Synoptic Composite Maps of Non-Tropical Severe Surface Wind Events at Corpus Christi, Texas

 

James A. Reynolds

 

1. Introduction

 

Similar to the rest of the United States, one of the greatest weather hazards that has a significant adverse affect on the Corpus Christi, Texas region is severe wind events. Severe wind events, which by National Weather Service (NWS) definition produce wind gusts greater than, or equal to 58 miles per hour (mph)/50 knots, create or enhance fire danger, impact marine and aviation operations, and cause thousands of dollars of damage each year to residential and commercial properties. Because of these reasons, forecasters at the Corpus Christi, Texas NWS office regularly strive to improve upon their detection of and reaction to this hazard. Synoptic pattern recognition is one way in which severe wind forecasts can be improved. In this study, composite mapping was used to help identify synoptic patterns associated with severe wind events that occurred at Corpus Christi, Texas. Previous research using composites to identify synoptic patterns associated with significant wind events in the Medford, Oregon NWS county warning forecast area (Reynolds and Obrien, 2001) has shown the usefulness of this technique.

 

2. Methodology

 

The composites used in this study were generated on the Physical Sciences Division (PSD) portion of the National Oceanic and Atmospheric Administration's Earth System Research Laboratory web site (2009) using the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis dataset (Kalnay et al., 1996). This dataset contains daily data for different atmospheric parameters and significant upper air levels since January, 1948. The PSD composite-generating web page allows users to generate composite maps after inputting an individual date, or a collective of dates, then selecting atmospheric parameters of choice.

 

To determine the dates of severe wind events at Corpus Christi for which composite maps could be generated, peak wind gust data for the Corpus Christi International Airport (CRP) weather station obtained from the National Climatic Data Center (NCDC) were examined. Peak wind gust data for CRP were only available from December, 1969 to November, 1995 and the temporal span of this study was limited to these dates. Eighteen days having severe wind events were identified in this time frame. However, a peak wind gust of 161 mph/140 knots associated with Hurricane Celia (8/3/70), a peak wind gust of 92 mph/80 knots associated with Hurricane Allen (8/10/80), peak wind gusts of 58 mph/50 knots (9/16/88) and 61 mph/53 knots (9/17/88) associated with Hurricane Gilbert, and peak wind gusts of 70 mph/61 knots (9/10/71) and 64 mph/56 knots (9/11/71) associated with Hurricane Fern were excluded from this study, which left a total of 12 events. Peak wind gusts fell into a rather small range from 58 mph/50 knots to 67 mph/58 knots for all of the events in this study.

 

3. Data

 

An analysis of CRP Local Climatological Data (LCD) monthly summaries covering the dates of the severe wind events identified in this study indicated that these events could be sorted into 2 categories due to convective or non-convective origins. If the LCD entry for any of the days in this study showed a weather code of "3" for thunderstorms, then a peak wind gust for that day was assumed to be of a convective origin. Of the 12 severe wind events in this study, 6 were found to have convective origins (Table 1.), while 6 did not (Table 2.).

 

Table 1.

 

Dates of Severe Wind Events with Convective Origins,

Peak Wind Gust Speed (mph/knots) and Direction

 

4/28/72	60/52	Southeast
5/26/74	62/54	North
2/27/87	60/52	Northwest
6/3/88	61/53	North
2/21/90	58/50	Northwest
5/5/93	58/50	North

 

Table 2.

 

Dates of Severe Wind Events with Non-Convective Origins,

Peak Wind Gust Speed (mph/knots) and Direction

 

2/9/81	61/53	Southeast
5/12/82	59/51	South
2/26/84	58/50	Northwest
11/16/87	60/52	Northwest
2/3/90	59/51	Southeast
4/15/93	67/58	Northwest

 

Further analysis of the 12 individual severe wind events demonstrated that the events could be organized into 2 smaller categories under both origin types relative to annual cool (October through April) (Table 3.) and warm seasons (May through September) (Table 4.). This division served to limit the masking of significant meteorological signatures found within the various severe wind events of this study. Considering annual meteorological norms at CRP, it was suspected that the May 12, 1982 severe wind event case might have been of convective origins even though the CRP LCD did not indicate the occurrence of thunderstorms on this day. In an effort to better understand what meteorological processes may have been active during this initially deemed non-convective warm event, sounding data for Victoria, Texas¹ were examined (not shown).

 

¹ The upper air site for southeast Texas was moved from Victoria, Texas to Corpus Christi in 1989, with the first sounding taken at CRP on 0000 UTC 11 November.

 

Sounding data for the May 12, 1982 case at 1200 UTC indicated a very unstable, high shear/high cape environment. While no thunderstorm activity was recorded at CRP, the Victoria sounding indicated a high possibility for the occurrence of convective activity somewhere along the mid Texas coast. Because it is highly likely that the severe wind gust recorded at CRP on this day was due to strong outflow from convective activity in the region, but outside of the CRP observing area, this case was reclassified as a convective warm event (Table 3.).

 

Table 3.

 

Dates of Severe Wind Events with Convective Origins

Peak Wind Gust Speed (mph/knots) and Direction

 

Cool Events

 

2/21/90	58/50	Northwest
2/27/87	60/52	Northwest
4/28/72	60/52	Southeast

 

Warm Events

 

5/26/74	62/54	North
5/05/93	58/50	North
6/03/88	61/53	North
5/12/82	59/51	South

 

Table 4.

 

Dates of Cool Severe Wind Events with Non-Convective Origins,

Peak Wind Gust Speed (mph/knots) and Direction

 

4/15/93	67/58	Northwest
2/9/81	61/53	Southeast
2/26/84	58/50	Northwest
11/16/87	60/52	Northwest
2/3/90	59/51	Southeast

 

4. Results

 

4.1 Severe Wind Events with Convective Origins

 

4.1.1 Cool Events

 

The 3 cool events in this category occurred from February to April (Table 3.). Composite maps of geopotential heights for these events at 250 mb, 500mb and 700 mb (Figs. 1, 3, and 5) showed a nearly vertically-stacked and neutrally-oriented high amplitude trough extending from the northern Plains to the southern Rockies. Vector winds over Texas throughout the atmospheric column from 700 mb and above showed a southwest flow. An upper jet of 91 knots² at 250 mb (Fig. 2) stretched from northeast Texas to northeast Oklahoma and northwest Arkansas, with the right-rear quadrant of the jet located near the Dallas-Fort Worth metroplex. Broad cyclonic swaths of wind at 500 mb and 700 mb (Figs. 4 and 6) stretched from northeast Mexico to the southern Plains with maximum values of 39 knots and 19 knots, respectively. A large bulls-eye of -0.15 Pascals/second (Pa/s) of omega, which indicated moderate upward momentum, stretched across much of southeast Texas, including Corpus Christi (Fig. 7). At the surface, a trough of low pressure was situated across much of the western Gulf of Mexico, northeast Mexico and south Texas (Fig. 8). The surface pressure at Corpus Christi was around 1012 mb.

 

² Vector wind speeds in the composite maps are shown in meters per second. These values were converted to knots, when quantified in writing, for uniformity of measure throughout this paper.

 

4.1.2 Warm Events

 

The 4 warm events in this category occurred in May and June (Table 3.). Composite maps of geotpotential heights at 250 mb and 500 mb (Figs. 9 and 11) for this collective of events showed a zonal flow over south Texas with the pattern at 700 mb (Fig. 13) becoming west-southwest. Vector winds at these specific levels were 52 knots, 23 knots, and 10 knots, respectively (Figs. 10, 12, and 14). A broad area of weak to moderate upward omega sprawled across much of northern Mexico, as well as the central and southern Rockies and Plains (Fig. 15). Omega values were closer to -0.05 Pa/s over Corpus Christi, which correlated to weak upward momentum. A surface trough was found over northeast Mexico and the Big Bend area of Texas, with the trough extending into the southern High Plains (Fig. 16). The pressure at Corpus Christi was around 1010 mb.

 

4.2 Cool Severe Wind Events with Non-Convective Origins

 

The 5 cool events in this category occurred from November to April (Table 4.). Composite maps of geopotential heights for these events at 250 mb, 500 mb and 700 mb (Figs. 17, 19 and 21) showed a somewhat positively-tilted trough extending from the Dakotas to the southern High Plains. An upper jet of 78 knots at 250 mb stretched from northwest Mexico into east Texas (Fig. 18). Similar swaths of strong wind were found at 500 mb and 700 mb (Figs. 20 and 22), with speeds of 49 knots and 33 knots, respectively.

 

A 500 mb wind maximum of 50 knots was found over northeast Mexico and south central Texas. A 700 mb omega bulls-eye of -0.25 to -0.30 Pa/s, which indicated strong upward momentum, was situated over much of far east Texas, Arkansas and Louisiana (Fig. 23). At Corpus Christi, 700 mb omega values were -0.07 Pa/s. At the surface, a trough axis extended from the western Gulf of Mexico into southern Oklahoma with a pressure of 1009 mb at Corpus Christi (Fig. 24).

 

4.3 Discussion

 

Regardless of whether cool events had either convective or non-convective origins, synoptic composite maps for both groupings showed numerous similarities. Most notably, a vigorous upper trough was present over the central Rockies and northern Plains states with much of Texas located in the warm sector of the trough. A strong band of wind extended upward through much of the atmospheric column in the warm sector, and interestingly, wind speed values associated with the non-convective events were approximately 25 to 50 percent greater at 500 mb and below than winds correlated to the convective events at the same levels. Surface wind gust speed values were very similar for the two separate groupings of cool events (Tables 3. and 4.), and it appeared that severe wind events associated with non-convective origins relied more upon the surfacing of strong winds aloft due to deep mixing in the boundary layer as opposed to the downward transport of weaker winds aloft by thunderstorm downdraft processes as evidenced in the convective events.

 

At least within the time span of this study, non-tropical severe wind events did not occur from July to September when the atmosphere over southeast Texas and the northwest Gulf was in a barotropic state. As a whole, wind speeds throughout the atmospheric column in the grouping of convective warm events were much weaker than wind speeds observed in the cool season groupings. Although not completely conclusive, severe wind events that occurred during the warm season at CRP appeared to always have convective origins. Upon inspection of the individual surface pressure patterns for the convective warm events, it was determined that convective outflows were sometimes enhanced through the combination of a strong, low level onshore pressure gradient and a well-defined sea breeze (not shown).

 

5. Conclusions

 

Severe wind events of a non-tropical nature at CRP are a relatively rare event, which makes the detection of and reaction to this hazard difficult. Composite mapping has made it possible to identify mean synoptic patterns associated with severe wind events at CRP. Formative processes leading to the generation of severe wind events at CRP are seasonally dependent. The composites in this study show that severe wind events over the area during the cool season are the result of either the surfacing of strong winds from aloft through deep mixing in the boundary layer or thunderstorm downdraft transport processes. Severe wind events during the warm season are generally a result of thunderstorm downdraft transport processes, which may be enhanced by a strong low level onshore pressure gradient and an active sea breeze over the southern Texas coast.

 

The composite maps in this study should only be considered as a method to provide forecasters with an indication for the potential of receiving a severe wind event. It would be necessary to determine the number of "null" events, if any, with a similar pattern that did not produce significant wind events before these composites could be used as a true forecasting aid. This was not done for this study, but certainly provides an opportunity for further exploration. Otherwise, the composite maps, and an ingredients-based approach, could give an even better indication for the potential development of severe non-tropical wind events in the area.

 

6. Acknowledgments

 

Thanks to Bernard Meisner, Southern Region Scientific Services Division Branch Chief, and John Metz, WFO Corpus Christi, for suggesting the use of the NCEP/NCAR reanalysis datasets and composite map plotting capabilities available through the Earth System Research Laboratory web site. Additional thanks to Alex Tardy, WFO Corpus Christi, Val MacBlain, WFO El Paso, and Amber Reynolds, American Electric Power, for providing suggestions and insights that led to the improvement of this paper.

 

7. Reference

 

NOAA/ESRL Physical Sciences Division, Boulder Colorado web page

7 January 2009 https://www.cdc.noaa.gov/data/composites/day/

 

Kalnay, et al. 1996: The NCEP/NCAR Reanalysis 40-year Project., Bull. Amer. Meteor. Soc.,

77, 437-471.

 

Reynolds, J. A. and M. Obrien, 2001: Synoptic Composite Maps of Selected Surface

Wind Events over South-Central Oregon.