Wednesday, September 14, 2011

First rainy weekend in Seattle in a while?

As anyone who has been in Seattle over the past few days knows that we've returned to more "normal" conditions for fall.  Which includes a sky that perpetually looks like this:
Webcam image looking across Hood Canal to the west on Sept. 14, 2011.  From Dr. Dale Ireland's page.
The marine layer is in constantly and so far we haven't had enough heating to erode it away.  Upper-level troughing is helping keep an onshore flow with cooler temperatures aloft, both good conditions for maintaining this cloud cover.

However, we haven't seen rain with this onslaught of cloudiness.  In fact, Seattle hasn't had any rain for a while.  We've been enjoying the usual summer "drought".
Climatological norms, records, and observed values for temperature and rainfall year-to-date at KSEA.
You can see in the rainfall chart that while Seattle is wetter than usual so far this year, there hasn't been much of a change in rainfall for quite some time.  In fact, aside from a .12 inch event on August 22nd and a few scattered "trace" readings, Seattle hasn't seen any real rain since the end of July.

That may be about to change, though.  Here's the UW-WRF 12-km extended forecast image showing 24-hour accumulated precipitation as of next Sunday evening:
108-hour UW-WRF 12-km forecast of previous 24-hour accumulated precipitation, valid 00Z, Monday, Sept. 19, 2011.
The units on the color bar an in hundredths of an inch.  This map shows around half an inch of rain in the south sound area, with maybe a tenth to a quarter of an inch further north in the Seattle area.  Higher amounts are forecast further south.  You can also see the area of lower precipitation forecast directly to the east of the Olympic Mountains--a rain shadow effect.

The trend in most of the major models has been to slowly increase our precipitation amounts this weekend.  Some factors seem to support this.  For instance, water vapor imagery shows a nice plume of upper-level tropical moisture headed our way over the next few days.
GOES-W water vapor imagery at 18Z, Sept. 14, 2011.
And there does appear to be a bit of a 300mb shortwave trough with associated jet streaks popping into the upper-left corner of the model domain by Sunday.
108-hour UW-WRF 12 km forecast of 300mb winds and heights, valid 00Z, Monday, Sept. 19, 2011.
Such a setup should generate some strong onshore flow.  This will definitely keep the cloud cover in place, and added lift as the air moves over the Cascades (or the Olympics) could help to squeeze some showers out of the moist layer aloft.  A vorticity maximum is also forecast to move over this weekend in advance of the upper-level trough, as seen on this 500mb vorticity forecast map:
87-hour UW-WRF 12 km forecast of 500mb absolute vorticity and heights, valid 03Z, Sunday, Sept. 18, 2011.
Positive vorticity advection (yes, if we're being technical, differential advection, but generally we have lower absolute vorticity at the surface...) generally supports stronger vertical motions, so this would also aid in getting enough lift for their to be some rain.

And the forecast rainfall amounts in the models just keep growing...

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