Autumn Has Arrived

Wednesday, September 24, 2014
10:07 pm

Good evening everybody, I just got back from my first day of school. I'm actually pretty exhausted, so this will be a quickie. Only nine more months to go...

We officially had our Autumnal Equinox here in Seattle at 7:29 pm on Monday, September 22. I'm sure you all think you know how seasons work, but take some time and think about it, and I'm sure your confidence will fall. (haha, get it? fall? hahah I'm so funnnnyyyy)

The Earth takes 365.25 days to revolve around the sun. That's why we have a leap year every four years [(365 + 365 + 365 + 366)/4 = 365.25]. The Earth is also tilted at a 23.5 degree angle as it rotates in a more-or-less circular orbit (not perfectly circular, actually ellipsoidal, with the Earth being closer to the sun in the Northern Hemisphere winter). If we assume that our ellipse is a circle and the rotational speed of the Earth around the sun is constant, then due to the axial tilt of the Earth, one hemisphere will be pointed more directly at the sun, and the other one will receive more of a glancing blow. Check out the visuals below, a picture's worth a trillion words when trying to explain this stuff. Don't worry about the dates of the equinoxes not matching in the two pictures below; they can vary from year to year.

A Reason For The Seasons.

You can see that on the Northern Hemisphere summer solstice (June 21), the Northern Hemisphere is pointed toward the sun, and that the North Pole remains in sunshine all day long as the earth spins about its axis. Conversely, on December 21, the winter solstice for the Northern Hemisphere, the Northern Hemisphere is tilted away from the sun, and the Southern Hemisphere is tilted towards it. It's now Antarctica's turn to experience the midnight sun.

These two events happen when the sun is over the Tropic of Cancer (23° 26' N) and Tropic of Capricorn (23° 26' S), respectively. Somehow, the sun's gotta travel between these two places, so it crosses the equator twice a year doing so. These are the vernal (spring) and autumnal equinoxes. During these times, both hemispheres are equally pointed towards the sun, and the day is 12 hours long throughout the entire globe. Cool fact for math nerds: rate of change of day length varies sinusoidally throughout the year, with day lengths changing slowest at the solstices and fastest at the equinoxes.

The picture below shows the hours of daylight for different latitudes north of the equator and how they vary throughout the year. Note the sinusoidal shape, and the "midnight sun from June to mid-July at 70° N.

physicalgeography.net

I love seasons, so if you have any more questions about seasons, throw 'em at me.

Weather-wise, the transition from fall to winter is much faster than winter to spring. In other words, we cool off faster than we warm up. And we sure got a early rainstorm to jump-start our autumn. It should be noted that "meteorological winter" here is actually the November-January period, even though geographical/astronomical winter goes through March, so it won't be long before it feels like winter. After this record-breaking summer, doesn't that sound a little abrupt? I sure think so.

Look at all the rain that fell from this past storm over the last 48 hours. The purples denote over two inches of rain. Sea-Tac got 1.54 inches. My weather station up on South Whidbey Island got 0.88 inches. And Sequim was dry.

Seattle RainWatch

Our winter is supposed to be warm and dry, but this is sure a way to kick off fall!

Charlie

Odile's Rain

Wednesday, September 17, 2014
5:54 pm

Hurricane Odile at peak intensity on 9/14/14 as a category 4 hurricane with 140 mph winds and a central pressure near 922 mb. 

Cabo San Lucas is your typical Baha-Californian tropical paradise (technically it's subtropical, but subtropical paradise just sounds... not as good as a tropical paradise, and Cabo San Lucas is about as exquisite as paradises come, so it deserves to be recognized as such). It is known for its fishing (never been there, but I really wanna catch a marlin), balnearios (Latin American resort towns), and scuba diving locations. If I had to guess, I'd also say it's known for its margaritas and continuous Jimmy Buffett songs playing outside open-aired bars. Jimmy Buffett's had a pretty powerful influence in resort towns.

One thing that Cabo is NOT known for are its major hurricanes. However, a couple days back, they experienced just that. Hurricane Odile slammed into Cabo San Lucas as a category 3 hurricane with 125 mph winds late Sunday, the strongest hurricane on record for the souhtern Baja Peninsula. That's as strong as Hurricane Katrina when it hit New Orleans.

Here's the track of the hurricane. Cabo is at the very southern tip of Baha California, which is on the Pacific side. As you can see, it experienced a direct hit from Odile. Odile started out as a tropical depression early September 10, as given by the blue triangles. Blue circles denote tropical storm strength, beige = category 1, light orange = category 2, darker orange = cat 3, and darkest orange = cat 4.

The track of Odile.

Odile caused a whole bunch of damage not just in Cabo, but the entire Baja Peninsula, and it degraded more slowly than your typical hurricane after encountering the mountainous terrain of Baja California.

Odile damage. Retrived from NASA.

Odile has now weakened to a "tropical rainstorm" with a central pressure of 1003 mb and sustained winds of 25 mph. However, don't let the pressure statistics fool you, as it is currently wreaking havoc on parts of the desert southwest. Chaparral, in Southern New Mexico, has already gotten over 3 ½ inches of rain, and amounts like these can be found throughout southern Arizona and New Mexico. Flash flood watches are in effect from southwest Arizona to southern New Mexico to extreme western Texas near El Paso for this event due to the heavy rainfall rates associated with the tropical low and embedded thunderstorms within it. Additionally, the topography of the area makes it even more prone to flash flooding, as the mountains simply act to collect all the rainfall into a valley and create a very fast-moving torrent of water... a flash flood. As if that wasn't enough, the dusty, arid landscape is not very conducive to absorbing rainfall, so any rain that does fall is more likely to end up in a stream instead of soaking into the soil.

As the below radar from the National Weather Service shows, the heaviest rainfall is currently centered over southwestern New Mexico. As the week goes on, this blob of rain will travel to the east, and places like central Texas will see some rain. However, as it does so, it will weaken, and places further east will receive less precipitation. This, combined with a change to flat topography and more grassy soils, should stamp out the flash flood threat in all but the heaviest thunderstorms.

Compliments to the NWS Doppler Radar National Mosaic

I start school on the 24th, so I have less than a week left of summer. I hope you can understand the lack of blogs, I've been too busy playing outside, trying to soak up these last precious moments of summer sunshine.

Charlie

Summer's Last Stand?

Thursday, September 4, 2014
4:14 pm

Custer's Last Stand - Mort Kunstler

I had to do some refreshing on my American history to remember what Custer's Last Stand was. I did some research, and it turns out that this Custer guy really hated Indians, so he decided to fight some (specifically the Lakota, Northern Cheyenne, and Arapaho). He died. In fact, much of his puny cavalry died, as did his family. It was truly his Last Stand.

Do I think that Friday and Saturday will be weekend's last stand? No. Even though they will be extraordinarily nice, I've got a feeling that September as a whole going to be warm and sunny.

Let's take a look at our setup right now, at 8 pm (03z). The map below is very hard to see at the given size, so click on it to increase the resolution. The lines drawn are called isobars, and delineate lines of constant pressure. You can see a ridge of high pressure in the Eastern Pacific and a thermal trough centered over the Central Valley of California. Winds are generally coming from the north-northwest off the coast and from the north in the interior. This is a very typical summertime setup for the Pacific Northwest.

Retrieved from "surface analysis" @ www.weather.gov/forecastmaps

Here's a look at the UW's WRF-GFS model 9 hours out from the 12z initialization this morning. Since it is so close to the initialization time, the large scale features are very similar to the analysis above. For all practical purposes, it just shows an alternative view of our setup.

In addition to showing the isobars, the above chart shows "thickness" contours. When the atmosphere is warmer, it becomes less dense, and the height between two levels of atmospheric pressure increases. This chart uses decameters and measures the thickness between 1000 and 500 mb. The larger the thickness, the less dense the atmosphere (and therefore the higher average temperature). As a side note, I often use thicknesses as a predictor of whether it is cold enough to snow in Seattle. I only start to seriously consider a snow when thicknesses drop below 522, and I'd rather have them in the 510s. There are exceptions of course (a relatively warm upper atmosphere with a cold lower atmosphere allowing thicknesses to be relatively high yet snow to still occur), but the 522 line is something respected by most meteorologists in the area.

As we go into the weekend, we are expected to warm up as the ridge to our west gets nudged to the southwest, weakens, and any onshore flow gets the boot. In its place, the thermal trough to our south will extend northward and open the door for warm, offshore flow to take hold over the Pacific Northwest.

Valid 08:00 pm PDT, Fri 05 Sep 2014 - 3hr Fcst   Retrieved from UW mm5rt Modeling Website

Sunday will be our warmest day, with highs reaching the mid to upper 80s. Highs will be even warmer near the foothills due to the adiabatic compression of air as sinks off the Cascades. In other words, the total amount of heat energy in the air does not change, but since a given mass of air takes up less space near sea level than at elevation, this energy is compressed into a smaller parcel of air. Therefore, no energy is exchanged from any separate parcels, but the air still changes in temperature. Pretty cool. Also, when air rises, it expands and the temperature cools, but the amount of energy stays the same

After Sunday, we will get a brief respite from the heat. I say "respite" because this 2-month period of July and August was the hottest in history, and I've not become acclimated to the heat; if anything, I've just been weakened by it. We could even see a little rain on Tuesday, but things clear up Wednesday, and a new ridge is back in full force by Thursday.

Valid 11:00 pm PDT, Thu 11 Sep 2014 - 150hr Fcst   Retrieved from UW mm5rt Modeling Website

This ridge won't extend quite as far up north, and the heights over our area will be slightly lower (567 instead of 570), meaning our temperatures will be lower as well (lower 80s if this scenario plays out). We should still have blazing sunshine though. For all intents and purposes, it will feel like our record breaking summer again.

Charlie

A Convergence Zone to End All Convergence Zones

Wednesday, September 3, 20142:14 pm

Someday, somehow...

Sorry, I was listening to Nickelback. But that question has often graced my mind when thinking about convergence zones. I've been wondering when we'd get a "zone to end all zones," and this is the best one since the morning of December 18, 2008. I don't think anything can top that one... I got 4-6 inches of snow and school was canceled. Our house also almost got struck by lightning around 5 a.m. It was bloody fantastic. That convergence zone represented the leading edge of the arctic air mass that settled in afterwards for nearly a week and wreaked havoc over the entire city.

05:08 am PST, Thu 18 Dec 2008

This zone, while not snowy and having quite as vicious lightning, had much heavier precipitation and much more lightning to go with it. Let's take a look at some screenshots at certain times, and then some animations of the entire evolution of the convergence zone as a whole. All of these were retrieved from the UW Atmos Online Weather Data Archive.

04:40 pm PDT, Tue 02 Sep 2014

We started out with a strong squall line heading through Snohomish and Skagit Counties. It even had the signature of a "bow echo," something commonly associated with very strong squall lines and mesoscale convective systems over the Great Plains, and something that is generally indicative of very high straight-line winds.

05:25 pm PDT, Tue 02 Sep 2014

45 minutes later, the squall line turned southeast and weakened.

05:59 pm PDT, Tue 02 Sep 2014

30 minutes later, we see more of a SW-to-NE oriented convergence zone begin to develop. The area of rain begins to broaden, and it continues to move southward. Northern Seattle is getting thunder and lightning.

06:57 pm PDT, Tue 02 Sep 2014

By 7, much of downtown has gotten thunder and lightning or is getting some at the moment. The real action, however, is further east. The zone becomes more east-west oriented.

07:26 pm PDT, Tue 02 Sep 2014

By now, only a few select places west of Lake Washington are getting hammered with precip. The vast majority of precipitation is falling east of the lake.

08:57 pm PDT, Tue 02 Sep 2014

Later on that evening, a secondary zone developed over Seattle as the main zone moved further southward.

Here are some animations.

The thing about convective precipitation (precipitation driven by rising air and instability) is that it is much more localized than the stratiform precipitation we usually get around here during our fall and winter months when our great Pacific storms roll in. Take a look at Seattle's Rainwatch rainfall estimates, and notice how much the precipitation varies based on your location. This is over the past 48 hours, but keep in mind that most of this precipitation fell within the space of just a few hours. Some places got the majority of their rain in a few minutes.

Retrieved from Seattle RainWatch

Over 6 inches east of Maple Valley! Simply incredible. Meanwhile, places like Mountlake Terrace hardly got anything.  That's how thunderstorms work.

Unfortunately, I can’t show any pictures of the flooding that occurred around the area because all the ones I’ve found are from commercial websites and I don't have copyright permissions. I've emailed a site to ask where I can find some ones available for public use/if I can use some of theirs. If I get my hands on some pictures, I'll put them up!

Charlie