Clearly, our model forecasts have had some problems and now the situation is now becoming clearer. Precipitation has spread was more intense and spread farther north than predicted. But the biggest threat was never this morning... all the models had only a few flurries over us. The 1-3 inch NWS forecast was for this afternoon and that is what we have to watch now. And a power glitch took out the UW modeling system last night..bad timing.
This is when forecasters make mistakes for psychological reasons...miss one way and overcompensate the other.
The key feature is a low center along the coast. Last night's model either had it too weak or mispositioned. Here is the latest surface observations at 10 AM. Click on the image to expand. A low center (roughly 1004 mb) is along the NW coast. If it moves SE we could get the set up for big lowland snow...moisture off the ocean circulates inland, being met by cold air coming down from the north. This cold air is driven by the difference in pressure between the cold, high over BC and the low pressure center.
The NWS is clearly worried about this scenario and have put out a heavy snow warning for the lowlands. The latest high-resolution run--which has the coastal low better but not perfect-- shows the following:
We are talking about 2-4 inches south of the city, with roughly 1-2 inches on the north side. More as you head towards the Cascades and south.Can you believe this short-term forecast? The coastal low is clearly stronger and farther north than predicted by ANY of the models. I really wish we had the coastal radar now...it would provide a clear view of what we are dealing with...would have made a huge difference now.
The 11 AM surface map...just available shows a 1002 mb low over the NW tip of the Olympic Peninsula and the latest visible image show VERY unstable air offshore. If the low goes south of us and draws some of that cold, unstable air in...and it meets the cold stream from the north, we are talking about serious snow (6-12 inches). Or if the low moves farther north we could get a Puget Sound convergence zone over the central Sound and a huge amount of snow in a narrow band (a la Dec 18, 1990). The system is moving slower than the models predicted and the real threat is the middle and latter parts of this afternoon.
In short, a major threat is there, but there is a lot of uncertainty. You can watch the radar imagery to see this unfold...and know whether there is any threat. Here is the NWS radar link:
http://radar.weather.gov/radar.php?rid=atx&product=N0R&overlay=11101111&loop=no
Or you can view the evolving surface observations and track the low! One think is sure...it will get much colder tonight. It is critical for all local DOTs to get as much of this stuff off the road as possible or hit it with deicer. All untreated slush and water will freeze solid tonight.
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