Friday, November 29, 2013

Tired of the Weather? Ready for a Change?

The Inland Northwest has had a stretch of rather strange weather, for late November that is: Dry and Sunny.  While some areas have been seeing more fog and low clouds the past few days, the overall weather for the region has been markedly different than what we typically see at this time of year.  November is one of our most active weather months, with precipitation typically falling on 2 of every 3 days.  Not so for the past 2 weeks.  But that's about to change.

The image below is a water vapor satellite image, along with the pressure at an altitude around 15,000' (blue lines).  You can see an area of low pressure currently located over Alaska.  This low will drop southeastward to the Pacific Northwest by Sunday.  It will also merge with the low you see to the west of it, just south of the Aleutian Islands.  This second low will provide the moisture, while the Alaskan low will provide the wind and cold.

Here's a computer forecast of the same pressure (yellow lines), valid on Sunday afternoon.  The green and blue shading is the computer forecast of precipitation.  While the low is still over the BC coast, the active weather will develop south of the low (i.e. over our area) on Sunday.  

The wind tends to blow along (parallel) to these pressure lines, and counter-clockwise around low pressure systems.  This means that on Sunday, we'll see westerly winds across the Pacific Northwest.  This wind direction tends to produce a rain-shadow in the Columbia Basin, with heavy precipitation in the Cascades and ID Panhandle mountains, and you can see this in the computer forecast above.

So what does this mean for us?  The storm has 3 phases to it.  The first is on Sunday.  The strong westerly flow (see the image above) will raise our snow levels to around 4000-5000'.  This means rain for the valleys.  The exception could been the northern valleys (e.g. Republic, Colville, Bonners Ferry), which could see some snow before changing to rain. Snoqualmie Pass will have rain, while Stevens and Lookout Passes could see a rain/snow mix.  All in all, not bad travel weather.  

By Monday morning, a Pacific cold front will sweep through the area.  The image below shows a forecast for Monday morning, with a surface low near Omak and a cold front (blue line) extending down into northwest Oregon.  As this cold front moves through the region, temperatures (and snow levels) will drop.  Typically with these cold fronts, the precipitation ends in the valleys before the temperature can get cold enough for snow.  But the mountain passes will all change to heavy snow on Monday morning.  So if you have to drive over the passes, Sunday evening will be better than Monday morning.

But take a look at the image above again.  See the other blue line just north of the US/Canadian border?  That is an arctic front, with much colder air to the north of it.  And that's phase 3 of this system.  This arctic front will move into our area Monday night, so by Tuesday morning, the forecast looks like the image below.

And by Wednesday, the arctic front continues it's southward plunge into Oregon and southern Idaho.

The blue H over western Canada in the figure above is an arctic high, which is a really cold air mass.  As this air moves into our area on Tuesday, the valleys could see a bit of light snow.  This isn't a good pattern for snow for our area, so don't expect much if we get any at all.

What we will get is a bitterly cold northerly wind.  While the forecast high temperatures on Tuesday are in the low 20s, the wind chill will be in the single digits as winds gust to 30 mph.  And Tuesday night the wind chill will potentially drop into the zero to -10F range.

For the rest of the week, temperatures will stay in the teens and single digits.  Last time we saw weather this cold was in late February of 2011, nearly 3 years ago.  The exact temperatures are hard to forecast for two reasons:  First, we're not sure how much (if any) snow will fall in the valleys.  If there's a snow cover on the ground, temperatures will probably be colder than forecast.  Second, high clouds moving over the area are hard to forecast.  Even a thin high cloud will keep nighttime temperatures a bit warmer than clear skies.  So, stay tuned to the forecast.

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