WeatherWatcher: Reviewing Month of December, Seasonal Trend Outlook

• December in Review
• Seasonal Trend Outlook

December in Review: So far this winter has turned out to be relatively uneventful. Particularly the month of December. We had a morning snowfall with accumulations of about 1-2 inches before it melted off mid-day on December 20, 2013. Outside of that we have been dry, with temperature inversions, all thanks to an upper level ridge of high pressure that has more or less dominated our region since last February (2013). This ridge has become more entrenched in late summer and fall, and so far this winter as well. December was no exception, with high pressure dominating the area every day except the first couple days of the month and the 20th (snow morning). If you feel like this fall and winter has had a lot more fog and burn bans than usual, you are feeling correctly. I will talk about why it has been so dry more in my seasonal outlook below.

I want to show you some statistics for December.

Average total rainfall for December (not including December 2013): 5.88 inches

December 2012 total rainfall: 8.69 inches

December 2013 total rainfall: 1.36 inches

Below are two graphs of the daily rainfall for the month of December 2012 and 2013 respectively.

December 2012

December 2013

You will notice the scale on the left, that's in inches, 2012's scale goes to 1.60 inches. 2013's scale goes to 0.35 inches.

Now the reason for our dry weather. Below are the graphs of the daily average barometric pressure in inches of mercury (inHg) for December 2012 and 2013. There are some gaps in the data due to glitches with the weather station on specific days. These glitches only block temperature and barometric pressure, not rainfall.

December 2012

December 2013

Again you will have to observe the scale on the left hand side. December 2012 ranges from 28.4inHg to 29.8inHg. December 2013 ranged from 29.1inHg and 30.1inHg. You can see how most of our days in 2012 ranged in the 29.0-29.4inHg range. No storm track blocking upper level ridge there. 2013 however you can see most days were far above 29.7inHg. 

High pressure blocks storms from moving into the area quite effectively and directs the jet stream around the bubble of high pressure. This year it has been directing the jet stream north of us into British Columbia and Alaska. The jet stream is a river of strong winds in the upper atmosphere. Most significant storms travel along the path of the jet stream. This has resulted in the calmer, drier than normal weather pattern we have been in for the past few months.

Here's the average daily temperature graphs, compared to station historic average.

December 2013 - compared to station average

Seasonal Outlook/Trend: No one really knows why this upper level ridge has been so persistent this year. The Pacific El Niño Southern Oscillation has been in a neutral phase all summer to present, and is expected to remain there through the summer of 2014. This is normally a strong indicator for meteorologist to expect a storm track over the region for a normal wet, stormy winter with a good chance of a possible snow event mixed in once in a while and near normal temperatures.

What has actually been the weather pattern is an upper level ridge covering California, Oregon and Washington ending about mid-way up British Columbia. This has been shoving the storm track well to the north with the exception of a couple days here and there where the ridge weakened slightly allowing a couple of storms to pass through Washington state. This same ridge is responsible for the severe drought in California. I've been researching more information about this pattern and no one really knows why it has been so persistent, so for now I will continue to watch things and see what happens.

For the remainder of the winter, I'm expecting that this ridging will continue. There was a hint of weakening in the upper level ridge when we had our weekend wind storms move through last weekend, however it has rebuilt stronger than it was before. There is some possibility that this could break down again early to mid February. 

What this means for Shoreline and Lake Forest Park: More fog, more air stagnation, cold nights, burn bans likely for the remainder of the month of January. If the upper level ridge does break down a little in February we will get some much needed mountain snow and lowland rain. During times when storms are able to make it into the area there is still a slight chance of a minor, or moderate snow event if we happen to get a shot of cold air, or if the ridge moves west. I wouldn't put the snow tires on the car, but I might keep them handy just in case, unless of course you are traveling into the mountain passes.

Bottom line: Winter has been calm so far, but if this upper level ridge does indeed start to break down in February it will open up the doors to rain storms, wind storms, and a possible but not so likely snow event. I will post an update in February when I do the January month in review when we are closer to that time frame to see if ridging will continue or not. Of course I will also post any updates during my weekly reports.

My long range trends are based on my own research and experience with the local Northwest weather and how it has interacted in the past compared to global oscillations and sea surface temperature patterns. All graphs and data unless otherwise noted are sourced from my own personal weather station located in Northeast Shoreline, Washington. For winter storm updates or current conditions, check out my weather station web page.