A Stormy Sunday Night Ahead

December 15, 2012

Now that the snow showed up as expected, we can move on to the next weather forecast issue.  A strong wind event or storm, depending on where you are located.

First, I’m amazed at how well the snow worked out this morning.  Almost everyone saw snow in the air, and we had some decent accumulations; 2-3″ up on Bald/Chehalem Mtns, I had 1″ or so at home, and many areas in the 500-1000′ elevation had at least “all-whiteness” for an hour or so.  Most important, mesoscale models absolutely nailed the sudden cooling atmosphere overhead with the steady precip band.  That was enough to change rain to snow in most areas.  And the timing (between 9-11am) was right on too.

We now have excellent agreement among the various models on what’s going to take place in the next 30 hours.  A developing low pressure system will deepen quickly as it heads towards the northwest tip of Washington.  It is forecast to reach somewhere between 972-980 millibars, then weaken quickly once it hits land.  All models look almost exactly like this at 4am Monday, the shiny new 00z ECMWF:

ecmwf_apcp_f36_nw

Several thoughts on this:

1. Pressure is nice and deep, but the low is beginning to fill as it heads inland.  If you want BIG wind, you want a system to still be deepening as it moves onshore.  That doesn’t mean you can’t get a damaging windstorm in this situation, but something to note.

2. Location of low pressure center is okay for a good wind event here in the Valley, but if we were expecting a significant windstorm, the low would need to be a bit closer, let’s say Hoquiam to Olympia or a little farther south.

3.  Pressure gradients are impressive with perfect isobar orientation (west-east) from sunset tomorrow through daybreak Monday.  Each model is different, but generally between 14-19 millibars EUG-OLM.  The old gauge for peak gusts in the north Willamette Valley is that gradient X 3.15.  That would give 45-60 mph gusts.  Interesting… 

4. I checked the Hannukah Eve Storm (2006), model forecasts were 20-22 mb in the same area.  That year we saw widespread gusts 45-60 mph in the metro area.  You may remember we were on the edge of that damaging storm, but even so, 250,000 PGE customers lost power at the height of the storm.  This storm should be weaker than that one.

5.  Model surface winds in the valley are not very impressive, BUT, I checked the blog entry just before the 2006 storm: http://fox12weather.wordpress.com/2006/12/13/windstorm_in_24/  That’s interesting isn’t it?  The same mesoscale models didn’t show real high winds in that case either.

6. The north coast really gets slammed by the “poisonous tail of the bent back occlusion” as the NWS says.  That is the surge of strong west-southwest wind whipping around the south side of the surface low.  This should happen around midnight tomorrow night.

So my forecast?

COAST:  Peak gusts 70-90 mph, two surges, one later tomorrow afternoon as the warm front passes, then the strongest as just mentioned around midnight tomorrow night.

INLAND VALLEYS:  Peak gusts 40-45 mph late tomorrow afternoon/evening, then 45-55 mph overnight tomorrow night.  If we actually see gusts around 50-55 mph at the official observing locations, we’re going to see 50-100,000 customers with outages.  How about we call it the POI (PGE Outage Index)?  So plan for at least brief power outages anytime tomorrow afternoon through daybreak Monday.

More snow Monday night and Tuesday?  It may be just barely cold enough, like this morning, but we might have a moisture limitation.  I sure wouldn’t say there’s a great chance to see sticking snow in the lowest elevations, but I’ll look at that more closely later; one exciting weather event at at time.

Chief Meteorologist Mark Nelsen


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