Ramblings of a Graduate Student

The National Oceanic and Atmospheric Administration’s National Weather Service Storm Prediction Center has issued a slight risk of severe thunderstorms tomorrow evening and overnight into Friday for a portion of the central United States (click here to view the outlook).  Conditions tomorrow afternoon are such that if, and that’s a big if, thunderstorms can develop in the central plains, they would have a good chance at producing severe weather.  So, how does a meteorologist go about trying to create a forecast for such a conditional threat?

To do so, we need to think about what ingredients are necessary for thunderstorms to develop.  Simply put, the necessary ingredients are moisture, lift, and instability.  In an effort to keep this blog post from being extremely long, I’ll go ahead and stipulate that, although not as great as it could be, enough instability will be present to support thunderstorms.  So what about the other two?

The image above is Wednesday morning’s 12 UTC North American Model (NAM) forecast of 2-meter dewpoint temperatures (a measure of moisture) valid at 21 UTC Thursday afternoon (4 PM CDT).  The greater the dewpoint temperature, the more moisture available in the atmosphere.  A quick rule of thumb is values greater than 50F are often needed for severe thunderstorms and a value greater than 60F are often needed for tornadoes.  (Remember, this is just a rule of thumb.  Severe thunderstorms and tornadoes can, and do, occur with dewpoint temperatures less than these rule of thumb values.) It is readily apparent by looking at the image that dewpoint values greater than 50F are forecast to be in place across a good portion of the southern and central plains.

However, on thing should stand out about this plot.  Notice the sharp edge on the western side of the higher moisture, whereas the eastern side of the moisture area is very diffuse.  This sharp gradient from moist air (east) to dry air (west) is known as a dryline.  A simple explanation as to why drylines develop is that on the dry side of a dryline, the wind advects dry air toward the moist air.  At the same time, on the moist side of a dryline, the wind advects moist air toward the dry air.  The more opposite the wind directions and the faster the wind speed, the sharper the moisture gradient.

Drylines are important in forecasting thunderstorm development because they can end up being a source of our second ingredient, lift.  This is a result of the wind on either side of the boundary converging at the same point.  When this happens, the air has two choices it can either go up or down.  Since the ground is below the converging air, it really can’t go down; this leaves “up” as the only option.  So, along a dryline, one can expect to find rising motion (lift), which is one of the main ingredients for thunderstorm development

The image above, valid at the same time as the previous dewpoint image, is of vertical velocity (also known as lift) at 850 milibars (about a kilometer above the ground over the central plains).  Areas shaded in warm colors indicate places with rising motion (ascent) and cool colors indicate areas of descent (sinking motion).  Notice how there is very little rising motion found along the area we identified as a dryline?  This is because the wind converging along the dryline isn’t particular strong at this time.  As such, only small pockets of rising motion would be expected.

The image above is the exact same as the previous image, except it is for 700 milibars (between two and three kilometers above the ground over the central United States) instead of 850 milibars.  Notice how the rising motion is a little bit stronger, but not by much.  Again, the wind convergence is not particularly strong.

So what does this mean for thunderstorm chances?  The image above is from an ensemble forecast (a bunch of models run together) run at around the same time as the NAM and is valid at 00 UTC Friday (7 PM CDT Thursday).  Areas contoured in red are places where at least one forecast model produced a thunderstorm during the last three horus, areas contoured in blue are places where every member produced a thunderstorm during the previous three hours, and areas in black are where the mean of all the forecasts produced a thunderstorm during the last three hours.  Notice how not a single model generated a thunderstorm over the dryline area between 21 UTC and 00 UTC (4PM CDT and 7 PM CDT).  This can almost certainly be attributed to the “lift” ingredient being insufficient to overcome a lack of really good moisture.

The next several images are valid 3 hours later than the images above (oo UTC / 7 PM CDT).

Notice how the dryline remains in roughly the same place.

However, notice how the rising motion at 850 milibars has increased along the dryline.  This implies that convergence along the dryline is increasing, resulting in greater lift.

We find the same thing at 700 milibars.

And consequently, some of the forecast models generate convection between 00 UTC and 03 UTC (7 PM CDT and 10 PM CDT).

Like last night’s post, this post is designed to demonstrate how a good meteorologist will interrogate model output, and won’t simply take it at face value.  A good meteorologist will attempt to understand why a model is doing what it is doing in an effort to have a better conceptual model about the event in question.

So, does this mean there won’t be storms tomorrow afternoon?  No.  The resolution of these models is such that it cannot resolve all the details of thunderstorm development.  What this forecast does indicate is that in order for thunderstorms to develop, some local processes are going to have to play a role.  This gives a good first guess as to what a meteorologist will need to watch during the day.

Note: The description of lift resulting from drylines given in this post is an oversimplification of what is actually occurring.  Meteorologists still have a lot to learn about the circulations and air-flow patterns along drylines.

Warning:  This post does contain somewhat technical information.

In preparing tonight’s blog post, which was originally going to be a forecast discussion regarding late week severe weather potential, I came across something that I thought would end up making a better blog post.  Especially when you consider that the severe weather prospects for Friday didn’t look good based on this morning’s 12 UTC model forecasts.

From time to time, meteorologists have to play detective.  What I mean is that numerical model forecasts often do things that at first glance don’t seem all that extraordinary.  However, thorough meteorologists will investigate why a model did what it did, and relying on theory, attempt to glean useful information that isn’t exactly obvious.  A good example of this can be found in examining the 12 UTC Global Forecast System (GFS) forecast for 12 UTC Thursday (7 AM CDT Thursday) through 00 UTC Friday (7 PM CDT Thursday).

The image above is valid 12 UTC Thursday (7 AM CDT), and displays the forecast 500 milibar heights and vorticity (top half) and 500 milibar heights and wind speeds (bottom half).

Through the course of these images, pay attention to the area of southern Colorado and northern New Mexico.  Watch for changes in wind speed and vorticity (both color fills).

The image above is valid 18 UTC Thursday (1 PM CDT).

The image above is valid 00 UTC Friday (7 PM CDT Thursday)

Did you see it?

Notice the model’s forecast wind speeds for northern New Mexico increase dramatically between 12 UTC and 18 UTC and then decrease dramatically between 18 UTC and 00 UTC.  In fact, the maximum seen in wind speeds at 18 UTC are greater than the surrounding magnitudes at 12 UTC and a lot greater than the surrounding magnitudes at 00 UTC.  Did you also notice the same thing happen with the vorticity?  Why might this be?

First, let me explain why the two images are linked together in a single frame.  Typically, meteorologists look for kinks in the 500 milibar height field to identify potential shortwave troughs and/or ridges.  In fact, the dip in the 500 milibar heights throughout the western United States are associated with a (larger) trough.  Troughs are associated with increases in (relative) vorticity and ridges are associated with decreases in (relative) vorticity.  Because increases in (relative) vorticity are associated with troughs, whenever the colors on the top-half of the image increase, one could infer the presence of a trough.  Without going through all the explanation as to why, an increase in wind speed can also be attributed to the presence of a trough, espcially downstream (in this case on the east side) of a trough.  (It has to do with  strengthening lower-level temperature gradient(s)).

In the vorticity images, one can track an area of higher vorticity values from southern Utah (image 1) to northwest Colorado (image 2) to southeast Wyoming (image 3).  This is associated with a shortwave trough breaking off from the southern portion of the trough and rapidly ejecting northeast.  This is known as a lead-shortwave trough as it “leads” the main shortwave trough.  As the lead shortwave trough races through the flow of the longer wavelength trough, it helps to increase 500 milibar wind speeds in northern New Mexico.

If we think about the main shortwave trough, we would expect to find stronger winds located downstream (east) of the trough axis.  The same can be said of the lead-shortwave trough.  We’d expect to find an increase in wind speed downstream from the trough axis.  When the expected area of increased wind speed from both troughs overlap, we get a cumulative effect where the winds increase to more than what they would without interaction of the two troughs.

If a meteorologist had simply examined the noisy vorticity plot above, the presence of the lead shortwave could have been missed.  The same if a forecaster had only examined the forecast wind speeds.  However, by examining the two together, and understanding that it is possible to indirectly link increases in vorticity to increases in wind speed, we can determine the presence of a lead-shortwave trough.  Why is this important? Downstream from a shortwave trough, rising motion is often found.  When a “cap” (discussed more in upcoming blog posts) is present, the rising motion associated with a shortwave trough will often help thunderstorms initiate, when otherwise thunderstorms wouldn’t.  This often results in what chasers refer to as “the day before the day” events.  That is, thunderstorms and tornadoes developed on the day before the main trough moves through, which most people consider to be “the day”.

Now, I’m not saying that Thursday will be “the day before the day”, however, if moisture return is a little better than forecast, and a lead shortwave trough moves through during the afternoon hours, I wouldn’t be surprised if an isolated severe thunderstorm or two developed out in the Texas panhandle.  I also wouldn’t be surprised if nothing happened, either.

A strong cyclone is currently located off the coast of the Pacific Northwest.  This cyclone is helping / will help to carve out a substantial trough across the western United States.  This trough will help to build a ridge downstream over the eastern United States, which will result in near normal to above normal temperatures throughout most of the week for the a large portion of the eastern two-thirds of the United States.  It has been a long time since the eastern two-thirds of the United States was under a ridge of any substance for any appreciable amount of time.  Hopefully this means that spring is here to stick around for awhile.

The image below (a favorite of mine) shows that the Gulf of Mexico is currently being scoured of moisture and the Caribbean doesn’t look much better.  Because of this, I’m concerned that moisture will be *relatively* hard to come by this weekend.  This would lead me to more of a potential widespread rain/thunderstorm event than a significant severe thunderstorm event.  However, far southern Texas could potentially see severe weather Friday evening.  Moisture won’t have nearly as far to travel…

Only time will tell.

Time permitting, I plan to do a more detailed post tomorrow looking at the forecast for Friday evening.

A big warm-up is coming to the southern plains and very well could extend northward into the central and northern plains as well.  The image above and below were generated from tonight’s 00 UTC (7 PM CDT) initialization of the North American Model (NAM).  The color fill in both of these images is the forecast surface temperatures at 21 UTC (4 PM CDT) on Monday (top) and Tuesday (bottom).

A big warm-up is underway, with temperatures approaching lower 80sF across far western Oklahoma and the eastern Texas panhandle on Monday.  By Tuesday, the warmth expands even more, and the NAM forecasts temperatures above 90F in the Childress, Texas area.  Temperatures could very well approach and even exceed 70F across portions of southern Canada!

This warm-up is the result of a strong low that is developing along the west coast of the United States.  This will help to warm the central United States, and should begin to draw up moisture by the end of the week.  As the west coast trough (low) moves into the central United States, severe weather chances should increase.

The spring severe weather season could very well be about to begin…

While browsing weather data this evening, I came across the following two images.  The image above is of 2-meter temperatures and the image below is of 2-meter relative humidity.  What is striking is how the shape of the lower temperatures is very similar to the shape of the higher relative humidities over eastern two-thirds of Oklahoma.  So, here is the question:

Are the lower temperatures the result of the higher relative humidity or is the higher relative humidity the result of lower temperatures?

My thought?  I believe that because dewpoint temperatures were relatively constant across the temperature gradients (not shown), and solar radiation was greater west of the colder temperatures (not shown), that the cold air resulted in the higher relative humidity and not the higher relative humidity resulting in lower temperatures.  What do you think?

The winds of change are blowing across Oklahoma this evening as a surface low pressure develops across western Texas.  Wind is the result of air moving from regions of higher pressure to areas of lower pressure.  The faster the pressure change over a given distance, the stronger the winds will be.  While not shown, there is a lot of change in pressure over a relatively small distance, and so the result is strong winds.

In addition to blowing from higher pressure to lower pressure, wind travels counter-clockwise around areas of lower pressure.  Thus, if you stand with your back to the wind, areas of lower pressure will be on your left.  This is known as Buys-Ballot’s law.  Applying this law to the image above, we would expect to find low pressure in western Texas, which is what we have.

The strong winds out of the southeast are allowing moist air from Texas (and ultimately the Gulf of Mexico) to be advected (blown) northwestward into Oklahoma.  The image above displays dewpoint temperatures, which can be understood as a measure of the amount of moisture at a given level.  The 2-meter dewpoint temperatures above (measured at 2 meters above ground level) are highest across southwest Oklahoma were the winds, and advection, are strongest.

Even though the dewpoint temperatures are highest across southwest Oklahoma, moisture is being advected northwestward by the southeast winds throughout all of Oklahoma.  If we examine the change in dewpoint temperatures over the last 3 hours we can see they have increased most everywhere in Oklahoma.

However, if we examine the change over the last 24 hours, it is much easier to see the dramatic increase in southwest Oklahoma.

The strong winds are doing more than bringing in moist air.  The winds are also bringing in warmer air from the south and helping to keep temperatures relatively warm tonight.  Again, notice how the warmest temperatures this evening are found where the winds are strongest.  This is not a coincidence.

Now, as the low pressure moves into and through the state of Oklahoma tomorrow, all the moisture above will be advected east into Arkansas.  This will cause dry air to move into the area, which will bring the change of wildfires to portions of southwest Oklahoma.  Caution is advised of those who might engage in any activity that could result in a spark.

Lastly, as we continue to move into spring, strong low pressure systems, such as this one, tend to bring with them the threat of severe thunderstorms.  Although thunderstorms are certainly possible tomorrow ahead of the low pressure, the lack of substantial moisture will severely limit the severe threat.  (The lack of severe weather this year may change in the coming 10-15 days.  Stay tuned…)

From time-to-time I try to write a post that doesn’t focus so much on the technical side of meteorology, and instead reaches a larger audience.  Tonight is one of those posts.  If you get nothing else out of tonight’s post, I simply hope you enjoy the two images.

Young meteorology students have a tendency to focus on small details, and lose sight of the larger picture.  In fact, one thing I try to stress to my students is to always start with the big picture and work in toward the small details.  The reason being is that if you don’t understand the large scale, then how can you expect to understand the small scale?  Keeping this in mind, the image above comes from the Geostationary Operational Environmental Satellite (GOES #12.  This infrared satellite image is measuring the temperature of the cloud tops and maps the resulting temperatures to a color scale.  In this case, the brighter the white, the colder the temperature (meaning the higher in the atmosphere).  Over the eastern United States you can see the clouds associated with the cyclone that moved through the southern United States (and brought Denver snow) earlier this week.  I should also mention that most of the “white” over the southern Rocky Mountains is not the result of cloud cover, but the aforementioned snow that fell.

In addition to being able to look at satellite imagery centered over the east coast (and much of the Atlantic), a separate satellite allows meteorologists to keep watch over the western portion of the United States (and much of the Pacific Ocean).  The image below comes from the the Geostationary Operational Environmental Satellite (GOES #11).  In this image, a cyclone can be seen moving onshore in the Pacific Northwest, and a very power cyclone is present in the Gulf of Alaska.  (Notice how in the comma head area it looks like a hurricane!)