WeatherWatcher: Very slight chance of the S word possible early Saturday morning

Saturday, December 8, 2018

A scenario that always catches forecast models in the Puget Sound region off guard is happening early Saturday morning. When you have cold, dry air in place with an approaching storm front off of the Pacific such as we do tonight, an experienced forecaster will look at temperature and dew points.

If you have an air temperature of 35°F and a dew point of 25°F, and suddenly throw precipitation into that air mass, the evaporation of the precipitation into the dry air cools the air until it saturates. As the air saturates, the dew point temperature will increase, and the air temperature will decrease until the two temperatures are the same, typically landing about halfway in between.

Using the above numbers of 35°F and 25°F, the air mass would saturate down to an air temperature of about 30°F and instead of rain, you'd get snow. Often this is overlooked, especially when today's weather forecasts depend so heavily on computer models.

I did a quick check on my weather stations for the Echo Lake area and Richmond Beach area. Both stations have a low dew point, 29°F at Northridge (Echo Lake), 23°F at Richmond Beach, with air temperatures at 37-38°F. It is marginal, but we could get very near 32°F when the precipitation arrives at about 1-2am. I checked the radar and it does show areas where precipitation has already started near Olympia as rain and snow mixed.

Light rain could start as rain and snow mixed, or even snow in the early morning hours in Shoreline and Lake Forest Park. 

I'm not expecting accumulation with this. At most it would be a show of wintery weather for the few that might be awake in the early hours of the morning. The storm front moving through is weak, but expected to warm us up slowly into the low-mid 40's tomorrow mid morning and afternoon.

Bottom line: Maybe some snow or rain and snow mixed overnight, all light rain by late morning. No accumulations expected. We have a stronger, all-rain storm moving in Sunday as well. I'll have more on the forecast late Saturday night.


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