Wednesday, October 28, 2009
Warm, wet and breezy
I am finally ready to talk about the upcoming wet weather--I had to wait until the solutions settled down. This week there were major shifts and changes in the forecasts, not only of the winds but for precipitation as well. The reason for the lack of predictability have been ETs--extratropical transitions. This is when tropical systems move northward into the midlatitude westerlies. As the tropical systems move poleward they transition into midlatitude or extratropical systems and this transition is very sensitive and can induce weather disturbances far afield. Sort of like a freeway entrance....hard to judge exactly what car you will be behind as you accelerate into the traffic.
Well, if you haven't been able to go to Hawaii, Hawaii is coming to you!
A warm front is now approaching our area, bringing clouds and light precipitation (see image above). Most of the rain will be done with by 10 AM and there should be breaks tomorrow afternoon. The 24-h precipitation ending 5 PM shows modest rain over the lowlands, with heavier amounts (1.5-2 inches) southwest of the Olympics and Mt Rainer. Nothing major. But you will notice the change in air temperature tomorrow as much warmer (like 20F warmer) air moves in aloft (see temps at 850 mb--roughly 5ooo ft). Bad news for snow levels. Major melting of today's snow will occur.
But then the second act will occur...the arrival of the premo moist, warm air from north of Hawaii and the cold front. A long thin conduit of moisture from Hawaii to the NW will be apparent on Friday afternoon--and at that time it will be headed for BC (Figure). The infamous PINEAPPLE EXPRESS.
Later on Friday and early Saturday the plume of moisture will sag down over Washington ahead of the cold front. So later on Friday night it will be our turn. Amounts will be substantial over the N. Cascades, perhaps producing minor flooding for rivers there, but not a really serious situation because the front will move through quickly (see graphic showing 24-h rain ending 5 AM Saturday). The worst flooding situations occur when the plume stalls over us. Amazing rainshadow over Sequim and vicinity. By Saturday afternoon it will be through, with postfrontal showers. No major threat for the Green River valley.
Posted by Cliff Mass at 7:28 PM