Saturday, October 3, 2009
The Meteorological F Word and Improving Weather
Temperatures over the region have been generally a bit cooler than normal during the past week (see graphic) as a cold, occasionally showery, upper trough has influenced our weather. But things are going to change in a big way--towards extraordinarily fine autumn weather.
The upper level weather map for later today shows the upper trough starting to move south of us, and as a result most of the showers are found over Oregon and southeastern Washington. The visible satellite image this morning shows those shower areas and some low clouds and fog over parts of western and eastern Washington (image). But don't worry about that. The computer forecast models are in agreement that the trough will progressively move south and a HUGE ridge (high pressure) will develop over the eastern Pacific. I have included below the upper level charts for late Sunday and Monday--not only is there a ridge...but northeasterly and easterly flow develops over...and such offshore flow produces rapid drying and west side warming.
But now the F word for all gardeners...especially those of us with the hopes of gathering the last of the season vine-ripened tomatoes. Frost. Nights are getting much longer now and when the skies clear there can be strong and extended radiational cooling to space. Yes, you and everything else emits infrared radiation and thus the ground can cool overnight this time of the year. Clear skies and low humidity promotes this.
Take a look at this morning's temps at 7 AM. Temperatures at some locations both east and west of the Cascades dropped into the 30s. Remember these are air temperatures at 2 meters...around 6 ft. It can be cooler at the surface! And as skies clear the next few days and the atmosphere dries, the radiational cooling will increase...so there is certainly a substantial risk of frost in the coolest places (away from water, low spot where cold air drains, etc). So if you have some cold sensitive plants on both sides of the Cascades...protect them.
Posted by Cliff Mass at 8:31 AM