Wednesday, October 22, 2014

A Tropical Storm for the Inland Northwest?

In the past we've mentioned how much of the moisture that falls on our heads in the Inland Northwest actually traces it's origins back to the tropics.  The rain that is currently falling on the Northwest US is actually a perfect example of this.  The image below shows a global view of atmospheric moisture. 

Global Precipitable Water - 22 Oct 2014

The red/orange colors show the highest moisture is located in the tropics.  But there are lines of moisture (yellow and light blue colors) that extend away from the tropics in both hemispheres.  These are now being called Atmospheric Rivers.  Storms moving across the globe in the mid-latitudes "tap" into the tropical moisture and bring it poleward.  This is all a part of the global atmospheric circulation.  The current imagery shows that the moisture hitting the West Coast of the U.S. is an east-west "river" of moisture that originated in the central Pacific tropics.

But there's a more subtle feature hidden in these images that may impact our weather early next week.  If you look near the Hawaiian Islands (center of the image), you'll see a swirl of orange.  This is Tropical Storm Ana.  She passed just south of the Big Island late last week and it now west of the Islands.

There's a large storm further to the west (look at the large swirl east of Japan).  The computer forecast models expect this storm to merge with Ana as it moves eastward across the Pacific.  Where will it go?

The GFS model has an interesting forecast.  First, here's the weather in the Pacific on Wednesday afternoon.

GFS forecast of Sea Level Pressure (contours) and Precipitation (shading) Wed 22 Oct 2014

You can clearly see Tropical Storm Ana, as well as the Pacific Storm that we referred to in the previous satellite image.  Also you can see the large area of rain that will impact the Northwest Wednesday and Thursday.

By Saturday (image below), the Pacific storm is starting to pull the remnants of Ana to the north.  Meanwhile, a rather strong storm is forecast to hit the Pacific Northwest for the weekend.  The Washington and Oregon coasts will see strong winds with this storm Friday night.  By Saturday night and Sunday, the strong winds will move into the Inland Northwest, so be ready for that.

GFS forecast of Sea Level Pressure (contours) and Precipitation (shading) Sat 25 Oct 2014

For early next week, the GFS has the remnants of Ana moving into the Pac NW.  

GFS forecast of Sea Level Pressure (contours) and Precipitation (shading) Tue 28 Oct 2014
Let's be clear.  This is just one model forecast, and it's still 7 days away.  And even if it comes to pass, this will not be a Tropical Storm by this point.  It is merely the remnants of Ana.  Even so, it could still be a very wet and windy storm.

Here's another model forecast.  This image show the forecast moisture.  The black arrows represent the movement of the moisture.  It's a nice depiction of the potential from this storm.  The first image is the forecast for Saturday. It shows the moisture from Ana merging with the Pacific storm on the left side of the image.

GFS Integrated Vapor Transport Forecast for Saturday 25 Oct 2014

And by the middle of next week, there's a lot of moisture heading for the West Coast.
GFS Integrated Vapor Transport Forecast for Wed 29 Oct 2014

However, as mentioned, the GFS is just one forecast model.  Here's the ECMWF model forecast for this storm Sunday.  The Pacific storm is entering the Gulf of Alaska, with the remnants of Ana to it's south.

But by next week, the ECMWF has the low much farther north, impacting the west coast of British Columbia.  We could still see some rain from this forecast, but not as much as what the GFS is forecasting.

So as usual, stay tuned to the forecasts.  As this situation evolves, we'll try to update this blog to give you a better idea of what to expect.

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