Challenging Icy Forecast

December 10, 2009

Whew, an exhausting look at current conditions, forecast models, and satellite imagery this afternoon which means…no one really knows what’s going to happen! Actually that sounds a bit negative.

Here’s what I feel good about now and what that means:

1. There is no sudden warming with a south or west wind from now through Saturday evening west of the Cascades. So Saturday we won’t get above freezing (okay maybe 33) if precipitation continues most of the day. Then with onshore flow Saturday night and Sunday we just gradually warm up with clouds and light showers. We’re calling for 33 Saturday and 38 Sunday.

2. There is not a strong Gorge wind transporting large amounts of cold air through the Gorge through the period. It’s more of a drift of cold/dry air. This isn’t going to be a Jan. 2004 or Dec. 2008 setup with snow falling sideways and blowing/drifting snow all over the metro area. Areas near and in the Gorge stay frozen through at least Sunday morning when a westerly flow (supposedly) kicks in.

3. No model has significant moisture north of a line from Astoria to Longview. What if it never makes it here? An error of 50 miles could mean snow/no snow.

4. The “window of opportunity” is from Friday night through Saturday evening. After that it’s all over except in the Gorge.

What we don’t know, or have a very good handle on:

1. How much moisture? Anything from almost dry to .50″. That’s the reason for the “Trace to 4” in our forecast graphic. I’m sure we’ll change that later tonight or tomorrow, but it’s a good first pin of the tail on the weather donkey (or first whack at the pinata?).

2. Freezing rain or snow? Models would say mainly freezing rain, but they also appear to be overdoing the warm advection AND underdoing the current (cold) state of the atmosphere overhead. So we’re leaning more towards snow up until Saturday morning at least.

Chief Meteorologist Mark Nelsen