoint (lower left). You will note the dewpoints are very, very low (single digits)--a sign of continental, arctic air. Look at the winds taken by the sensors on Washington State Ferries- strong NE winds...with 38 knots in one channel. Wa Ferries does a very important public service by having such observations....and there are available for you to view on the web site...Ferry Weather (http://i90.atmos.washington.edu/ferry/Ferryjs/mainframe1.htm)
Finally, it will be very cold tonight and Tuesday morning...lots of places will see lower 20s and teens west of the Cascades....so be ready!

By the way, several of you have asked about a line of clouds stretching southwest of the Olympics (see figure). You know what this...the anti-Puget Sound convergence zone! The winds aloft are from the NW and the winds are passing around the Olympics and then converging in the lee of the mountains. Normally the winds are westerly when we get a convergence zone over Puget Sound, but now the winds are coming from the opposite direction...so the convergence switches side. Converging flow causes upward motion and clouds! You notice all the clouds offshore..that is cold air that has passed offshore and the atmosphere going unstable at low levels, as cold, dense moves over warmer water. Big change in temperature with height produces convection (cumulus to cumulonimbus).