Tuesday, February 1, 2011

More storm discussion

The plot thickens. Not only are subsequent model forecasts increasing the snow for central OK, but the global model has backed way off. The SREF was going crazy with anywhere from 1inch to 22 inches of snow for Norman as of 2100 UTC. The operational NAM was going for 24 inches, and the  GFS was going for 1 inch. Talk about uncertainty. The funny thing is that the NAM gets its boundary conditions from the previous GFS model run.

I speculate that the width and depth of the cold air helped get the GFS a good initialization and as a result a good depiction for the NAM. But maybe that cold air depiction was projected onto too large scale such that the ingredients for this event were misaligned. Also, the NAM ingredients may be too well aligned such that convective feedback may have been playing a role in creating a lot of precipitation but was able to simulate the banded features well.

As of 9:25pm there is the lovely, heart pumping sound of thunder, big lightning streaking across the sky, and ice pellets! Some of which could count as hail. I had to take a break so I could enter some observations for the W-PING project at NSSL. On a related note station KPVJ in OK reported 0.25" of liquid equivalent in an hour (6.3 mm/hr). Impressive rate. If only we had data for the precipitation rates at all mesonet stations!

It looks like the main issues of the day were (in no particular order):
1. where would cold air be, its depth, and the depth and magnitude of the warm air layer aloft,
2. where would thunderstorms initiate and then travel over,
3. as a result of the position of the cold air, where would the surface low be,
4. and related to 3 where would the maximum divergence aloft be (horizontally coupled jets).

I looked at the SREF placement of the Upper Level Jet (ULJ) and looked at the diagnostics from SUNYA* (irrotational and non-divergent wind at 300 hPa; the 850 q vectors with the along and cross front components of the vertical motion). This event looks very similar to east coast snow storms with coupled jets. In this case the irrotational wind center was intense over OK nearly tripling in value over a 6 hour period (18 to 24 hr forecast) from this mornings NAM. The along front forcing for ascent was forecast to come through in 2 waves ... the leading one much weaker than the 2nd. Aloft the forcing for ascent is forecast to be more widespread and last longer into tomorrow late morning.

There were hints that a coupled jet structure was developing at 0000 UTC this evening. But closer inspection of the soundings suggested that a the true jet was near 200 hPa as evidenced by the 150 knot winds near 210 hPa at LZK and the 120 kts at SGF and 135 at LMN and OUN at about 145 kts. Only 0600 UTC balloons at TOP and LMN could help identify if a coupled jet feature did develop.

*Thanks to Kevin Tyle for trying to keep SUNYPAK alive!