Tuesday, October 22, 2019

Heavy Rain, Strong Warm Front, Flooding, and Model Problems

The last 24-h has brought some record-breaking rain to parts of Washington, with flooding on some rivers.   And it reveals a problem with our best weather forecasting models.  The cause of all the weather action?  A strong warm followed by an atmospheric river with lots of moisture and strong winds.

First, let's consider some of the amazing rainfall totals (from 1 AM Monday to 5 AM this morning).  Click on the  image to enlarge.

More than five inches at the cost at Quillayute, over DOUBLE the previous record for the date.   6.37 inches near Lake Quinault.  With strong westerly (form the west) winds aloft, much of the Puget Sound lowlands were in the Olympic rainshadow, with some locations only getting about a tenth of an inch.  But as the moist, warm air approached the Cascades, it was forced to rise, resulting in HUGE amounts on the western slopes of the Cascades, with one location getting nearly 8 inches.  Wow.


The action began yesterday afternoon with the passage of a potent warm front that pushed the freezing level to well over 10,000 ft.    If you were in western WA yesterday the warming during the day was pretty obvious--temperatures rising and staying around 60F. 

Below is a plot of the temperatures (red lines) and wind above Seattle Tacoma Airport.  Height is in pressure (700 is about 10,000 ft) and time increases to the left (this is called a time-height cross section).  Look at the 0C line--freezing level.   Heads straight up around 21/18-- 11 AM yesterday.  And if you can read the wind barbs, you can see the winds got very strong from the west--50 knots.

 So behind the warm front, there was a current of strong flow associated with lots of moisture and warm air--an atmospheric river.  To see this graphically, here is a model plot of the total moisture in the atmosphere at 11 PM last night.  Blue and white indicates the most moisture, followed by red.  You can clearly see the current of moisture heading into our region--that is the atmospheric river.


With such huge amounts of precipitation falling in a short period, the rivers had to respond. The latest information from the NWS River Forecast Center in Portland is predicting moderate flooding on several rivers on the west side of the Cascades, like the Snoqualmie.


To illustrate, here is the hydrograph, the prediction of river level, for the Snoqualmie near Carnation.  The river is already at flood stage and soon at moderate flood.  Make sure you don't drive on flooded roads if you live in the area.  But since this a basically a pulse of heavy rain, the river will rapidly settle down.   If you want a show today, head to Snoqualmie Falls.


The National Weather Service has flood watches and warnings out for a number of rivers of our region:

Finally, now the model problem part of the blog.    As many of you know, I am an atmospheric modeler, using computer simulation for understanding both weather and climate phenomena.  I, of course, like to see the models do well, but my most important job and that of UW graduate students and research scientists, is to note model failures and try to fix them.   The model forecast of accumulated precipitation for the event started at 5 PM Sunday (see below) got the pattern quite correct, particularly the huge amounts on the western side of the Cascades and the rainshadow over Puget Sound.  But it underplayed the amounts at Quillayute and the western side of the Olympics.


What is wrong?    A graduate student (Robert Conrick) and I are working on this problem, and we believe there is a deficiency in the physics of warm rain in the model (WRF).   This is the topic of his Ph.D. thesis and I am hopeful that we will make important improvements.  This is where science is both fun and satisfying.

9 comments:

  1. The Bow WA Rock-on-a-string weather reporter had more than 2" of dribble during the day. Apple picking postponed.

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  2. Nice...thanks for sharing the intricacies of your research. I find it increases understanding!

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  3. "...science is both fun and satisfying" and all absorbing too; excellent brain food! Thanks for writing about it!

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  4. Can you post about our tornado in Mason County last Friday? We are all very curious down here how such an event happened. I was wondering if it had anything to do with the wind coming down Spencer Lake first.

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    1. I personally imagine that it was probably not all that different from the EF2 that ambushed Port Orchard last December. That storm was so shallow in height, that radar couldn't even see it as a tornado until the reports came in. I suspect since cloud tops don't get particularly high around here, that there were probably more tornados in Western Washington than actually reported due to radar limitations as well as video become so much more prolific with cellphones able to shoot really good video.

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  5. Just yesterday I was trying to relay issues related to the narrow band of high-intensity warm rainfall at the level of individual river drainage, and how these events are not well captured by present modeling. Specifically, is the desire to build as close to a mountain stream as possible for aesthetic reasons or just above the official 1% annual flood event level. From subjective observations of over 40 years that 1% in many Western and coastal drainages is more like a 3% annual event with the likely hood that at least one of these flood events will exceed published 1% flood elevations at some point.

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  6. Snoqualmie river was above 40,000 cfs, which looks like a daily record.
    https://waterdata.usgs.gov/wa/nwis/uv/?site_no=12144500

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  7. Broke the 1990 record for this day in Port Angeles. And for once the 1-1/3 km nailed it for us, dead on -- just barely broke the 128 mark. Even the fine resolution doesn't typically handle the rain shadow very well; other hand, P.A. is right on the edge of it.

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  8. It's good to know you and Robert are enjoying your work and helping the nation at the same time, Cliff.

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