Ken Clark

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Update on Miriam

September 25, 2012; 12:36 PM

I have only a short time for an update on Miriam today. There are some things that have become clearer than yesterday and some things that have not.

In general, Miriam will continue to weaken as it is drawn north the next couple of days, but it still will carry a lot of moisture even into Friday. I expect Miriam to be below hurricane strength on Thursday and could be down to a weak tropical storm or depression on Friday.

The overall synoptic pattern is about the same on the models now. By Friday, especially later in the day, a closed low will develop about 600 miles west of San Diego and spin in place into next week. A ridge will persist to the east as well as to the north of that low. The rub is always with the details, and here is where the models diverge. The GFS captures the weakening low that will be Miriam around the upper-level low spreading moisture north and northwest even as the low-level circulation falls apart. The European takes the upper-level vort max associated with Miriam and sends it east across the Baja later Friday and into Mexico for Saturday while the remnant low-level low and some moisture remain behind with the moisture moving north and northwest. So what is the difference? The GFS would have much more moisture available than the European. Even so both models bring QPF out to at least the coastal waters west of Southern California Saturday and Sunday on the GFS and mainly Sunday on the European. With a low spinning out where it will be, and that is agreed on by all models, one has to be somewhat more concerned about rain in Southern California in some place. However, I don't think it's time to stick one's neck out too far just yet, but at least it's time to bring up the possibility that this could occur. Time is our friend right now, and this will certainly be an interesting forecast. So for another day, the eggs I referred to yesterday will remain set aside. This is going to prove interesting to watch. Models are usually horrible in predicting what really happens with this kind of situation. At times, they carry too much moisture north and other times not enough. Stay tuned for more tomorrow.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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Ken Clark
Ken Clark's Western U.S. weather blog tackles daily weather events with commentary from one of the most experienced and trusted Western U.S. weather experts.