Monday, March 3, 2014

A Good Soaking

Monday, March 3, 2014
9:57 p.m.

I don't want to say winter is over. Heck, Snoqualmie pass was closed from North Bend to Ellensburg today, which I believe is a 70 mile stretch or so. Bellingham got clobbered with snow twice in the past two weeks. But, we face an unfortunate reality - it is March, and we are warming up. In fact, we are warming up fast. Highs this week are expected to be in the mid-50s down here, and there will be plenty of times in which the passes will see liquid precipitation instead of the white stuff for a change. These temperatures aren't just affiliated with warm, subtropical patterns; as the sun gets higher in the sky, surface temperatures warm up further during the day. Our December lows average around 36, and our current lows average around 38... a two-degree difference. Our highs, on the other hand, average 52 degrees for this time of year as compared to 45 for much of December... a seven-degree difference. The pool of arctic air to our north is becoming warmer and weaker, and it is getting harder and harder for us to cool down to a point where we can get sea-level snowfall. In Seattle, snowfall chances tend to drop off dramatically after mid-February.

Highs this week will be in the mid-50s, but these temperatures will not principally be because of increased solar insolation. In fact, our lows will hover around the mid-40s all week long. What we have is a fairly continuous flow of moisture flowing off the Pacific coming into our area interspersed with brief periods of sunshine. I talked about this in my previous post. But now I want to focus on two specific storms that could be quite a doozy,

The ironic part is that these cyclones are actually relatively weak. They are dissipating as they approach the coast, and as you can see below, by the time the Wednesday night storm makes landfall on the coast Thursday afternoon, the low will have degraded to 996 millibars. That is a very weak low considering how much rain is involved with this system. However, it goes to show that you don't need a deep low pressure system to get flooding rains. On the other hand, you need sharp pressure gradients to get wind associated with a windstorm, and these sharp pressure gradients are more common with deep, powerful cyclones than small, weak ones.

Valid 01:00 pm PST, Thu 06 Mar 2014 - 69hr Fcst:

I should mention that there is a fair amount of disagreement between the models in terms of the timing of the rainfall. The GFS model brings periods of rain in Tuesday night and Wednesday night, with the cold front sweeping through on Thursday. The EURO gives us a similar picture but also gives us some rain Tuesday morning. The NAM on the other hand is different entirely, bringing periods of rain Tuesday afternoon and Wednesday morning, with the cold front sweeping through Thursday afternoon. Right now, I'm sticking with a blend between the Euro and the GFS. Majority rules, right? Also, these models are generally more accurate than the NAM.

Take a look at the picture below. It shows the precipitation predicted over the past 72 hours from 4 a.m Tuesday to 4 a.m. Friday. If you look closely, you can see a region of 10-20 inches on the windward side of Mt. Rainier. That is a tremendous amount of rain. This won't be a record flooding event by any means, but any river in Western Washington could flood, with rivers off the Olympics and the Snoqualmie Basin at the highest risk. The Skokomish will flood. I would bet my life on that.

Valid 04:00 am PST, Fri 07 Mar 2014 - 84hr Fcst:

We'll need to keep an eye out for landslides. The National Weather Service actually issued a Special Weather Statement highlighting this risk. We've had quite a bit of rain recently, and this additional heavy rain on saturated soil. I remember when there was a massive landslide near our house not too long ago. It seems unlikely that an entire hill would give way, but it does happen. If you live on a hill or steep bluff, take note of this. The landslide risk will diminish Thursday onward as the rainfall lightens up.

Stay dry! Or at least make a decent attempt. :)


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