Severe Weather Discussion for October 12 -13, 2012.
– A few severe thunderstorms are possible in far southwest Oklahoma later today, however the main impacts will be from general thunderstorms across western Oklahoma.
– Active severe weather day is expected on Saturday with tornadoes, destructive hail, and wind all possible. Main issue for Saturday will be early morning convection and subsequent surface heating during the day.
Already messy situation for Saturday just got a heck of a lot more complex. However, potential remains for a medium-end event across parts of Oklahoma.
For today, the severe weather threat is mostly confined to the Texas panhandle. 19z satellite images shows the long awaited southern stream closed low meandering eastward through far southern Nevada. In Oklahoma, an outflow boundary has moved south to I-40 from Oklahoma City westward to the Texas panhandle. 19z mesoanalysis shows CAPE values AOA 2000 J/Kg south of this boundary and in eastern Oklahoma. Mesonet indicates a tightening temperature gradient as clouds clear south of the boundary. Tower camera shows the boundary just south of downtown Oklahoma City and appears the southward push is decreasing. However, the boundary still appears to oscillate north/south as the cloud deck lowers and then increases per time lapse video. This outflow boundary was not in model progs, although last night’s convection was well noted. My thinking is this boundary will be a major player tonight into tomorrow.
The forecast from this point forward seems to point to elevated hailers in the Texas panhandle, north of the boundary with any surface based activity near Lubbock. Now, the boundary should weaken and start lifting northward, which may allow for surface based convection a little farther north than is currently indicated. SPC seems to be allowing for this given the current placement of the slight risk. 20z Day 1 is essentially unchanged and looks good. They did nudge it into far southwest Oklahoma, probably owing to the previously mentioned boundary. Primary threats remain damaging hail and wind, but a few tornadoes are possible in far west Texas.
Looking into Saturday, the forecast models are of little help. The upper system has now been well sampled in the upper air network and this is likely the reason for such good agreement between the NAM/GFS. This agreement yields the upper system moving into western Oklahoma after 7pm. Prior forecasts had the system in eastern Kansas at this time. Along with the much slower movement of the upper system is also much slower movement of the surface features. Aside from the ongoing outflow boundary, the main players should be a dryline along the OK/TX panhandle border tomorrow afternoon and an incoming cold front early Sunday.
The new placement of the upper system and timing puts all of Oklahoma under a threat for severe weather on Saturday. The usual players will appear on-stage: directional/speed shear, lift, and plenty of moisture. The iffy part has been and continues to be early morning convection and subsequent impacts on surface heating. The round of thunderstorms this morning and associated cloud cover continues to reinforce the boundary over I-40. Indeed, recent Mesonet data shows 74F at Crossroads Mall and 69F at the Capitol Complex. Models do want to lift this boundary northward out of the state but overnight convection or attempts therein may provide microscale boundaries on Saturday.
After all of that the standard disclaimer applies, if the atmosphere can warm on Saturday then an active severe weather day is likely. On the other side, if widespread convection occurs, then severe thunderstorms remain likely but the threat will be modulated downward to large hail and damaging wind. Both, as the 18z NAM is now hinting at, we may have two rounds. Round one, lights off around 11am and moves quickly northeast, reaching northeast Oklahoma by 4pm. Back west along the dryline, where there should be sun, CAPE values approach 1750 J/Kg and round two seemingly begins in western Oklahoma.
All this being typed, the best opportunity for chasing will be southwest Oklahoma. Could end up being northwest Texas but not sure I’m gonna make that leap yet. Tomorrow is one of those days I’m glad not to work for SPC. Good luck to them on how to handle the risk area. Obviously a slight risk is a given, if they go to a moderate, really tough. I’d think we’ll see a large slight risk with higher probabilities on the early outlook and then fine tuning during the day. If an upgrade happens, it will likely be the 16:30z outlook.
And, apparently I was amiss for not mentioning the OU/Texas game tomorrow on yesterday’s discussion. Models agree that Dallas should remain dry during the game. Storm chances increase during the evening hours with severe weather likely.
Risk area for any part of OK/TX (valid: 10/13 Day 1):
Level 2 – Looking to head southwest on Saturday, maybe Lawton/west of there. Mesoscale analysis Saturday afternoon will be the key on what we do.by