Snowfall opportunities in the coming days

With an Arctic air mass solidly entrenched over the northwest US, we're looking at the potential for snow for the Inland Northwest in the coming days.  As usual, these forecasts are going to be tricky.  So keep checking the forecast to stay up to date.

First, we'll start off with a look at this morning's minimum temperatures in the Pacific Northwest.

Low Temperatures Thursday 6 February 2014

Sub-zero temperatures were widespread in eastern Washington and northern Idaho. There were several locations which reported readings of -20°F, the coldest readings this winter.   Western Montana saw temperatures as cold as -40 degrees!  But notice that it was quite cold in western Washington and Oregon as well.

The first storm is already moving onshore today (Thursday).  Snow is falling over western Oregon, all the way to the coast.  Here's the webcam capture from the Oregon coast in the town of Pacific City.

Web camera showing snow at Pacific City Oregon, 6 Feb 2014

However, this storm will track across northern Oregon into southwest Montana.  This will leave most of the Inland Northwest on the northern fringe of the snowfall.  Below is the current forecast of snow for Thursday night through Friday:

Snowfall forecast for Thursday night and Friday morning (Feb 6-7 2014)

As you can see, those folks near the Canadian border will see the least amount of snow, while locations in southeast Washington the southern Idaho Panhandle will see the most.  The Spokane metro area is looking to get around 1".  The very cold temperatures will result in a light fluffy snow.  That coupled with the cold dry roads means that driving conditions shouldn't be too bad on Friday. However, there is some potential for blowing and drifting snow, especially over the Palouse and southeast Columbia Basin where northeast winds of 10-15 mph could stir things up considerably.  

The next storm will follow a similar track, and will move through the region on Friday night and Saturday.  The chart below shows the expected snow fall from this storm.

Snowfall forecast for late Friday night through  Sunday (Feb 8-9 2014)

After a break on Sunday, a weak front will bring light snow to most of the area on Monday.  At this point, this doesn't look like a big snow event.

Storm number four looks like it will move through the area by the middle of the week.  The computer models don't have great agreement on this system, so our confidence is low.

Throughout all of this, our temperatures will gradually be warming.  The image below gives you an idea of what we're looking at for Spokane.

7-day forecast for Spokane

As the graph shows, by the middle of next week, our temperatures will have returned to near normal for this time of year, which means highs in the upper 30s and lows in the mid 20s.

One thing to note.  Typically in these warm ups after a cold arctic intrusion, the cold air near the surface is tough to scrub out.  This can set the stage for freezing rain.  At this point it's too far in the future to predict where and when this will occur, if at all.  But most likely it would be between Monday and Wednesday of next week.

The outlook through the third week of February suggests the warming will continue with more wet weather. It's quite conceivable the weather we just experienced will be the coldest of the winter. The sun angle is getting higher by the day, while the days are steadily lengthening. By February 20th, the length of daylight will be 10 hours and 38 minutes, a gain of 47 minutes compared to today. 

  

8-14 day temperature outlook

8-14 day precipitation outlook

Weird Temperatures

  In previous blogs, we've noted how there are a few ingredients that affect temperatures. We're all familiar with the affects of sunshine and its resultant heating. In meteorological terms we like to call this the "diurnal cycle", since it corresponds to the daily sunrise and sunset cycle. Here's a graph of what we're talking about:

Hourly temperature graph for Deer Park, WA. 

This shows the temperature at Deer Park, Washington for an "average" winter day, one that wasn't affected by clouds, precipitation, wind, etc.  With time starting on the left, the temperature warms during the day shortly after sunrise. The temperature then reaches it peak around 3pm after continuous solar heating. The temperature then rapidly plummets after sunset, continuing to cool through the remainder of the night until the sun rises again and the cycle repeats itself. Sunrise and sunset for this time of year are around 730 am and 400 pm. 

On the past few nights, we've seen some temperatures that haven't "behaved" like the average diurnal cycle.  The image below shows Deer Park on the 7th through the 9th.  

Hourly temperatures for Deer Park, WA 12/7-12/9 2013

On the morning of the 7th (left part of the graph), temperatures started off around -5°F, and then gradually warmed to 13°F in the afternoon under sunny skies.  And then the temperature quickly plummeted back to 0°F by 6pm.  But then a strange thing happened; the temperature warmed overnight, and actually reached 13°F again at sunrise on the 9th.  In other words, the temperature at 8 am on the 9th was the same as during the previous afternoon.  How could this happen?

Let's take a step back and discuss how the sun actually warms us.  The suns rays don't warm the air directly.  Instead, the suns rays largely pass through our atmosphere and reach the earth's surface, where it is absorbed by the ground, roads, buildings, etc. This energy then warms the air above it.  

Earth-Atmosphere Energy Balance

After the sun sets, there is no incoming solar energy.  But the earth's surface still radiates it's energy (i.e. it cools).  If the skies are clear and the air is dry, this process is very efficient, and the cooling is rapid.  But if the skies are cloudy, the warmth that is radiated by the earth is trapped, and this acts to warm the air, even at night.

Nighttime heat radiation 

So in the case on the 8th/9th of December, under sunny skies, the incoming solar radiation warmed Deer Park up to 13°F.  But as soon as the sun set, the earth quickly cooled under the clear skies.  At 6pm, clouds moved into the area.  Not only did this stop the cooling process, it actually warmed the air.  The clouds "catch" the energy emitted from the earth and re-emit it back to the surface, just like a blanket on your bed.

A similar event took place last night.  Here's the temperature trace at Deer Park again, this time for the 10th and 11th.

As before, temperatures reached their high around 3pm (25°F on Tuesday), cooled quickly to 10°F by 7:30pm, and then warmed overnight.  All of this warming could be solely attributed to the cloud cover.  There is no "warm wind", warm front,  or anything like that.

All of this leads to the question:  In the winter,  at our latitude, what's better, a sunny day and clear night, or a cloudy day and night?  If you're not a fan of cold temperatures, then as nice as a sunny day is, it's probably better to hope for the clouds.  The prevalent low clouds and fog typically found in the Inland Northwest during the winter may seem dreary at times.  But it keeps our temperatures on average much milder than other locations at our latitude (e.g. Great Falls, Duluth, and Caribou, ME). The clouds have the most apparent impact on our low temperatures. See the chart below for details.