The Big Wet Chill

Well, we were all getting a little smug about this spring.

Day after day of warmer than normal temperatures, bulbs and trees blooming roughly a month early, and drier than normal conditions. El Nino winter was here and Seattle City Light and others worry about the low snowpack and lack of water for power generation. Summer had to be just around the corner.

Not so fast! Mother nature has other ideas...a big, wet chill is approaching and some of you, particularly those at higher elevations, may even see snow showers.

Lets review where we are. The following graphs show the temperature and precipitation conditions at Sea Tac Airport, with comparisons to normal conditions, for the past 4 weeks. You will notice that for nearly every day temperature was above normal (Saturday got to 59F, normal is 51) and rarely have nighttime lows fallen to the average values. Observed precipitation (red) is roughly two inches below normal.

The reason for our benign situation? The jet stream has been splitting with most of the action going south of us. Classic El Nino configuration...and this has been an El Nino year. Not global warming.

But this delightful situation is all going to change in a major way.

Sunday a weak front will approach and you can see it clearly on the latest satellite image.We will progressively cloud up during the day, with showers later in the afternoon and and tomorrow evening over the interior lowlands. Behind the front is MUCH colder air (see graphic of low level temperature--blue is cold--and sea level pressure). Thus, the snow level will plummet and by tomorrow evening it will be all snow in the mountains. There is the possibility of a weak Puget Sound convergence zone tomorrow night and Monday AM, bringing showers and perhaps a few snow showers to the lowlands. Monday and Tuesday will have showers before the next frontal system comes in late Tuesday. Temperatures rise with that Tuesday front and fall in its aftermath. The models even show some snow showers reaching the surface in a few locations on Wednesday (see graphic)...but I wouldn't bet on that now. Later in the week we get into a warmer, wetter pattern with southwest flow aiming at our region (see flow at midlevels below, winds are parallel to the lines).

What we do have now is a much stronger sun, so even with cold air over us, our maximum temperatures will be able to reach the forties. Remember, our sun now is equivalent to that of late September!

Let me note that what is happening is not that unusual. A number of El Nino years have had warm and relatively dry late winters, only to be followed by a cool, wet spring that saves us. That is why it never pays to panic about water supply in March. But it is of course prudent now for dam operators to save all the water they can--little risk of floods at this point.


Bottom line: cool and wet week. January in March. Protect your delicate plants. Skiing will improve by mid-week. And perhaps Seattle City Light will get enough water to stop the threatened rate increase.

Why was my flight so bumpy? Part II

Well, as long as I am terrorizing all of you with tales of flight turbulence, I might as well be complete!

In my last blog I talked about the number-one source of turbulence in the skies, shear-induced turbulence. When such turbulence occurs outside of clouds it is often called clear air turbulence or CAT.

But there is another source of aircraft bumps that is often felt around here...mountain wave turbulence. Ever notice that when flying across the Rockies that sometimes the plane is rocking over and immediately to the east of the mountain crest? Or have you experienced lots of bumps flying into Denver? Mountain Wave Turbulence could be the cause!

As shown in the figure, when air moves over mountains, waves can form. Quite simply, when air is pushed up it often oscillates up and down. These oscillations can spread downwind or can propagate vertically. As these waves move up into less dense air then can amplify causing increased shear (and turbulence) or they can break (producing strong turbulence). One sign of such mountain waves are lenticular clouds (see picture at the top). Mountain wave turbulence can sometimes become severe, but generally it is light to moderate in intensity. But every once in a while you hear about jets crossing the Rockies that are jolted by the strong stuff, with people and belongings being thrown to the ceiling.

Interestingly, the Cascades generally don't produce strong mountain wave turbulence because they are too narrow, but there are exceptions to the rule. During strong easterly flow and Enumclaw-style windstorms (see my book for details), moderate to severe turbulence can occur over and to the immediate west of the Cascades.

Another important source of turbulence...but one not as frequent here in the Northwest... is convection....big cumulus and thunderstorms. With strong up and downward motion and large wind shears, such convection can produce moderate to severe turbulence. But pilots know this and generally keep a safe distance, something made easier by weather radar on most big planes.
Remember the NW Weather Workshop!!
http://www.atmos.washington.edu/pnww/