Thursday, February 16, 2012

The Northwest Death Zone for Pacific Fronts

Living in the interior of western Washington or Oregon, or anywhere east of the Cascades, we must live with a reality:  our frontal passages are generally really wimpy.  You live on the central Plains of the U.S. or even much of the East Coast, when a front goes by, you know it.  Big wind shifts, large (10-20F or more) temperature drops, heavy precipitation with the front, and after the front goes through things dry and clear out quickly.
Approaching strong cold front over the central U.S.
Frontal passage over the Puget Sound region?   Very weak cooling after frontal passage (generally not more than a few degrees), small wind shift from SE to SW, and rain showers continue forever after frontal passage.  Similarly wimpy over western Oregon and in eastern WA.

On the other hand, frontal passage is far more fun along the coast and the offshore waters:   big windshifts with strong winds, are not uncommon.  But even there temperature changes are minor.

So why do our fronts have such weak temperature changes, unlike their eastern brethren?
Answer: The Pacific.  Fronts moving off of Asia and Alaska might start with large temperature contrasts across them, but the relatively warm waters of the Pacific warm the cold air over time, so by the time the fronts reach us, the temperature differences are well, wimpy.  In fact, cold, dense air moving over warm water is very unstable and you get lines of cumulus called cloud streets.  Below is a satellite picture of the process.  Such instability is a very effective mixing machine.  Often there are large temperature contrasts across fronts at say 5000 ft than at the surface.
Cold air pouring out of Asia and Alaska produce lines of instability clouds.
Over the western Pacific, with large temperature contrasts between the cold air from Asia and warm waters off the ocean (including warm currents like the Kuroshio), there is a lot of "juice" for rapid cyclone development and intense frontal development.   Weather systems mature over the central Pacific and then decline over the eastern Pacific and Gulf of Alaska.  Yes, we are getting retirement home fronts that are generally losing their vigor.

But there is something else...our mountains.   Terrain tear fronts apart at low levels, and mountain effects greatly perturb the wind and pressure fields associated with fronts.  We can see this today!.   Here is a simulation of the front that came in this morning.  The first image shows the front offshore--nice pressure trough and wind shift.  The second shows the situation after landfall....can you see the front?

And here is the temperature record for the last day or so at a buoy off of Tillamook, Oregon.  Not much temperature change with this front or any other!

So we live in the land of weak fronts...

The next few days we will see a relatively strong low center move through, but by the end of the weekend another major ridge will develop in the eastern Pacific.  The mountains will get some snow, which is good, but it is looking increasingly unlikely that we will see lowland snow for the remainder of this winter.   

 The draft agenda of the Northwest Weather Workshop is now online at:

http://www.atmos.washington.edu/pnww/index2012.php?page=agenda

and the general meeting page is at:

http://www.atmos.washington.edu/pnww/

1 comment:

Ron said...

Dr. Mass, you made the great observation in one of your journal articles awhile back that the biggest day-to-day temperature changes in Seattle don't come from winter cold fronts, but from summer marine pushes/surges.

There's also one other factor that the Cascades play. In eastern Washington we often have trapped cold air in the low levels ahead of the front. Westerly post-frontal winds result in downslope adiabatic warming and mechanical mixing, removing the trapped cold air. So Pacific cold fronts sometimes bring warmer temperatures to eastern Washington.