"The ice storm is an event, and it is not an event which one is careless about"
- Mark Twain
The winter of our discontent continues. We have not only failed to mix out the cold pool and pollution, but temperatures last night remained warm enough aloft that we have light freezing rain falling across the Wasatch Front. Freezing rain is very rare in Utah, and this event is shaping up to be a doozy.
The culprit in all this is the cold pool/inversion combined with a wedge of warm air aloft. Temperatures in the shallow cold pool at the surface are below freezing (18ºF at the airport when the sounding was taken at about 4:30 am). Aloft, temperatures are well above freezing within the warm wedge (in this graph, known as a Skew-T, the 0º line is slightly skewed and indicated by a solid black line – everything to the right of this line is above freezing). Snow falling from the cloud deck aloft melts in the warm wedge, turns to rain, and then freezes on contact when it hits cold surfaces in the cold pool.
|Skew-T source: NOAA/SPC|
The current situation is probably a worst-case scenario for northern Utah. If precipitation rates were higher, the warm wedge aloft might cool enough that the snow would be able to reach the valley floor. The blue line in the image above shows the temperature that the atmosphere would cool to from evaporation and you can see that except in a very shallow layer it is at or below freezing. However, with light precipitation, there's not enough water evaporating to fully cool that layer and we're stuck with freezing rain. The National Weather Service deserves a lot of credit for calling for a winter mix and getting the word out last night, although the impacts are still severe.
Looking at the radar, I don't see this event ending soon. By Utah standards, this may shape up to be one of the worst ice storms in many many years. I already can't remember anything this bad. In 1000+ blog posts, I don't even have a label for "ice storm." I just had to add it!