Ramblings of a Graduate Student

The picture says it all. The low temperature in Norman this morning was in the upper 50s. Why can’t every summer morning be like this?

After a relatively slow start to the Atlantic hurricane season, the Atlantic Ocean is showing signs of becoming more active. The National Hurricane Center (NHC) is currently issuing advisories on two tropical cyclones, and monitoring a third area of showers and thunderstorms for (low probability) potential for tropical cyclone development.

Hurricane Danielle (contained in yellow circle below) is currently located in the central Atlantic (southeast of Bermuda). Danielle is currently undergoing intensification, but poses no threat to the United States and only an indirect threat to Bermuda at this point. This is the result of a large trough located over the eastern United States. (Thick white dashed lines below indicate the mean long-wave trough axis.) In addition to this large trough, several smaller troughs are rotating through the larger scale flow (indicated by thin white dashed lines). This trough will tend to act like a protective barrier for the east coast. This is because of the southwesterly wind and increased wind shear that can be found on the east side of the trough. The southwesterly wind will help steer Danielle back to the north and ultimately northeast over the next few days, as was discussed on Day 234.

To the south and east of Hurricane Danielle, Tropical Storm Earl (forecast track contained in red circle) is also strengthening. This tropical cyclone is far enough removed from the previously mentioned trough, that it will have an opportunity to avoid being steered out to sea. All model guidance develop Earl into a hurricane over the next few days, and I see no reason to doubt this. Interests in the Gulf and East Coasts need to be aware of Earl as the potential will exist for a US landfall late next week. (It’s still too early to know any specifics, this is just one of many possibilities.)

In the southwestern Gulf of Mexico, the NHC is monitoring showers and thunderstorms along a remanent frontal boundary. It appears that the showers and thunderstorms will make landfall in either far south Texas or Mexico before having a change to develop into a tropical cyclone. However, to play it safe, the NHC has given a 10% chance of tropical cyclone development in this general area.

(Note: Since I didn’t describe it above, the orange contours above are 500mb height contours from the latest RUC model analysis.)

No, no. This post isn’t about the 1980′s. It’s about high temperatures in the 80Fs. The image above (courtesy of the Oklahoma Mesonet) depicts the high temperatures for Tuesday. The high temperature of 85F (actually 84.6F) in Norman, OK actually occurred at 12:45 AM, not during the day as normally is expected. The reason for this is because of the strong cold front that moved through the state this morning. The maximum daytime temperature occurred around 2:10 PM and was 80.8F. This is significantly cooler than the high temperature the day before of 101F.

I needed the cooler weather today; it’s helped me (temporarily) shake a persistent migraine. If only I could make this cooler weather last…

Well, it’s the start of another academic school year, which means my favorite meteorology class is once again back in session — Synoptic Meteorology. Although I’m not a TA for the course this year, I will once again be helping where and when I can. I also hope to use this blog to supplement the lecture materials by having my nightly post tie into whatever was discussed in class that day. (We’ll see how long this lasts…)

Today we had several brave students get up and do a spot map discussion. (The students did very well for the first day. This bodes well for the rest of the semester!) One of the maps we had the students discuss was today’s water vapor imagery. While the students were discussing the large dry region over much of the eastern US apparent in the water vapor imagery, I noticed something interesting in the high plains near the Rocky Mountains. If one examines a loop of today’s water vapor imagery, it is possible to watch the progression of a surface cold front as it surges south. Although water vapor imagery tends to depict moisture in what meteorologists refer to as the “mid and upper troposphere”, it can actually “see” very near the ground in regions of high elevation (such as mountains and the high plains). This is because in these locations the ground is actually near the bottom of what we consider the mid troposphere (700mb or so).

The cold front shows up in water vapor imagery as a thin, dark, curved line extending southward along the ridge of the Rocky Mountains and then curves east and then northeastward as it extends out over the lower plains. The satellite is detecting the density gradient along the leading edge of the cold front. (Remember, cold air is more dense than warm air!) The satellite is most likely detecting the subsidence portion of the mesoscale circulations within the frontal zone. The reason for the “surge” southward nearer the peaks of the Rocky Mountains can be attributed to the location of the surface high pressure (not shown). As the surface high pressure moves southeastward out of western Canada, the circulation around the high pressure results in a northeast surface wind to the southeast of the center of the high pressure.

This northeast wind advects the “cold” air behind the font into the Rocky Mountains. As the cold air piles up along the Rocky Mountains one of two things has to happen in order to conserve mass: 1) the cold air can flow over the Rocky Mountains, or 2) the air must spread out horizontally along the front range. Since cold air is dense (heavy) it is very hard to lift this air over the Rocky Mountains, which means the air tends to spread out along the eastern slopes of the Rocky Mountains. Because of the north-to-south component of the surface wind, the cold air does not spread out equally to the north and south. it predominantly moves south. Thus, the cold front surges southward faster the closer to the peaks of the Rocky Mountains. (In actuality, the fastest surge occurs just east of the Rocky Mountain peaks due to frictional effects of the mountains…)

Below is a series of water vapor images in sets of two. The top most image has no annotation whereas the bottom image has the hand-drawn location of the surface cold front. Notice in the last few images higher moisture is detected by the satellite. This is because thunderstorms developed in response to the surface convergence along the advancing cold front. Also, in the first few images, a second cold front is detected to the north of the primary cold front. This cold front appears to dissipate as the day progresses, most likely the result of daytime heating weakening the temperature/density gradient along this secondary front.

Water Vapor Imagery Valid 1215 UTC (7:15 AM CDT)

Water Vapor Imagery Valid 1515 UTC (10:15 AM CDT)

Water Vapor Imagery Valid 1815 UTC (1:15 PM CDT)

Water Vapor Imagery Valid 2115 UTC (4:15 PM CDT)

Water Vapor Imagery Valid 0015 UTC (7:15 PM CDT)

The National Hurricane Center (NHC) has been tracking an area of showers and thunderstorms in the eastern north Atlantic Ocean the past few days for possible signs of tropical cyclone development. Yesterday, the NHC classified this area of convection as Tropical Depression #6. Today, the NHC updated Tropical Depression #6 to Tropical Storm Danielle.

Above is the forecast track of Danielle. It is expected to remain at sea, well away from land. This is due in large part to a large trough forecast to develop across the central north Atlantic Ocean. This trough will increase the southwesterly winds ahead of Danielle and help “curve” Danielle away from any land. Should this trough fail to materialize as expected, Bermuda would currently have the greatest chance at experiencing direct impacts of Danielle. (However, a lot can change over the next 5 days.)

This evening a small thunderstorm developed over Norman, OK and went on to produce a small downburst/microburst over parts of the city. This thunderstorm was very small and (for the most part) missed the observation network located at the Norman airport. As such, much of the eastern half of the city received a soaking rain whereas the official report from Norman was much less. The National Weather Service’s reporting station is very near the Oklahoma Climate Survey’s Mesonet station, so the mesonet station is a good first guess as to what the official report was. As you can see, the mesonet site recorded 0.01″ of rain, which is much less than what I observed personally. It’s amazing what a small change in position can mean when it comes to measuring precipitation.

Last night I mentioned that central Oklahoma missed out on the rain bonanza of the past week. It turns out that Mother Nature is attempting to provide a little bit of rain to central Oklahoma. Showers and thunderstorms have developed along a weak cold front discussed two days ago.