Tuesday, March 17, 2009
Over the weekend and Monday I was busy giving public talks...but I am back again. Nice to meet several of you.
Many have complained about the cold temps...and in general we have seen substantially below normal temps during the past two weeks in western Washington. Above you see the temps for Seattle, Yakima, and Spokane...with the normal highs and lows shown. Except for one day, the Seattle max temps have all been below normal--most well below normal. Eastern Washington was cold early in the period, but the last few days have been near normal. On the 12th Spokane's temp fell to near zero, and Yakima had several days in the teens. It does look that a moderating period is ahead though.
On the road, several people asked me about the interpretation of the weather radar images...so let me try that a bit (see image). What is shown is reflectivity...a measure of how much the target (precipitation) scatters the radar signal (which is in the microwave part of the spectrum). Grey is drizzle (5-10), reds are light rain (15-25), green is moderate rain (30-35), and above that is either absolutely pouring...or wet hail. The concentric range circles are 100 and 200 km from the radar. If you look at the upper left corner you see some additional information. This radar picture is for an elevation angle of .5 degrees, which means the radar is canted slightly up from the horizontal. Thus, the beam is sweeping out a conical surface as it rotates...or in other words...the beam gets higher and higher the farther out it gets from the radar. How how? The second row of numbers tell you. The radar is at 196 meters, and the beam gets to 1068 and 1941 meters by the first and second range circles. That beam is way overhead by the time it gets to the south Sound for example. So shallow drizzle can be missed for such locations.
Now why don't we see the mountains on the radar? Special software remembers where the mountains out and removes that part. In the right corner you see max=44 ..that means the maximum reflectivity anywhere in the domain was 44.
Rain shows up better than snow on weather radar. And big raindrops show up better than small ones. Anyway, enough radar 101.
On Thursday night at 10 PM KIRO TV is repeating their hour-long special on the Dec 3-4, 2007 windstorm/rainstorm, if you are interested.
Posted by Cliff Mass at 7:35 PM