Sunday, April 7, 2013

4/7/13 - All about the details

Today is the beginning of multi-day severe weather episode. Thus, it is a good opportunity to take a look at the Storm Scale Ensemble of Opportunity (SSEO) and do a #science. The SSEO is a 7 member WRF-ARW (NSSLWRF 4km, 2 5.1 km HiResWindow) , WRF-NMM (1 CONUS, 2 HiResWindow all 4km) , and NMMB (New NAM 4km) convection allowing ensemble. My recent work (unpublished) suggests that the NMMB is similar in effective resolution to the 5.1 km ARW membership.

Todays forecast features the not-so-mythical cap: that love-to-hate it feature which keeps storms from forming OR makes them isolated and pristine to photograph. So it is not surprising that the first take-home message is that not all members even initiate storms in western OK. NSSL-WRF has a hard time generating storms for the southern half of OK. It appears it develops at least isolated, shallow convection: the models equivalent to turkey towers. Two NMMs develop convection further south, one of which has an isolated storm down by the Altus area. And this is the second take-home message: This behavior in the individual members suggests the models are picking up on storms that they themselves cannot resolve (i.e. storm coverage is low). In situations like this it may mean that isolated storms will be the only mode, and that they will be short lived the further south you go. It is my interpretation that at least one good storm will develop off the dryline by the surface low in SW OK, though it might be short-lived by our standards, long enough to pose a solid hail threat if it is supercellular (sometimes it takes time to ramp up these storms in tough to thrive in capped environments).

The result of this limited storm coverage is reduced probabilities for severe weather proxies. In this case, we use model derived Updraft Helicity as a signal for storm intensity. Our sensible threshold for stronger than most is UH > 25 m2s-2. To calibrate you, storms that get up to 100 are considerably stronger. Storms in NSSL-WRF have generated upwards of 400, but those tend to be single point maxima.

The forecast UH > 25 m2s-2 probabilities (3 hour variety) show the maximum chance for severe weather in the models is northern OK and southern KS. Quickly tonight the threat shifts to MO for any line segments along a front.

Storm motion estimates (southern OK) for right moving supercells are slow and to the southeast, while left moving storms will be quick to the northeast. For the north, somewhat slow and to the east, with left splits quicker to the northeast.

So the uncertainty is relatively large in OK, but I guess I would say that is normal in a capped environment. That models have any signal at all suggests I should favor it. But being wrong by one or two storms means ... well it may mean that nothing happens (i.e. risk is there but not realized) or that something does indeed happen (but is no severe because it doesnt last long enough).

Best chances for today appear to be in KS, with isolated supercells, eventually merging along that boundary and moving eastward. No doubt KS has the bigger tornado threat but you cant rule it out in OK.

The bottom line is that, on a day like today, it is highly unlikely that I could draw for you what the Radar Screen would look like. The devil is in the details and today is ALL about the details in OK.

UPDATED 4/8/13:
 The above is verification for yesterday from my experimental processing of the SSEO relying on Updraft Helicity tracks essentially using 4 passes. The strongest tracks (UH > 75) get the new fancy magenta contours (same levels as the color bar). Looking back at the evolution in the NSSL WRF this morning, it was apparent that it did a fantastic job with both mode, location, and evolution of the storms. It had some flaws in timing across northeast KS and overdid the MO MCS. I also looked at the experimental 12UTC version of the NSSL WRF and it had some differences but generally looked pretty similar. I guess that means I have to start processing the 12Z experimental SSEO version!

The ensemble mean probabilities takes on the shape of the dominant contributing members of the NMM variety but does bot beat the skill of the NSSL WRF at the 15 percent probability level. Looking carefully, each member overlaps with, and thus contributes to, at least one of the storm report local maxima.