Wednesday, May 12, 2010

warm front MCS

So I fell asleep last night and missed out on real time watching of the Mesoscale Convective Systems that formed. May 11-12 2010 .

I am always fascinated by the different evolutionary modes of convective initiation. Yesterday's mode was scattered convection under a moderate shear-high instability day. But it was effectively capped, given that multiple elevated supercells formed and quickly dissipated much to the chagrin of VORTEX2 participants. It was a classic Iowa warm front event, with little to no forcing but shear and instability abound.

at 2200 UTC, the first supercell was in the process of initiating in SW OK and after 1 hour falls apart. It skirts an area to the north just south of woodward I believe, and another set of CI takes place between 2345 and 0015 UTC. Again short lived supercells. However, the anvil aloft is cold and the cloud does not dissipate. So it looks like a storm is there, but on radar it was a useless blob of 30 dbz echo.

By 0145 the warm front in SE KS gets hot and explodes into the primary MCS. This initial convection gets going and after perhaps 45 minutes to an hour backbuilds to the west along the front while that old cloud shield drifts in by 0402. Not much longer later the old cloud shield is rapidly dissipated around the MCS as it swells to maximum size virtually covering MO by 1100 UTC.

at 0345 UTC SW KS sees another MCS initiate and suddenly from KS into NE convection is firing up. This MCS looks asymmetric almost half circle like but looks very similar to an MCC. Once these two systems get close enough to each other the eastern MCS appears to dissipate, at least its western end. The radar reflectivity pattern was chaotic in nature. This is a different breed of cloud system most notably MCC like. It doesnt come with much severe weather oddly enough given its environment. But the cloud shields are huge.

After looping tonights water vapor imagery .... the eastern MCS traveled from IL through VA out over the Atlantic. I noticed a pretty compelling anticyclonic gyre aloft. very classic long lived system.

Also note that CI occurred on both sides of the water vapor dry slot ... to the east failed and to the west got going.

The surface data doesn't look that compelling .. where the warm front convergence was occuring in SE KS is where CI took place along the front. beyond that moisture was slowly working north (implied moisture convergence). Both systems were on the anticyclonic shear side of the upper level jet which was just by position only since the wind field aloft was pretty homogenous.

Soundings from LMN indicated the cap had weakened boasting CAPE of between 2-3000 J kg-1 at 2100 UTC. The wind sbacked at 0000 UTC and the low levels became saturated as the cap layer seemed to strengthen a bit. though perhaps the lapse rate steepened under slight cooling aloft. 6 hours later the layer below 850 had a nasty 50 kt LLJ and warmed and moistened appreciably. moisture depth to the east was high, as the low levels appeared to be saturated.

visible imagery indicated some sort of cloud boundaries, like elevated outflow boundaries that might be implicated here. But I dont have access to the OK mesonet data to see if anything showed up there. In my experience it doesnt show up on the sfc winds, but it might be in the pressure field.

Interesting day. Maybe V2 will have some photogrammetry of these supercell initiations.