Pacific Storm Train Update

October 18, 2010
4:04 P.M.

So, that strong surface low that looked like it was going to plow right into Western Washington has gone away, but the models are still showing an active pattern with a lot of rain possible. Here are some shots from this morning's MM5-GFS run (12 UTC cycle).

You can see the beginnings of a storm on the left (where all the precipitation is). I don't know if this moisture is fed from tropical sources but it looks like it is at the very least subtropical. Of course, it is being driven by a very strong jet stream directly westward and that is what will allow it to strengthen.

The next image shows it in the process of what is called "explosive cyclogenesis," or as seasoned meteorologists around here say, "bombing." This basically means that the central pressure of the low is dropping rapidly and the system is quickly increasing in intensity.

As you can see, the occluded front is well-formed, there is a very strong and moist warm front out ahead of
the system, and there is a long cold front trailing behind it. And look where the bulk of the moisture goes!

... you guessed it - right into Western Washington. In fact, from 5 P.M. Saturday to 5 P.M. Sunday, some places could see 2-5 inches of rain, perhaps up to 8 inches in the wettest spots, as the red in the graphic below shows.

Seattle may be rain-shadowed though, especially north Seattle, and might not see much at all. It is VERY important to point that out - weather forecasters often do not communicate things like rain shadows very well to the public, so when Seattle ends up getting very little rain they are not surprised while the rest of the city just thinks the weather forecasters got it wrong... again. That's something all weather forecasters need to do. However, from this graphic, it looks like Seattle will still get a good amount of rain, but you can see a sizable shadow with a bullseye around the Kingston area.

Remember, this is only one possible scenario. Other models are not showing this situation quite yet, so we might end up with a fairly dry day (though we would probably still see some rain). But it is never fun to analyze those models. I guess we will just have to wait and see how everything unfolds! It's that time of year...
Charlie