Overnight Snow? Freezing Rain? Both?
9:15pm Update —
I have been poring over all of the latest data / maps this evening and it is looking like we are going to be in for a very interesting overnight period. Temperatures across the metro area as of 9pm are right at freezing and the air above the metro area is at or slightly above freezing. The warmest I could find in the coast range, yet still nearest to the deepest pool of cold air, was 34 degrees out near Timber Junction. I saw nothing higher than 34 degrees up to almost 3000ft. It is important to note that none of these locations had east winds nor were they right over the metro area. What’s my point? Dew points are even colder over Portland / Vancouver, with most running about 22-23 degrees as of 9pm. So some evaporative cooling is likely, but it will be minimal.
The layer of warm air above the metro area is thin enough to where precipitation falling in to it will not likely have the chance to completely melt back to rain, but rather stay at least 50/50 snow and freezing rain all of the way down to the valley floor. There will also likely be a mix of sleet. Let’s just call it a big bowl of weather soup! As the layer of warm air above the metro area expands overnight, the combination of snow, sleet and freezing rain will change over to mostly rain. Latest model data just out late this evening suggests that the low level flow at the surface will remain calm or slightly easterly through 6am. That worries me a little. Why? Because historically models transition us out of these cold air masses quicker than what really happens. Meaning, the cold air at the surface could stay in place longer that what is currently shown by the models. I have seen it many times before. The closer you are to the gorge, the deeper the layer of colder air is, so you are likely to have snow at points deeper east in the gorge. No matter what, it looks like we will break out of this by sunrise tomorrow morning.
Bottom line for the metro area tonight — anyone traveling overnight tonight should be aware that roads will likely become slick in areas that have been shaded for the past few days and have remained below freezing. Snow, sleet and even freezing rain are all likely to begin across the area anytime after 10pm and continue through daybreak Wednesday. Precipitation amounts will be light. But it does not take much to make a mess of the roads, especially with freezing rain. Then rain changes to showers tomorrow morning. For those of you who like to look ahead a bit, it looks like we will again flirt with low snow levels this weekend. But we will focus on that after we get through tonight first. Please feel free to post your current conditions here throughout the night. If you have any pictures to share, please do!
P.S. Special thanks to my new “blog intern” for assisting in this post.
Good morning! Welcome to the first Tuesday of the new year. So, will it snow tonight? How about that nasty “freezing rain” word? Maybe just plain rain? Well, I just got a good look at the morning suite of models and I have to say that any number of things could happen tonight.
As of 10am, here is what I am sure of — precip will move in sometime between 3am and sunrise tomorrow (Wednesday) morning. Taken at face value, model soundings show a nice slab of warmer air will move in above us this afternoon and evening and will cause any snow that falls to melt before it hits the ground.
Here is the tricky part — will it be below 32 degrees when the precip gets here later tonight? At this point it will be a battle between what local observations are showing us and what the modeling data suggests will take place as the day progresses. At this point almost all models warm up the Portland / Vancouver area enough to see nothing more than light rain overnight tonight. However, if models are not correctly accounting for the early arrival of cloud cover this morning, than our heating may be capped for the day and that could make for a more interesting overnight period tonight. The best thing I do in close calls like this is monitor the Ch 12 TV tower temps over in Portland and see where they are at today. You can click KPTV tower temps here. If they pop up to 35 degrees or higher at all three sensors, then we can eliminate snow from the forecast tonight. If they remain below 32 and the temp at the surface here in the city stays below 34 or so today, then snow is back in the forecast. If all locations warm into the mid to upper 30’s as suggested by the models, then it will be all rain.
So, as of 10am I recommend flipping a 3-sided coin. Then check back here about 7pm tonight and I should have some additional thoughts and should be able to pinpoint things by then. So, what do you want to see happen?