Monday, January 26, 2009
Rainshadow and snow/rain
I will update in more depth later...but another Pacific disturbance is approaching (see figure) and we will get precipitation...the question is the how much and whether it will be rain or snow. Because the disturbance is moving towards us from the northwest, there will be a strong westerly component as is approaches and moves over us. Westerly flow produces a rainshadow over the central Puget Sound region...as demonstrated last month. The air over us is cold and dry, dry enough the there will be lots of initial evaporative cooling as the precipitation falls into it from above. Evaporative cooling only acts for a short while..until the air becomes saturated. And then the movement of warm air into the region associated with the system will take over and turn the precipitation into rain.
The current run of the high-resolution local model provides the following 24-h snowfall ending 4 PM tomorrow (see figure). The mountains do well (6-18 inches) and light snow is found over portions of NW and SW Washington, and over the Kitsap/Hood Canal area. But little over Seattle. By mid-day all the precipitation over the lowlands should turn to rain. Strong winds will also occur over Puget Sound and NW Washington as low pressure moves north of us tomorrow morning and afternoon.
Here is the latest 24-h snow from the Monday night run...but from the coarser 12-km resolution version. Pretty much the same story.
Posted by Cliff Mass at 5:50 PM