Wednesday, October 12, 2011

A warming week for the west?

Showers and thunderstorms are currently moving across the central US with another area of rain in the northeast.  These areas of rain are associated with two different surface lows and we're expecting clearing skies in their wake.

However, I want to look at the longer-range forecasts from the global weather models for the rest of this week and into next week.  Let's start by setting the stage.  Here's a map of 500mb heights this morning across the entire northern hemisphere.

We can see two types of waves in this pattern.  One is "longwaves".  These waves have a wavelength of tens of thousands of kilometers.  See the big troughs over the central US, the north-central Pacific and the north-central Atlantic?  Those big waves are the longwaves.  Embedded in these larger scale patterns are smaller wiggles in the 500mb height contours.  These represent "shortwaves" which only have wavelengths on the order of 100s of kilometers.  For example, that little bulge in the 500mb height lines over the central Appalachians is a little shortwave.  That little shortwave is what's helping to support the rain currently over the northeastern United States.

I bring this map to your attention because I want to point out where the lowest 500mb heights are located.  These are areas shaded in white and blue.  You can see that those regions are generally on the opposite side of the planet from the United States--they're generally over northern Asia, Greenland and north of Europe. Since lower 500mb heights are generally associated with colder air, this means that we shouldn't have to worry about any strong cold air masses moving southward into the US, at least for a few days.

Here's the GFS forecast for the 500mb pattern on Friday:
The lowest 500mb heights are still off over Asia and Greenland.  We see that the GFS is developing an unusually deep cut-off low in the middle of the Pacific.  Since this is well offshore, I wouldn't be concerned about this yet.  Another trough is present over the northeastern US.  However, most of the central and western part of the country seems to be under a ridge between the two troughs.  Ridges are generally associated with higher pressure (and often warmer temperatures).  So, this gives some hope for warming weather going into the end of the week.

Now if we get into the really long range forecasts for next week, the pattern looks like it's going to get more amplified.  Here's the forecast 500mb heights for next Tuesday:
Notice that a pocket of lower 500mb heights (and consequently, cooler air) has broken off and is moving southward through northern Canada.  This is leading toward more height falls and general troughing over the northeast.   However, that cut-off low in the Pacific is still forecast to be hanging around (though weakening).  The result--with a cut-off low and longwave trough in the pacific and a deepening trough over the eastern US--implies a rather strong ridge in between.  You can see here that there is forecast to be a very big ridge over the western US.

And it's not only the GFS model--here's the ECMWF forecast of 500mb heights for the same day:

A very dynamic pattern, with a big trough over the eastern US and a soaring ridge over the western US.

So how will this affect the temperature?  I still and somewhat leery about looking at surface temperatures from a 130-160 hour forecast.  But, just as a hint, here's the ECMWF 850mb temperatures for next Tuesday:

The temperatures are represented by the colors in degrees Celsius. Notice how the temperature pattern closely matches the height pattern in the maps above--a trough of cooler temperatures over the eastern US and a ridge of warmer temperatures over the western US.  You can see that even as far north as Seattle, the ECMWF is forecast 850mb temperatures around 20 degrees Celsius--that's 68 degrees Fahrenheit.  Since surface temperatures are usually somewhat warmer than the 850mb temperatures, we may see a warm beginning to next week out here.  That would be a nice break from the cool rainy weather we've had all week...

1 comment:

  1. This is an awesome blog! Where do you get those GFS 500mb heights pictures? Are they in any way related to the WRF mesoscale models run at the UW?