Thursday, July 29, 2010

multiscale convection initiation 29 July 2010

Tis the season for CI close to the mountains. I have been saving the water vapor image for 0645 UTC so I can look at the monsoon season over AZ and MCSs over the midwest for the season in its entirety. The other evening I saw that the WV imagery depicted some weak mountain generated convection in Northern CO and Southern WY. This is a late season hot spot for convection (proof required, so this is anecdotal at the moment). The story unfolded at the surface as simply southeasterly flow with dew points in the upper 60's. At 850 hPa the situation could be described  as a moisture pool along a warm front.

In any case, it didnt look dynamic until 300 hPa, where an upper level anticyclonic shear zone was situated along the NE SD border, with an anticyclonically curved jet streak approaching. So the situation unfolded as short-lived convection in a pretty humid pouch failed ... drifted eastward and by 0845 UTC CI was underway in this pouch. It was not a big event but did drop copious amounts of rain in the FSD area in eastern SD over into Storm Lake in Iowa.

This appears to be an established corridor for storms this evening. Some could argue it is intimately tied to the developing monsoon moisture stream. Tonights storms initiated over the Rapid City area (mountain generated). They have currently split with a slow moving portion of the high cloud tops over NE and drifting slowly SE, while further to the east in SD the storms are just slightly north of last nights storms.

Storm reports are absent in this corridor with the exception of tonight, so this type of event would only stand out as being different in terms of the precip. The NSSL model run handled this quite nicely though the second diurnal cycle appears to be extreme ... this case could be examined quite well.

Hopefully these types of events don't get ignored since the storm report numbers were low. How was convection initiated? What were the dynamical controls, and how much did the upslope, moist flow at the surface contribute, and what role did pre-existing convection play and how did it manifest itself (humidification or boundary development)?