Saturday, July 11, 2015

Why Such a Hot Summer?

Saturday, July 11, 2015
5:04 pm

Check them out! http://www.hvc-wa.com/

Hello everybody! I've been living in the woods for the past 3+ weeks up near Granite Falls, Washington, and there ain't no WIFI in the woods, so that's why the blogs have temporarily ceased. But right now, I am just relaxing at home, as I have a day off before I head back up to Hidden Valley Camp tomorrow. There are three camp "sessions"... the first one ended Friday afternoon, and the second one starts Sunday afternoon and ends in early August. For 1st session, I was a group counselor for a bunch of rowdy but lovable 10-year-olds, and 2nd session I will be taking care of 13-14 year-olds. I learned a ton during 1st session and had a great time, and I hope to have an even better time 2nd session. 

Anyway, let's talk about weather. It has been extremely dry and hot. Campfires are a big part of HVC, but we haven't been able to have them in many areas because of how hot and dry it is. I'm not getting my tan on this summer, but I am definitely getting my "dust" on, and when I go to take a shower at night, you can see a clear color change on either side of my sock, with lighter color below and darker above. The darker hues are not due to sun exposure. It is unbelievably dusty up there right now.

Why has this been happening? Well, there are two main reasons. The primary reason is that we have had a massive ridge of high pressure over our area. As my TV meteorologist friend Matthew Leach says, “high means dry”, and it also equals warm, as high pressure is associated with sinking air, and as a given parcel of this air sinks, it compresses and increases in temperature (note: because the temperature of this sinking air parcel increases but its volume decreases, the total amount of heat in the sinking air parcel remains unchanged). This process is known as adiabatic warming.

To illustrate how prominent this ridge has been, let's take a look at the 500 mb level heights from July 2nd. Sea-Tac had a high of 93 on this day (they had a high of 92 the previous day and had highs above 90 the next three days, tying the record for the most consecutive 90+ days (I believe it was set back in the early 80s)).  There is a huge ridge over the entire Intermountain West and Pacific Northwest.  

Valid 05:00 pm PDT, Thu 02 Jul 2015. Retrieved from UW mm5rt modeling site.

The secondary effect that contributes to our warmth is a large mass of unusually warm water off our coast. When we have onshore flow, the Pacific moderates our climate substantially, but if the air coming off the Pacific is passing over warmer-than-normal water, it will not cool off as much before it gets here, and we will see warmer conditions than one would expect without this warm water. As you can see, this warm water in the NE Pacific is quite expansive. This big mass of warm water appeared during last summer as well, and last summer was super warm, so when you consider that this summer is off to an even warmer start, it is not surprising to see the warm water reappear. Nick Bond, who is the state climatologist and a UW professor (I took a class from him and he is truly a first-class teacher and human being) casually named this big area of warm water "The Blob" last summer, and the name stuck. The Blob is back, and it is making us even warmer. The Blob tends to form under calm conditions where there are persistent ridges of high pressure that prevent warm surface water from mixing with cooler water from the depths below, and boy oh boy have we had our share of persistent ridges over the past couple weeks.

Retrieved from IRI Global SST Maproom

Was this predicted? You betcha! Look what NOAA's CFS model predicted earlier this spring. I couldn't find past CFSv2 simulations so I found this from Cliff Mass' blog post "Drought Misinformation," which I highly suggest you read. Contrary to popular belief, the Pacific Northwest didn't experience a precipitation drought this past winter. However, we are off to a hot and dry start to summer, so things could get worse for us if we don't cool off or get some rainfall.


Finally, the Pacific Northwest has been the hottest place (compared to normal) in the U.S. over the past month. Eastern Washington has been 6-7 degrees above normal. I remember reading the paper one morning late June and seeing the temperature forecast for the day. Walla Walla: 108. Riyadh, Baghdad, and Cairo: 105.

CPC Temperature Analysis

Thankfully, things have cooled off, and we will see some light shower tonight and tomorrow. It won't be long before the sun gives any remaining showers on Monday the boot, giving us mostly sunny skies and highs in the upper 70s for Tuesday-Thursday. There's a chance of more (gasp!) showers on Friday and Saturday, so the mega-heat looks to be gone for now. Thank goodness.

Thanks for reading! I'll update as often as possible.
Charlie

Monday, June 1, 2015

The Nepal Earthquake Part 2: Earthquake Types

Tuesday, May 26, 2015
6:38 pm

Comparison photographs taken from the same location on 4th Avenue near the intersection with C Street and looking west. Red arrows point to the west wall of the Army Navy store in both images. (Top photograph [1964] from U.S. Geological Survey Photographic Library, ID aeq00045; bottom photograph [2013] taken by R.G. McGimsey, U.S. Geological Survey, 2013.)

On Good Friday, Mary 27, 1964, Southeast Alaska was rattled by a 9.2 magnitude earthquake with a hypocenter some 15 miles below Prince William Sound. The damage throughout the region was simply astounding. 4th Avenue in Anchorage was absolutely destroyed by the quake, but a couple buildings on the south side of the street remained standing. Among them was Big Ray's Army-Navy Store (distinguished by the red arrow). My grandpa co-founded this store and my uncle and his family still run it today! Pretty cool, huh?

There are four general categories of earthquakes - tectonic, volcanic, explosion, and collapse. Tectonic earthquakes are associated with plates, volcanic earthquakes are associated with magma moving up from the mantle to the surface and causing earthquakes as it does so., explosion earthquakes are formed by, well, explosions, and collapse earthquakes are formed when something (say, an underground cave) collapses and makes a small earthquake The massive landslide that took off the north face of Mt. St. Helens was triggered by a volcanic earthquake The world had a lot explosion earthquakes during the 50s and 60s when atomic bombs were being tested, especially those underground. I'll stick to tectonic earthquakes here, since they are the largest and most destructive ones.

In the previous blog post, I talked about the three plate boundaries: convergent (and the three subtypes: oceanic-oceanic, continental-oceanic, and continental-continental), divergent, and transform. Not all tectonic earthquakes occur on fault zones, but the vast majority do. As the pictures below show, most of the earthquakes are centered across convergent, divergent, or transform plate boundaries. These "interplate" earthquakes contrast with "intraplate" earthquakes, which occur in the interior of plates and are relatively rare. The concentrated zone of earthquakes in East Africa is due to the East African Rift, which is a divergent boundary but is not illustrated in the diagram of the world's plates below. Additionally, a significant number of earthquakes are centered on "hot spots," which are locations of volcanic activity not affiliated with plate tectonics (Hawaii and Yellowstone are two examples of hot spots in the United States).

Credit: NASA.

The tectonic plates of the world. Credit: USGS.

These plate boundaries are manifested as faults, which are discontinuities or fractures in a volume of rock due to different plate movements that stretch, shear, or compress that rock. Not every single fault is a plate boundary, but all plate boundaries are faults, and it is these faults that are responsible for earthquakes. There are three main types of faults: normal, reverse, and strike-slip, and they each create different types of earthquakes. As you will see, these fault structures are very similar to the plate boundary structures I talked about in my previous post.

Normal Fault:

Anatomy of a normal and reverse fault. Retrieved from Wikipedia.

Normal fault. Retrieved from mamontoff.org

Normal faults occur when two masses are being pulled apart and one slides down the other. This is analogous to the divergent plate boundaries that form mid-ocean ridges. Earthquakes with these types of faults can be frequent, but they are weaker than the earthquakes caused by reverse or strike-slip faults because the diverging motion prevents large amounts of stress from building up.

Reverse Fault:


A reverse fault. Retrieved from San Dieguito Union High School District page on Faults

Reverse faults occur when two masses converge and one subducts under the other. These are analogous to the oceanic-oceanic, continental-oceanic, and continental-continental plate boundaries talked about in the last post, but remember, not all faults are plate boundaries. Extremely long reverse faults at subduction zones between plates can cause the largest earthquakes in the world due to the enormous stress that builds up as one plate is "thrust" under the other. Earthquakes occur when this stress is released. The largest of these earthquakes are called megathrust earthquakes. The one in Alaska in 1964 was a megathrust earthquake, and two more recent events were the 2004 Indian Ocean earthquake and tsunami and the 2011 Japan earthquake and tsunami. These megathrust earthquakes often cause tsunamis because subduction zones are generally located underwater, and these tsunamis can impact areas thousands of miles away from the epicenter of the earthquake. The marinas in Crescent City, California were heavily damaged from the Japan tsunami due to local bathymetry enhancing the effects of the tsunami. Most of the West Coast of the U.S. escaped unscathed though.

A reverse fault in the Grands Causses near Bédarieux, France. The right side moves up while the left side moves down. Retrieved from Wikipedia.

Strike-Slip Fault:

Anatomy of a normal and reverse fault. Retrieved from Wikipedia.

Strike slip faults are faults where masses of rock slide laterally past each other. The transform fault talked about in the previous blog is a special type of strike-slip fault in that the fault separates two continents. Fast moving strike-slip faults can cause frequent earthquakes, and the earthquakes that do occur are often very shallow and can therefore be extremely intense. The great San Francisco Earthquake of 1906 not only had a moment magnitude of 7.8 but occurred at a depth of 5 miles, and as a result, 80% of San Francisco was destroyed, with much of the damage due to fires throughout the city initiated by the earthquake that roared on for several days. This earthquake occurred along the most famous transform/strike-slip fault in the world, the San Andreas Fault.

San Francisco on fire in 1906. Credit: Library of Congress.

The San Andreas Fault along the Carrizo Plain. Credit: USGS.
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In case you were sleeping too easily at night, Seattle lies directly under a reverse fault called the "Seattle Fault." This fault last shook approximately 1,100 years ago and delivered an earthquake having a moment magnitude of at least 7. While the Cascadia subduction zone off our coast can deliver much larger earthquakes (magnitude 9 or greater), the fact that this fault is right under our city makes it a far more dangerous fault for us. 

An approximate location of the Seattle Fault as looking from the west side of Puget Sound. Retrieved from Wikipedia

Reverse fault structures near Vasa Park. Retrieved from 2005 Seattle Fault Earthquake Scenario Conference

Predicted intensity of Seattle Fault earthquake throughout region. Retrieved from 2005 Seattle Fault Earthquake Scenario Conference

Everybody always talks about "the big one" waiting for us off the coast, but "the big one" is actually right below our feet!

Thanks for reading!

Charlie

Thursday, May 21, 2015

The Nepal Earthquake Part 1: Plate Tectonics

May 7, 2015
12:28 pm

I still remember the Nisqually Earthquake of 2001. Ironically enough, we had just finished an "earthquake drill" when our second-grade teacher shouted "Earthquake! For real!" We all immediately bolted under our desks, not necessarily because we were concerned about falling debris, but because we just liked bolting under desks. You could say it was a guilty pleasure of ours, I guess.

Many of those who have not been in an earthquake or who have not studied them equate them to the shaking and rattling of the Earth, but when I was under my desk, the movement seemed more akin to organized waves propagating throughout the room. When an earthquake occurs, it sends seismic waves from its hypocenter that travel thousands of miles per hour through the ground, so I have a feeling I was sensing the motion of these waves. It was actually pretty fun... I clearly remember having this vision of being in a boat on the water navigating through heavy seas, and these 'seas' were the seismic waves. Nothing was being damaged or falling down... it was just a little 15 second roller-coaster ride. Afterwards, we resumed school as if nothing had happened. The only place in Seattle that had sustained significant damage was Pioneer Square, which has old buildings that are not built to the same engineering standards as the other, more modern buildings in our area.

The Nisqually Earthquake may have been fun for 8-year-old Charlie, but the Nepal earthquake is horrible for everybody. There have been a number of severe earthquakes that the world has experienced over the past decade (Indonesia, Haiti, and Japan are just a few that come to mind), and the Nepal earthquake is right up there with them. Although it did not have the destructive tsunami that the earthquakes in Indonesia and Japan had, the damage that it has caused is simply unbelievable. Whole villages have been turned to rubble. Langtang Village, a small settlement of 435 people in the Langtang Valley north of Kathmandu, was buried under a landslide that, according to evewitness reports, was 2-3 kilometers wide and 100 meters deep. Seeing some of the destruction through pictures online makes me very sad, and I can't imagine how the friends and families of those affected must be feeling right now. A girl I grew up with and her friend are still missing from the quake and are presumed to have died (I wrote a blog post about it here), and the earthquake has directly impacted my community in a way I never thought it would.

I've always been not only a weather buff but an earth science buff in general, and that extends to geology. I think we can all agree that the Nepal Earthquake is terrible and we wish it never happened. But earthquakes themselves do not have any regard for life or property; they are just a natural consequence of plate tectonics. In that aspect, I think the Nepal Earthquake is a fascinating geological event, and that's what I'd like to blog about here since you hear so much about the death and destruction caused by the earthquake in the media. I hope you can join me in taking an interest in the geological nature of the earthquake and the processes that led to it.

This will be a three-part blog; the first part will explain some of the basics of plate tectonics, the second will explain the basics of earthquakes, and the third will look specifically at the earthquake in Nepal and how it compares to some recent earthquakes around the world.
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Mt. Everest from Gokyo Ri, November 5, 2012. Retrieved from Wikimedia Commons.

In order to understand the Nepal earthquake, we first have to understand the geology of the Himalayas. The Himalayas are the world's tallest mountain range; out of the world's 10 tallest mountains, 9 reside in the Himalayas. The Himalayas are so tall for a variety of reasons, and not all of them are related to plate tectonics. They are young and located at low latitudes, and as such, they have not been heavily eroded by glaciers. The dominant reason why they are so tall, however, is due to the type of plate boundary that forms them... a continental-continental convergent boundary.

There are two general types of plates: oceanic plates and continental plates. Oceanic plates are thinner, denser, and comprised of basalt. Continental plates are thicker, less dense, and are mainly composed of granite. When one of these plates hits another plate, the less dense plate generally goes under the more dense plate. The characteristics of all plates are not the same, so some oceanic plates are denser than others, and the same is true for continental plates. There are many types of earthquakes, but the largest and most destructive ones are generally caused due to different plates smashing into each other. There are three kinds of these "convergent plate boundaries:" oceanic/oceanic, continental/oceanic, and continental/continental, so let's take a brief look at each of them and how they differ from each other.

The Aleutian Island chain is a great example of a convergent oceanic/oceanic plate boundary. The Pacific Plate is being subducted under the North American plate, creating a beautiful arc of volcanic islands and a deep ocean trench in the process. Note that the plates themselves are NOT the oceanic/continental crust; they are subdivisions of the lithosphere, which is the crust and the upper, rocky portion of the mantle. The movement of these plates is driven by convection currents on the asthenosphere, which, while still solid, is more viscous and deformable, and is liquid at certain locations, like mid-ocean ridges (more about those later).

Oceanic/oceanic convergent plate boundary. Retrieved from USGS.

Map of the Aleutian Trench and Islands. Retrieved from Wikipedia Page on Aleutian Trench.

The continental/oceanic convergent plate boundary is the one that we see along the West Coast of not only North America but South America as well. Our Cascades were formed by this plate boundary, and we will certainly have an extremely powerful earthquake within the next several hundred years, likely on par with the Japan earthquake back in 2011, and perhaps even stronger. The Andes are a textbook example of this type of plate boundary, as they have an extremely deep trench and very high (over 20,000 feet!) mountains in close proximity to each other. They have the strongest earthquakes in the world; the 1960 Nazca Earthquake was a 9.5 magnitude earthquake, which is approximately 800 times more powerful than our relatively puny Nisqually earthquake in 2001 (remember, earthquakes are measured on a logarithmic scale). These cataclysmic earthquakes are not just limited to continental-oceanic plate boundaries though, they occur at oceanic-oceanic convergent plate boundaries too. A 7.9 earthquake actually just struck the Aleutian Islands less than a year ago.

Continental/oceanic convergent plate boundary. Retrieved from USGS.

Continental-continental convergent boundaries create very high mountain regions for two reasons: no subduction takes place because the plates are too buoyant, and the plates have much more mass and are far thicker than oceanic plates. This leads to extremely high mountain ranges like the Himalayas of Nepal/Tibet and the Karakorum of Pakistan. These boundaries often create a massive plateau behind the main mountain range due to rock being pushed upwards instead of subducted. The Tibetan Plateau, which is five times the size of France and, on average, taller than Mt. Rainier, is a textbook example. These boundaries do not tend to have volcanoes and "megathrust" earthquakes (earthquakes approaching a magnitude of 9.0 or greater), but as we have seen in Nepal, they can still generate extremely powerful quakes.



Continental/continental convergent plate boundary. Retrieved from USGS.

These convergent boundaries are responsible for creating most of our mountain ranges. It makes sense intuitively; if you have a plate subducting underneath another, it's going to push that other plate up, and in the case of a continental-continental plate boundary, when subduction is not an option, the ground has nowhere to go but up!

As well as I'm discussing plate boundaries, I might as well briefly touch on the other two, divergent and transform, as earthquakes occur on all plate boundaries, not just convergent ones. Divergent plate boundaries arise when hot masses of the asthenosphere rise and burst through the lithosphere. This mass then cools, creating a new basalt crust and lithosphere. This formation of new lithosphere pushes each plate away from each other, causing them to diverge; hence the name divergent plate boundary.

Divergent plate boundary. Retrieved from USGS.


Ironically enough, divergent boundaries can create mountain ranges as well due to the magma rising through the lithosphere and cooling, forming a range. However, most of these ranges are underwater, because when plates diverge, seawater inevitably flows in after the rift between them has gotten large enough that seawater can flow through. Ever notice how the east coast of South America and west coast of Africa look like they fit with each other? Well, it is widely accepted that they were once connected in Pangea, which was a "supercontinent" composed of all the current continents, and that they began to break up 180 million years ago. In the middle of the Atlantic between these continents is the Mid-Atlantic Ridge. The Mid-Atlantic ridge is only one of many; the ocean is filled with these ridges.


Not all divergent plate boundaries occur underwater though. The East African Rift is a divergent boundary along equatorial Eastern Africa from Ethiopia to Malawi. There are several very deep lakes and stunning volcanoes in this area, including one that's becoming famous for its stark decrease in snow coverage (note: it's not due to global warming, it's due to deforestation).

Kilimanjaro's retreating glaciers. Credit: Penn State Department of Meteorology

Transform boundaries neither converge nor diverge; they slide right past each other. They differ from convergent and divergent boundaries in they are conservative plate boundaries, meaning lithosphere is neither created (divergent plate boundary) or destroyed (subduction zones). When plates are subducted or created, they are generally not done so in a uniform fashion, and rather have many individual transform faults connecting the overall subducting or diverging structure of the plate boundary itself. While they are most common around mid-ocean ridges, the most destructive ones occur near subduction zones on land. The San Andreas Fault in California is one of the most famous transform faults in the world, and is responsible for delivering many catastrophic earthquakes to the sunshine state.

The San Andreas Fault, an example of a transform boundary. Retrieved from USGS.

The San Andreas Fault has a striking structure and is something I would like to see in real life. Take a look at this photo below!

Aerial photo of the San Andreas Fault in the Carrizo Plain. Retrieved from Wikipedia

Southern California and Baja California are currently sliding northwards, and in approximately 50 million years, they will become subducted under the Aleutian Trench. So in the very very slight possibility that Los Angeles is still a bustling city 50 million years from now, citizens there will have to move somewhere else.

Charlie

Sunday, May 10, 2015

Tropical Storm "Dolphin" Near Pohnpei, Micronesia!

Saturday, May 9, 2015
11:46 pm

Me in a mangrove forest as part of a UW study abroad trip in Pohnpei, Micronesia during the summer of 2013.

Almost two years ago, I went on a UW study abroad trip to Pohnpei in the Federated States of Micronesia to learn about coastal ecosystems, and it was truly life-changing, in more ways than one. I had an amazing time while I was there, but I actually had seizures (I have epilepsy) and was ordered to leave prematurely by the UW. My mom got a free plane ride over as part of our travel insurance, and we traveled back, but not before spending over a week in Kona, Hawaii, and having amazing experiences there. We had really shoddy internet access while we were in Pohnpei, but I was still able to thoroughly document my experiences via blogs. If you look through the June/July blogs from 2013 (see the archive on the left), you'll see all my Micronesia and Hawaii posts. These are my favorite posts I've ever made - by a long shot.

Before I start, let me just clear up one misconception: "Micronesia" and the "Federated States of Micronesia (FSM)" are not the same. Micronesia is a geographic region encompassing tens of thousands of islands and 6 sovereign nations. Guam is in Micronesia. The Federated States of Micronesia are a group of four island states; from west to east (and, coincidentally, north to south), they are they are Yap, Chuuk, Pohnpei, and Kosrae. The coordinates range from 9.5° N, 138° E (Yap) to 5.3° N, 163.0° E (Kosrae). Pohnpei is around 6.8° N, 158° E.

One thing we talked about as soon as we got there was that Pohnpei rarely experienced hurricanes. The reason was its proximity to the equator. The Coriolis force, which causes everything from moving parcels of air to Felix Hernandez' devastating changeup to deflect to the right in the Northern Hemisphere and left in the Southern Hemisphere, is at a maximum at the poles and a minimum (zero) at the equator, so when you are within 5 degrees of the equator, air tends to not rotate around a storm, which is what you need for cyclogenesis.

One map that really shows this is a simple map of all the cyclones tracks from 1985-2005 over all ocean basins. You can clearly see that there are no cyclones forming within approximately 5 degrees of the equator. The black star I put on the map is the approximate location of Pohnpei. Notice how it is just on the edge of the area where cyclones start to form there, and that the cyclones that do impact it are generally pretty weak (stronger ones are in orange and red, weaker ones are in blue).

Data from the Joint Typhoon Warning Center and the NOAA. Retrieved from Wiki Commons

The above picture also clearly shows the 7 cyclone 'basins' - the North Atlantic, the Northeast Pacific, the Northwest Pacific, the Southwest Pacific, the Southwestern Indian, the Southeastern Indian, and the Northern Indian. The Northwest Pacific is by far the most active basin, averaging 26 cyclones a year. The North Atlantic Basin only averages 11.

The seven cyclone basins. Credit: NOAA

Let's take a look at what the satellite over Pohnpei looks like right now. This is the most recent visible satellite from the Aqua polar orbiter satellite, taken around 7 P.M. Sunday FSM time. Pohnpei is that small island in the middle.

Retrieved from NASA's Worldview Satellite Page

You can see there is a tropical storm - Tropical Storm Dolphin - that is really bearing down on Pohnpei. Thankfully, the maximum sustained winds are currently around 40 mph, barely tropical-storm-force. However, I wouldn't be as concerned about the winds or waves as I would be about the rain. Pohnpei is a very mountainous island, and heavy rains over the mountains could result in the inundation of piggeries in rural areas or flash flooding in cities that are within close proximity to rivers. Pohnpei is one of the wettest places on earth... the rainfall ranges from 140 inches per year near the airport to over 325 inches per year over the mountains. When I was there, I experienced some of this rainfall first-hand.

In fact, here's a little video I took of a rainstorm as it passed by while I was at our hotel on the Soundau Estuary.


Why does Pohnpei get extraordinary rainfall rates? It's because they get incredibly tall thunderstorms due to tons of heat energy and a very high tropopause. When a 70,000 foot cloud passes over you, you are going to get dumped on.


One more video... not only to show you how stunningly beautiful Pohnpei is, but how mountainous it is. Many islands in the Pacific are in danger of becoming completely submerged due to rising sea levels over the next century. Pohnpei is not one of them. These mountains significantly amplify precipitation not only due to orographic effects but due to upslope flow converging over the mountain peaks and causing convection over the mountains to cause those 70,000 foot thunderheads.

Meanwhile... we might get a few hundredths of an inch of rain on Monday. If you live between Olympia and Portland though, be on the lookout for some heavy rain Monday night and Tuesday night.

Enjoy the rest of your weekend!
Charlie

Wednesday, May 6, 2015

Major Tornado Near Norman

Tuesday, May 6, 2015
4:05 pm

I recently posted about several massive tornadoes and hailstorms in Texas. Well, it hasn't even been two weeks, but it looks like an even bigger tornado is bearing down on Norman,Oklahoma, a suburb 20 miles south of Oklahoma City. Norman is home to NOAA's NSSL (National Severe Storms Laboratory), and this is no accident, as they are ground zero for some of the most destructive tornadoes in the world. In fact, the strongest tornado ever recorded actually hit Moore, Oklahoma, another suburb of Oklahoma City that is about 10 miles north of Norman, in 1999. They had another extremely strong EF-5 tornado in 2013. I haven't heard any spotter reports yet, but just by looking at the radar, this tornado looks like it could be one of the strongest of the year. That's a pretty bold statement, but it's definitely not unreasonable when you look at some of the radar imagery coming in.

Radar showing hook echo, 9:45 pm CDT, April 26, 2015. Retrieved from Dallas-Fort Worth Scanner Twitter Page.

In my last post, we were talking about "hook echoes." Supercell thunderstorms are thunderstorms that have mesocyclones, or rotating updrafts, and the strongest of these supercells have a very prominent "hook" signature, where rain, hail, and debris are wrapped around this updraft. The picture above shows the hook echo from the storm I was following on my last blog, which was one of the best examples of a hook echo I had ever seen. That is, until I saw this image, taken 90 minutes ago.

Radar showing supercell with extremely well-defined hook echo heading towards Norman area. Credit: a Facebook thread.

Look at how tight that circulation hook is around the mesocyclone. I have never seen anything like that. This hook weakened momentarily, but strengthened right back up, and the tornado is continuing to move northeastward towards northern Norman/southern Moore. The following images were taken from GR2Analyst, which is a weather software that, while not free, is relatively cheap and extremely useful for analyzing severe weather.

Radar image at 5:50 pm CDT showing reformed hook echo heading towards Norman/Moore. The red polygon denotes the area under a tornado warning, the yellow diamond shows the tornado location.

Many of us have heard the term "Doppler Radar," but most of the images you see on TV have nothing to do with Doppler Radar. They simply give an approximation of how heavy the precipitation is by measuring the how much of the radar signal broadcast is reflected back to the radar site. Usually, the objects that do the reflecting are different types of precipitation, but they can be many different things. Back here, we sometimes see a radar signal at night even under clear skies due to birds migrating. In fact, as a little aside, let me show you a comparison between two radar images taken this past Sunday. The one at 8:45 pm has no signal (the stuff you see is just random noise), while the one at 09:52, just an hour later, is completely lit up. And it's all because of birds migrating north for the summer.

08:45 pm PDT, Sun 03 May 2015. Credit: UW Atmospheric Sciences Online Weather Data Archive

09:52 pm PDT, Sun 03 May 2015. Credit: UW Atmospheric Sciences Online Weather Data Archive

But back to Doppler Radar. All of the NWS radars across the countries are equipped with Doppler capabilities, meaning they can make use of the Doppler effect to find out the velocities of particles in the atmosphere and whether they are moving towards or away the radar. The Doppler Effect refers to how an object's apparent frequency to an observer is higher while approaching and lower while receding than its actual frequency. We've all experienced this with trains and ice cream trucks... as soon as they pass by, the pitch lowers. Doppler radars make use of this same effect, and are able to not only decipher whether an object is approaching or receding, but how fast it is moving. They make use of pulse-Doppler techniques, which are quite complicated and will be discussed in another blog.

In any event, the radar below shows a clear hook echo signature. Not only that; the velocities are extremely high. The maximum negative velocity (i.e. the max speed of the particles traveling away from the radar at Norman) is 124 knots, while the maximum positive velocity (the max speed of the particles towards the Norman radar) is 146.2 knots. These are incredibly high values, and are indicative of an extremely strong mesocyclone and tornado.

Doppler radar image taken around 6:05 pm CDT.

Other tornado warnings are continuing to pop up around the region, and I have a feeling we'll continue to see more tornadoes throughout the night. The latest warning from the NWS is for a "large and extremely dangerous tornado" near Lake Chickasha, which is around 30 miles west of Norman. If you have any friends or family in that region, alert them now. The entire region from northern Nebraska southward to central Texas is under a tornado watch until 9 pm and storms are continuing to fire up across the region. Let's hope that none of them hit any populated areas.

Charlie