Monday, 7 November 2016
Broadway Rooms (Hilton Portland )
In the late afternoon on the evening of May 11, 2016, a pair of supercells formed in relatively dry easterly upslope flow (temperature in the 70s F and dewpoints < 50F) along an area of surface ridging and intensified into southwest Oklahoma into the late evening hours, culminating with damaging swaths of hail up to baseball size and 75mph winds around 730z. The unusual large and highly contorted high-reflectivity radar echoes (70-80 dBZ) with surprisingly weak storm-top divergence evolved similarly throughout their lifetime, and suggested this environment favored strong low-mid level updrafts, broad moderate mid-level rotation, and a unique type of storm structure. The magnitude of this event was difficult to anticipate, as evidenced by the Storm Prediction Center marginal risk of severe wind and hail probabilities (less than 15% with no significant wind or hail expected). While the 40-50kt 0-6km effective shear clearly supported supercells, the marginal instability (MLCAPE 250-1000 J/kg during the daytime and the elevated nature of the MUCAPE < 500 J/kg during nighttime above north-northeasterly surface winds) made this an unusually challenging nocturnal forecast. While numerous mesoscale discussions were issued on this event, no severe weather watch was issued due to the isolated threat and marginal nature of the environment. With the strong hail and wind signatures observed by radar, the severe thunderstorm warning decision making was more straightforward, though providing lead time on the significant severe hail and wind was also challenging. The U.S. climatological reports record of significant nocturnal hail baseball size and larger suggests the peak intensity of this event coincided with the start of the 10-hour diurnal minimum in hail reports which further illustrates the unusual nature of this event. Reviewing the available online SPC mesoanalysis from 2006-2015 for 23 storms from 6-7z (time of May 11, 2016 significant severe hail reports) shows a range of environments with some similarities (elevated lifted parcel levels > 1000m, MLCIN > 100 J/kg, 0-6km shear > 40kts, most associated with a boundary that is sometimes strong, some with no significant severe anticipated by SPC) and some notable differences (most other events have convective initiation within 3hrs of the report and are not long-track daytime-initiated supercells as in this event). This study focuses on a rarely studied and challenging to forecast part of the significant severe weather parameter space and climatological time of day, and it provides some insight into some of the unique radar and environmental characteristics observed.
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