The Other Weather Disaster Last Week: A Downslope Windstorm

Although the big local weather story this week has been the heavy rain and serious flooding over the Pacific Northwest, there WAS another extreme weather feature:   powerful downslope winds on Wednesday that caused massive tree damage and power outages over the eastern slopes of the Cascades.

Some of the winds exceeded 100 mph from the west.

Leavenworth, on the eastern slopes of the Washington Cascades, was hit particularly hard, with the entire town losing power, forcing the cancellation of the Christmas Lights festival.  Here are a few samples of the destruction:

The maximum wind gusts on Wednesday (from the limited collection of locations with power) are shown below. 106 mph above Lake Wenachee, 112 mph at Mission Ridge, 77 mph on the slopes just above Leavenworth. Much lighter winds over the lower Columbia Basin.

The winds at Mission Ridge (6730 ft) are shown below (the dark blue line indicates the sustained wind, the top of the light blue area indicates the gusts, the bottom of the light blue indicates the low winds during the hour).

Very windy from early Wednesday through early Thursday, with many gusts over 100 mph.   And the winds were exceedingly gusty.

 Were these winds predicted ahead of time?   You bet they were.  Below is the forecast surface winds from the UW WRF modeling system for 1 PM Wednesday.  

Reds are over 70 knots (81 mph).  You can see bands of localized strong winds on the eastern side of the Cascades.  

The NOAA high-resolution model (HRRR, High Resolution Rapid Refresh) was doing the same thing, as shown by the forecast wind gusts at 7 PM Wednesday.

So why such localized winds?    

Answer: There was a downslope wind event in which strong winds approaching a mountain barrier are accelerated as they descend the lee slopes (see schematic below).

Favorable conditions include strong winds approaching the barrier with sufficient vertical stability, conditions that can occur with the passage of the weather system from off the Pacific or the approach of a strong atmospheric river, which occurred during that day.

The eastern slopes of the Cascades are well known for strong downslope winds. In my Northwest weather book, I note several incidents.

Finally, many of you would like to know whether another major atmospheric river is headed to our region.   The answer is yes.

Here is the atmospheric river diagnostic for Monday morning.  A juicy plume heading right into us.

Consider the predicted precipitation totals through next Friday.

Yikes!  Some places will get over 10 inches. On top of saturated soils and rivers that are running high. Some rivers will flood again.

Will discuss in future blogs.

The Atmospheric River Event is Ending--- And ANOTHER One is Predicted

The heavy rainfall is over, but some rivers are still rising.   The most impactful atmospheric river event of the past few decades.

Consider the 5-day rainfall totals below, and be prepared to be impressed.  

Many of the windward slopes of the Cascades and Olympics received more than 10 inches, with some experiencing 15-17 inches.

You will also notice extreme rain shadow areas in the lee (east) of major barriers, which received less than an inch.

Numerical weather prediction models were STUNNINGLY good, as shown by the predicted 5-day precipitation totals by the UW WRF model made last Sunday (see below).  

Close correspondence between predicted and observed rainfall.

The weather prediction world has changed dramatically during the past decades, with virtually every major extreme weather event predicted days before.

The problem is that governments, local agencies, and others are not sufficiently taking advantage of these skillful forecasts to save lives and property.  The media do not understand the profound impact of improved weather prediction technology.  

This problem is compounded by the National Weather Service, whose forecasts and warnings have declined substantially compared to the state of the art.  

 I can provide a half-dozen cases of the NWS failing to provide timely warnings even when model forecasts are excellent, such as this event and the Seattle ice storm of December 2022.

I hate to reveal this, but  ANOTHER significant atmospheric river/precipitation event is in the forecast.

Consider the predicted 72-h precipitation total ending 4 AM on Thursday.  Yikes...pretty wet, with some mountain areas getting 10 inches.  This is problematic, falling on saturated soils and rivers already well above normal.

To address the reliability of this forecast, one should consult ensembles of many forecasts.  Below are plots of forecast precipitation at Stampede Pass in the central Cascades from many forecasts.

All are going for a major precipitation event starting in the middle of next week.  You can take this to the bank.

All of this is a reminder that the most impactful severe weather of the Pacific Northwest is flooding.  Not heatwaves, not wildfires, not windstorms.

The Second Atmospheric River Will Bring Far More Severe Impacts

In act one of this meteorological drama, yesterday and this morning brought heavy rain and flooding to many sections of western Washington.  Well-predicted days in advance.

But the weather prediction models indicated that this would be a two-act event, with the second act being far more serious.   The curtain is about to rise.

Take a look at the precipitation totals of the first act (last two days, below).  Some areas received 7-8 inches.

Many local rivers are now at flood stage, with some at record levels (black dots are record levels, blue are 90th percentile or greater, with normal being 50th percentile)

The media is full of pictures of flooded roads, such as in the Snoqualmie Valley.  A once-in-1-3-year event.

But this is just the meteorological appetizer course.  The real action is about to begin:  a second, stronger event that will throw massive amounts of rain on saturated surfaces.

Consider the total precipitation from the UW ultra-high resolution modeling system for the 72 h start this morning at 4 AM (shown below).

Wow.  There are going to be some happy ducks.  Large areas will get 10 inches of rain or more, particularly in the Olympics and northern/central Cascades.

Serious flooding is inevitable.   

How can I be so sure?  Because of the use of ensembles of many high-resolution forecasts.  If they are all on board with heavy rain, my confidence grows.

Let me demonstrate this to you.   Here is the ensemble of accumulated precipitation at Humptulips on the southwest side of the Olympics.  A huge amount of precipitation (about 6 inches), most over about 12h.   Not much variance in the forecasts.

Stampede Pass in the central Cascades (below)?  A similar soggy story.

Every reservoir in the region will reach normal or above normal levels.  Consider the Seattle Times' favorite drought reservoir system (the Yakima), which they featured in a recent story.

The Yakima River is now at flood stage (see below), and in a few days the entire system will be full to normal levels.

The Torrent Has Begun: The Philippine Connection

The predicted rain has begun.  Rain associated with a potent atmospheric river, with some of the water originating thousands of miles away near the Philippines! 

Consider the situation on Sunday afternoon, as shown by special satellites that observe atmospheric moisture from space (below).  

You can see the plume of atmospheric moisture extending from the western Pacific to the Northwest.

But it is not quite that simple.  There is substantial three-dimensionality to atmospheric rivers.

To prove this to you, below are air trajectories--the three-dimensional motion in space--of the air reaching the Cascades at 10,000, 5000, and 2000 meters above the surface.   At high altitudes, the moisture is coming from the western Pacific, but at lower elevations, the moisture is coming off the Pacific to our south.

Atmospheric rivers are like real rivers, with tributaries contributing moisture along the way.

The big precipitation has started.  Below are the precipitation totals for the past 24h.  

 A number of locations have already received 3-4.5 inches.   And more is on the way.

Our rivers are rising very rapidly.  For example, the levels of the Cowlitz River in SW Washington are zooming upward (see below).   Soon it will be at flood stage.

Remember...this is a two-step event...and this is only the first part!

The Seattle NWS Forecast Office has FINALLY put out flooding warnings over large areas of western WA (see below).  Quite honestly, they waited way too long to do this.  

This is a serious event, and people need time to prepare.

A Monster Moisture Plume is About to Hit the Pacific Northwest

 The latest satellite imagery is stunning....and concerning.  A huge plume of water vapor, extending thousands of miles to the southwest, is about to hit our region, resulting in massive rainfall and localized flooding.  

Consider the water vapor imagery late this afternoon (below). Green and yellow are the highest values,

Look at a wider view, and you can see this atmospheric river of moisture extends many thousands of miles to the southwest.

We are now close enough in time to the event, and forecast models are similar enough, that we can have great confidence in the forecasts.  

The forecast totals through Wednesday morning are stunning, with over ten inches over the Cascades and coastal mountains. Some locations in SW Washington and NW Oregon will receive 15 inches.

Very concerning.

But it doesn't end there.  The next 24 hours will bring even more, particularly over the southern Cascades.  Major rivers in SW Washington (e.g., the Cowlitz) are going to flood.

Don't take my word for it:  here are the latest predictions from the NOAA NW River Forecast Center:

TEN Rivers are predicted to go into major flood, up from two predicted a few days ago.

Some rivers, like the Collitz, are predicted to approach all-time record levels (black line below).

Stunning.

I am more than surprised that the National Weather Service does not have flood warnings out for drainages in the central and southern Cascades (see below).  They should.

Flooding Ahead

It is now clear that we will have a major precipitation event on Monday and Tuesday, with flooding over vulnerable areas of western Washington, such as the Snoqualmie Valley, on Tuesday and Wednesday.  Later in this event, substantial snowfall is expected in the mountains.

Overall, this will be a very positive event for water resources in our region.  

The NWS has now released a flood watch over much of western Washington, which is good (see below).  Soon they should upgrade this to a warning. 

Models, such as the UW WRF forecast system, are still going for a big event, although the totals have backed off slightly.  Still predicting ten inches or more over large sections of the Cascades and Olympics (see total precipitation through 4 PM Wednesday below). 

Perhaps what is just as impressive is where it is not precipitating.  Look more closely at the predicted totals, and you will see a profound rain shadow northeast of the Olympics.

We are talking about less than a tenth of an inch.  So ONE HUNDRED times more rainfall in the mountains than in Sequim.  Much less rain in north Seattle than at SeaTac.

Back to flooding potential, it is now highly probable for vulnerable locations such as the Snoqualmie Valley.   

The situation near Carnation is stunning, with the river reaching major flood level for THREE days (see below).    You don't see that very often.

Please...if you live in local flood plains, avoid driving through water.  And complete your preparations for flooding by early Monday.

Finally, there is a lot of talk on social media and online about the importance of rain versus snow and snowpack versus reservoir storage.   

A lot of it is not correct.  Some of the AI information is not accurate either.     I will discuss the water situation more in a future blog.

Major Flooding Coming to Western Washington

With very heavy precipitation expected next week, particularly on the western side of regional mountains, major flooding is being forecast.

Over the next five days, the UW WRF model is forecasting more than ten inches over the western side of the Cascades (see below).

 

Are you sitting down?  Ready for a shock?  Below is the ten-day accumulated precipitation forecast for the region.  Some locations are expected to receive 15-20 inches in total.  

That means flooding.

The European Center model is doing the same thing, as shown by the predicted totals through next Saturday (13 December).  For all models, the wettest day is on Tuesday.

The super-heavy precipitation on Tuesday will be associated with this region being hit by one of the strongest atmospheric rivers in a long time (Tuesday at 7 PM shown):

The Gogzilla Atmospheric River.  Trained meteorologists call this a GAR.

The Northwest River Forecast Center in Portland is predicting moderate flooding on several northwest rivers (see red dots on the map below).

For example, the Snoquamie River near Carnation will get to MAJOR flood stage (purple line) on two days (10-1lth), with levels reaching all-time high river stages on two days.

The Snohomish will reach major flood stage (red line) during the same period.

And the Skagit will approach major flood stage on December 20th.

This is a serious event, and those of you in vulnerable locations should make preparations. 

A good effect of this major precipitation event is that local reservoirs will gain an enormous amounts of water.  All the western Washington reservoirs should fill to at least normal levels and a major filling of the Yakima system will occur.

Massive La Nina Precipitation Coming to the Northwest

Heavy rain and snow?  Check.

Skiing before the holidays?  Check.

Bring reservoirs to near normal levels?  Check.

Threaten flooding on some local rivers?  Check and check again!

The next few weeks are going to be an exciting meteorological ride, so buckle up.

A series of atmospheric rivers will make landfall over our region, resulting in bountiful precipitation, including snow in the mountains.  Let me show you.

The next week will include three atmospheric rivers (water vapor transport is shown below...the key measure of atmospheric river activity)

A modest atmospheric river on Thursday,

A strong one next Monday.

And a GODZILLA atmospheric river next Wednesday.  Directed right towards us.  

Precipitation totals over our region?

Through Friday morning, a good wetting, with the mountains soaked by 3-5 inches.  Just the start.

By Sunday morning, very large precipitation totals will have accumulated in the mountains, with some locations receiving 5-7 inches.

But then the third, crazy-strong atmospheric river hits, and by next Friday, the totals become insane..10 inches and more in much of the mountain areas.

The above are forecasts from the UW modeling system, driven by the NOAA/NWS GFS model.

The European Center modeling system is doing the same thing, which gives us confidence that a major event is ahead.  Below are the totals through next Monday morning.  Mountains are hit very hard.

By next Friday, crazy high totals:

The rivers will rise rapidly, some to flood stage.  Consider the situation for the Snoqualmie River near Carnation (below).  The first two atmospheric rivers will bring the level up to flood stage (see below).  The third, stronger river will cause substantial flooding (not shown below).

Snow in the mountains?  Huge amounts will fall.  Here is the forecast through mid-December by the European Center model.  Yards of snow in some locations.  

The ski season will begin.  Our water resources will be greatly enhanced.  Winter recreation will beckon.