Friday, January 21, 2011

Update on Seattle Weather This Weekend with Ensembles

Quick post to just update the outlook for the weather here in Seattle this weekend.

Interestingly, the 500mb analyses this morning really don't pick up much on the shortwave we were forecasting (on Tuesday) to move through last night.  Neither the RUC 500mb analysis:
Fig 1 -- RUC 500mb analysis of geopotential height (contoured) and dewpoint depression (shaded) at 12Z, Jan 21, 2011.  From the HOOT website.
Nor the initialization of the 00Z UW WRF ensemble last night:
Fig 2 -- UW WRF Ensemble 500mb geopotential height mean (contours) and spread (shading) at 00Z, Jan. 21, 2011.
Seem to have much of a shortwave signal at 500mb near the Pacific northwest.  However, this morning at 850mb we do see a weak trough moving through the area:
Fig 3 -- UW WRF ensemble 12 hour forecast of 850mb geopotential height (mean) and spread (shaded) for 12Z, Jan 21, 2011.
All this is saying is that this shortwave trough came onshore shallower and weaker than we had thought on Tuesday.  However, it still provided enough lift to kick off some rain.  This morning it was raining in Seattle and the UW ensemble model confirmed this:
Fig 4 -- UW Ensemble WRF 18 hour forecast of probability of measureable precipitation in the previous 3 hours, valid 18Z, Jan. 12, 2011.
However, the rain has now ended in Seattle and looks to be gone for a while.  The same ensemble model run shows slight precipitation chances mostly confined to the mountains tomorrow, though a few showers could pop up in the lowlands:
Fig 5 -- UW Ensemble WRF 45 hour forecast of probability of measureable precipitation in the previous three hours at 21Z, Sat., Jan 22, 2011.
That blob of precipitation probabilities in the central Puget Sound region has pretty low probabilites--at most, 50%.  The location suspiciously looks like a Puget Sound convergence zone event (something else I saw potentially hinted at back on Tuesday).  If this is so, the fact that convergence zone precipitation is usually in a very narrow band whose location can drift might explain that broad brush of low probabilities in that area.  Hopefully we'll stick with the odds and stay dry.  It will probably be somewhat cloudy, though.

On Sunday things no longer look as clear as they used to:
Fig 6 -- UW Ensemble WRF 69 hour forecast of probability of measureable precipitation in the previous three hours at 21Z, Sun., Jan 23, 2011.
On Tuesday we saw the highest precipitation amounts being forecasted on Vancouver Island and this probability forecast confirms that.  However, some of the ensemble members are bringing showers down into the Seattle area.  In the graphic above, it's only a 30% chance of measureable precipitation for the Seattle area, though that increases the further north you go.  I note this really says nothing about the intensity of the precipitation, just the probability that rain will occur.  Often in Seattle we just get very light mists or showers which really aren't enough to slow you down...

Anyhow, I have to get back to work and preparations.  But as people start making their way out here, I just though I'd let people know a bit of what to expect...

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