Saturday, April 12, 2014

Will the Weather let us see the Lunar Eclipse?

In case you hadn't heard, there's a lunar eclipse coming up.  It will occur on Monday night.  You can read more about it at:

Here's a great figure from  that shows the lunar eclipse times for those of us in the Pacific Time Zone:

So as you can see, the moon will start to disappear around 11pm PDT.  Complete eclipse will take place between 12:07am and 1:25am, or 0707 to 0825 UTC.  The moon will be high in the sky for this eclipse. 

The question then becomes, will we be able to see it, or will clouds get in the way?  The answer isn't a slam dunk either way.  Here's one graphic that we look at when making forecasts for a single point.  It shows the forecast relative humidity in the atmosphere for the next 3 days at Spokane.  The horizontal axis is time (increasing time goes from right to left), and the vertical axis is height.  We call it a time-height chart.

NAM Time-Height of RH at Spokane, WA
The time window of the eclipse is shown with the red lines.  Higher RH (green shading) means clouds, while brown shading indicates dry cloud-free air.  As you can see, we'll have some clouds on Saturday and Saturday night.  But then Sunday will be very dry with abundant sunshine.  For Monday, a few high clouds will move over.  By the time of the eclipse, more clouds are moving into the region.  In other words, it's gonna be close.

Let's look at another kind of forecast from the University of Washington WRF model.  In these graphics, they attempt to take the computer forecast and make it look like what a satellite picture would look like.  Areas of white are clouds, dark colors are the ground.  Here's the forecast valid at 8pm Monday evening:

WRF cloud cover forecast for 8pm 14 April 2014

Note the well defined band of clouds extending from southern BC, across eastern Washington and Oregon.  But then there's a break in the clouds over central Washington, with more clouds over western Washington.  Now let's look at the forecast for 11pm, which  is when the eclipse starts:
WRF cloud cover forecast for 11pm 14 April 2014
We can see that the features in the previous forecast have all translated eastward.  Now the Idaho Panhandle has the clouds, eastern Washington is in the clear slot, with more clouds coming over the Cascades.  And the forecast for 2am, at the end of the eclipse, looks like this:

WRF Cloud Cover forecast for 2am 15 April 2014
Most of eastern Washington has clouds, while the Idaho Panhandle is now in the clear slot.

So what does all this mean?  All indications are that there will be some high wispy clouds on Monday evening around sunset.  There could be a break in these clouds at about the time of the eclipse, depending on where you live in the Inland Northwest.  Thicker clouds are expected to move into the region later in the night.

All of this said, it's a bit far into the future to trust the computer forecasts for this detail.  It won't take much for the weather system responsible for these clouds to wind up being 3 hours faster or slower than depicted here.  At this point, it's a safe bet that it won't be a completely clear sky, but the clouds might be sparse enough and thin enough to still get a good viewing of the eclipse.  And if it turns out that the clouds are too thick, well, we only have to wait until October 8th for the next lunar eclipse.

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