Sunday, February 15, 2009
When lack of weather is unusual
We are now in the midst of a fairly unusual weather situation...an extended midwinter drought extending well over a month (see figure..red is this year, blue is climo average). And it is not over yet. But lets be clear...such droughts have occurred before and some worst then this year's. If you are really careful about picking period--mid January to mid-February--this is the driest year on record...but that really is artificial. If you started two week earlier, the heavy precipitation in early January would be included and we would be far from a record (an example of how one can.
The persistent upper level flow with a ridge offshore and a trough over the western U.S. (see figure) is predicted to persist, both by the National Weather Service GFS forecast, a collection (ensemble) of such forecasts, and the forecasts of other forecasting centers. We should have a dry week with partial sun in the lowlands, with little snow accumulation over the mountains. The latest snowpack observations indicates that some locations in the north Cascades and the Okanogan have dropped to roughly two-thirds of normal (figure).
Today, the situation will be very different in the northern and southern portion of the state. The southern half is in clouds associated with a weak ban associated with the low to the south (image). Light rain has made its way up as far north as the Portland metro area, while Seattle and north are sunny. This band should slowly move north as it dissipates . Light showers could move into southern WA...but I don't expect it to move further northward (see forecast for 4 PM this afternoon).
And there is some good news for weather watchers. A key weather observing station that had been offline...Stampede Pass at 4000 ft in the mountains--is back online. It provides the most high quality observations in the mountains..so many of us missed it.
Finally, let me note my talk on the 25th at Town Hall....it will be a comprehensive review of what we have learned about forecasting NW weather...a link about the talk is found to the right.
Posted by Cliff Mass at 8:59 AM