**P1.4** WEST TEXAS MESONET OBSERVATIONS OF WAKE LOWS

> AND HEAT BURSTS ACROSS NORTHWEST TEXAS

Mark R. Conder\*, Steven R. Cobb, and Gary D. Skwira\
National Weather Service Forecast Office, Lubbock, Texas

1.  **INTRODUCTION**

Wake lows (WLs) and heat bursts (HBs) are mesoscale phenomena associated
with thunderstorms that can result in strong and sometimes severe
(\>25.5 m s^-1)^, near-surface winds. The operational detection of
severe winds is problematic because the associated convection can appear
quite innocuous via WSR-88D data. The thermodynamic environment that
supports WL and HB development is similar, and is often compared to the
classic "onion" sounding structure as described by Zipser (1977).
Observational studies such as Johnson et al. (1989) show that nearly
dry-adiabatic lapse rates are prevalent in the lower to mid-troposphere,
with a shallow stable layer near ground level. This type of environment
is most commonly observed in the high plains during the warm season.
While documented heat bursts were infrequent in the past, the recent
expansion of surface mesonetworks are increasing the likelihood that
these events will be sampled. One such network is the West Texas Mesonet
(WTXM), a collection of over 40 meteorological stations spread across
the Panhandle and South Plains of northwest Texas. During the period 1
June 2004 to 30 August 2006, the authors have recorded 10 WL/HB events
that have been sampled by stations of the WTXM. Some of the more extreme
measurements include a 15 degrees Celsius increase in temperature at the
Pampa and McClean WTXM sites in one event and 35 m s^-1^ wind gusts at
sites in Brownfield and Jayton on separate occasions. Additionally,
several of the events have caused considerable property damage.

This paper will survey the mesoscale environment of 10 WLs/HBs through
analyses of the WTXM augmented by supporting radar and satellite data.
Although only limited statistical analysis is provided, it is hoped that
the collection of events in this study will help establish some of the
typical surface characteristics of WLs/HBs and aid forecasters in
recognizing their onset and evolution.

2.  **DATA AND ANALYSIS**

The study area encompassed the current domain of WTXM stations (Fig. 1).

\* Corresponding author address: Mark R. Conder\
National Weather Service, 2579 S. Loop 289, Suite 100, Lubbock, TX
79423-1400

email: <mark.conder@noaa.gov>

![](media/image1.png){width="3.022222222222222in"
height="3.2159722222222222in"}

> FIG. 1. The Study area including the WTXM station locations. Not all
> stations were available for every event.

Meteorological data from each WTXM station is recorded in 5-minute
intervals, representing average values during that period plus the peak
3-second gust for the wind speed. Schroeder et al. (2005) provides
further information on the WTXM. Objective analysis grids of
temperature, dewpoint, pressure, pressure tendency (30-min and 60-min),
and mass divergence were produced for each WL/HB event. Radar data from
the Lubbock (KLBB) WSR-88D, satellite data from GOES East, and
radiosonde data from Amarillo (KAMA) and Midland (KMAF) were also
utilized.

3.  **WAKE LOWS**

Previous studies (e.g. Johnson, 2001) have shown that WLs typically form
during the decaying stage of a mesoscale convective system (MCS) near
the back edge of the trailing stratiform precipitation region (TSR). In
this area, parcel descent is aided by a dry subcloud layer where
evaporative cooling is inadequate to maintain saturation or offset
adiabatic warming; and the parcel continues to the surface despite being
warmer than its environment. Additionally, rear-to-front flow aloft
within the stratiform region, often in the form of a descending
rear-inflow jet, may enhance subsidence warming. WLs are commonly
associated with pressure falls of 4 mb hr^-1^ and can be tracked through
the pattern of pressure falls or surface divergence (Johnson et al.,
1989). The pressure difference between the WL and the mesohigh can also
lead to a strong pressure gradient between the two regions. A surface
pressure fall of 10 mb in 20 minutes was measured in one such case
(Johnson et al., 1996). Table 1 shows the WL cases detected from the
WTXM data for this study.

TABLE 1. Wake low cases in this study along with the maximum pressure
drop and wind gust for each.

  ------------------------- ------------------------ ---------------------
  **Date (UTC)**            **Maximum\               **Maximum\
                            Pressure Drop**          Wind Gust**

  7 July 2004               -1.7 mb                  29.3 m s^-1^

  4 June 2005               -6.4 mb                  28.8 m s^-1^

  6 June 2005               -4.4 mb                  35.1 m s^-1^

  18 Sept 2005              -4.1 mb                  25.6 m s^-1^

  27 May 2006               -3.7 mb                  34.2 m s^-1^

  21 June 2006              -5.8 mb                  28.4 m s^-1^
  ------------------------- ------------------------ ---------------------

4.  **THE WAKE LOW OF 7 JULY 2004**

This WL resulted in two severe wind gusts reports of 30.2 m s^-1^ at
MACY and 27.0 m s^-1^ at TAHO between 0500 and 0700 UTC. Radiosonde
observations at 0000 and 1200 UTC from KAMA and KMAF (Fig. 2a and 2b)
revealed nearly dry adiabatic lapse rates from 700 mb to 500 mb. The
KAMA sounding also exhibited a substantial capping inversion at 0000
UTC. Scattered thunderstorms that developed during the evening hours
across the western and central portions of the domain evolved into a
squall line exhibiting a typical symmetric trailing stratiform MCS
structure by 0400 UTC (not shown). By 0556 UTC, composite reflectivity
showed several indications that the MCS had entered the dissipating
stage, including a weak echo region extending from the southwest flank
of the MCS into the center of the TSR and a shift to a more asymmetric
appearance to the stratiform region, with greater areal coverage to the
north of the MCS axis (Fig. 3). One to 2 mb hr^-1^ pressure falls began
to develop in the vicinity of TAHO on the back edge of the TSR, behind
an area of pressure rises associated with the mesohigh. The pressure
fall center moved southeast during the period 0555 to 0625 UTC and
weakened. About this time a secondary fall center developed near LAMS.
The fall center extended east to near MACY, but weakened considerably by
0640 UTC. Meanwhile, the pressure rise center to the southeast
strengthened to near 6 mb hr^-1^ in the vicinity of SNYD by 0640 UTC.
The reflectivity minimum also continued to move southeast during this
period and was located in the vicinity of MACY at 0625 UTC, the time of
the peak wind gust. Despite the weak changes observed in the pressure
tendency fields, the divergence fields (Fig. 4a and 4b) proved useful to
track the mesolow/mesohigh couplet.

5.  **HEAT BURSTS**

Numerous previous HB studies have resolved many of the properties of
this phenomenon. While WLs can occur any time of day in conjunction with
MCS evolution, the HB is primarily a nocturnal phenomenon. In one of the
earliest works, Byers and Braham (1949) noted a relative humidity "dip"
in their mesonet data during certain thunderstorm events. This dip was
accompanied by a temperature rise of one to two degrees Celsius. The
authors suggested that this was in part due to an insufficient rate of
evaporation of the water droplets in the downdraft to maintain
saturation during their descent to the surface. This hypothesis was
supported by Fujita (1985) who also noted that a drop in dewpoint
temperature could be caused by the entrainment of drier environmental
air into the downdraft. Both of these studies examined downdrafts
resulting from individual thunderstorm cells. In a similar manner as
WLs, HBs may also result from downdrafts in the TSR of a MCS. Bernstein
and Johnson (1994) concluded that HBs in their study were a result of
strong, large-scale descent caused by rear inflow jets in the TSR. The
speed of the downdraft and the depth of the stable layer will determine
whether or not the downdraft is sufficient to deform the stable layer,
allowing warm, dry air to reach the surface. Operational determination,
however, of these two parameters is problematical due to the spatial and
temporal resolutions involved.

Through examination of the WTXM data the onset of the HB was determined
to occur when there was a simultaneous rise in temperature and decrease
in dewpoint of any magnitude that preceded a HB event. The first
10-minute period (three observations) was examined to study the initial
gradients associated with the HB while a 60-minute period was chosen in
order to capture the full extent of each event. Interestingly, the
majority of the HB events in this study occurred around 0600 and 1200
UTC. Table 2 highlights the extremes found from all the station data for
each event.

TABLE 2. Heat burst cases in this study along with the maximum
temperature and dewpoint changes and wind gust for each.

  ----------------------- ------------- --------------- ------------------
  **Date (UTC)**          **Max\        **Max\          **Max\
                          ****ΔT**      ΔT~d~**         Wind Gust**

  23 May 2005             7.8 °C        -11.0 °C        19.9 m s^-1^

  4 June 2005             3.3 °C        -9.4 °C         28.5 m s^-1^

  6 June 2005             5.8 °C        -7.2 °C         35.1 m s^-1^

  18 Sept 2005            6.1 °C        -9.1 °C         25.6 m s^-1^

  27 May 2006             7.2 °C        -15.0 °C        34.20 m s^-1^

  21 June 2006            6.7 °C        -13.3 °C        28.35 m s^-1^

  12 July 2006            5.6 °C        -1.7 °C         28.80 m s^-1^
  ----------------------- ------------- --------------- ------------------

6.  **THE HEAT BURST OF 18 SEPTEMBER 2005**

This HB is of particular interest because it produced widespread winds
in the range of 20 to 30 m s^-1^ and damage in the city of Lubbock. The
evolution of this HB was representative of those found in previous
studies (Johnson, 1983). In late afternoon, thunderstorms developed over
the western and central portions of the study area. During the period
0000 to 0600 UTC on 18 September, several thunderstorms over the western
area evolved into a small MCS which then gradually dissipated while
moving northeast. The vertical profile of the atmosphere showed
characteristics typical of the WL/HB environment. Manual interpolation
from the 0000 and 1200 UTC radiosonde observations from KAMA and KMAF
(Fig. 6a and 6b) showed a dry and warm mixed layer with nearly dry
adiabatic lapse rates to 500 mb. Additionally, a shallow inversion
developed approximately 100 mb (1000 m) deep. Objective analysis of the
WTXM data showed an area of temperature maximum/dewpoint minimum located
in the vicinity of the damaging wind gusts at 0510 UTC (Fig. 7). This
area was also located within the TSR of the decaying MCS as indicated by
the KLBB WSR-88D (not shown). The maximum temperature rise with the HB
was 6.1 degrees Celsius while the maximum wind gust was 30.2 m s^-1^ as
recorded at the National Weather Service (NWS) office in south Lubbock.
Table 3 shows salient data from the WTXM stations that were primarily
affected by the HB. During the first 10-minutes of the event, indicators
of a HB were subtle at best. At the western stations, the HB signature
was of shorter duration, lasting between 15 and 30 minutes, while the
signature was discernable up to 70 minutes across the central stations.
The meteogram from the WTXM site in west central Lubbock (LBBW) depicts
a non-typical gradual change in the dry-bulb and dewpoint temperatures
with the first surge of strong winds arriving approximately 15 minutes
after the HB onset and the strongest winds 25 minutes later (Fig. 5).
**\
**

7.  **SUMMARY AND CONCLUSIONS\
    **

WTXM observations of WL and HB events with associated strong/severe
winds were compiled for this study. Despite these detailed observations,
no completely reliable method of high wind prediction was discovered
through the use of the surface data alone. Approximately 50 percent of
the severe wind gusts were observed concurrently or slightly in advance
of pressure falls in the case of WLs or with rises in temperature and
decreases in dewpoint in the case of HBs. However, a real-time algorithm
to detect the wind gusts using the five-minute WTXM data has the
potential to provide some lead time in the other half of the cases. An
algorithm currently being developed will be tested at the NWS Lubbock
office to determine its viability. Additionally, given a suitable
thermodynamic environment, radar and satellite data have the potential
to show areas of subsidence warming associated with the development of
WLs and possible HBs.

References and supplemental information will be available at the
following address:

http:// www.srh.noaa.gov/lub/science/23sls

![](media/image2.png){width="3.1305555555555555in"
height="3.6770833333333335in"}

![](media/image3.png){width="3.1347222222222224in"
height="3.6770833333333335in"}

> FIG. 2. Comparisons of the 0000 and 1200 UTC, 7 Jul 2004 skew-*T*
> plots for a) Amarillo, and b) Midland.

![](media/image4.png){width="5.729166666666667in"
height="4.614583333333333in"}

> FIG. 3. KLBB WSR-88D 0556 UTC, 7 Jul 2004 composite reflectivity with
> overlay of surface observations. Note the wedge of lower reflectivity
> in the vicinity of Tahoka around the time of maximum wind gust.

![](media/image5.png){width="3.125in" height="2.6666666666666665in"}
![](media/image6.png){width="3.125in" height="2.7083333333333335in"}

> FIG. 4. Divergence of the wind objective analysis plots at times of
> maximum observed wind at TAHO and MACY for a) 0555 UTC, and b) 0625
> UTC, 7 Jul 2004. Divergence contours are in units of 10^-4^ s^-1^.
>
> TABLE 3. Summary of West Texas Mesonet Station data for 17 Sep 2005.
> Stations are listed in order\
> of west to east. **ΔT** and **ΔT~d~** refer to the difference between
> the initial temperatures and the highest\
> (in the case of **ΔT**) or the lowest (in the case of **ΔT~d~**)
> values in the subsequent 10 or 60-minute\
> period. **ΔWS** refers to the difference between the 5-minute average
> wind speed at the HB onset\
> time and the peak 5-minute average wind speed recorded in the 10 or 60
> minute period. **Gust\
> **refers to the peak 3-second gust recorded during the 10 or 60-minute
> period.

<table style="width:100%;">
<colgroup>
<col style="width: 11%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
</colgroup>
<tbody>
<tr class="odd">
<td rowspan="2"><strong>Station</strong> <em>(refer to Figure
1)</em></td>
<td rowspan="2"><strong>Time of HB Onset (UTC)</strong></td>
<td colspan="4"><strong>10 minute maximum values</strong></td>
<td colspan="4"><strong>60 minute maximum values</strong></td>
</tr>
<tr class="even">
<td><p><strong>ΔT</strong></p>
<p>(°C)</p></td>
<td><p><strong>ΔT<sub>d </sub></strong></p>
<p>(°C)</p></td>
<td><p><strong>ΔWS</strong></p>
<p>m s<sup>-1</sup></p></td>
<td><p><strong>Gust</strong></p>
<p>m s<sup>-1</sup></p></td>
<td><p><strong>ΔT</strong></p>
<p>(°C)</p></td>
<td><p><strong>ΔT<sub>d</sub></strong></p>
<p>(°C)</p></td>
<td><p><strong>ΔWS</strong></p>
<p>m s<sup>-1</sup></p></td>
<td><p><strong>Gust</strong></p>
<p>m s<sup>-1</sup></p></td>
</tr>
<tr class="odd">
<td>LEVE</td>
<td>03:40</td>
<td>3.2</td>
<td>-3.2</td>
<td>5.0</td>
<td>12.9</td>
<td>3.4</td>
<td>-3.2</td>
<td>11.8</td>
<td>19.8</td>
</tr>
<tr class="even">
<td>REES</td>
<td>03:55</td>
<td>1.4</td>
<td>-3.4</td>
<td>1.9</td>
<td>10.7</td>
<td>4.3</td>
<td>-8.5</td>
<td>6.8</td>
<td>14.0</td>
</tr>
<tr class="odd">
<td>LBBW</td>
<td>04:20</td>
<td>1.8</td>
<td>-4.0</td>
<td>-1.0</td>
<td>7.5</td>
<td>5.4</td>
<td>-8.8</td>
<td>11.7</td>
<td>23.6</td>
</tr>
<tr class="even">
<td>ANTO</td>
<td>04:30</td>
<td>0.8</td>
<td>-1.0</td>
<td>2.4</td>
<td>8.0</td>
<td>6.1</td>
<td>-7.7</td>
<td>12.7</td>
<td>18.9</td>
</tr>
<tr class="odd">
<td>ABER</td>
<td>04:30</td>
<td>2.4</td>
<td>-3.9</td>
<td>3.9</td>
<td>9.4</td>
<td>3.0</td>
<td>-5.2</td>
<td>3.9</td>
<td>9.4</td>
</tr>
<tr class="even">
<td>SLAT</td>
<td>04:30</td>
<td>5.0</td>
<td>-8.3</td>
<td>10.5</td>
<td>17.0</td>
<td>5.2</td>
<td>-9.1</td>
<td>15.6</td>
<td>25.0</td>
</tr>
<tr class="odd">
<td>RALL</td>
<td>05:40</td>
<td>1.9</td>
<td>-3.9</td>
<td>6.3</td>
<td>18.5</td>
<td>4.4</td>
<td>-7.1</td>
<td>14.5</td>
<td>25.6</td>
</tr>
<tr class="even">
<td>WHIT</td>
<td>05:45</td>
<td>0.7</td>
<td>-1.3</td>
<td>2.4</td>
<td>12.2</td>
<td>3.2</td>
<td>-7.4</td>
<td>5.4</td>
<td>16.7</td>
</tr>
</tbody>
</table>

![](media/image7.png){width="6.496527777777778in"
height="4.270833333333333in"}

> FIG. 5. Time series of five-minute observations from the WTXM LBBW
> station for 0300 to 0700 UTC, 18\
> Sep 2005 (10 p.m. to 2 a.m. local time). All variables are 5-minute
> averages except for\
> "GUMS", which is the peak 3-second gust in each 5-minute period.

![](media/image8.png){width="2.963888888888889in"
height="3.504166666666667in"}
![](media/image9.png){width="2.9715277777777778in"
height="3.4993055555555554in"}

> FIG. 6. Comparisons of the 0000 UTC and 1200 UTC, 18 Sep 2005 skew-*T*
> plots for a) Amarillo, and b) Midland.

![](media/image10.png){width="5.6in" height="4.329861111111111in"}

> FIG. 7. Objective analysis of temperature (°C, solid yellow contours)
> and dewpoint (°C, dashed blue contours) for 0510 UTC, 18 Sep 2005.