Tuesday, March 29, 2011

Heavy Rain for the Pacific Northwest

We currently have relatively weak flow across the Rocky Mountains and central US, so it's a bad time to be looking for lee-side cyclogenesis.  So, in the meantime, I'll turn to more current weather to discuss.

Spring has definitely arrived here in the Pacific Northwest--the trees are blooming, leaves are starting to appear, and high temperatures are consistently in the 50s.  However, with spring comes bouts of heavy rain which continue the feel of winter well into June.

The jet stream is on its way north and with it, lots of moisture.  Here's the water vapor satellite image from this morning:
Fig 1 -- GOES-W WV satellite image at 17Z, March 29, 2011.  From the HOOT website.
That's a lot of upper-level moisture pouring in from the tropics up to Washington and southern British Columbia.  It's interesting to note just how long this particular jet streak happens to be--here's the northern hemispheric analysis of 250mb winds from 12Z this morning:
Fig 2 -- 250mb winds (colors) and geopotential height (contours) at 12Z, March 29, 2011.  From the HOOT website.
 This particular jet streak begins all the way back in eastern China and crosses the entire Pacific before ending over the northwest coast.  This sets the stage for a very long-duration rainfall event as moisture is continually advected into the region.

How much rainfall are we looking at?  Here is this morning's 24-hour rainfall forecast for the end of the day on Wednesday from the 12km UW WRF model.
Fig 3 -- Previous 24-hours of rainfall at 00Z, Thursday, March 31, 2011.  From the UW Modeling page.
The units on these maps are somewhat odd--"cin" stands for "centi-inches" or hundredths of an inch, I believe.  Therefore the black colors in the northern and central Cascades that correspond to "256" on the color bar would actually be 2.56 inches--which is a lot.  You can really see the orographic enhancement of the precipitation like I talked about in my last blog post.  The strongest bands of precipitation are on the windward side of the Cascades and the Bitterroots on the Montana-Idaho border.  On the leeward (eastern) side, the precipitation drops off considerably.  Also note how dry it is over the northern Puget Sound area.  This is a rain shadow behind the Olympic Mountains on the Olympic peninsula.  Rain continues through Thursday as well:
Fig 4 -- Previous 24-hours of rainfall at 00Z, Friday, April 1, 2011.  From the UW Modeling page.
You can tell by comparing Thursday's rainfall totals (figure 4) with Wednesday's rainfall totals (figure 3) that the axis of the jet stream is expected to move further north during the time between.  Oregon and points further south aren't seeing nearly as much precipitation on Thursday as they did on Wednesday.  There's still some black shadings in the North Cascades, though, indicating another 2.5 inches or so on Thursday.  That's over 5 inches total, according to these models.  Will these amounts verify?  The models do tend to over-estimate mountain precipitation in this area, so probably not.  But still--quite a bit of water.

Such extended rainfall events being forecast often raises the specter of flooding in the mountain rivers.  The Northwest River Forecast Center (NWRFC) runs many hydrologic models using the input from atmospheric models (like the ones we've been looking at) to try and predict how much water will enter each river basin.  They can use this information to try and forecast river height.  Here's a chart from the NWRFC showing the observed and predicted levels of the Snoqualmie River near the city of Snoqualmie east of Seattle.
Fig 5 -- Observed (blue line), model forecast (green line) and extended trend (cyan line) of river stage level for the Snoqualmie River near Snoqualmie, WA.  From the NWRFC.
On the above chart, the current time (late Tuesday morning) is shown by the vertical purple line.  Flood stage is shown by the horizontal red line.  You can see that for the past several days river levels have remained relatively constant at around 5 feet.  However, with rains expected to begin Tuesday afternoon and continue through Thursday, the level of the river is forecast to rise sharply, peaking around 17 feet on Thursday, which is 4-5 feet above flood stage.  However, the river is also forecast to quickly drain out, dropping below flood stage once again by the weekend.  I'm curious as to the timing of this rise--I'd expect the peak to occur on Friday or so, after the rain had stopped.  Granted, the plot above was made around 9Z this morning, before the new 12Z model data had come in.  So maybe this forecast will be amended in the near future.  Regardless, flood watches have been posted for a lot of the central and northern Cascades.

It's going to be a wet week in Seattle.

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