How Unusual Has This Winter Been?

It is now officially spring, and a good time to check on how unusual this winter has been. 

Meteorologists often do so by reviewing conditions over the water year (October-now), and I will do the same.

 First, consider Seattle's accumulated precipitation during the present water year (green line, observed; red line, climatological average).  

We have been a bit above normal overall!   Wet periods in December and March.

Yakima, on the east,  is similar, with a water year total near normal.  There is a reason I have been discouraging the drought talk.

Temperature?  See below, with blue being observed, tan showing the normal range, and red/light below showing the record highs and lows.

At Seattle, temperatures have been unremarkable, with periods of above and below normal warmth.  No records broken.

At Yakima, there is a clear warm bias this year, particularly in December and recently.  

These two warm periods are the reason the snowpack is now below normal for the Yakima Basin, but why the reservoirs are well above normal in water stored.    

I know some folks are interested in climate change, so what do the long-term trends for water year temperatures and precipitation look like? (October through February shown below)

Consider the situation for Western Washington from 1896 to the present.

For temperature, this year was warmer than normal but not a record. Over the past 130 years, there has been a slow warming of roughly 1.5 F.  

A modest global warming signal.

In contrast, a slight increase in precipitation....too small to be noticeable or significant.

Because of the two warm spells, the snowpack over the region is about 60% of normal (see below).   

Let me stress, this is mainly about the warm/wet periods associated with atmospheric rivers and Kona Storms, NOT global warming, as being claimed by some media and amateur YouTube channels. 

The Revenge of Kona Hits BOTH Hawaii and Washington State

 There has always been a close connection between Hawaii and Washington, and that relationship was on display over the past few days.

Kona does not kid around

A strong Kona storm, with powerful, southwesterly flow, rammed into the Hawaiian Islands, producing intense rainfall.   Check out the totals over the past 48 h  around Oahu (below)

Many locations received more than 10 inches, with some reaching as high as 20 inches.   Major flooding has resulted.

This moisture continued toward the Pacific Northwest (see the total atmospheric moisture from yesterday morning below).   As you know, these moisture plumes are also called atmospheric rivers.

When that moisture reached our region, it was forced to rise on our terrain, resulting in heavy precipitation.  To see how much...check out the 72 hr totals shown below.   

Wow.  Several locations received over 6 inches,

As a result, several of our rivers are at major flood stage, with the National Weather Service issuing serious flood warnings on BOTH sides of the Cascades.

The NOAA River Forecast Center is predicting serious flooding on many local rivers (see below).

For example, the Snoqualmie River near Carnation is now at major flood stage (see below), something that is unusual this late in the season.

According to the U.S. Drought Monitor, much of the heavy rain/flooding area is "abnormally dry."     I have some colorful language to describe their analysis, but this is a family-friendly blog.   I will Kona deal with them 😁

How Far into the Future Do Weather Prediction Models Have Skill?

30 years ago, providing a forecast of greater than 4-5 days would result in smirks from responsible forecasters.

But times have changed, and today there is substantial skill in the second week.

Let me prove that to you.  The plot below shows how skill declines over time for the American Global Model (the GFS in blue) over the Northern Hemisphere.    1.0 indicates perfect skill by the measure considered (something called anomaly correlation).  Anything above roughly 0.6 indicates useful skill.

So according ot this measure, there is skill through 8 days!   More than a week!

Buit is better than that.  Today, we have enough computer power to run many forecast runs each cycle (called ensembles), and the average of these many runs is even more skillful. 

 The US ensemble (GEFS, red color) has skill out to 10 days!

The European Center forecasts are even better, adding about another day of useful skill--11 days.  Machine learning prediction adds another day.

So what has contributed to the improved skill?  The most important contribution is three dimensional obserservations over the entire planet! No data voids to ruin forecasting skill.  

Plus, more computer power leads to higher resolution simulations, better physics descriptions, and the ability to run ensembles of many forecasts.

Now that I have convinced you of the potential for extended prediction, what is the forecast for Seattle over the next ten days?   Boring highs in the 50s every day (see below).  If you want real warmth, you will have to head to Southern California.

The Northwest's Kona Connection

The Hawaiian Islands are experiencing one of the wettest Marches on record, and some of that moisture is reaching the Pacific Northwest.   Day after day for an extended period.

Just to wet your appetite, here are the precipitation totals for the last 72 h. Huge totals, exceeding 20 inches, over the southern portions of the Island of Hawaii.  On location got to 31 inches.   Substantial flooding and other damage have occurred.

This has been a Kona storm event with strong, moist southwesterly flow reaching the islands, instead of the normal northeast (from the NE) trades.   This is associated with a deep, high anomalous low to the northwest of  Hawaii, known as a Kona Low.  The upper level map below from Saturday illustrates this feature (the purple colors indicate the highs/pressures are much lower than normal.

 

This type of pattern directs tropical moisture to the northwest, as illustrated by the water vapor pattern tomorrow morning (red indicates large amounts of water vapor in the column of air).

This pattern is going away.   

Want to be impressed?  Below is the total precipitation over the next ten days.  You won't have to go to Hawaii.....Hawaiian moisture and showers will be coming to us.

Snowy Washington State

The snow event yesterday had major impacts (generally negative) for transportation, but major benefits for water resources and late-season skiing.

It provides a highly educational lesson about snow and freezing levels.

As I write this Saturday morning, both Snoqualmie and Stevens Passes have reopened after being closed much of yesterday.   There have been major snow dumps in the mountains, with Snoqualmie getting 42.5 inches and Stevens receiving 34.5 inches on Thursday and Friday.

A big problem for the ski areas has been the major power outages, which are now resolved at Snoqualmie.

To get an idea of how the regional snow water situation has changed, here is the percentage of normal (1991-2020) of snow water equivalent of the regional snowpack.  A month ago, there were significant areas of red (less than 50% of normal) over the northern portion of the state.

Now the red is gone. Western Washington has gone from 54% of normal to 66%, and the Yakima Basin from 41% to 63%.    There has been a large and significant improvement in the amount of water stored in our regional snowpack.

This snowpack improvement, coupled with our above-normal reservoir levels, bodes well for water supplied this summer.  For example, the Yakima Basin storage is hugely ahead of schedule and will easily fill (see levels below, blue is the current year, red is normal).

Yesterday's lowland snow over western Washington was fascinating.    Before I discuss it, I want to remind you of some snow science 101:  the difference between freezing level and snow level (see below)

Nearly all of our precipitation starts as snow aloft.  It falls until it hits the freezing (or melting) level at which the temperature is 32F.  It takes about 1000 ft for all the snow to melt below the freezing level.  The elevation of total melting is called the snow level, below which the precipitation is all rain.

Between the freezing level and snow level, there is a mixture of melting snow and rain.   On Friday, much of western Washington was in this layer, with a lot of wet snow falling.

Snow that melted rapidly on the relatively warm roadways.

During Friday (and this morning), the freezing level has ranged from 1200 ft to 300 ft, which resulted in a lot of mixed precipitation reaching the surface.    If you were lucky enough to live above 1000 ft in the hills above Bellevue, you enjoyed a foot of snow (see below).

Picture courtesy of Peter Bender

The rapid melting of snow between roughly 500 and sea level was evident if you looked at some of our hills:  very white on the top, with little snow at the bottom (see some images below).

Just for fun, I drove from near sea level to around 350 feet in north Seattle on Friday afternoon.

Near sea level, there was no snow on the grass and a few flakes mixed in with some rain.

A different story at 350 ft.   Grass and roofs were covered with snow, but most of the snow melted on the warm pavement.

Elevation was not only a key element.  Where the precipitation was heavier, the greater amount of snow falling from aloft caused more cooling (it takes energy to melt snow), which drove the snow level lower, resulting in more snow at the surface.   

There was a band of heavier precipitation over the South Sound that produced a band of heavier snow (see below, showing snow depth measurements on Friday).  As much as 4 inches in Federal Way!    Not very impressive by mountain standards.😉

Lowland Snow over Western Washington Tonight? American versus European Models

 Update at 10 PM.  Looks like some mixed rain /snow and wet snow in the South Sound away from the water, increasing with elevation.  Some accumulation above 500 ft.

_________________

A very interesting forecast dilemma is before local meteorologists tonight.

On one hand, the U.S. high-resolution model (HRRR) is predicting as much as 8 inches over the South Sound and 5 inches at SeaTac Airport by 11 AM Friday morning. The U.S. GFS model is doing the same.  Most of the snow will fall overnight.

 

In contrast, the European Center model, which is generally the most accurate (remember the missing bomb cyclone in December?), is NOT going for lowland snow.  Zippo.

Keep in mind, this is snowfall, not snow depth.   This is going to be very wet, melting snow in any case, and snow depths will be MUCH, MUCH less.

An interesting aspect of this event is the large north-south temperature change forecast over the region.  Below is the forecast temperatures around 5000 ft  (850 hPa pressure) at 5 AM tomorrow morning.   Huge contrast over southern Washington (blue cold, green warmer).  A small error in position would be significant.

Currently, the freezing level is around 1600 ft at SeaTac (see below), much too high for snow reaching sea level.   The snow level (where all the snow has melted) is about 1000 ft below the freezing level.  But it should decline slowly.

What about uncertainty in the forecasts?  It is HUGE.   The UW high-resolution model, forced by many global models, shows a wide range of possibilities. Many forecasts are for zero, others as much as 4 inches of snowfall (NOT SNOW DEPTH).

I suspect the European Center projections will be closer to the truth.  Near sea level, some of you may see some flakes tomorrow morning in the central and south Sound near sea level (500 ft and below elevation).  But nothing significant.  Snowfall will increase rapidly above 500 ft.

In contrast, the central and south Cascades will be hit hard (see accumulated snow through Sunday morning).  Feet of fresh snow.  The mountains of SE Washington will also get plentiful powder.

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.