Amarillo Wake Low

An unusually strong period of damaging winds occurred during the early morning hours of June 28, 2011 across the southern Texas Panhandle. What was even more unusual was the fact that these winds occurred in the stratiform precipitation region behind a convective complex that stretched from near Pampa to southwest of Clarendon. Damaging wind gusts as high as 69 mph were measured at the Amarillo ASOS, and these strong winds even produced damage across parts of the city. Quite possibly the most remarkable stat from this event is that sustained severe wind gusts occurred continuously for almost an hour. It is believed that these damaging winds were associated with a wake low event.

0600Z Surface Observations

0700Z Surface Observations

0800Z Surface Observations

Observations at 0600Z indicate a rain cooled air mass associated with convection moving across the southwestern Texas Panhandle. The temperature and dew point at Amarillo were 78 and 56, respectively along with a MSLP of 1014.7 mb. By 0700Z, strong convection was moving across Amarillo with the evaportive cooling taking place as evidence by the temperature dropping to 64 and the dew point rising to 64. It was during this time that a mesohigh had developed immediately behind the strongest convection. In fact, the MSLP had increased 4.1 mb in a hour to 1018.8 mb. By 0800Z, inexplicably, the temperature jumped to 71 and the dew point dropped to 56. At the same time, the pressure dropped a whopping 5.5 mb to 1013.3 mb and winds increased significantly in a small corridor just east of Amarillo.

Pressure & Wind Time Series

Temperature & Dew Point Time Series

The changes in temperature, pressure, and wind are best seen through these time series charts. The rapid fall drop in pressure between 0700Z and 0800Z corresponds to a sudden increase in sustained winds and gusts. The wind gust peak at 69 mph around 0753Z and again at 0841Z. At the same time, the temperature rose 7 degrees and the dew point dropped 7 degrees.

This data is consistent with other case studies (Handel and Santos, 2005) and numerical modeling of wake lows (Johnson, 2001). As described by Johnson and Hamilton (1988), wake lows form within the stratiform precipitation region behind the strongest convection. Within this area, evaporational cooling is not able to balance the adiabatic warming that occurs due to the descending rear inflow jet. This can be explained further by comparing the rainfall rate in the region of strongest convection to the stratiform region. Rainfall rates will likely be much higher in the strongest convection, thus increasing the potential for strong evaporative cooling. Meanwhile, the rainfall rate in the stratiform region is much less, which results in less potential for evaporative cooling. In addition, the ambient air within the stratiform region has usually already been evaporatively cooled to a significant degree by the preceding convection. Therefore, the effects of evaporational cooling within the stratiform region are much smaller than compared to the strongest convective region. Within the stratiform region, evaporational cooling is unable to offset the effects of adiabatic warming incurred by any descending air, which can produce locally enhanced dynamic pressure gradients, increased wind speeds, and warmer temperatures.

0600Z LAPS Sounding from Amarillo

0700Z LAPS Sounding from Amarillo

0800Z LAPS Sounding from Amarillo

LAPS soundings show the lower tropospheric warming associated with the wake low between 0600Z and 0800Z. Between 0600Z and 0700Z, this warming is most pronounced between 825 mb and 720 mb with temperatures around 800 mb almost 2 degrees C warmer by 0700Z. Additional low-level warming takes place at 0800Z, and temperatures around 800 mb are almost 3.5 degrees C warmer than at 0600Z.

0801Z IR Satellite Image & Surface Observations

It is interesting to observe infrared satellite imagery where a localized area of warmer cloud tops was most pronounced from southern Randall County to western Carson County. This warm pocket also coincided with an area of higher temperatures, lower dew points, and stronger winds.

0739Z 0.5 deg KAMA Base Reflectivity

Base Reflectivity Cross Section

Base Velocity Cross Section

0747Z 0.5 deg KAMA Base Velocity

Even within the stratiform region, very little precipitation was occurring at the surface or aloft in the area of strongest winds. However, base velocities were extremely impressive, especially in the lowest 2,000 feet ARL. In the bottom image above, 60-68 kt inbound velocities were even sampled around 120 feet ARL. These base velocity winds were likely quite accurate as target motion was moving parallel to a radial.

How can wake lows be forecast? Well, it's virtually impossible to predict the formation of these phenomena in advance, but an understanding of how they form and the circumstances they form in can help forecasters to quickly recognize the potential for wake low development. Attentive monitoring of observational and radar data is the most critical element to the detection and diagnosis of wake lows.

Forecasting Severe Hail

This blog post will attempt to describe thermodynamic and dynamic characteristics I look for when forecasting severe hail. Some factors, such as storm microphysics and kinematics, are currently impossible to analyze/forecast but play an equally large role in governing the threat for severe hail.


Buoyancy: The environment was moderately unstable with a 2,435 J/kg of SBCAPE and very little SBCIN (-2 J/kg). Although the CAPE profile is fairly deep, it is rather thin. Within the hail growth zone, there appears to be about 820 J/kg of CAPE.

Deep layer shear: Despite relatively weak winds in the lowest 4 km, the degree of veering and the stronger winds near 6 km more than make up for this. 0-6 km shear of 37 kt is more than adequate. Even stronger winds are noted at 7 and 9 km.

Mid-level lapse rates: A 700-500 mb lapse rate of 6.2 C/km is respectable for a tropical environment. However, the lapse rate within the hail growth zone is closer to moist adiabatic.

Freezing level: 14,381 feet is rather high and likely allowed for more melting. The deep moist layer from the surface through ~650 mb also increased the melting potential of hail stones.

Overall threat: The threat for severe hail is pretty high based on this sounding. The buoyancy alone should allow for at least some threat for severe hail. However, the decent deep layer shear and large CAPE in the hail growth zone certainly increase the hail threat. Some negative factors for significant hail (2" or larger in diameter) are the weaker lapse rates in the hail growth zone, the high freezing level, and the moist profile from the surface to about 650 mb.

Summary: A few reports of hail up the golf ball size were reported in Broward County. This is extremely rare for the middle of June, but the thermodynamic and dynamic environment was supportive. Additionally, storm microphysics and kinematics were likely very favorable for severe hail production.

It should also be mentioned that this sounding profile was also quite favorable for wet microburst production. Indeed, several severe wind gusts were reported, including a 64 mph gust measured at Weston, FL.


Severe Thunderstorm Rocks Western Broward Metro -- WFO Miami