First Big Storm of the Season

Wednesday, September 4, 2013
8:49 p.m.

I remember when the above picture went viral all over the internet last year. I took one look at it and knew that it was fake; the cloud in the back is a LP (low-precipitation) supercell thunderstorm, and New York was not in an atmospheric configuration that would allow a storm like this to form. This storm was actually over Nebraska, which commonly receives supercell storms as it is located in “Tornado Alley” over the Great Plains of the United States.

We won’t see anything remotely close to Hurricane Sandy over our neck of the woods, but we will see the first organized storm system we have seen in a long, long time. We saw those intense thunderstorms on the 29th over the area that brought quite a bit of rain to parts of the area, but as the Seattle RainWatch picture below shows, far more rain fell in some locales than others. Renton got pounded with nearly two inches, with most of this rain occurring in one hour. My house in Seattle picked up a measly 0.1 – 0.2 inches of rain. 0.1 to 0.2 inches of rain per hour is actually pretty heavy rain by Seattle standards, but two inches of rain is extreme and something that is hardly ever seen here.

I got the below picture from Cliff Mass’ blog. I went to the Seattle RainWatch website to try and get the picture myself, but I couldn’t find past data older than 48 hours.

We had a similar type of situation a couple nights back. Take a look at the chart below. 3+ inches southwest of the Kitsap Peninsula, with much less on the peninsula itself. Southern Seattle hardly got anything.

I retrieved this from the actual Seattle Public Utilities Rainwatch Website myself.

Today, we will see some thunderstorms, and the Seattle RainWatch pics will probably look similar to the ones above. If you look at the model below, you can see that there is not really a uniform band of precipitation over a large area. Instead, you have these convective showers that pop up everywhere. These are not being marked as terribly strong in this chart, but I believe that some could be. We’ve made a lot of progress with numerical weather prediction, but we aren’t at the point where we can consistently predict individual convective showers.

My belief that some may be quite strong is due to the CAPE (Convective Available Potential Energy) over our area at the same time.

CAPE is essentially a measure of how buoyant a parcel of air is and, by extension, the overall instability of the atmosphere. We usually do not see CAPE values this high over Western Washington. We’ve also got quite a bit of moisture in the air, so this unstable, rising air will have the opportunity to condense and form cumulus clouds which may later become cumulonimbus. If these showers are as intense as last time, certain areas could pick up extreme amounts of rain in a short period of time while nearby areas stay dry.

But unlike the previous events shown by the RainWatch charts above, Western Washington (and much of the Pacific Northwest for that matter) will see steady, heavy, November-esque rain later tonight into Friday as a deformation zone sets up over the area. A deformation zone is an area where a mass with fluid characteristics is stretched and sheared due to variations in wind. This stretching of the air mass aloft creates a vacuum of sorts, causing air from the surface to rise in an attempt to equalize the pressure differences. This rising of air causes clouds and precipitation. The picture below shows how air converging in one place often results in it diverging from another place.

Even though the air rises to create clouds and precipitation, deformation bands/zones actually tend to form in a pretty stable atmosphere. Take a look at the CAPE over our area.

Much lower CAPE over the state, but as the picture below shows, much higher precipitation where the deformation band is.

.This precipitation will be more stratiform in nature, and because the CAPE is low and the atmosphere isn’t too unstable, we will not see thunderstorms associated with the band.The band will sweep over Seattle early Friday morning. When it is all said and done, Seattle will probably have around an inch of rain, and certain spots in the Cascades could reach 4+ inches. Storm totals could be highly variable based on the nature of the showers and thunderstorms today, but once we enter the nighttime hours, much of the state will get smacked.

Deformation bands can be pretty hard to accurately predict, so the rainfall totals shown above should be taken with a grain of salt. One thing looks to be certain though: after some showers today, some of which could be quite intense, some part of the state will get a good soaking. We will return to a sunny pattern after this low pressure system moves on through. By next week, you’ll probably forget that this storm ever happened.Still, when was the last time you saw this?

Have fun!
Charlie