Effects of Texas Panhandle Topography on Dryline Movement
NWSFO Amarillo, Texas
On the afternoon of May 15, 1996, the Amarillo WSR-88D detected a dryline in the eastern Panhandle. The dryline configuration, as depicted by the radar, coincided with the Caprock. Such radar images illustrate that topographical features in West Texas indeed play a role in localized weather events. The idea that features such as the Caprock and the Llano Estacado play a major role in West Texas weather is not original. Fankhauser (1971) and Marshall (1980) suggest that the local storm climatology, including the precipitation climatology, correlates with rising plumes of air associated with the Caprock.
This paper will examine the May 15 dryline to suggest situations in which the topography of the Panhandle affects dryline movement. Considering these effects, a relationship can be drawn between topographical features and convective activity in the Panhandle. A significant result lies in the Panhandle tornado climatology. The final section of this paper will explore the impact of topographical effects on dryline behavior to the local tornado climatology.
2. West Texas Topographical Features
The Caprock of West Texas (as described by Doswell 1982) is an extensive north-south plateau rising abruptly about 100 to 300 m from the plains, in a horizontal distance of a few kilometers. Doswell also noted the escarpment is cut by several canyons, with two being the White River Canyon to the east of Lubbock, and the Palo Duro Canyon to the south of Amarillo.
Another major topographical feature in West Texas is the Llano Estacado, a gradual upslope of the terrain which crests in the extreme southwest Panhandle and extends into east-central and southeast New Mexico. Elevations up to 4,150 ft are located in the southwest Panhandle, associated with the Llano Estacado (Figs. 1 and 2).
On May 15, 1996, the 1908 UTC Amarillo radar base reflectivity product at 0.5 deg elevation (VCP 31) indicated strong reflectivity values along the dryline. The line of maximum reflectivity values associated with the dryline was coincident with the outline of the Caprock (Fig. 3).
Composite Reflectivity at 1858 UTC over the southeast Panhandle also showed high reflectivity values along the dryline. Figure 4 shows an overlay of terrain clearly indicating that the dryline was indeed positioned along the Caprock. Significant details in the escarpment configuration appeared to be traced by the dryline. The most significant detail noted was a small bulge in the dryline which correlates with the mouth of Palo Duro Canyon.
4. Meteorological Data
The 1200 UTC surface analysis of the southern plains (Fig. 5) showed a dryline extending to the southwest from a low in southwest Kansas. Dew points to the east of the dryline were in the mid and upper 50s, with 60 dew points in northern Texas and central Oklahoma. Very dry air existed west of the dryline, with dew points in the upper teens in northeast New Mexico. By 1500 UTC, the dryline was located in the eastern Panhandle and continued to extend to the southwest across West Texas.
The 1800 UTC surface analysis (Fig. 6) showed the portion of the dryline over the eastern Panhandle had remained stationary since the 1500 UTC analysis, but eastward movement of the dryline was still occurring to the south over West Texas. The dryline then held a position along Beaver, Oklahoma, east of Borger, Texas, to just east of Lubbock line through 0000 UTC.
The 1200 UTC Amarillo sounding indicated a shallow moist layer which extended roughly through the first thousand feet above ground level (Fig. 7). Extensive drying was noted just above this shallow layer. The shallowness of this moist layer allowed the dryline to rapidly advance eastward across the western and central Panhandle between 1200 and 1500 UTC.
To more accurately represent the depth of the moist layer to the east of the Caprock, the Norman, Oklahoma, 1200 UTC sounding was examined (Fig. 8). The Norman sounding indicated that the moist layer over the lower elevations east of the Caprock was significantly deeper, extending to about 5,000 ft AGL. Therefore, the increased depth of the moist layer hindered the eastward advance of the dryline once it encountered the escarpment between 1500 and 1800 UTC.
The WSR-88D wind profile from the Amarillo radar on the afternoon of May 15 (not shown) indicated "ND" (Non Detected) through 3,000 ft. Southwest to westerly 15 kt winds were indicated at 4,000 ft with 20 kt at roughly 5,000 to 8,000 ft. Winds in the lowest 3,000 ft at 1900 UTC measured by the wind profiler at Gatton, southeast of Lubbock, were southwesterly at 5 kt in the first 3,000 ft at 1900 UTC.
Both the Amarillo radar wind profiler and the Gatton profiler indicated that winds just above the surface were weak, 5 kt or less. This suggests that mixing processes were not sufficient to mix the deeper moist layer to the east of the Caprock.
Schaefer (1973, 1974a,b) has suggested that the eastward movement of the dryline during the day is a result of strong vertical mixing. Schaefer states that the apparent movement is too rapid to be explained by advection alone. Instead, warm dry air is entrained from aloft into the moist layer near the surface. Schaefer, along with Benjamin and Lanicci (1985), also suggested that dryline movement typically slows as the terrain elevation decreases and depth of the moist layer increases to the east across Texas. The Norman sounding, showing a moist layer to 5,000 ft, represented a significantly deeper moist layer to the east of the Caprock.
A topographical map of West Texas (Fig. 2) shows the height gradients are very tight along the Caprock from the Canadian River southward to the Palo Duro Canyon. South of Palo Duro Canyon, the gradient begins to lessen, with the greater height contours extending southwestward toward the Llano Estacado. Considering a terrain with such lesser degree of slope, it can be determined that the surface heating and weak winds in the lowest 3,000 ft were sufficient to continue the mixing process south of the Panhandle. This resulted in the continued eastward movement of the dryline across this area well after the dryline had become stationary in the eastern Panhandle.
6. Topography/Dryline Phenomena Effects on Tornado Climatology
The local tornado climatology of West Texas is highlighted by a well-defined tornado maximum (Fig. 9) stretching from the east-central Panhandle to the southwest, toward the extreme northwest portions of West Texas. The existence of this tornado maximum was also noted by Kelley, et al. (1978).
A strong correlation exists between the West Texas tornado maximum and changes in elevation associated with local topographical features. The tornado maximum appears to run very close to and roughly parallel to the 3,500 ft contour. Out of 119 significant tornadoes in the Panhandle and the extreme northwest part of West Texas (Grazulis 1993), only eight percent occurred north and west of this line as it extends southwest from Palo Duro Canyon to the southeast side of the Llano Estacado. Most of the tornadoes which occur north and west of this contour are associated with higher elevations near the plateau of the Llano Estacado in extreme northwest West Texas. This seems to verify that severe, and sometimes tornadic, convection commonly initiates near the leading edge of the deep moisture layer along the Caprock and the Llano Estacado.
The area of the dryline is a favored location for thunderstorm initiation (Rhea 1966). Typically, the dryline moves from west to east during the day. Although no thunderstorms developed on May 15, storms often develop near the dryline by late afternoon. During situations similar to the one presented in this paper, the dryline is incapable of advancing beyond the increased depth of the moist layer present east of the Caprock.
As the convective season progresses, low-level moisture becomes more abundant, and the dryline retreats west into New Mexico. The winds above the surface typically weaken later in the season. This makes it difficult for the dryline to mix beyond the gradual slopes of the Llano Estacado. It is also interesting to note that regardless of the typical weakening of the upper-level winds in the latter portions of the convective season, approximately 50 percent of all the Panhandle significant tornadoes occur after May 15.
Movement of the dryline is affected by local topographical features such as the Llano Estacado and the Caprock. In this case, the dryline eastward movement halted upon encountering the deep, moist layer east of the Caprock on the afternoon of May 15, 1996. Detailed inspection of Amarillo radar data confirms that the dryline had indeed stalled along the rim of the escarpment. This is frequently observed when a surface dryline is present.
The impact of the above mentioned geographical features is evident in the local tornado climatology. The tendency of the dryline to not mix east beyond the steep elevation gradients along the Caprock and the Llano Estacado corresponds to the existence of a well-defined tornado maximum. The maximum is located very close and parallel to the 3,500 ft contour which extends south along the escarpment and then southwest to the Llano Estacado.
The influence on tornado climatology is likely one of many effects that West Texas topography has on local climatology. Other less notable features may play significant roles in local weather events elsewhere in the country. The reader is invited to pursue similar studies in his or her region.
The author would like to thank Rich Wynne (Science and Operations Officer) for his review and input into this paper.
Benjamin, Stanley G., Lanicci, John M, 1985: Effects on dryline behavior due to soil moisture and topography: numerical results from SESAME I and SESAME V, Preprints, 14th Conf. On Severe Local Storms, Amer. Meteor. Soc., Boston, 9-12.
Doswell, Charles A. III, 1982: The operational meteorology of convective weather, Volume I: Operational mesoanalysis, NOAA Technical Memorandum NWS NSSFC-5, III-61.
Fankhauser, J.C., 1971: Thunderstorm-environment interactions determined from aircraft and radar observations. Mon. Wea. Rev., 99, 171-192.
Grazulis, Thomas P., 1993: Significant tornadoes, 1680-1991, Environmental Films, St. Johnsbury, Vermont, July 1993, 431.
Kelley, D.L., J.T. Schaefer, R.P. McNulty, C.A. Doswell III and R.F. Abbey, Jr., 1978: An augmented tornado climatology. Mon. Wea. Rev., 106, 1172-1183.
Marshall, Timothy P., 1980: Topographical influences on Amarillo radar echo climatology. Master's Thesis, Texas Tech. Univ., Lubbock, TX, 65.
Rhea, J.O., 1966: A study of thunderstorm formation along drylines. J. Appl. Meteor., 5, 58-63.
Schaefer, J.T., 1973: The motion and morphology of the dryline. NOAA Technical Memorandum ERL NSSL-66, 80.
Schaefer, J.T., 1974a: A simulative model of dryline motion. J. Atmos. Sci., 31, 956-964.
Schaefer, J.T., 1974b: The life cycle of the dryline. J. Appl. Meteor., 13, 444-449.
Figure 1. Map of the Panhandle and surrounding
areas showing the Caprock and the Llano Estacado.
Figure 2. Topographical map of the Panhandle and northern
West Texas shows tight gradients relax somewhat south of
Palo Duro Canyon, as the greater isoheights slope southwest
toward the Llano Estacado.
Figure 3. The Amarillo WSR-88D 0.5 deg base reflectivity at 1908 UTC
on May 15, 1996 shows the dryline in correspondence with the Caprock.
Figure 4. The Amarillo WSR-88D 0.5 deg composite reflectivity at 1858 UTC
on May 15, 1996 magnified at 4X and overlaid with a road map showing the
Caprock (black shade), shows significant detail in the dryline correspondence
with the escarpment in the vicinity of Palo Duro Canyon.
Figure 5. The 1200 UTC surface analysis from May 15,
1996, shows a low in southwest Kansas with a dryline
extending to the southwest into southeast New Mexico.
Figure 6. The 1800 UTC surface analysis showing the
low shifting southeast with the dryline extending to the
south through the Panhandle and into West Texas.
Figure 7. The 1200 UTC Amarillo sounding indicating
a shallow moist layer, to approximately 1,000 ft AGL.
Figure 8. The 1200 UTC Norman sounding showing
a moist layer extending up to approximately 5,000 ft
AGL, indicating a significantly deeper moist layer
east of the Caprock.
Figure 9. A map of the Panhandle and northern West Texas showing
the paths of all significant tornadoes from 1950-1989 (Grazulis 1993).
A maximum area of tornado occurrence is evident, oriented northeast
to southwest, generally parallel to the 3,500 ft isoheight.