Autumn on Your Skin

Thursday, August 29, 2013
12:00 midnight

It's raining, it's pouring, my father, is snoring...

It's been a long time since it has rained. Sure, we saw some rain earlier this month, but with only a quarter-of-an-inch, things dried up extremely quickly. Our garden's been doing fine because we have been diligent in watering it, but all those other plants in the nooks and crannies of our yard are probably figuratively jumping for joy and literally ecstatically existing right now with this rain that has moved into our area. And our grass is far from green. It's pretty hardy though, and even if rain wasn't occurring and nothing was forecast, it'd still find a way of hanging in there.

But the good news is that it is raining. Let's take a look at the most recent composite radar image composed of the Langley Hill and Camano Island Radars.

12:24 a.m. PDT, Thu 29 Aug 2013. UW Northwest Radar Loop.

The rain isn't particularly heavy, but it's significant when you consider how dry we've been lately. And it could get heavier later today.

Let's take a look at our current setup. It's quite complex. Anything would be complex after the weather we've seen recently (even the thunderstorms had a fairly typical setup), but this setup is one I'm not familiar with. With an opening sentence of "WE HAVE QUITE THE SET UP" in their Wednesday 8:40 p.m. forecast discussion, it doesn't sound like the forecasters at the Seattle NWS branch in Sandpoint have a mastery of the current situation either. Don't mind the caps, the forecasters use all caps for everything. Weather really is that important.

 Just like every other NWS forecast office that I am aware of, the Seattle NWS office produces a forecast discussion four times daily, and does so at 3 a.m, 9 a.m., 3 p.m., and 9 p.m. What makes our discussions stand out are our forecasters usage of graphics to help explain their discussions, thus making graphical area forecast discussions.

Check out the picture below. It shows our 500mb height chart overlaid upon a satellite image of the current setup off the NE Pacific. We've got two main upper-level troughs in the jet stream - one off our coast, and one crossing the Aleutians. The one over the Aleutians will act to nudge the trough off our coast further inshore, bringing with it clouds and precipitation. In addition, the air associated with this trough is quite moist and unstable, and convective activity cannot be ruled out across the area as a result. 

Retrieved from 8:40 p.m. 8/29/2013 NWS SEW Graphical AFD

As this trough moves onshore, the height lines will arrange themselves in a way in which the air will tend to diverge aloft. When air diverges aloft, air converges at the surface and rises in an attempt to equalize the change in air pressure due to the divergence. Take a look at the picture below and notice how the lines are not parallel over Washington. Because air rotates counter-clockwise around lows in the Northern Hemisphere, the air ahead of the storm flows from southwest to northeast and does so in a manner that is parallel to the height lines as shown on this map. As the air moves northeast, it diverges in different directions, with the western portions turning northerly and the eastern portions turning easterly.

Valid 02:00 am PDT, Thu 29 Aug 2013 - 9hr Fcst: 500mb Heights, Relative Humidity. UW WRF-GFS 12km Resolution: Initialized 00z Thur 29 Aug 2013. Retrieved from the (University of Washington) Pacific Northwest Environmental Forecasts and Observations website.

Also, notice how high the relative humidities are. According to this chart, relative humidities are pretty darn close to 100 in our neck of the woods as I write this blog at God-o'clock in the morning.

Valid 00z Thu 29 Aug 2013. Quillayute Stuve Radiosonde Sounding. Retrieved from the (University of Washington) Weather Information - framed version website.

The above plot shows how the temperature, dewpoint, and winds vary throughout the atmosphere as measured by a radiosonde launched from the Quillayute Prairie Airport west of Forks Wednesday afternoon. I'm not super good with my correlations between atmospheric pressure and approximate altitude, but I do know that this is a pretty steep drop in temperature with elevation and that the air is pretty humid. Combine this with a significant amount of wind shear, and it becomes noticeable to the trained eye that the air is pretty unstable.

If you can remember from my The Graceful Apocalypse: Part 1 - Basic Thunderstorm Knowledge post, instability is needed for convection, and convection is needed for cumulonimbus clouds and thunderstorms to occur. We may in fact get some thunderstorms today. But before we get into the finer details of how our precipitation will be delivered to us, let's just take a look at how much rain we are supposed to get.

Valid 05:00 am PDT, Fri 30 Aug 2013 - 36hr Fcst: 24-hour Precipitation, 10-meter Wind Speed (Knots). UW WRF-GFS 4km Resolution: Initialized 00z Thur 29 Aug 2013. Retrieved from the Pacific Northwest Environmental Forecasts and Observations website.

Amounts above and below a half inch are forecast over a wide swath of Western Washington lowlands, with the highest amounts inland and south between Portland and Seattle. With the showery nature of this system, some places could receive much higher amounts, while some places may see nary a sprinkle. The orographic effects of the Cascades and, to a lesser extent, the Olympics, will cause heavier precipitation amounts there. According to this model, Mt. Rainier will take home the gold with over 2.5 inches of precipitation. I wonder what the freezing level is.

CAPE (Convective Available Potential Energy) values over the area don't look too impressive per the UW models, but the NWS is calling for values of 1000 Joules/kg over the area this afternoon. I'm not too familiar with CAPE, but apparently 1000 is large for Seattle. I'm sure Oklahomans would laugh at us, but we'll take what we can get.

Here is the NWS' "Active Weather Story." These things sure are helpful.

I have to say though... I'm not entirely sold on thunderstorms happening tomorrow, especially after seeing the CAPE as predicted by the UW models. I wouldn't be surprised if I heard a rumble of thunder, but I'd be bewildered, confused, and exhilarated if all hell broke loose in the afternoon.

The NWS forgot the last part of their last sentence about thunder roaring. The real saying is "When thunder roars, go indoors, get a camera, and go back outdoors." I'm not trying to encourage reckless behavior, I'm just trying to give advice on what I would do. That said, if you sailing, swimming, or just doing something that feels like it might be dangerous in a thunderstorm, better safe than sorry.

~ Charlie

Finished 2:08 a.m.