Why we're forecasting snow with temperatures in the mid-upper 30s
With Thursday's challenging snow forecast looming, with timing and snow total amounts already being tweaked as better projections come in, you might be wondering: If you're calling for a high of 37-38 in Seattle, why won't it just be rain?
In addition to watching the radar on Thursday, it'll be important to watch your area's dew point, which is more important than the temperature. (See latest hourly temperature/dew point here: ) The air is not only cold over the region, but it is dry.
Drier weather means when precipitation falls, it will initially evaporate into the drier air -- a process called evaporative cooling. Since the air has plenty of room to hold the moisture, the rain or snow will evaporate into water vapor. However, evaporating water requires heat and energy. As more and more drops evaporate, the air is using up more of its heat and energy, thus making the air cooler.
You can get a sense if this is possible by looking at the current temperature and dew point. (Dew points are the temperature at which the air would be saturated.)
As a general rule of thumb, if you take the mid point of the dew point and temperature, and it’s 32 degrees or less, then the precipitation could begin as snow. For example, if it's 40 degrees with a 20 dew point, that's a midpoint of 30 degrees). It occurs because the temperature drops as the air cools, but as the air gains more moisture, the dew point rises. They generally meet in the middle.
Heavier showers can enhance this process and cool the air even more. This is what occurred a bit on Monday morning when we had temps in the upper 30s and rain started to fall in the Seattle area -- but then switched over to snow once the evaporative cooling process got under way and temperatures dropped to around 33.
With the cool, dry air coming out of British Columbia this week and more dry air expected to push in from Eastern Washington through the gaps in the Cascade Mountains, dew points are expected to remain in the low-mid 20s when the moisture arrives (whenever it does) on Thursday. Thus even if the temperature is 36-38 degrees when you see the blob advancing on the radar, it'll likely very quickly cool to around freezing as the precipitation begins and be snow.
There is high confidence in that part of the current forecast. The uncertainty remains in the timing of the snow, and the amount of moisture available, which current projections are leaning later and lighter. Eventually enough warmer air will move in and push both the dew points and temperatures higher to where we change back over to rain.