Thursday, November 13, 2014

Winter 2014/15 Outlook

One of our most asked questions is "What is the winter going to be like?"  In this blog, we'll try to answer that question as best we can.  As most people know, looking this far into the future is more of an "Outlook" than a "Forecast".  

Most everyone has heard of El Nino and La Nina, especially when we're talking about the winter outlook.  We often refer to this as ENSO (El Nino Southern Oscillation)  There are other "oscillations" in the atmosphere that affect our weather.  However, ENSO is the only one that is linked to the ocean temperatures.  This is important for two reason.  First, we have some skill at forecasting ocean temperatures. Second, the ocean temperatures don't suddenly change.  These two facts allow us to predict the atmosphere several months into the future.  Most of the other oscillations are purely atmospheric.  As such, we can predict them only 1-2 weeks into the future with any skill.  What does all of this mean?  ENSO is just one of the influences on our winter weather, and the only one that we can predict with any skill.  The other oscillations are less predictable, and could alter or even override the effects of ENSO.  So let's first take a look at what is the state of ENSO.

The area of the Pacific Ocean that is monitored for El Nino is shown in the figure below.
ENSO Monitored Regions

The Nino 3.4 is the most critical area, although the other regions also play a role.  What is monitored is the Sea Surface Temperatures (SSTs) in this area.  And the main way to look at these is to observe how the temperature compares to what we "normally" observe in this area.  This comparison is referred to as an "anomaly".  For El Nino, the anomaly must be 0.5C warmer than normal for 5 months.  La Nina is defined as an anomaly of 0.5C cooler than normal.

Currently, the SST anomaly in the 3.4 region is very close to 0.5C.  It's actually been close to that value since early summer, but decline a bit in August before warming this autumn.

Sea Surface Temperature Anomalies in the 4 NINO regions

So the Nino3.4 region is already close to the 0.5C criteria for El Nino.  So what is the forecast for the SSTs?  Here's a figure that shows a number of numerical predictions from several different countries.

This graph shows that the majority of forecasts lie between the 0.5C and 1.0C lines through the winter and next spring.  This would equate to a "weak" El Nino.  There are a few that expect a somewhat stronger El Nino, as well as a few that are predicting no El Nino.  

Here's the prediction from a number of runs from the U.S. CFS model:

Again, the majority (but not all) of the forecasts are between 0.5 and 1.0C, with additional warming during the summer.  The European climate model has a similar forecast:

So the take-away from all of this is that the majority of computer forecast are expecting a weak El Nino to persist through the winter and into the Spring of 2015.  The latest NOAA Climate Prediction Center discussion states that there is a 58% chance of an El Nino.  

The affects of El Nino on North America weather are fairly well understood.  And these are reflected in the official winter outlook from NOAA:

For the Pacific Northwest, the expectation is for warmer and drier conditions than normal.  And this fits what we commonly see in our area during an El Nino winter.

Here's a bar chart showing the observed temperatures at Spokane for the winters since 1949/50.  The bars are colored in red (El Nino), blue (La Nina), and white (neutral).  The black horizontal line represents a "normal" winter.

As you can see, just about every red bar (El Nino winters) is above the black line, meaning that the El Nino winters are warmer than normal.  

A similar graph for precipitation is found below:

In this instance, you can see more variability with the red bars.  Some El Nino winters are wetter than normal, others are drier than normal.

The third graph shows the occurrence of snow.

This is a fairly clear signal.  We haven't seen an above-normal snowfall El Nino winter since 1977/78.  Most El Ninos bring below normal snowfall to the Inland Northwest.

So the outlook for this winter in the Inland Northwest is for above-normal temperatures, with below-normal snowfall.  But as was earlier stated, this is by no means a done deal.  The other atmospheric oscillations could still alter the weather beyond the affects of ENSO.  Additionally, this is an outlook for the entire winter.  So there could still be a frigid or snowy week or two.  But when the winter is all said and done, we'll probably look back at it as a milder winter.

Friday, November 7, 2014

Winter Arrives Tuesday

We've had unprecedented warmth this Autumn season.  But all that is about to end.  First, let's take a look at how warm it's been.  

October was a record warm month is some locations.  Here's a quick table showing the warmth:

  •  Wenatchee
    • Oct 2014: 56.6F - 2nd warmest October
    • Record: 57.3F in 1988
  • Spokane
    • Oct 2014: 53.3F - 5th warmest October
    • Record: 54.5F in 1952
  • Lewiston
    • Oct 2014: 57.9F - 3rd warmest October
    • Record: 58.7F in 1988
The warmth actually extends back into mid-September.  Here's a graph of the temperatures at Spokane Airport for the last 2 months:

The blue bars are the observed high and low temperature this year.  The brown shading is what we typically see for temperatures on these days.  And the blue and red lines are the coldest/warmest temperatures ever observed.  So you can see that we didn't set any daily records at Spokane.  But notice how the observed temperatures were consistently above normal, with just a few cool spells.  Add to that, Spokane didn't reach the freezing mark until Nov 2nd, which ties with 2005 as the latest ever for the Airport first freeze.  

But as we said, all of that is about to change, and in a big way.  Here's the weather pattern that is going to do it.  Below is a depiction of the temperatures at about 5000' above sea level for this afternoon (Friday 7 November):

850mb Temperatures Friday afternoon 7 Nov 2014

This image shows that the coldest air is over north-central Canada with mild air over all of the western US.  Below is a forecast for Monday morning:

850mb Temperatures for Monday morning 10 Nov 2014

Now the cold Canadian air has penetrated into the northern tier of the US.  You can see that the Inland Northwest is right on the edge of the cold air.  By Wednesday morning, even colder air has made its way into the lower 48:

850mb Temperatures for Wednesday morning 13 Nov 2014

By this time, cold air has completely moved into all of Washington and Oregon east of the Cascades.  Even Portland will probably see some cold air seep through the I-84 Gorge.  Also note how far south the cold air will move in the central US, all the way down to the Texas Panhandle.  The coldest air will be over Montana into the Dakota, but the Idaho Panhandle will still be plenty cold.

What's causing this pattern shift?  In part, it's Super-Typhoon Nuri in the western Pacific.  Nuri developed east of the Philippines on Halloween, rapidly strengthened, and "recurved" into the westerlies.  The CIMMS blog has some great satellite images of it.   

As Nuri moved northward, it converted to an extra-tropical cyclone, and has currently become a massive storm in the Bering Sea.  So why does this affect our weather?  First, he's a graphic showing the jet stream today.

Jet Stream analysis Friday 7 Nov 2014

The yellow line represents the jet stream.  As you can see, the jet stream is coming at us from a very southern latitude, just north of Hawaii.  But as Nuri's remnants develop and move into the Bering Sea, the weather pattern will shift.  

Jet Stream forecast Monday 10 Nov 2014

By Monday, the jet stream will be directed from the central Pacific into Alaska, where it will pick up cold Canadian air, and bring it southward into the lower 48 states.

So how cold will it get?  Here's the forecast high temperatures for Tuesday.

That's right.  High temperatures on Veteran's Day will be below freezing for much of the Inland Northwest.  And here's the forecast low temperatures for Wednesday morning:

Yes, you're reading that right.  Low temperatures in the teens in many places.  This is way colder than anything we've seen this Autumn.  If you've been putting off those Fall Chores (e.g. blowing out sprinklers, putting away hoses, etc), this weekend will be your last chance to do that.

And with these temperatures, people will naturally be wondering if there's any snow that will come with it.  About the only chance for snow will be in the northern Panhandle and northeast Washington on Sunday Night and Monday Morning.  Here's the current forecast: