Thursday, January 22, 2015

Spring Skiing in January?

If you've looked at the forecast lately, you'll notice that we're expecting warmer temperatures next week.  Temperatures are forecast to be in the 40s, with even some lower 50s in places (i.e. the Palouse and Lewiston area).

Why will it be so warm?  Very strong high pressure will develop over the western US in the next few days.  But we often see that.  So what's so special about this?

The image below shows the temperatures at about 5000' above sea-level this morning.

Temperature at 5000 feet MSL on Thursday.

You'll notice that the warmest air (red shading) is over California, as well as in the lee of the Canadian Rockies.  Now here's the same image on Saturday afternoon:

Temperature forecast at 5000 feet MSL Saturday

Notice those bright red colors off the coast of northern California?  Where did that come from?  The wind will be "offshore" at this time, which means it will be blowing from the northeast, coming from Nevada and eastern Oregon, to the Pacific Ocean.  As it does this, it descends from the Sierra Nevada mountains down to sea level.  Air that descends warms due to compression.  So this warm air didn't move here from somewhere else, it was created by the wind flowing from the Sierra's (e.g. 8000 feet elevation) down to sea level.

Now here's the forecast of what will happen to that warm air on Monday:

Temperature forecast at 5000 feet MSL Monday morning

You can see that the warm air has spread northward, along the coast, and into the Inland Northwest.  

OK, so just how warm are we talking?  Well, in addition to keeping track of all of the record high and low temperatures at various cities, we also keep track of historical temperatures on our weather balloons.  So here's the January record high temperatures at 5000' MSL for various western locations, along with the computer model forecasts:

Max 5000’ MSL Temperature
Model 1
Model 2
Model 3

The three models differ a bit in their forecast, but they all have very warm temperatures at 5000'.  Some records (going back to 1948) could be broken.  How warm are these?  The forecast of 14.1°C at Spokane, if it verifies, would be the same as what we normally see on April 6th!!!

So what does this mean?  The answer is somewhat complicated.  Here's the forecast high temperatures for Monday:

These are about 10 degree above normal for the end of January.  But this forecast could easily be very wrong.  You'll notice that much of the western Basin (i.e. Moses Lake), and the northern valleys (e.g. Omak, Colville, Bonners Ferry) are in light blue (40-45F) while the mountains (e.g. the area between Republic and Colville) are in green (45-50F).  What gives?

The northern and Cascade valleys have snow on the ground.  Any melting of snow could lead to fog and low cloud formation.  And this would keep the valley temperatures cooler (i.e. little or now sunshine).  But if that fog doesn't form, then the temperatures could easily be 10 degrees warmer than forecast.

Typically, for a mid-winter warm spell, we need wind.  The wind helps to mix the atmosphere, taking warm air aloft and mixing it with the cold air near the surface.  But in this case, there won't be much if any wind.  So we won't have any "mechanical mixing" to stir the atmosphere. The sun will have to do all the work by itself to warm up the near-surface temperatures.  With the low sun angle of late January, this is a tough task.  Not impossible, but difficult.

Meanwhile, the mountains will be above any fog formation and they should see abundant sunshine.  So confidence is high that they will be warm.   Here's the minimum temperature forecast:

Notice that the mountains are as warm as the valleys?  The mountains won't just be warm during the day, but they'll probably be above freezing during the nighttime hours as well.  And this will likely last for 3 or 4 days.  A week cold front in the middle of next week will cool things down some, but it will still be warmer than normal for late January.

So we'll get a taste of early spring next week, especially in the mountains.  There's a big bust potential for the valley temperatures, especially north of I-90.  But if you're going skiing, make sure you bring the sunscreen and don't over-dress.

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