8.5 A Preliminary Look at Well-Developed Vorticies near and behind the Monsoon Frontal Boundary Area during the Mid-July through Early August 2017 Monsoon Outbreak in Southern California

Wednesday, 10 January 2018: 2:45 PM
Room 19AB (ACC) (Austin, Texas)
Ivory J. Small, NOAA/NWS, San Diego, CA
Manuscript (4.3 MB)

For much of the period 19 July through 4 August 2017 an “East-West Ridge Axis to the North” pattern developed, which easily allowed a multitude of convection-enhancing features to develop and move westward beneath it into Southern California. During this time, the southwest edge of the monsoon frontal boundary and moisture gradient extended through California, resulting in severe weather and flash flooding. On 19 July, 23 July, and again on 1 August rather impressive-looking small vortices as well as the larger Mesoscale Convective Vortices (MCVs) associated with night and morning convection, moved through California and northern Baja California, then out to sea. Inland, convection was developing on outflow boundaries during the nights and mornings, then moved southwest through Southern California. Outflow boundaries and various vortices seemed to race southwestward on convergence zones (some convergence zones were made up of convective cells), and outflow related cells also occurred in just the moisture-rich environment itself without encountering lines of cells to enhance. On most days prior to 1 August, moist unstable flow interaction with terrain occurred (including outflow boundaries), and fairly weak afternoon and evening surface heating/terrain driven showers and thunderstorms developed in the typical diurnal fashion, which basically became more and more prevalent with each event. Even more vortices were noted after 1 August.

On 19 July and 24 July, near the beginning of this monsoon period, a small wave and a deformation zone very close to the monsoonal front generated showers and thunderstorms that extended from just southwest of Southern California to over Southern California. A pair of lows off the coast and the small wave likely created optimal wind patterns and moisture gradients along and near the monsoonal boundary, so exceptionally well developed vortices occurred. Conditions may have been more favorable for even very small vortices to develop under relatively marginal conditions in Southern California (however, conditions seemed exceptionally strong for supporting convection and easily sufficient for thunderstorms based on the instability). On 1 August large inverted troughs began enhancing the monsoonal boundary moisture, instability, and forcing for severe weather and flash flooding. These inverted troughs showed up as huge circulations on the monsoonal boundary. This monsoonal event ended the evening of 4 August, when westerly flow pushed the monsoonal boundary to the east with drier air moving in. In this study, an inspection of the numerous waves, potential causes, and detection strategies will be presented. Although initially considered to be preliminary, non-operational data, some information can be gleaned from GOES-16 data, so some thoughts on GOES-16 possibilities will be included. Other tools will also be utilized to further analyze these very interesting weather phenomena and the conditions associated with them.

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