Friday, July 15, 2011

No End In Sight

The Pacific Northwest continues to be mired in a Spring-like run of weather in the middle of Summer. Today marks the first day that average temperatures should reach 80 degrees, yet we are currently in a stretch of 8 consecutive days below 80 degrees. And then comes even worse news: there isn't an end on the horizon. Let's see why:

Above is our trusty 500mb chart. This is a set up known as the "Omega Block" because the graph resembles the Greek letter Omega. It is caused by a large ridge of high pressure over the Central U.S. and troughs on either side of the ridge. Under a ridge this big, hot air is sinking to the ground, resulting in scorching temperatures that have brought several days of triple digits. All we are stuck with is cool, gray conditions and some rain this week. That huge ridge blocks systems that typically track north into Canada but are now forced down into our backyard. This Omega Block has been set up for the better part of a week now and things aren't changing a whole lot.
This map is an anomaly chart that shows the average 500 mb lines from multiple model runs with different starting parameters. It basically gives us a an idea of where ridges and troughs will be 8-14 days in the future. Notice the big ridge over the Central U.S. has subsided a bit but the general trough in the NW/ridge in the East pattern still exists. That's not a welcoming sign for a return to Summer!
Here is a temperature probability map that correlates with the 500mb map 8-14 days out. I think this is pretty self-explanatory. Blues mean below average, red above. Yadda yadda yadda. Summer appears to be on hold until further notice. Maybe it's time to take a vacation...

Wednesday, July 6, 2011

Arizona Duststorm

A huge and rare weather phenomena occurred this week in Phoenix, Arizona. The storm has an odd name too, one that we LOVE to say around the station here. The event is known as a "haboob". The haboob originates from Arabic and is common in arid regions of the world like Saudi Arabia and the Sahara desert. The giant cloud of dust is rather impressive and can cause near-black out conditions with zero visibility. Check out this video time-lapse on Scott Wood's photography page:

Really incredible! But how exactly do these "haboobs" develop? The geography of the Phoenix area is set up just right for these events. During the summer seasons, a monsoon develops. People often mistake monsoon with a very intense period of heavy rainfall. In fact, the monsoon is a seasonal reversal of wind patterns. The heavy rainfall is just a result. In Arizona, the Summer monsoon winds come off the moist Gulf of California from the Southwest and push up against the Colorado Plateau, to the Northeast of Phoenix. As this moist, gulf air is forced to rise up the plateau, thunderstorms are generated over the top of the dusty plateau. These thunderstorms can be very intense and produce lots of rainfall in a short period of time. That rainfall eventually cools the center of the thunderstorm and forces cold air to rush out of the bottom of the storm, known as a downdraft. This downdraft, which acts like a mini-cold front, picks up all that dust and sand on top of the plateau and forces it down towards the Phoenix area! Thus, a resulting haboob. Here is an image of the setup for the haboob:
Following the dust storm, Phoenix was rocked with a pretty good thunderstorm. Those storms were sparked from the continued moist SW flow colliding with the rain-cooled downdraft air rushing out of the older thunderstorms over the plateau. They converge right over Phoenix and huge thunderstorms result! Again, this all results from the Summer monsoon in the Southwest! Monsoons occur all over the world, the most famous being the South Asian monsoon in India and Myanmar. Summer monsoons are critical in delivering rain for the growing seasons in that part of the world. Winter monsoons exist as well in South Asia. High pressure over the Tibetan Plateau keeps things cold and dry. All over the world, the monsoon creates unique and wild weather!