February 20, 2024

Substantial Snow Returning to the Pacific Northwest

During even the strongest El Nino years there is often a period at the tail end of winter when the atmospheric circulation associated with El Nino collapses.

When the atmosphere adjusts to considerably more solar radiation over the northern latitudes the "lock" of the tropical Pacific is lost.

This re-adjustment will likely occur next week, leaving the door open to cool wet flow over our region.

This means lots of mountain snow and even the chance of flakes near sea level.

Let me show you!

The forecast for total snowfall through 1 PM Friday is hardly impressive, with a few inches in the mountains.  Yawn.


By 1 PM Sunday, the totals in the north Cascades and southern BC are healthier, reaching nearly a foot in favored locations.


Fast forward to 1 PM Tuesday, and the Washington Cascades and Olympics have gained several feet.  Considerable lowland snow is evident.



And by 10 PM Wednesday, February 29th. some mountain areas have gained 4 feet and parts of the western Washington lowlands have a thin veneer of snow.


So what is going on? 

 Examining the situation at around 18,000 ft (500 hPa) this morning, a strong trough of low pressure is found off the California coast. This is the kind of pattern that has been dominant over the last few months.


Fast forward to the end of the week (Sunday evening) and a very strong trough of low pressure is moving in.   Classic snow producer for the NW.


But the shocker is the prediction for March 1 (below).  Very cold, powerful trough off our coast.  Very wet, very cold, and very snowy.


All of this is a bit far out in time to be sure, but most of the solutions of ensemble forecast systems, in which the model is run many times with slight changes, produces similar predictions.  You may not want to put away your winter gear quite yet.

Keep tuned.




February 18, 2024

Dry Air Storm Hits Western Washington

 How do I put this tactfully?

Have any of you noticed any dry or cracking skin?

Or perhaps a parched mouth at night, requiring a sip of water?

I have and I know exactly why it is happening:

Desiccating, easterly downslope flow moving from eastern Washington into the coastal regions has occurred repeatedly during the past week.

A dry storm if you will.  The Northwest version of the Sahara.

Consider the outdoor relative humidity at Seattle Tacoma Airport during the past week.  Under 50% for nearly a day--dropping as low as 30%!  A day before it got down to similar levels.

Portland (below) was even worse,  with additional low-humidity episodes earlier in the week. There is a reason Portland is so bad...and it has to do with the Columbia Gorge (more later).

But these are just point values, let me now show you the lowest relative humidities that occurred in Washington and British Columbia on Friday (below).  

Amazing.  A large portion of northwest Washington and British Columbia had relative humidities dropping into the 20s!      No wonder skin my need some ointment!


As you might expect, the uber-skillful UW WRF modeling system predicted the dry storm well in advance, as shown by the relative humidity prediction for 10 PM Thursday (below)


So why such dry air?

The map below shows sea level pressure, near-surface winds, and lower-atmosphere temperatures at around 2500 ft ASL(color shading, blue and purple are cold) at 7 AM last Friday morning.  Cold, dense air to the east produced high pressure, while a Pacific low-pressure center was offshore.  There was an intense pressure difference over the Cascades due to cold air banking up on the eastern slopes of the barrier.


Over terrain, air near the surface tends to move nearly directly from high to low pressure, and thus the pressure difference forces strong easterly (from the east) winds.    Some of the winds moved directly over the mountains, while in places with gaps (like the Fraser River Valley and Columbia Gorge), cold air moved westward within sea level in the gaps.
 
The cold air has low amounts of moisture since cold air holds far less moisture than warm air.  As that air from eastern Washington crossed the Cascades and then descended the western slopes of the barrier, the air was warmed by compression, which caused the relative humidity to plummet.

Why? 

 Because relative humidity is the ratio of the amount of water vapor in a sample of air divided by the maximum amount it can hold.   Warm air can hold more water vapor than cold air.  Thus,  relative humidity plummets for warming, sinking air.


The effects of downslope flow is the reason why the lowest relative humidities were downstream (west) of the terrain crest in the figure above.

The future outlook for relative humidity?   Good news....no more dry storms on the horizon.



Substantial Snow Returning to the Pacific Northwest

During even the strongest El Nino years there is often a period at the tail end of winter when the atmospheric circulation associated with E...