Winter is NOT Over: Strong Winds, Heavy Rain, and Substantial Mountain Snow Ahead

Tomorrow and Thursday should bring very active weather to our region, including bountiful precipitation, heavy mountain snow, and gusty winds.

Local ski areas don't mince words.  They are terming this "Miracle March":

But many old weather hands expected this transition, and the models have been hinting at it for a while.

First, the winds.   A modest low-pressure center will make landfall on Vancouver Island, producing a strong north-south pressure difference over western Washington ( see map below)

That means strong southerly winds.  Turning to the Seattle WindWatch system, the forecast maximum wind gusts will accelerate to around 40 mph over Seattle late tomorrow afternoon.  Not the end of the world, but expect a few scattered power outages.

South of the low, a potent atmospheric river will approach the Northwest coast, bringing heavy precipitation.

The forecast accumulated precipitation through Saturday morning is shown below, with up to ten inches (of liquid water) in the southern Cascades.  EXACTLY where it is needed most. Plenty elsewhere.

And temperatures will be sufficiently cold so that the Cascades will get a massive snow dump (total through Friday afternoon shown below).  Yellow and white colors are two feet or more. Ski folds will be happy.

Again, just where it was needed the most.

The National Weather Service has a blizzard warning out for the Cascades and Olympics, plus a wind advisory for most of western Washington.

This winter started as a lion (heavy rain and flooding), turned into a lamb, and reverted back to a lion again. 

An Unusually Strong Cold Cold Front Is Moving Through The Northwest

Over the eastern U.S., the passage of a strong cold front, with a rapid decline of surface temperature, is a frequent winter treat.

In contrast, here in the Northwest, low-level cold fronts are generally weak because the cold air, starting over Alaska and vicinity, passes over relatively warm water.  To illustrate, the current water temperatures (in °C) today are shown below.

The water is well above freezing the whole way (note 4°C is 39°F).

Today, an unusually strong Pacific cold front will be moving through, and you will notice its impact, including substantial snow in the mountains and even a few flakes reaching sea level in "favored" locations this week.

The visible satellite image this morning shows the story (below).  Arctic air is moving southeastward from Alaska, and as it warms from below, it becomes unstable, producing huge fields of cumulus convection (the black circle shows an example).  

The location of the cold front is shown by the red arrow, and the position of the clouds/light showers from a Puget Sound Convergence Zone is indicated by the black arrow.

As the cold front moves through today and cold air sweeps in, the temperature of the air aloft will decline rapidly.

Near sea-level, as at Olympia Airport (below), temperatures will plummet from the mid-50s today to highs in the low to mid-40s tomorrow and Tuesday, with lows reaching freezing.

But go a bit higher, where the low-level warming influence of the ocean is less noticeable, and the cooling will be much greater.  For example, at the Fire Training Academy, just east of North Bend, WA, temperatures will barely get about freezing during the day and decline into the mid-20s on Tuesday.

The forecast temperatures over Washington for Tuesday morning (shown below) will be very cold for onshore flow, with higher terrain dropping into the teens and nearly the entire state below freezing (blue and purple colors).  Only near the water will temperatures get to freezing and above.

You want snow in the mountains?  

You're in luck.  By Wednesday morning, the Olympics and the north Cascades will get a few feet, but this is just the appetizer.  

The main course is later in the week.  Here are the snow totals from the European Center model through Saturday morning (below).  As much as TEN FEET in some locations.  Many locations have a yard or more of snow.

Anyway, the influx of cool/moist air this week is very welcome and will go a long way to help the water situation this summer.

Could it be True? Lowland Snow in March?

A few social media sites are predicting as much as a foot of snow in Seattle next week.

Why such excitement?

Some weather prediction systems are predicting something quite unusual for mid-March:  lowland snow.

Not everywhere, but in favored locations.  And not every weather modeling system.

Consider the predicted snow total from the NOAA/NWS model for the 24-h  ending 2 AM on Thursday (below).  Lots of snow in the mountains, with a snow band across Puget Sound, probably from a Convergence Zone.

Plotting the GFS model snowfall prediction for Seattle from its ensemble system of many forecasts shows that several of the predictions included several inches of snow, although there is a lot of disagreement.  Disagreement means uncertainty.

 

Now, let me prove that I lack promise as a YouTube channel host.  

I suspect that such forecasts of bountiful lowland snow are problematic.    

Let me start by noting that the climatology of significant snow in Seattle plummets after the first week of March, as shown below.  Yes, late March snow has occurred, but it has never been significant.

Another issue is the temperature of the air, as shown by the forecast for Tuesday morning below.  The air over us will be cool, but not the primo Arctic air required for serious lowland snow in our region.   And there is onshore flow from off the mild Pacific.   But cold enough for PLENTY of snow in the mountains.    Get your skis out!.

The US GFS model has a long history of overdoing lowland snow in our area....its forecasts are generally too cold.   In contrast, the UW WRF model shows substantial snowfall in the mountains, but essentially nothing in the lowlands (accumulated snowfall through 5 PM Wednesday shown below).

The European Center's predictions are similar:

In short, plenty of snow in the mountains (which should address many concerns about this issue), but virtually nothing in the lowlands, although a few of you might see some melting snowflakes.

Precipitation totals during the next weeks should be impressive, providing water exactly when it is most valuable before the dry season starts in 2-3 months.

Announcement

I will hold a special online Zoom session at 10 AM on Saturday for Patreon supporters.  Will answer questions and talk more about the snow situation.

The Most Valuable Precipitation of the Year

Precipitation at some times of the year is more valuable than at others.

Here in the Pacific Northwest, March and April precipitation is by far the most prized and valued.

Fortunately, the latest forecasts suggest we will enjoy bountiful amounts during this period, which should be a relief to those worried about the water supply.

Why is spring precipitation so important?

It can moisten the soil and help fill the rivers before our dry summers.

It can build up water behind our dams and in our reservoirs, providing water and power during the summer.

For years without good snowpack (e.g., 2024), it can provide needed water.   

It doesn't make much difference if we fill our reservoirs with melted snow or spring precipitation.

As you can see from the average cumulative precipitation at Olympia (below), quite a bit of the annual precipitation at this station occurs in March and April (between the red bars).  

Bottom line:  it is not too late to get substantial precipitation, even in a normal year.

Although there has been a lot of gnashing of teeth about this year's precipitation, the water year totals (from October 1 to now) are actually near normal, something shown by the cumulative precipitation at Seattle and Yakima below (red is normal and green is this year).

The issue is that the snowpack is about 50% of normal as of today.  However, reservoirs are in very good shape, with some well above normal (e.g., the Yakima reservoir system).

As I have noted before, the prediction models have been emphatic about our progression into a far wetter pattern.

To illustrate, below is the total precipitation forecast from the European model through Friday, March 20th, over the Northwest.   Quite a lot over the Cascades!

Much of that will be snow, as shown by the totals through March 19th

The latest extended forecast from the NOAA Climate Prediction Center is for wetter than normal conditions for the second half of the month.

With La Niña rapidly weakening, there is no reason to expect that we will get a persistent dry pattern, as in January.  

I suspect our summer water situation will be fine.

A "Dirty Ridge" is Coming to the Northwest

No, this is not some kind of meteorological pornography.

A "Dirty Ridge" is when moisture, often accompanied by clouds and precipitation, makes its way around the northern portion of a high-pressure area ( ridge is another name for high pressure).    

Wet conditions with high pressure, not some type of obscenity.

Before I start, some review. Low-level winds typically rotate clockwise around higher pressure areas (see below). Thus, winds are southerly on the western side of the high and westerly (from the west) on the northern side.

You can see this windfield around the forecast high at 7 PM Friday night (below, for the 850 hPa level, about 5000 ft above the surface).

Consider a forecast map for 7 PM on Friday of sea level pressure, surface wind (arrows), and the transport of water vapor by the wind (shading, called integrated water vapor transport).

Strong winds from the southwest and west on the western and northern sides of the high draw moist air northward from the subtropics around the western flank of the high and moves it eastward over the northern flank of the storm.   The clouds and rain are the "dirt" of the dirty ridge.

Fast forward to Tuesday morning (March 10), and the high is still offshore, and moisture continues to move around its western and northern flanks  (see below).

This circulating moisture will be forced upward by local mountains, resulting in precipitation (the 72-h precipitation ending 5PM Sunday is shown below).  

Considering the relatively low snowpack over the region (about half of normal right now), this precipitation will be welcome, even if it is associated with a dirty ridge

With Rapidly Strengthening Sunshine: High Temperatures are Possible in March

 If the sun feels stronger to you right now, it is not your imagination.   

And this stronger sun and longer days can result in spikes of warmth during the month of March in our region.

First, let's consider the average solar radiation in Seattle over the year (below).  Late February, March, and April are times of rapidly increasing solar radiation at our latitude.

Of course, there are clouds, which reduce surface warming, but even so, March is a time of rapidly increasing solar radiation reaching the surface, as shown by the observed solar energy in 2019 (below).   

As a result of the increased sun (and generally decreasing cloudiness), we can get some amazingly warm days in March.  The plot below shows this year's temperatures (blue bars) and record highs (red) and lows (light blue).

Some March days have gotten as high as 80F, with many in the 70s.    You will also note that the chances of getting really cold (below 20F) end by March 10th.

So, will you get a taste of this welcome warmth this week?   Monday will be the warmest day, with rain and cooler conditions by midweek.

The European Center model is going for 58F on Monday, similar to the UW system.

But the warmth will vary over the region, as shown by the temperature forecast for 4 PM on Monday over Washington State (see below).  Temperatures will exceed 55F over central and southern Puget Sound country and in the lower elevations of the Columbia Basin.

Some isolated lucky folks (e.g., near Olympia) will get to 60F.    

I am planning a nice long run late Monday afternoon...which should be wonderful

Rainshadow on the Wrong Side of the Olympics

As shown below, the Olympic rainshadow is normally on the northeast side of the Olympics, while heavy precipitation is found on the south and southwest side of the barrier.

This configuration is due to the typical southwesterly (from the southwest) winds approaching the barrier during the cool season, resulting in upward motion (and precipitation) on the windward (SW) side and sinking motion (and little precipitation) on the leeward (NE) side.

But something unexpected happened on Tuesday morning.   

The weather radar indicated precipitation in the rainshadow area, with nothing apparent on the southwest side (see below, there is some ground clutter on the SW side from the radar beam hitting some peaks).

A cloud band was quite evident to the north of the Olympics that morning.

Rain gauges picked up modest rainfall north of the Olympics, with precipitation on the dry-sky lovers in Sequim and vicinity.  Surely unwelcome for those hitting the links that morning.

The origin of this backwards situation?   The low-level winds had reversed, with northerly wind approaching the Olympics from the north (see surface wind below).

This situation was associated with high pressure to the north and low pressure to the south (see below), with the wind blowing from high to low pressure.

We will be dry until the middle of next week, after which the atmospheric spigot turns on in a major way.

La Nina is Collapsing: Implications for Late Winter and Spring

As predicted, La Niña is collapsing, which should be good news for those worried about water supplies.

La Niña is associated with cooler-than-normal surface ocean temperatures in the central tropic Pacific:  what is known as the Nino 3.4 region (see below).

Here is a plot of the temperatures in the area (actually, the difference from normal). Most of this winter, these temperatures were .6 to .9 C below normal: a weak La Niña.  But during the past month, these ocean temperatures have started to warm, resulting in "neutral" conditions.

A plot of the sub-surface temperatures across the central tropic Pacific shows the warming (more red colors) and loss of cold surface waters (less blue colors)

The latest NOAA forecast of temperatures in the Nino 3.4 area shows rapid warming, with the cool surface waters gone by April.  Neutral conditions will be in place then.

So why should you care? 

 La Niña tends to produce big ridges of high pressure in the Gulf of Alaska, with occasional breakthroughs of moisture to the south (see schematic below).   We saw a lot of this configuration after January 1; however, the pattern shifted a bit to our south.    

In January, the coastal high pressure produced dry conditions over the NW, and the precipitation was mainly directed into central and southern CA.   A modest shift in the pattern has made a big difference for the Northwest.

But as the La Nina weakened this month, the ridge weakened, and low pressure replaced it, producing the cooler/wetter conditions of the past weeks.   

You can see this transition in the figure below, which shows upper-level maps for January 4 to a few days ago.   Red indicated higher than normal neights (pressures)

January 7-21 had a huge ridge located off of southern BC....no wonder we were dry!

January 11-Feb 9 was transitional, with the ridge weakening.

Feb 6-21 was VERY different, with troughing (low pressure over the West Coast).

With La Niña disappearing, there is no reason to expect any locked-in upper-level patterns.

 Thus, no reason to expect a return of the high-pressure ridging that kept us dry and warm.