Environment
As May 30th progressed, a strong area of low pressure moved northward through northern California and into northwest Nevada. Winds aloft increased across the Pacific Northwest, including eastern Idaho as the day progressed. By the early afternoon hours, showers and thunderstorms over northern Utah began to produce outflow winds in the 60-75 mph range. This activity then moved into southern Idaho in 2 waves, with the winds from the first round moving north up the Rockland, Arbon and Raft River Valleys in both waves. Despite a marginal environment for widespread, organized severe thunderstorms, multiple reports of severe thunderstorm winds (defined as 58 mph or greater) were observed. So despite the somewhat unfavorable environment for widespread, organized convective activity, why were such widespread, severe winds observed, even from weaker activity? Plentiful dry air in the low to mid levels of the atmosphere was available, and mixing heights increased through the day to allow for the stronger winds aloft to be made available to mix to the surface. The dry air available in the low and mid-levels of the atmosphere helped facilitate this process. In addition, once showers and thunderstorms developed, the cooling effects from evaporating raindrops helped add velocity to the thunderstorm outflow winds. The relatively flat, tree-less surface of the Snake Plain is a fairly low-friction surface, so thunderstorm outflow winds can continue for miles before slowing down. The discrepancy between the somewhat marginal environmental conditions for widespread, organized thunderstorms and the resultant widespread, severe thunderstorm winds produced by the showers and thunderstorms on May 30 made this an intriguing event to document.
Near-storm Environment Data:
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Figure 1: 12z Salt Lake sounding.
Inverted V, dry air in lower areas,
with 40 knots aloft above the
inverted V. Does show a nocturnal
inversion |
Figure 2: 00z Salt Lake Sounding.
Stronger winds aloft. Inverted V with
nocturnal inversion gone. |
Figure 3: Downcape observed
values of 1600. Anything over
1000 is significant. |
Synoptic Environment Data:
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| Figure 1: 12z 300mb winds |
Figure 2: 00z 300mb winds |
Figure 3: Bulk Shear over
40 is good. They were
below 30. |
Additional Environmental data:
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| Figure 1: 12z 500mb winds |
Figure 2: 00z 500 mb winds |
Figure 3: LCL (Lifted Condensation
Level) Anything over 4000 was
significant. |
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| Figure 4: Low-level Lapse rates over 9. |
Figure 5: Mid-level Lapse rates |
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