Tornado Central

I uploaded some pictures and a brief narrative of the recent ice storm in Oklahoma at http://www.tornadocentral.com/events/icestorm07.php . Alternatively, this page can be accessed by clicking on "Other" on the left, then clicking the appropriate link. Just FYI.

I continue to see mixed freezing rain and sleet. Most of the day, I'd say the predominant precipitation type at my house (on Waterloo E of I35 along the Oklahoma / Logan county border) has been sleet (IP), though we did have heavy enough liquid precip about 30 minutes ago that it started overflowing the gutters (granted, they're clogged with leaves that I haven't had time to remove, but it's interesting nonetheless). I'd say there's ~0.4" of glaze on the grass and branches, but the solid surfaces (i.e. my concrete driveway) has a good deal of sleet on it, so walking isn't too bad (the sleet acts to bite into the glaze ice a bit, giving a little bit of traction).

The Missing or Calm wind obs on the Mesonet can most likely be attributed to frozen anemometers. I just took a stroll down the road, and I'd say sustained winds are 5-10 mph.

I'm quite fascinated by the persistance of the quasi-stationary band of precip that started about 24 hours ago along I44. Asphalt roads are still largely just wet, not surprising given 2" and 4" bare soil temps in the 34-36F range (see http://agweather.mesonet.org/soil/default.html ). The freezing of the roads will be slowed a bit as the freezing process itself, as I'm sure we all know, releases energy (i.e. latent heat of fusion slows the freezing process). I'm still looking to get out tomorrow morning to take some pictures. Some of my favorite winter landscape pics involve glaze ice.

FWIW, news is reporting 100 auto crashed since last night, with 8 fatalities (a count that doesn't include a multi-fatality collision in Okemah along I40 that closed that interstate). Enhanced rainrates expected given possibility of convection yet again tonight. The 00z OUN sounding (http://www.spc.noaa.gov/exper/soundings/07121000_OBS/OUN.gif ) showed ~700 j/kg MUCAPE, with a very impressive warm nose (~10 C at 850mb... in December!). Interestingly, there's also ~45-50 kts of effective shear (~35kts at updraft base / 850mb / increasing to 85kts at the equilibrium level / ~300mb) which may promote a little organization to some of the storms.

KTLX base velocity data at 0300 UTC indicate that the cold air is back down to near 2800-2900 feet in depth. If I recall correctly, the cold air deepened earlier this morning to near 4000 feet, so it's decreased in time lately. This is not surprising given the strength of warm air advection just above the cold dome. Eventually (most likely by tomorrow afternoon), the freezing line should move back northward. Of course, it's important to note that models have a strong tendency to underforecast the speed and intensity of these shallow arctic air intrusions, so I wouldn't be surprised to see subfreezing temps hold on a little longer than models forecast.

I'm always fascinated to see moderate convection when surface temperatures are below freezing. A look at the Spencer, OK, mesonet meteogram (Spencer is just northeast of OKC), shows that it picked up 0.5" of rainfall in a 20 minutes time span near 1am. The temperature at the time was around 30F, and the temp has continue to fall to a current value of 28F. Not surprisingly, the anemometer appears to have been frozen, given that it's currently not reporting any winds. For ASOS/AWOS and mesonet sites, calm or missing winds are usually good ways to find where the precip has been falling as freezing rain (assuming ambient/environmental winds are >3 mph). A glance at surface obs on the OK mesonet show about half a dozen sites with missing winds (at a time when most wind obs in central and northern OK are 10-15 mph sustained).

FWIW, temperatures have also fallen into the teens in far northwestern OK, with 8-15 mph NNW winds likely continuing to bring in low-mid 20F temps closer to I40. These very shallow arctic fronts have a tendency to be underforecast by most models (both in intensity and speed), so this isn't too surprising.

07 UTC SPC/RUC mesoanalysis also continues to indicate >500 j/kg MUCAPE, supportive of a continued risk of thunderstorms and upright convection. The band of heaviest precip just N of I44 east of I35 in northeastern OK appears to be in an area of decent frontogenetical forcing (and the fun ageostrophic accelerations associated with such forcing). The 850mb cold front may be a good marker for the freezing rain - snow transition zone (or liquid - frozen transition zone).

The VWP from KTLX valid 0720 UTC suggests that the top of the cold layer is ~1800 ft AGL (I'm using the 2.5 degree base velocity data to pick that out easily; KINX / Tulsa / data suggests a similar depth). The cold air deepens to >3000 ft above the KVNX radar, pretty much in line with velocity data from KICT. Given this, I wouldn't be surprised to see some sleet mix in with the freezing rain closer to the OK/KS border west of I35. Well, maybe not, but perhaps close... Same mesoanalysis shows the 850mb front located near a line from Shamrock TX to Emporia KS to the NE MO / SE IA border area.

EDIT: It's important to note that, per OK Mesonet (see HERE), current 2-in and 4-in bare soil temperatures are in the mid 30s in most of northern OK. As such, the precip is not likely to freeze upon impact of roads. Elevated surfaces that tend to cool to ambient temperature more quickly (e.g. powerlines, trees, bridges) are likely icing up rather rapidly right now N of I44. I imagine that it may take until tomorrow night for most areas to see ground temperatures (at least 2" or 4" below bare soil) reach the freezing mark.

EDIT 2: OGE System watch (HERE) is showing >1000 customers w/o power in eastern OKC. Of course, in the scope of things, this isn't a large number. However, I think this is only a hint of what's likely to occur in N of Tulsa through the morning. Latest TLX data shows convection and showers are redeveloping in the quasi-stationary zone along I44, from near Chickashaw to Norman to Stroud and Bristow. My house in Arcadia is currently between this band to the south and the intermittent activity nearer Perry and Guthrie. *shrug*