Getting a good estimate of forecast cloud-top heights is fairly easy using the regional MM5 model from the University of Washington. Follow this link to the forecast soundings site.
www.atmos.washington.edu/mm5rt/rt/soundi....cgi?GFS+current_gfs
Mouse over to a site close to your desired trip location.
You will then see a forecast “sounding” for every 3 hours out to 84 hours. The horizontal bar near the top of the page gives buttons for forecast time. The forecast time for the displayed data is given in PDT. The sounding is displayed graphically, and in a table. In the graph, the Y-axis is the the height in a pressure coordinate (Mb), and the X-axis is temperature in degrees C. There are two curves in the graph. The left hand curve is the dewpoint or a measure of humidity as a function of altitude. The right hand curve is the temperature as a function of altitude. When the two curves are close together, the air is humid or cloudy. When the curves are far apart, the air is dry or clear.
The table displays the same data. Look for elevations where the temperature and dewpoint are within a degree or two of each other. At those locations, the air is likely mostly cloudy or cloudy. The table gives heights in meters. The graph and table are a nice way to get cloud base and cloud tops as a function of time and location. Also available is a forecast of winds as a function of altitude.
Remember, these are forecasts from a high resolution regional model and are generally pretty accurate. As you step out in time, forecast confidence decreases.
The marine cloudiness is prevalent in later spring in the NW. To get a good idea of the likelihood of a “marine push”, check the pressure gradient between North Bend, OR and Seattle (OTH-SEA) late in the afternoon. (
www.atmos.washington.edu/marka/pd.cgi
). If it is greater than 4 mb, a marine push is likely. Expect cloud tops between 6000 – 8000 ft. If the pressure difference between Seattle and Yakima (SEA-YKM) is greater than 4 mb, expect windy conditions on the east slopes of the Cascades.
Bryan
