The dryline, and other boundaries radar can see

A dryline is a boundary at the ground between moist air and much drier air, most common running north to south across the southern and central High Plains in spring and early summer, where it is a favored starting point for severe thunderstorms.

The outlooks and forecast models guide gives the professional shorthand for a model forecast: “discrete cells developing along the dryline in the late afternoon.” This page covers the boundary in that sentence, and the other low-level boundaries, outflow boundaries above all, that decide where storms start.

What a dryline is

The National Weather Service glossary defines a dry line as “a boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains.” Hoch and Markowski (2005), in the Journal of Climate, trace the two sides to their sources: moist air from the Gulf to the east, dry air from northern Mexico, eastern New Mexico and western Texas to the west. They describe the dryline as essentially the line where the top of the moist boundary layer meets the ground, which is why it runs roughly north to south, across the slope of terrain rising toward the Rockies.

How sharp it is varies a great deal. The same paper notes dewpoint gradients above 10 K in 1 km have been observed, while on other days the change is spread over 100 km. Using a moisture gradient of at least 3 g/kg per 100 km, they counted how often drylines appear on 0000 UTC surface analyses:

The definitions differ, so this is not a trend. Hoch and Markowski found the frequency peaks in mid to late May, above 40% of days, and falls to about 20% by late June. The favored position at 0000 UTC is near 101°W, drifting west as spring goes on.

Why it moves east by day and west by night

The NWS spotter glossary: “The dry line typically advances eastward during the afternoon and retreats westward at night.” Joseph Schaefer’s 1974 life-cycle study placed the moving dryline at the surface projection of the western edge of the low-level inversion, and his numerical model the same year reproduced observed dryline motion and concluded that “vertical mixing is the primary cause.” As the ground heats, turbulent mixing reaches down through the moist layer where it is shallowest and replaces it with dry air from above, so the line shifts east. Forecasters use the verb. The Storm Prediction Center’s 1630 UTC Day 1 outlook on 6 May 2024 expected that “a dryline will mix east into western OK by late this afternoon.”

Two qualifications. First, the glossary adds that a strong storm system can sweep the dryline into the Mississippi Valley or beyond at any hour; Hoch and Markowski found virtually all dryline passages east of about 97°W come with a migrating cyclone. Second, the line does not advance evenly. A dry-line bulge is “a bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels,” and the glossary notes that “severe weather potential is increased near and ahead of a dry line bulge.”

Why storms form on or near it

The glossary states the pattern: “Severe and sometimes tornadic thunderstorms often develop along a dry line or in the moist air just to the east of it, especially when it begins moving eastward.” Whether the lift along the boundary makes a storm is the hard part.

Ziegler and Rasmussen (1998), from aircraft and mobile observations on three field-project days, found cumulus most likely about 15 km east of the dryline. Their conclusion was that moist air must be lifted to its condensation level and its level of free convection before it leaves the mesoscale updraft, so vertical shear and dry layers aloft can stop clouds that the instability numbers say should grow. A sounding read for CAPE and inhibition alone, they found, has “less prognostic value than is conventionally assumed.”

Atkins, Wakimoto and Ziegler (1998) examined a Texas Panhandle dryline on 6 May 1995 during VORTEX. Horizontal convective rolls west of the line crossed it at intervals, producing reflectivity and vertical velocity maxima, and “clouds initiated at these intersection points.”

In surface observations

The Hydrometeorological Prediction Center’s Unified Surface Analysis Manual uses a dewpoint gradient of 14 °C (25 °F) over a tight distance, or 17 °C (30 °F) more broadly, to help decide a dryline exists, and labels it DRYLINE on the map. The NWS spotter glossary lists what a passage looks like at one station: “a sharp drop in humidity (hence the name), clearing skies, and a wind shift from south or southeasterly to west or southwesterly,” with blowing dust and rising temperature possible in daytime. When the line retreats, the changes come in reverse.

On radar: the fine line

With no rain nearby, boundaries often appear in reflectivity as a narrow line of weak echo. Wilson and Schreiber (1986), studying convergence lines east of the Colorado Rockies, described them on Doppler radar as “thin lines of enhanced reflectivity between 0 and 20 dBZe,” with a strong gradient in Doppler velocity, and “observed even in clear air in the absence of any clouds.” The convergence was concentrated in a zone 0.5 to 5 km wide. What scatters the energy, mostly insects gathered by the converging wind, is covered in the shelf cloud guide.

Drylines produce the same kind of line. In the 1995 case, the Amarillo WSR-88D recorded the along-line variation Atkins and colleagues analyzed. Range works against the signature: the scatterers are concentrated in the boundary layer near the ground, and by a 4/3-earth calculation the 0.5° beam centre is already about 1 km up near 76 km from the radar.

Outflow boundaries

The NWS spotter glossary defines an outflow boundary as “a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front.” Outflow boundaries “may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles.” The gust front is the leading edge of that outflow while it is still fresh and gusty; its cloud and its radar signature are set out in the shelf cloud guide. The glossary adds: “New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary.”

The evidence that boundaries decide initiation is strong. Of 418 storms that formed in Wilson and Schreiber’s study area, 79% began close to a radar-observed convergence line, rising to 95% for storms of 60 dBZe or more. The most common identified origin of those lines was storm outflow. Where two lines collided, new storms formed or existing ones intensified in 71% of cases, and a new storm took a median 24 minutes to reach 30 dBZ. The glossary names the intersection a triple point, “often a focus for thunderstorm development.”

Boundaries matter for tornadoes too. Markowski, Rasmussen and Straka (1998) found that during VORTEX-95 “nearly 70% of the significant tornadoes occurred near low-level boundaries” that were not the storms’ own flank downdrafts. Most occurred on the cool side, generally within 30 km. They suggest the boundaries likely augmented the horizontal vorticity the storms drew in. A supercell crossing an old outflow boundary is a reason to look harder at its mesocyclone, not a tornado detection.

WeatherStory Radar decodes reflectivity and velocity from full-resolution Level II and shows METAR surface observations over the map, so a fine line can be compared with the stations on either side of it.