Flash Weather: April 27-29 Skew-T Analysis

Anyone have a quick minute for a meteorology lesson?

The graph below is the 18z GFS Skew-T for Nashville valid 21z (or 3 pm) Monday afternoon. This is what is called a "loaded gun" sounding. For those who like football, these also known as "goalpost" soundings (probably serves as a better visual, right? =). These soundings are common with mid-south severe weather episodes, like the one we'll experience early next week. They typically represent storm systems with a large supply of moisture in the boundary layer provided by a low-level southerly flow and low-level moist convergence on the nose of a low-level jet at 850mb.

Sorry just had to get the mumbo-jumbo out of the way.

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There are many lines on a Skew-T, but we're just going to focus on the main ones for now:

First, let's identify the x-axis and y-axis contours. On the x-axis, we have temperatures in degrees Celsius; on the y-axis, we have the height and pressure of the atmospheric column (relative to meters and mb respectively).

Next, we have our isobars (lines of equal barometric pressure), which run horizontally from left to right and are provided in increments of 100 mb ranging from 1050 to 100 mb. In conjunction, we have our isotherms (solid lines of equal temperature), which run from the southwest to the northeast across the diagram in increments of 10° Celsius (more on dry/moist adiabats and saturation mixing ratio lines mean in a later post).

Now for our two primary colored lines:

The Red Line -- This is the plot of the temperature measurements that were taken from the rawinsonde (weather balloon) as it was increasing in height. This curve will always be
to the right of the dewpoint curve as you are facing a Skew-T. It is usually drawn in red but can be other colors.

The Green Line -- This is the plot of the dewpoint measurements increasing with height. This curve will always be to the left of the temperature curve as you are facing a Skew-T. It is usually drawn in green but can be other colors.

The plots of these lines can tell us a great deal about the atmosphere above us. As for this Slew-T, we see very strong instability with a breakable cap (cap = a stable region of the lower troposphere that limits storm development; see how the red line increases between 1500-2000 meters?), meaning we can expect explosive thunderstorm development once the cap weakens.

Now, let's look at the “slope” (or angles from the horizontal contour (the isobars) clockwise to a section of the T curve.

For stability, the smaller the angle, the greater stability there will be. The larger this angle, the more instability there will be. Basically, the stability of air parcels in an atmospheric layer is indicated by comparing the slope of the temperature line (red line) to the slope of the dewpoint line (green line). If we analyze the Skew-T below, we note the potential for high instability.

Speaking of high instability, check out the theta-e values for our regions next Monday. As defined by the National Weather Service, "Theta-e (or Equivalent Potential Temperature) is the temperature a parcel of air would have if  a) it was lifted until it became saturated, b) all water vapor was condensed out, and c) it was returned adiabatically (i.e., without transfer of heat or mass) to a pressure of 1000 millibars. Theta-e, which typically is expressed in degrees Kelvin, is directly related to the amount of heat present in an air parcel. Thus, it is useful in diagnosing atmospheric instability."

Back to the Skew-T, see those black flags on the right side column of the graph? These represent the strength and directional profiles of the wind. Here, we see the difference in directional wind shear between low and mid-levels is modest, but not terribly impressive; however, the difference in actual wind speed between low and mid-levels is high enough for tornadoes to be a threat.

Bottom line: At this time, all severe-weather criteria (large hail, straight-line winds, isolated tornadoes, isolated flooding, etc.) will be possible as we prepare for the worst severe weather system to impact middle Tennessee since April 2009. Details are still forming, but anytime we have a HWO (Hazardous Weather Outlook) issued with the following words, we know we have to keep our antennas up:

From OHX: "A POTENT STORM SYSTEM WILL BEGIN TO IMPACT THE TENNESSEE VALLEY BEGINNING LATE SUNDAY AFTERNOON AND THROUGH TUESDAY MORNING. THE MOST SIGNIFICANT PERIOD WILL BE MONDAY AFTERNOON THROUGH LATE MONDAY NIGHT AS POSSIBLE SUPERCELL DEVELOPMENT COULD RESULT IN LARGE HAIL...DAMAGING WINDS...AND ISOLATED TORNADOES."

To see what the Storm Prediction Center is saying about next week's severe weather outbreak for middle Tennessee, visit: http://www.spc.noaa.gov/products/exper/day4-8/

Also, for a more comprehensive explanation on what all the Skew-T lines mean, check out Jeff Haby's write-up @ The Weather Prediction.

Note: Graphics received from tennesseewx.com forum discussion

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