Th 7 May 2009 Noteworthy aspects

Centerpoint for tda’s activity is KGMJ in extreme ne OK. With the last 2 days activity being farther E, this area was quiet yesterday, so no significant preexisting convection was present to complicate matters. The exception to this was some midlevel convection that at 12Z today was over ern KS into MO.

Early on we identified at the surface 2 weak transition zones (these were too diffuse to call them “boundaries”. The first of these was along the Red River, separating shallow cool, moist air over OK (partly the consequence of the persistent cloud cover and pcpn the previous 2d) from more humid air (dew pt > 21C) over n TX. The second was a weak e-w zone of confluence marking a weak sfc front from roughly IL, srn IA across much of NE. The parent sfc low with this feature was over srn MB this mrng, if memory serves correctly.

Aloft, strong zonal flow crosses the W coast and the wrn 2/3 of the CONUS. This is providing a shear environment plenty adequate to support supercells. The temperature contrast up and down the W coast of the CONUS at 12Z this mrng was noteworthy: +13C at KNKX (San Diego) to -11C at KUIL (Quilliute). Only weak disturbances are present in this flow, revealed by 6.7 micron water-vapor imagery primarily.
There aprs to be a weak upper lvl PV perturbation in the flow at 12z this mrng centered nrn NV, srn ID and aligned E-W. Associated with this is enhanced N to S temp gradient across NV at 700mb and evidence (from METARs) of a weak surge of slightly cooler surface air heading ewd along I-80 in srn WY. This surge can be argued to have some relationship to the cooler air that this mrng was over the Nrn high plains behind the aformentioned sfc front.

So, we had in place early this mrng warming conditions over the Southwest, very moist, high CAPE air over the southern plains, and no distinct sfc boundaries or marked upper-air features. This made for another challenging forecast day for today.

Our outlooks for the 2000 – 0000Z and 0000 – 0400Z periods this mrng focused on two areas. The first and most important was KS and MO, where indications were from the 00Z Th 7 May initialized hi-res models (CAPS ensembles, NMM4 from NCEP, NSSL ARW3) that activity would initiate in the general vcty of Great Bend KS in late aftn and move into MO durg the evening. The proximate cause of initiation appeared likely to be [based on 10-m wind field from the CAPS control runs and the other so-called “deterministic” (poor term, but I use it for lack of a better one) models] convergence alg a wind shift derived from the frontal confluent zone noted earlier in NE. Updraft-helicity and CAPE-shear parameters argued that there was a chc of sig severe with this stuff.
During the forecast praparation period it became more apparent that the frontal confluent region at the surface noted earlier was going to be a focus of activity, if there was going to be activity in this area. Complicating the picture somewhat was the mid-level stuff noted earlier as being over ern KS into MO.
We noted mesoscale pressure perturbations and fluctuations with this, suggesting that there was some associated wind perturbation, probably just above the surface in early morning. Falling pressure in ern KS and rising pressure in MO appeared to be associated with acceleration northward of the OK low level moisture into KS during the morning, aprnt on visible satellite imagery as a tongue of the OK Sc advancing into central and e KS. The possibility of outflow cooling at low levels over MO reinforced the decision to go with initiation over central to ern KS.

Another factor was the behavior of both the NCAR 3km (initialized from RUC at 00Z and using GFS LBCs), and the 12Z initialized HRRR. Both these models had shown initiation of radar reflectivity over central KS before 15Z, and that the resulting storms would move eastward. Since there was no evidence for such initiation by then in the observations, and it seemed unlikely that such initiation would occur before, say, 1900 to 2000Z, the west team, at least, largely discounted these fcsts as providing useful guidance.

(As an aside, in the Sc over OK and KS mid-morning through mid aftn there were southward-propagating waves in this low-level Sc. I speculate that these were manifestations of low-level bore-like features, initiated by penetrative downdrafts from the earlier convection, though that these continued obvious into mid-afternoon raises some questions about this.)

Both teams, then, pretty much bought off on the scenario of the 00Z NAM-based intialized hi-res models in their forecasts, tho the teams differed in details. Both teams figured that this was worth a 15% chance of svr … no higher due to a lot of concern that storms would not even form … and a 10% conditional probability of sig severe.

The second area of concern was along the Red River in AR and SE OK, where the aforementioned 00Z NAM-based initialized models were indicating moderately intense storms forming in aftn, but that storms this area would become weaker after 00Z. The W team put a 5% chc of svr this area, but the E team did not consider this area worth even that much.

These fcsts were completed by 1600-1620Z.

By 1800Z or so clumping of sfc-rooted Cu was occurring along I-80 WSW of KOMA along a WSW-ENE line. These cells were partly flagged by the GOES-R algorithm for detecting growing clouds along boundaries. (I would have to question whether this algorithm added anything to what an experienced forecaster knowledgeable about the overall convective environment could provide.) This convective development contributed to the final forecasts for the 20 – 24Z period and the 24 (00)Z – 04Z period having the areas of forecast severe shifted Nwd from the preliminary forecasts, touching the nrn border of our forecast domain near the IA-MO border. This was confirmed by the 12z-initialized forecasts, as well as teh 16Z HRRR. The 12Z NMM from EMC in particular had initiation very close to where it actually occurred, and subsequent movement of the activity into MO with rotating updrafts. However, we tended to discount the massive outflow generated by this large cell.

We had less confidence in the liklihood of sig severe in preparing our final forecasts. The temp-dew-point spreads in the air farther N were roughly mid 80s to mid 50s, arguing that strong tornadoes would be very unlikely. So, the main threat for sig severe would have to come from wind gusts or hail. Accordingly the sig severe areas were reduced somewhat and the E team might have eliminated theirs altogether (don’t remember attm).

A couple of observations.
The NMM 4km from NCEP initialized at 12Z today also initiated a cell near CDS that put out a circularly spreading outflow that covered much of OK after a few hours.
The HRRR seems to be producing overall too much coverage of radar reflectivity, though in terms of forecast reflectivity I believe it is doing a little better than the CAPS forecasts initialized same time. (NOTE that these forecasts are really not directly comparable since different model configurations (phyics options, lateral boundary conditions, grid spacings, domain sizes, etc.)

Comments and corrections welcome …
John Brown

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