Why Do Strong Southerly Winds and Western Washington Rainshadows Come Together?

There were two singular aspects of the weather on Saturday.  First, there were extreme variations in rainfall across the region, with an amazing rain shadow over Puget Sound.

As shown below, for the 24 hours ending 10 PM yesterday, there was nearly 5 inches of rain over the western slopes of the Olympics, but only 0.01 inches near north Kitsap County in the Olympic rain shadow.

You read that right, a factor of FIVE HUNDRED in rainfall.

If you want to be further impressed, consider southeastern Vancouver Island, where a peak rainfall of 11.17 inches was observed over the coastal side while only 0.57 inches fell in the lee.   

And then there were the winds.

Strong, gusty winds from the south and southeast were found across western Washington, with Paine Field winds gusting to 62 mph, with 50-55 mph observed at several sites (below, max gust yesterday).

As shown by the wind observations at Paine Field in Everett, the winds really revved up between 2 and 5 AM on Saturday and then stayed strong for hours.

Paine Field sustained winds (cyan) and gusts (purple)

And now the interesting part.  The rain shadow and the strong winds were directly related.  

The flow that produces a strong rain shadow also results in powerful southerly winds.

Let me explain.

The heavy precipitation on the western side of the local terrain and the rain shadow to the east of the mountains are associated with strong moist westerly flow (winds from the west) reaching our region.   

To show this, the winds around 5000 ft (850 hPa pressure) yesterday afternoon are found below.  The lines are like pressure (called heights), and the winds aloft tend to be parallel to the height lines.  The closer the lines are together, the stronger the winds.  Lower heights are to the north and higher heights to the south, which is associated with higher pressure to the south and lower pressure to the north.  Remember this.

Such strong incoming flow rises on the western side of local causing precipitation enhancement and then sinks on the eastern slopes producing a rain shadow (as shown in the schematic below).

The windward enhancement was obvious in the weather radar yesterday afternoon (see below)

The accumulated precipitation during the event simulated by the UW WRF model, is below.  We have an amazing ability to simulate these precipitation contrasts.

But this wind field is also associated with strong winds from the south over Puget Sound and the Georgia Strait.

As noted above, to get strong winds from the west or southwest approaching the mountains aloft, there needs to be lower pressure to the north and higher pressure to the south.  In meteorological classes, we explain this fact, which is associated with geostrophic wind balance.   

Now, let me show you the predicted sea level pressure pattern yesterday afternoon.  

You will note low pressure over British Columbia and higher pressure over Oregon.  There is a large north-south pressure difference over western Washington State  At low levels in the presence of terrain, air moves directly from high to low pressure, which explains the strong southerly winds.   Offshore, away from terrain, the low-level winds are more parallel to the sea-level pressure lines, which are called isobars.

One final thing, why were winds particularly strong over the north Puget Sound area, such as Paine Field?

Because the pressure differences (a.k.a. pressure gradients) are particularly strong there, something shown by a high-resolution simulation for nearly the same time (see below).    

Why is the pressure difference large there?   Because of the tendency to produce a low-pressure area in the lee (downstream) of the Olympics.   Analogous to an eddy in the lee of big rock in a stream.

Strong southerly winds also bring up warm air.....and yesterday's high hit 65F at SeaTac, 6 degrees above normal.

Today should be warm and dry for most of western Washington until late afternoon when a moist Pacific front will cross the region.

Profound Rainshadowing over the Western Washington Lowlands

If you like extreme differences in precipitation, the Pacific Northwest is the place for you!

A moderate atmospheric river is heading into our region: some locations will get huge amounts of precipitation, while others will enjoy only a few sprinkles.

At lower elevations in western Washington, you will hardly need to use an umbrella.

Let's start by looking at the predicted 24-h precipitation totals ending at 5 AM Saturday morning (below).

Parts of the Cascades will get 2-7 inches, while southwest Vancouver Island will be drenched with nearly a foot of rain.   In contrast, many western Washington locations will receive 0.05 inches or less.  Near Sequim, almost nothing.

The next 24 hours, ending 5 AM Sunday?  Virtually the same story. Wet Olympics and the North Cascades, with generally dry lowlands (see below).   I plan to work in my garden tomorrow and go for a long run.

All this will occur as a moderate atmospheric river slams into our region.  To illustrate, below is a plot of the key measure of atmospheric river activities, integrated water vapor transport, a measure of how quickly the wind is moving water vapor horizontally.  The magnitude is indicated by the shading (purple is the most) and the direction and magnitude by the arrows.

A strong plume of moisture (and wind) is moving from the southwest to the northeast at the time shown (8 PM tonight, Friday).  Most of the weekend will have the same pattern.

With winds from the southwest, air rises on the western side of the regional terrain (producing more precipitation) and sinks on the eastern slopes (where this is a rainshadow).

That pattern of windward enhancement and leeward rain shadowing explains the complex precipitation pattern shown above.

You can see the precipitation/cloud pattern setting up this afternoon.

Less cloudy areas are developing over the lowlands (see visible satellite image at 3 PM, arrows show the areas)

And the rainshadow is clearly evident in the regional weather radar imagery.

So stay out of the mountains and you will be relatively dry.

A Very Unstable Atmosphere

 The view looking northward over Lake Washington early this evening was dramatic and included towering cumulonimbus towers, some reaching 20-30 thousand feet (see my picture below).

A sure sign of a very unstable atmosphere.

Monet would have loved the scene

An unstable atmosphere has a lot of vertical mixing, with rising air associated with cumulus clouds and sinking air in between them, marked by clear skies.  An unstable atmosphere is associated with large decline of temperature with height, with warmer temperatures near the surface and cooler temperatures aloft.

And that is what we had this afternoon.    The temperatures of the eastern Pacific are relatively warm, with offshore weather temperatures around 60F.  Not Hawaii, but warm for us, following summer and early fall solar heating.

At the same time, a slug of cold air moved in aloft associated with an upper-level trough moving in off the Pacific (see temperature map for around 5000 ft for 11 PM today).  The colors show the difference in the temperatures from normal, with light green being the coldest.

With a relatively warm surface and cold air aloft, there was a large decrease in temperature with height, which resulted in the lower atmosphere convecting, with upward and downward motion.  The upward motions produced clouds, such as cumulus, cumulus congestus, and cumulonimbus, the latter including rain.

You could see the instability clouds this afternoon on the visible satellite imagery (see below, the star is over Seattle).  A HUGE field of instability clouds over western WA and the eastern Pacific.    Best so far this year.

Radar imagery picked up on the shower activity associated with the instability (see radar image at 5:45 PM below)

The instability was so great that the resulting thunderstorms produced substantial lightning (see below, x indicates a lighting stroke).

The instability and showers should decline tomorrow.

A (Typical) Soggy Future Ahead

In the Northwest, October is the transition month to the wet, winter regime that extends through mid-February.   And the next few weeks should reflect the typical changes.

But before I show you the latest forecasts, let's look at the typical precipitation of climatology at Seattle.  

The figure below shows the climatological probability of getting at least 0.01 inch of precipitation in a day at SeaTac.  After the dry minimum around August 1 (about 8% probability), there is a plateau (around 30%) in September, and then the big move upward in October to around 55% (October is delineated by the vertical lines).  

For more meaningful rain (a quarter inch in a day), there is a substantial increase in October, but the heaviest rain period is still a month away, centered in November.

This week will be a good one to keep your umbrella handy.  

There will be a slow, steady accumulation of precipitation, as shown by the accumulated rain at Seattle (below) through October 22.   This plot shows the precipitation accumulating from many model runs, something called an ensemble.

The map of the total accumulation through 5 AM next Wednesday shows rain over the entire region, with particularly heavy totals (as much as ten inches) over the western side of the terrain in BC.  The Olympics and North Cascades get a piece of it.

The official NOAA 8-14 day outlook is for cool and wet over our region (see below).

No complaints will be allowed.  We have been drier than normal for the past few months and soils are dry.    This rain is good news and will be modest.

Comet Viewing Tonight

 Tonight may be the best night this week to view Comet Comet Tsuchinshan-ATLAS in the western sky just after sunset.

The biggest issue will be high clouds.  Today there are some high ice clouds over the region (see the infrared satellite image around 11 PM).

The simulated cloud field from the European Center model suggests the potential for a break in the high clouds around sunset over central Puget Sound.  Clear skies over eastern Oregon and SW Washington.

In any case, worth a look, particularly 15-45 minutes after sunset, which is about 6:24 PM.  You need a clear view to the west.  

Good comet watching!

Is the Top of Mount Rainier Shrinking Due to Global Warming?

The Seattle Times had a startling revelation on its front page last Sunday:  the top of Mount Rainier is shrinking due to global warming (see below).  

According to this article, the frozen ice cap at the summit is melting "as the atmosphere warms."   Unfortunately, this claim is false and can easily be shown to be untrue.  Another error-filled article by the Seattle Times Climate Lab, a journalistic enterprise funded by climate activists.

Why is this claim obviously false?  

Because the top of Mount Rainier is so high that temperatures at the crest are consistently below freezing, even with a warming planet.   

In addition, the added moisture associated with global warming would tend to INCREASE the amount of snow at that elevation (warm air can contain more water vapor than cooler air).

Let me demonstrate that the top of Rainier is not melting from global warming.

The crest of Mount Rainer is 14,410 ft above sea level.   The nearest upstream upper-air observation location is the NOAA/NWS radiosonde launching site at Quillayute, located on the northern Washington coast.  Below is the climatology of the melting level at this location, with the melting level being the elevation at which the temperature falls to freezing.  Above the melting level, temperatures are below freezing.  The red line is the elevation at the top of Mt. Rainier.  This graph is based on over 50 years of data and is the quality controlled data provided by the NOAA/NWS Storm Prediction Center.  Some data is rejected by the quality control algorithms.

Notice something in the figure?   The freezing level is virtually always below the top of Mt. Rainier (the crest is below freezing nearly all the time).

Has the freezing level ever reached the top of Rainier?   Yes, but not recently.

The following figure shows the daily climatology of the freezing level of the air approaching western Washington (again based on the radiosonde sounding at Quillayute).  The horizontal red line shows the elevation of the top of Mt. Rainier.  The light red line shows the daily highest freezing level.  The black line is the daily average freezing level, which is around 2000 ft during the winter and 6000-8000ft in summer.

Between November 1 and April 1, the snow accumulation season, the freezing level has only reached the crest of Mt. Rainier twice.  Over the summer about 20 times.

 

Now the key point,  All these above-freezing temperatures at the creest were in the historical past, with NO RECENT cases.   Let me prove this to you.

Here are all the dates when the freezingly level at Quillayute ascended above the top of Mt. Rainier.

Twenty-six dates.   The most recent was 2006, 18 years ago.     

Most of the "warm" events were in the 1950s and 1960s.  There is no trend toward more above-freezing temperatures at the top of Mt. Rainier.  In fact, this data suggests just the opposite.   There is the potential that the NOAA quality control software could have rejected a few extreme outliers.  This may have happened during the June 2021 event.

Even at lower elevations, the temperature changes on Mt. Rainier are not impressive.    Let's consider the situation at Paradise Ranger Station, at around 5500 ft.

The annual highest temperatures had had little trend upward over the past century.

What about the annual average temperatures?  The same thing.

Snowfall at Paradise during the global warming periods since 1980? Perhaps a very slight decline.

Thus, warming temperatures are not the cause of reduced snow at the top of our favorite mountain.

Furthermore, global warming will tend to increase the precipitation (snow) at the top of Mt. Rainier, since warmer air "holds" more water vapor.   

My group is doing regional climate simulations to study these effects.  Assuming large increases in CO2 (the RCP 8.5 scenario), precipitation increases at the top of Mt. Rainier by the end of the century (see map showing precipitation change below).  That means more, not less, snow.

The claims of the Seattle Times front-page article are demonstrably false.  Global warming is NOT causing loss of snow at the top of Mt. Rainier.

Now, let me be clear, warming lower down on Mt. Rainier (say at 6000 ft) is happening and that could impact snow and glaciers well down the mountain.  BUT NOT THE CREST AS DESCRIBED IN THE ARTICLE. 

If snow/ice is being lost at the top of Rainier it has to be for other reasons, such as less precipitation, strong winds, or human impacts.   

The Seattle Times Climate Lab repeatedly publishes climate-related articles that are incorrect.  Articles that  hype or misinform about climate change.  The Seattle TimesClimate Lab is supported by outside money, groups with a climate advocacy agenda.

Our region deserves factual, rigorous journalism on this important subject. Climate Lab is failing to provide it.  

The Aurora Shows Itself!

Even with pesky clouds, a colorful aurora was apparent over Washington State last night.

Here is the view from north Kitsap County from the wonderful Skunk Bay weather website run by Greg Johnson (click on the arrow to watch an impressive animation).

I went to the kite hill at Magnuson Park in Seattle and my camera picked up some of the aurora action (see below).

What is so frustrating was how much better the smartphone camera was at picking up the colors compared to the human eye.  And the clouds were a significant detractor as well.

The key index, Kp, has dropped substantially today (see below), so I am not optimistic about tonight.  Plus, there are lots of clouds.

Potential for Major Aurora Tonight!

You might go outside after dark tonight and look north--a significant aurora may be visible over Washington.

A severe geomagnetic storm is occurring following a major coronal mass ejection (CME).

The GOES weather satellite orbiting Earth noted a powerful proton flux produced by the solar storm two days ago...so we knew it was coming.

An important measure of the potential for an aurora is the Planetary K index (Kp).  My own experience is that we don't see much auroral activity around here until the index gets to 7.  At 8, the changes are good.  9 it is certain.

As shown below, we are now at 8.5 (see below)

And the NOAA predictions suggest relatively high values this evening (see below).

The NOAA Space Prediction Center predict a decent aurora over us:

The big issue is clouds.  Right now, the skies are fairly clear above us (4 PM)--see below.  There is some high cirrus to the south and west--which may be an issue.

The high clouds to the southwest are clearly shown on the latest infrared satellite image (see below).

Because of the clouds, there are no guarantees....but I suspect there is a real chance tonight. 

 So once it is decently dark (after 9 PM), go to a dark place with a view to the north.  In Seattle, I recommend the kite hill at Magnuson Park.....that is where I will be.   

Seattle of Seattle Parks Department, please don't close the gate!