IT'S RELEVANCE ON DALLAS/ FORT WORTH
AND NORTH TEXAS WEATHER
Here is the scenario- it is early May in Dallas and it is hot and humid. The temperature is approaching
90 degrees and the dew point is 70 degrees. Surface winds are out of the SE at 25-30 knots, pumping up
rich Gulf moisture, veering and strengthening with height, and pressure is falling. There is a surface
low in SW Oklahoma, an attached warm front running along the Red River, a dryline extending south into
west central Texas, and a strong, protruding cold front extending from the low southwesterly, back
behind the dryline.
The National Weather Service has issued a Tornado Watch for entire North Texas area. The latest 12Z sounding
out of KDFW indicates CAPE values are at 2700 and the Lifted Index is at -12. All the parameters are here
for a major severe weather outbreak. As an avid storm chaser, I look forward to these days. My adrenaline
starts pumping in, as I race out to my car in search of the perfect storm. Storms are initiating out
west ahead of the dryline, and moving in my direction. I drive out to intercept them. However, by
the time I reach them, they have weakened or totally dissipated. What happened?
As I head back home, I take a more thorough glance at my Skew-T sounding; I noticed something strange on the
graph. From the surface to the 850 mb level, temperatures decrease with altitude, which is very normal.
However, once at the 850mb level, temperatures start to increase. What is the phenomenon that creates
this abnormal behavior? It is called a thermal inversion, or "Cap".
The cap is basically a lid that is put on the atmosphere. We can think of the planetary boundary layer
as a pot of water. As this water heats up, it begins to boil. If there is no lid, then energy (latent
instability) can escape without obstruction. If a lid is put on the pot, then all this energy will be
trapped within the pot. This potential energy will keep increasing with time, as long as the lid is
kept on. If the lid is removed, then all that energy will escape upward. However, if the pressure
within the pot increases to a point to where the lid can no longer be held down, then that energy
can punch through and explode violently.
Every spring and summer, the Dallas/ Fort Worth area forecasters are challenged with the prospect of severe
storms and how the cap will influence the outcome of their forecasts. Information on forecasting revolves
around processes that occur both vertically and horizontally in space and time. These criteria are
important when looking at overall weather patterns. One pattern revolves around weather to our south
and west. This is the best place to start describing the atmospheric processes that produce this cap,
with its birthplace, in Mexico.
The geography of Mexico, our neighbor to the south, is made up of coastal areas, mountain ranges, plateaus,
and deserts. Two mountain ranges, the Sierra Madre Occidental in the west and the Sierra Madre Oriental
in the east are located in the northern part of Mexico. In between these two mountain ranges lies a
higher plateau that include deserts. We can think of the mountain ranges as barriers as the desert
air can only move north or south. When this air gets advected to the north from the higher terrain
in response to mid-latitude cyclones, this creates the inversion that overlies the
Dallas/ Fort Worth area.
Differential advection of a deep mixing layer from the planetary boundary layer in this hot, arid, elevated
air mass over a shallow, more cooler, and moist mixing layer produces a mid-level front known as the
"Elevated Mixed Layer Front". This front is located in the mid levels of the troposphere and
overrides the cooler, moister air mass in the low levels. This front establishes the boundary
will produce the inversion, as this forms a barrier for lifting mechanisms to produce thunderstorms.
The location of this front and its movement revolves mainly around two criteria. The first has to deal
with wind vectors. Wind vectors describe the direction and magnitude of wind in relation to troughs
of low pressure that either move in or develop west of the area. A favorite location for this development
that concerns the Dallas/Fort Worth area is on the lee side of the Colorado Rocky Mountain ranges in SE
Colorado. The counter-clockwise rotation around the low allows for southerly and southwesterly winds
ahead of it. This will pull the elevated mixed layer northward. How far north depends on the magnitude
of the wind vector associated with the low.
The second has to with seasonality. As we go from May into June, the main belt of the Westerlies shifts
north. This allows the southern plains states to exhibit a more barotropic profile, allowing for
further poleward advancement of the elevated mixed layer. This enables the cap over the Dallas/Forth
area to be stronger and more solidified. As mid-latitude cyclones move across the northern tier
states during the summer, the Dallas area rain chances become scarce, as there are virtually no
synoptic scale storm systems to come along and erode the cap. Climatologically, the area experiences
five days of capping, from April through June, and then decreasing to two days by August.
The strength of the cap is once again determined by seasonality. Late May is once again the most difficult
time of year to break the cap. The reason for this is that the air within the elevated mixed layer and
the air in the moist low levels possess characteristics within their source region of potential
temperature. A formula to determine the strength is:
CAP STRENGTH= Saturation Wet Bulb Potential Temperature of the cap - Wet Bulb Potential
Temperature of the surface.
If the calculation is three or higher, then the cap is very strong and severe storms are unlikely. Negative
values will indicate a weak cap.
There are several ways, however, to erode the cap. This can best be illustrated by using
a Skew-T chart. The first is with daytime heating. An air parcel will rise adiabatically
from the surface if the air parcel temperature on a sounding is warmer than the surrounding
environmental air. In the case of the elevated mixed layer, the environmental temperature
will be warmer than the temperature of the air parcel. As the heating of the day occurs, the
air parcel temperature will begin to rise. If it raises enough to surpass the environmental
temperature, then the air parcel will be free to rise on its own.
Another way is synoptic scale forcing or lifting. This situation does not have to rely on surface
thermal heating. If a mechanism, such as a front, should come through, lifting of the air can be
forced up to where it will help erode the capping inversion. Temperature changes in the vertical
and mesoscale features can also aid in the erosion of the cap. All of these factors have to be
considered in forecasting weather in the Dallas/Fort Worth area.
The consequences or the ability to predict such situations is vital in forecasting for the area. The cap
can make or break a day for severe weather. It doesn't matter how much low level moisture is being pulled
in, if the cap is too strong, no storms will develop. If the cap weakens before the primary heating
of the day, then we might be looking at a linear squall line developing. If the cap breaks at the
right time during the afternoon, then severe storms with potential tornadic
evolution is possible.
It is also difficult to evaluate the condition of the cap during various parts of the day. Soundings
profiles are the best tool to determine the strength of the cap. However, in the Dallas/ Fort Worth
area, soundings are taken only twice a day. The first sounding is taken locally at 1200 UTC
(7:00 AM CDT/ 6:00 AM CST) and at 0000 UTC (7:00 PM CDT/ 6:00 PM CST). As a forecaster, it
is very difficult to know when and what the strength level of the cap will be in the future,
using information from a pre-existing sounding, hence, a real challenge.
One expert opinion comes from meteorologist Steve McCauley of WFAA-TV in Dallas. He calls this
phenomenon the "Mexican Plume", obviously due to source region. He uses this term frequently
in his forecasts and graphically outputs this data so that his audience can visually comprehend
this event. He also explains that two ways for the cap to go away is for the upper level winds
to "blow it away", or for an upper level disturbance to lift the cap vertically, causing it to
cool, weaken, and then eventually go away.
It is difficult to come up with ideas to better forecast this problem. There are numerous tools
that can be used to predict if and when the cap might break. The use of models, satellite imagery,
and data from other observation stations upstream from the forecast area can be used to determine
timing and strength of synoptic scale features that could influence the strength of the cap. Also,
more frequent launches of weather balloons can help forecast, using real time information in
determining the progression of processes that are influencing the cap.
The Cap: It's Boom or Bust!
Mid-Latitude Weather Systems
American Meteorological Society 1998
Chapter 16-pages 448-480