There will be some lowland snow in Western Washington

The model forecasts provided excellent over the past 24 hours, predicting the cooling and the localized snow in a Puget Sound convergence zone near Everett (see the radar image and model forecast this morning at 1 AM this morning).  Stunning.

The model forecast also suggested the potential for some snow at high elevations near Bellingham, and that occurred.

It is amazing how good the high-resolution models have become.  The same models nailed the Santa Ana winds during the recent wildfire event in LA, and the downslope winds that devastated Lahaina, Maui.  

This a very powerful technology that can save lives.  

Because of onshore flow at low levels, temperatures are marginal for lowland snow over western Washington AT THIS TIME.

Using aircraft observations coming in and out of Seattle, we know that the freezing level around 11 AM was about 1400 ft (see below).   Which means the snow level is about 400 ft.

In very heavy showers, the snow level (below which all the snow melts to rain) was able to reach sea level, which just happened here in Seattle (see below)

For local snow lovers, the problem is that the lower atmosphere is too warm a good snowfall because the air is coming over the relatively warm Pacific Ocean.  As shown below, the sea surface temperature off our coast is now about 10C--around 50 F.    A big problem if you like snow at sea level.

But something is going to happen tomorrow that will change things.  

A low-pressure center will move southward to a position off the northern Washington coast (10 PM Sunday shown).  In contrast, there is high pressure over the interior associated with very cold, Arctic air (red and white colors, blue is relatively warm).

As a result, there will be a surge of very cold air out of the Fraser River Valley and then over NW Washington.  Below are the forecast surface winds and temperatures a 4 PM Sunday and 10 AM Monday.

This has snow potential!

Now let's look at the accumulated snowfall (not snow depth) through 10 AM Monday.   

 Several inches around Bellingham and LOTS of snow on the NE side of the Olympics as the northeasterly Arctic air is lifted by the mountains.   Another band to the south where the Arctic air hits southwesterly flow coming around the southern Olympics.

But what you REALLY want to know is the predicted snow depth at this time (see below). 

 Not as impressive, but the Bellingham Area will enjoy a fairly good snow dump.  

They are used to it.  The lowlands south and east of the Olympics, where southerly air is forced upwards do well also.

Salting of some of the roads would be good and municipalities must get the homeless off the street or there will certainly be some unnecessary deaths from this event.

__________________

Steve Pool Scholarship Fund

We are now 65% to our goal of the $100K needed to create a permanent undergraduate scholarship in honor of Seattle's most well-known weather communicator,  the late Steve Pool.   

Please consider making a contribution (tax-deductible) to this University of Washington fund.   And thanks to the hundreds of folks who have already contributed.

Here is the info:

A Steve Pool memorial undergraduate scholarship has been established in his honor (see below)

Once funded by contributions, this new undergraduate scholarship will provide financial assistance to promising Atmospheric Sciences students, fostering the training of the next generation of meteorologists and atmospheric scientists.

If you want to contribute or learn more about Steve, check out the webpage below.   Thanks so much....cliff

https://together.uw.edu/Campaign/steve-pool

The Cold/Snowy Period Reveals Itself

As we get closer to the big transition, more powerful and higher-resolution modeling tools become available.

So what do they say?  

Almost everyone will see some snowflakes, but for many in the lowlands, it will not add to much.

For the 36-h through 4 AM Saturday, the mountains will be hit hard by snow and NE Washington will get several inches as well.  Nearly no snow near sea level.

In contrast, the snowfall during the next 24-h (through Sunday at 4 AM) starts to spread into the lowlands, with three main foci (below).  One snow band extends westward across Bellingham and the San Juans.  A weaker band is found just north of Everett and some snow is found over the higher elevations between Olympic and Portland.

Not much over eastern Washington for that period.  Almost everyone in western Washington will see a few flakes in the air by this time.

The snow is not over yet.  During the next 24 hours (ending 4 AM Monday), the entire region will experience some snow showers, with heavier amounts near sea level over northwest Washington and over the mountains.

Not satisfied?   The subsequent 24 hours bring light snow over much of the region, particularly over the northern Washington coast.

Add it all up, and the total snowfall through Tuesday morning is impressive-looking (below)

 But never forget that total snowfall, is NOT snow depth on the ground...which is generally much less for many reasons.

Below is the predicted snow depth over the region on Tuesday morning.  Not much over the Puget Sound lowlands.  Sorry kids....no snow days this time for most of you.  But skiers should be very happy.

Finally, let's not forget the cold.  

This is going to be the coldest period of the year so far.

 Here in Seattle, highs will only reach the mid-30s, and lows well down in the 20s.   Cold enough to kill unprotected folks living outside.

The Upcoming Period of Cold and Snow

 We are going into a very complex, difficult weather situation, with substantial uncertainty.

But some things are clear:

• There will be heavy snow in the mountains.
• There will be a return of serious precipitation on Friday, starting as rain
• Some areas near sea level will have snow starting on Saturday.
• Very much colder air.....cold enough to snow at low levels...will move in on Saturday and stick around for days.
• The surface and road surfaces have cooled to freezing and will not be warm enough to melt incoming snow.

Step 1:   Friday "Warm" Storm

On Friday, a moderate low-pressure center will sweep off the Pacific and move into British Columbia.  As shown by the sea level pressure forecast for 1 PM Friday, warmer southwesterly winds will move into the Northwest.  Yellows and light green colors (at around 800 meters ASL) are relatively toasty.

But over the Pacific, a STRONG cold front is moving in from the west (blue colors are cold).

The precipitation totals from this system through Saturday at 4 AM will be very significant, with the mountains getting 2-4 inches of precipitation (water equivalent of snow).  About a half inch in the lowlands except northeast of the Oympics in the rainshadow.

Mountain snowfall will be substantial, reaching  20-30 inches at higher elevations.

Stage 2:  The Cold Air Moves in

Behind the cold front, very cold air will move in.....cold enough to snow at relatively low elevations.  Near sea level, the warming by the Pacific will create a shallow layer above freezing.

Consider the temperatures above the surface...in this case about 5000 ft above sea level (850 hPa pressure).   When such temperatures drop below approximately -6°C (21 F) meteorologists worry about snow reaching sea level.   

Here is the temperature forecast above Seattle at this level from an ensemble of many forecasts.   Wow.  Cold enough to snow on Saturday! 

The European Center forecasts for surface air temperatures at Seattle for the next week show highs only in the mid-30s from Monday onward.

Stage 3:  Lowland Snow

   The problem for lowland snow lovers will be a lack of precipitation after the low and front move through.   Yes, there will be a few rain/snow showers moving through.....so many folks will see a few flakes.

Two local features may produce more localized snow near sea level over the weekend.  Below is the accumulated snowfall (NOT SNOW DEPTH WHICH IS MUCH LESS) through Monday morning at  4 AM.   

One snow "hot spot" is in a Puget Sound Convergence Zone near Seattle and the other is related to the outflow of cold air from the Fraser River Valley (near Bellingham to Vancouver BC). I have put arrows indicating these regions.  

Northeast Washington also gets some snow.  Not much around Portland.

As I noted above, we don't have the protection of warm ground, as in November and December.

The snow risk is not over on Monday....but that threat will await another blow.....I have to teach a class now!

Freezing Fog this Morning

 Just a quick note that there is substantial danger of icy roads and sidewalks this morning, not only from frost, but freezing fog.   Some of the fog is freezing on contact with the cold surface, leading to substantial deposition of ice.

 The surface air temperatures, taken at around 6 ft above the surface, at 7 AM we nearly all below freezing.   Except for higher hills above the inversion.

The cold has been conducted into the earth, with the soil temperatures 2 inches down way below freezing in eastern Washington.  Temperatures are just above freezing at that depth in Washington State.

Finally, the latest visible satellite image shows the low clouds/fog over the Puget Sound basin and NW Washington.  Not much in eastern WA and over higher terrain.

Be careful out there is morning.   And the cold ground will have major implications for what could happen this weekend....more on this later.

Why has air quality declined recently?

Air quality has been moderately degraded during the past few days.

This morning, you could see hints of haze in an image from the Seattle PanoCam.

Tonight, the PurpleAir air quality network shows marginal air quality (orange and red colors) over Puget Sound.

Why the poor quality?   Because of high-pressure overhead that produces a strong inversion in the lower atmosphere.

An inversion is a situation in which temperature warms with height, the opposite (inversion) of the normal cooling with elevation.

Inversions suppress mixing in the vertical, thus allowing pollutant concentrations to increase over time.

The temperature sounding this morning at Forks, on the northern WA coast, shows a profound inversion with temperature (red line) increasing by roughly 10°C (about 18F) over the lowest few thousand feet.

A similar inversion was noted at SeaTac Airport.

The inversion is the result of a strong high-pressure area (or ridge) aloft just to our northwest (see upper-level map this morning).

Such high pressure produces sinking air aloft, sinking that weakens towards the surface (see schematic below).  Sinking air warms by compression (like in your bike pump) and greater sinking aloft produces strong warming aloft.

This contributes to producing an inversion.

Yes, there is a HEAT DOME overhead (but don't tell the media about it).

 In addition to warming aloft, the sinking air suppresses clouds, allowing the surface to radiate heat to space, thus cooling the surface more than the air above.  This also produces an inversion.

Don't worry......we have only a few more days of this situation and then the meteorological configuration will change radically.  

And yes, there is a threat of lowland snow. A lot of uncertainty at this point.  Will blog about it on Wednesday.

The Direct Connection Between Northwest Weather and Los Angeles Santa Ana Winds (and Wildfires)

 It is not an accident that while Los Angeles experiences sustained Santa Ana winds, the Northwest enjoys sunny, dry weather.

As I will show, there is an intimate connection.

Considering the dates on which Burbank Airport (see map) had wind gusts greater than 50 mph and a relative humidity less than 30%, a strong indicator of a strong Santa Ana wind event.

Burbank was surrounded by the fires and is a good location to determine if strong winds getting to near sea level--a hallmark of the recent event.

This wind and humidity criterion found the following dates:

Then I found the the average sea-level pressure distribution on these Santa Ana dates (see below).  

Wow.  A very well-defined (and highly statistically significant) pattern.  Huge/strong high pressure over the Great Basin that extends into the Northwest.  Low pressure over northwest Mexico.     Big pressure change (gradient) over the Sierras.    This pressure pattern is associated with strong easterly flow over southern CA.

But let's not stop there.  Let's also calculate the difference in pressure from normal.  Something called the anomaly of pressure from climatology in the weather biz (see below).  

Double wow.   The biggest pressure anomaly from normal is over Washington State.

So there is an LA/Washington connection!

An interesting question is whether high-pressure periods are increasing over our region...something several of you have asked about.

To check this out, I have plotted the frequency of sea level pressure getting above 1035 hPa (a relatively high pressure) at Seattle-Tacoma Airport from 1948 to now (see below).   

Lots of cases in the 50s, fewer in the 60s and 70s, and higher numbers during the past two decades.   As noted by the thin dotted line, a small upward trend over the entire periods.  But not much trend if one considers the past two decades alone.   

An issue I will be doing more research on.

The latest forecasts suggest major...and I mean major...changes in store for the West Coast for the next two weeks, with substantial rain for southern CA and a potential for some flurries over the western lowlands.

More in a future blog.

The Highest Pressure in Ten Years

 Did you feel the pressure yesterday?   

Feel a bit weighted down?    If so, it may not be due to the political energy in the air.

Yesterday morning, sea level pressure around the Northwest was the greatest in ten years.

The average sea level pressure is approximately 1013 hPa (hPa is a unit of pressure).

Yesterday morning, several northwest locations experienced 1041-1045 hPa.   This is high...very high.

Higher than any other time since 2015 in our area.  To illustrate, here are the pressures observed at 10 AM yesterday (see below).  Around 1042 hPa near Seattle and higher to the north and east.

To see how unusual such pressures are, check out the sea level pressures at Boeing Field.  Yesterday barely beat out the highest pressure in 2023 and was slightly behind late 2024.  1045 hPa is about as high as it gets around here.  On the low side, some low centers have driven the pressure down to about 980 hPa. 

At the University of Washington, a similar story.

The pressure was above normal over a vast swatch of the west, as shown by the pressure map at 10 AM Wednesday. The shading shows the difference from normal.. with red being much above normal.

This high-pressure interlude is associated with a high amplitude upper-level wave pattern, with a ridge off the West Coast and a trough over the eastern U.S. (see upper level (500 hPa level) map at the same time).

The origin of this unusual situation is most probably the result of natural variability.   My group has done regional climate simulations assuming aggressive increases in CO2.  As shown below, global warming generally leads to lower pressures.  This makes sense:  warmer air is less dense, which leads to lower pressure below.  Global warming also does not lead to transient ridges over the West Coast, something I have researched and published on.

Announcement

I will hold a special online Zoom session at 10 AM on Saturday for Patreon supporters. I will talk about our dry weather and when it will end.