Factors that Lead to the Development of Elevated Daytime Thunderstorms - April 27, 2000

Introduction

Thunderstorms developed over Eastern Oregon the afternoon of April 27, 2000 and quickly spread across Eastern Washington, and North Idaho. The thunderstorms moved north at 50 mph and produced a tremendous amount of lightning, heavy rain, and pea to dime sized hail. Despite the rapid movement of storms, winds gusts associated with the thunderstorms were under 20 mph.

The weather pattern on April 27^th was typical for thunderstorm development in the Inland Northwest. The set up was the approach of a slightly negatively tilted trough with strong upper level forcing for rising motion. Even though the weather pattern was common for thunderstorm development, there were several factors that made this a unique storm system for the Inland Northwest. Among the more interesting aspects was the elevated nature of the thunderstorms, despite the development of storms during maximum heating. This overview will detail the forcing mechanisms that helped produce the storms as well as additional interesting aspects of the event.

Synoptic and Mesoscale Environment

A deep low pressure system was slowly moving northeast toward the British Columbia coast on April 27, 2000. A short wave or upper level disturbance was rounding the base of the trough the morning the 27^th. As this short wave swung around the trough and toward the coast, the upper level trough began to take on a slight negative tilt, which increased the rising motion due to strong upper level divergence. Meanwhile, a jet streak was heading north along the Oregon coast. The influence of this jet can be seen in water vapor imagery (right) by the enhancement of cirrus over Washington between 4/27/00 21z and 4/28/00 01Z. The passage of the jet streak through Washington ahead of the trough, enhanced the upward motion over Eastern Washington and North Idaho and lead to the development of thunderstorms. The lagging short wave trough eventually pin-wheeled into Eastern Washington and North Idaho overnight on the 27^th and brought widespread showers to the area.

At the surface, a cold front was pushing across the Cascade Crest around 4/28/00 00Z. A stationary surface trough was also situated across central Washington and generally extended from near Moses Lake, WA to Pendelton, OR around 4/28/00 00Z. Easterly flow on the east side of the surface trough produced weak cold air advection while good surface convergence existed along the trough axis. In the mid levels, strong southerly flow existed, which resulted in an impressive vertical wind shear profile in Eastern Washington.

Even with easterly surface flow and weak cold air advection at Spokane and Pullman, surface dew points rose steadily through the afternoon (see surface observations left). The relatively high dew points resulted in surface-based CAPE of 500 to near 1000 J/KG, with the highest CAPE values over the Washington Palouse. Despite strong surface convergence, relatively warm 850-700mb temperatures and weak surface cold air advection prevented the development of surface based cumulus clouds through the evening. As a result, the 500 to 1000 J/KG of surface-based CAPE was not utilized on the 27^th.

Instead, altocumulus castellanus developed. The development of these clouds were associated with steep mid level lapse rates and a jet streak that likely helped increase the instability via deep vertical motion. The castellanus quickly developed into thunderstorms with cloud bases near 10,000 feet AGL (cloud to ground lightning strikes are noted by the cyan markings on the visible imagery above). Thunderstorm development occurred and was aligned with the region of greatest middle level potential (convective) instability, as seen by the decrease in equivalent potential temperature between 700 and 500 mb (see AVN forecast of differential equivalent potential temperature). Surface based cumulus clouds were not observed in the vicinity of Spokane through the afternoon and evening (the picture at the top of the page shows the high based thunderstorms as they moved toward the NWS in Spokane, WA). Thunderstorms tops on the 27^th peaked around 33,000 feet AGL while cloud bases were around 10,000 feet AGL. The storms were steered by the 700-500 mb winds, which were southerly at 50 mph. Despite the rapid movement of the storms, strong surface wind gusts were not reported (another indication the thunderstorms were not surface based). Despite the fairly shallow nature of these storms, 0.25 to 0.50 inch sized hail was produced by several storms. One isolated thunderstorm produced 0.75 inch sized hail in eastern sections of Spokane. A modified sounding for Spokane, based on lifting from 700 mb, predicts the observed conditions (see modified RAOB and observed RAOB ).

Radar

Conclusions

The approach of a slightly negatively tilted the afternoon and evening of 4/27/00 was classic for thunderstorm development. This pattern provided Eastern Washington and North Idaho with good upper level forcing, including 200-300mb divergence associated with a jet streak and 500-300mb Q-vector convergence. Near surface conditions were not favorable though. The most significant inhibiting factor was low level cold air advection. This prevented deep, surfaced-based, moist convection (see 4/28/00 initialized 00Z Eta for depiction of significant features). Amazingly, the observed 4/28/00 00Z Spokane RAOB indicated the boundary layer was only 25 mb deep. Strong 700-300 mb upward motion was a critical factor that lead to the development of elevated thunderstorms. Mid level instability that was in place over Eastern Washington and North Idaho prior to thunderstorm development was another important parameter. However, the deep vertical motion likely played an important role in increasing the mid level instability further. Based on the evolution of events on 04/27/00, forecasters should realize that upper level forcing (associated with a negatively tilted trough) combined with middle level instability may be sufficient for the generation of strong to severe thunderstorms.

Ironically, the same 850-700mb temperatures that inhibited the development of surface based cumulus prevented moisture that was evaporating from the ground from mixing out. This is likely why surface dew points rose through the day while low level easterly flow existed.

By Don Moore and Paul Bos