Severe Weather 1/22/12

One of the things I wanted to do more this year was weather posts. However, it’s late and I’m busy all day today, so this post will not be very detailed. It’s really just going to briefly discuss some nuances about the tornado setup today. Actually, most of my posts will probably be like this. Whatever.

1. I think initiation will occur fairly early — around 21Z. We have an intensifying trough going negative tilt, and northern AR/southern MO will be on the left-exit region of a 120 kt upper jet. The SPC slight risk should be shifted west to account for this and the fact that the 06Z NAM continues to trend slower with the upper air energy. Initiation might even occur west of Little Rock.

2. Everything north of I-40 will likely line out quickly after initiation. 700 mb flow backs quicker than 500 mb flow on this run, meaning we get a veer-back-veer vertical wind profile which makes storm mode messy. Not to mention the geometry (low wavelength) of the trough is historically associated with less discrete storm modes.

Further south, there is less of the veer-back-veer, the forcing is weaker, and the cap is slightly stronger. I’d look between LA-AR border and I-40 for the best chance of “long-lived supercells” as is mentioned in the SPC Public Severe Weather Outlook.

3. The parameters are actually very good. The instability is there due to cold mid-level temperatures and the shear profiles are beautiful. If we can get a few supercells to remain discrete I expect significant tornadoes. Highest threat, given what I said in bullet #2, is in an area bounded by (just south of) Little Rock, AR – Crossett, AR – Columbus, MS – TN/AL/MS border. This roughly correlates to the southern 2/3 of SPC’s MDT risk, stretched a little to the west.

4. Big outbreaks usually have complex initiation modes. Almost never do all the storms initiate from one boundary, at least in the early season. Instead, the forcing is usually more diffuse and scattered through subtle confluence lines ahead of the main [cold front, or dryline, or other initiating boundary]. (See 2/5/08, 3/28/07, 4/27/11, etc. for examples).

Tomorrow’s setup has the look of subtle confluence lines. The E-W temperature gradient is weak, and the wind shifts are not especially pronounced. So I think we will get the subtle confluence lines ahead of the main cold front (which will be well to the west, actually). Moreover, the 700 mb vertical velocities do not smell like linear forcing. So this should be watched, and may throw Bullet Pt #2 out the window.

A lot of this however will hinge on the capping in the warm sector. The strength of the warm layer/cap at 700 mb will have a strong longitudinal dependence because of the high-amplitude nature of the trough – I’ll explain why in a future drawing maybe. Because of this, if the confluence lines are too far ahead of the strongest forcing and too far into the capped region, they will go to naught.

5. Watch out for the near-surface inversion that will be slow to erode N of I-40. If storms initiate before it does, or if low clouds/stratus/light rain hangs around for awhile, the threat area might shift even further south, to areas that are more strongly capped. This will tend to reduce the threat somewhat.

*Disclaimer* My explanations of nuances/subtleties end up being bullsh*t sometimes (i.e. irrelevant or flat out wrong – weather forecasting is a difficult business). Beware, and always use official forecasts for planning/safety/etc. etc. etc.

P.S. For reference, the SPC 06Z outlook that I refer to a million times can be found here.


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