THE WEATHER

1/9/2008 Weather Discussion: Snow should taper to showers today as the weak high-pressure ridge moves east of the crest. The next system will enter the mountains tonight. This system looks very similar to yesterdays storm. Snow with warmer temperatures will be possible through Friday. Long-term models show snow tapering off Friday with mountain snow showers through the weekend. Models are currently showing Saturday’s storm tracking north - this will bring warmer temps and some precipitation, but for now it’s not looking all that major. Early indications of a shift in the jet stream are showing up on long term jet stream models. This could be a sign of a change in our weather pattern sometime next week. Forecast from Wednesday 4pm to Thursday 4am Precipitation .25 -.5 in. Water 2 - 6 in. Snow Temperatures Low 20 High 24 Winds 0-5 E/SE Forecast for Thursday 4am to 4pm Precipitation .5 in. Water 4-8” in. Snow Temperatures Low 22 High 32 Winds 5-15 W gusty TRENDS AND TIMING Precipitation: The main front of moisture should reach the mountains after 10pm tonight. Snow expected to increase in intensity after 2am and continue through most of the day. Storm totals by Friday afternoon could be near 1-2” of water. Models are showing the potential for a Puget Sound Convergence Zone tomorrow afternoon and evening. Freezing levels: Freezing levels should hold steady through the night. A warming trend driven by isentropic lifting * will begin early tomorrow morning. Freezing level could push to 3500 -4000’ by noon tomorrow and hold there into the weekend. Winds: Westerly flow will give way to a short period of east flow as the front approaches the mountains tonight. West winds will then kick in by 7am tomorrow. Could see some periods of gusty westerly winds through the Pass tomorrow. *Todays weather lesson: Isentropic Lifting - This is the mechanism for the swing in winds from east flow and west flow, as well as, a way to achieve inversion layers. We could see this develop in the next few days as warm air from the west overrides the cold air "trapped" in the eastern basin. The science: The density varies between cold air and warm air with cold air being relatively more dense. Due to the higher density, cold dense air sinks to the surface and has a high resistance to being lifted in the vertical. In a differential advection situation, relatively warm air will lift over shallow surface cold air because the warm air is less dense. The idea of isentropic lifting is air "prefers" to move toward a region with the same density (potential temperature surfaces (a.k.a. along a constant theta surface)). Warm air resists undercutting cold air. To stay at the same density (by preserving potential temperature) it must override the cold air. The depth of cool air on the cool side of a warm front generally increases moving to the north of the surface warm front boundary. As warm air advects over colder air, it advects to a higher altitude above sea level as it moves north of the warm front boundary. The warm, less dense air rises gradually in the vertical as it overrides the sloping cold dense air (less potential temperature air).