Colder than Normal for End of February

 

Like it or not, weather changes are in the forecast.

Colder than normal temperatures are predicted for next week. The 6-10 Day Outlook issued by the Climate Prediction Center on 16 February calls for a 70-80% chance of below average temperatures for eastern Washington and northern Idaho.

Climate Prediction Center: 6-10 Day Outlook issued 16 Feb 2022
https://www.cpc.ncep.noaa.gov/

Compare the 6 to 10 Day Outlook (above) to the temperature anomaly map over the last 30 days (below). It looks like the atmosphere will undergo quite a flip-flop. Since mid January, much of the Western U.S. and High Plains experienced above average temperatures.

Climate Mapper: https://climatetoolbox.org/tool/Climate-Mapper

And since mid January, the persistence of a high pressure ridge along the Pacific Coast has lead to much drier than average conditions for Washington, Idaho, and the rest of the Western U.S.

Climate Mapper: https://climatetoolbox.org/tool/Climate-Mapper

Below is a map of the weather pattern we have been influenced by over the past 30 days (January 15th through February 14th).  The bulge along the Pacific Coast (red, orange and yellow colors) illustrate the mean position of the upper level ridge axis during the last 30 days. 

500mb Geopotential Height Composite Mean for 30 days (Jan 15 - Feb 14)
NOAA Physical Sciences Laboratory
https://psl.noaa.gov/data/composites/day/

This kind of pattern (ridge over the West and trough in the East) looks pretty close to the Typical La Nina pattern. Check out the illustration below. However if you look closely, the ridge axis on the 30 day map above is further east (closer to the coast) than the typical La Nina pattern. 

Typical La Nina Winter Pattern: Source NOAA

The map below probably demonstrates the orientation and amplitude of the 500 millibar ridge axis more clearly. This anomaly map depicts the 500 millibar difference from average for the last 30 days. The reds and oranges illustrate the strength of the upper level ridge along the coast of Washington and southern British Columbia.  Unusually high height anomalies along the coast of Washington and southern British Columbia imply that the Polar Jet Stream further north than we typically experience during the winter months. Subsequently, our typical "wetter" than average La Nina pattern has been interrupted since mid January.

500mb Geopotential Height Composite Anomaly for 30 days (Jan 15 - Feb 14)
NOAA Physical Sciences Laboratory
https://psl.noaa.gov/data/composites/day/

Changes are on the way! Over the weekend of 19-20 February, there is strong model agreement of a major pattern shift. By Monday (Feb 21st), the West Coast ridge of the last 30 days will be replaced by a cold upper level trough. The blue colors represented on this 500 millibar forecast from the Global Ensemble Forecast System (GEFS) represent anomalously low heights across the Western U.S. The north to south orientation of the black lines from the Northwest Territories into northern California indicate strong northerly flow aloft.

https://www.tropicaltidbits.com/

Northerly flow aloft means the Western U.S. will be vulnerable to cold air intrusions from the Arctic. By Monday evening the GEFS (GFS model ensembles) depicts strong cold anomalies plunging into the High Plains. While most of the coldest air is expected to remain east of the Continental Divide, the states of Idaho, Washington, and Oregon will likely receive a shot of a colder than average air mass.

https://www.tropicaltidbits.com/

The Global Ensemble Prediction System (Canadian) shows a similar surge of cold air into Montana, Wyoming and the Dakotas by Monday evening. However, anomalies are not progged to be quite as cold over Washington and north Idaho.

https://www.tropicaltidbits.com/

Cold air intrusions and periods of dry weather can be common during La Nina in the Pacific Northwest. The strength and position of high pressure over the eastern Pacific can vary through the winter. So far this winter, the Inland Northwest has experience periods of heavy precipitation in November and December, record snows in central Washington in January (nearly 2 feet in Wenatchee on January 6th), and unusually dry weather from mid January to mid February.

It's unclear at this time how long the below normal temperatures will last.  The coldest days are forecast to be between Feb 21-23rd, but below normal temperatures could last through the end of February.

I thought La Nina meant cold and snowy weather.

In our last blog entry, we were stuck in a dry ridge of high pressure in mid January, wondering if winter was over.  The computers were giving hints that the pattern would change right around the time the calendar changed from January to February.   And that actually panned out.  This was quite impressive for a 10 to 14 day forecast.  The problem is, the "cold snap" of early February only lasted a whopping 2 days.  (The blue bars in the image below show the daily high and low temperature.  Blue bars below the brown shading are below normal temperatures).

So instead of changing the weather pattern to colder/snowier for the Northwest, we only had a short cold snap. 

There's an index that forecasters monitor that's called the Arctic Oscillation.  It's pretty difficult to explain and even more difficult to see by looking at weather maps.  But it does tend to track pretty well with our weather.  Here's the AO for the current winter:

You can clearly see that when the AO index goes negative, we tend to have cooler weather here in the Northwest.  Even our 2 day cold snap shows up in the AO.  Unfortunately, it's difficult to forecast the AO out beyond more than a couple of weeks.  Here's the GEFS forecast (red lines) for the AO over the next two weeks.  Pretty much assured of a positive AO through mid-February.

Here's the GEFS pressure forecast for 9-14 February.  The red/orange shading over the West indicates dry and mild weather for our area, while the blue shading over the eastern US points to more winter weather for them.

By 15-20 February, the GEFS model suggests that the high pressure will shift a bit to the west, off the West Coast.  This shift could allow some weather systems to move through our area from the northwest.  But those are typically drier storms, so this pattern wouldn't be very wet for our area. 

If we really look far into the future (23-28 February), the GEFS would imply that the high pressure would move even farther offshore, allowing the cooler/stormier weather to impact the western US.  Still, this isn't a real wet pattern for the West.

The GEFS model has actually been showing this pattern shift for a few days now.  Additionally, the European model has been doing the same.  So there's a small amount of confidence that we could see stormier weather return by the last week of February.  But forecasting the weather 3 weeks in advance is risky business.

All of this brings up the question:  is our recent dry spell leading us back into a drought?  Here's the accumulated precipitation for Spokane since October 1st. The brown line is what our precipitation should look like in an average year.  The green line and shading shows our precipitation for this winter.  

You can see that we started off dry in October (green line is lower than the brown line).  But then we quickly caught up to normal and we were actually wetter than normal in November (green line is above the brown line).  Then our mild December dropped us back onto the dry side.  But our stormy weather after New Year's brought us back to normal.  Unfortunately, we've fallen behind again in the past 3 weeks.  Mind you, we're only 0.7" below normal, and we can make that up in a hurry.  But as we just showed you, we're probably not going to see much precipitation for the next 3 weeks.  By then our precipitation deficit will probably be about 1.5".

The image below is the same as what we were just looking at, except we've added the precipitation trace for the previous winter (2020-21) in purple.  It's sort of scary how similar it is to our current winter.  And we all remember how dry things were around here last spring and summer.

At this point in the winter, our mountain snow pack is looking good; right about where it should be (green colors indicates snowpack is normal).  Of course, we'll see these number drop a bit as we go through these next 3 dry weeks.

Looking farther into the spring, the average of the climate models expect cooler-than-normal temperatures for the Northwest US February-April.

And wetter than normal conditions for the same 3 months.  

This pattern is consistent with what we usually see for La Nina during the spring.  Obviously last year's La Nina didn't live up to those expectations.  We'll see how this La Nina round #2 fares.