Forecast experiment–May 12 thoughts

The forecast experiment centered on ABI (Abilene, TX) today to catch development along the dryline and other moisture gradients, including a warm front eroding the morning’s stratus northward. After analyzing and discussing the 12Z sounding plan views, we had about an hour on the schedule to create the 20-00 UTC and 00-04 UTC forecasts. It turns out we took a bit more than 90 minutes, since there were three scenarios we considered: (a) Convective initiation over the mountainous terrain of southwest Texas, coverage, and mode as forcing moved east; (b) Same initiation, coverage, and mode problem but over the South Plains and panhandle of Texas and extreme southwest Oklahoma, and; (c) what to think about some model members’ convection forecasts over southeastern Texas.

The forecast team I participated in wrestled with forcing mechanisms since the shear profiles were less than robust over the central and southern portions of west Texas and looked to be favorable in the panhandle, South Plains, and extreme southwest Oklahoma. We (there were six of us on the team today) settled on two initiation scenarios–the first in southwest Texas in the 20-21 UTC time frame as a shortwave moved toward El Paso and a second toward the space of Texas between Amarillo and Lubbock with a second jet streak moving into the area. The third area was discounted based on standard theories of organized severe convection.

For ensemble displays, we used

  1. The probability of 40 dBZ reflectivities. This helped focus our attention on the areas of concern and timing scenarios. This is probably the quickest way to assess the result of each model’s integration rather than interrogating multiple plan views, soundings, and postage stamp images of significant fields.
  2. Spaghetti outlines of 40 dBZ model-derived reflectivity. This is a noisy but useful depiction.
  3. Max reflectivity from the ensemble members to assess in a very rough manner storm instensity, and
  4. Max updraft helicity to assess the likelihood of severe weather.

From these we went to individual high-res (1-4 km) models and the observations to modify the threat area. It’s interesting to note that most participants prefer to use a single high-res or a familiar lower-res (e.g., 12 km WRF-NMM) as assess potential mode and than use the ensembles to place mental “possibilities” around what I’d call the “individual’s most probable” forecast.

The deterministic and ensemble runs suggested that southern storms would like be isolated and probably end short after 01 UTC. In the north we believed more organized convection, possibly a couple of clusters, was likely due to the presence of better 0-6 km shear. The was some issue as to how far east the convection would progress by 04 UTC, with the 1 km models suggesting propagation as far east as I-35. The operational SPC forecaster acting as our team’s guide tempered our enthusiasm with a little climatology, so the east edge of our forecast area was kept a little west (upstream) of the most aggressive model’s 04 UTC position for convection.

We believed there was a significant hail threat over this area given the shear, mid-level lapse rates, and NAM-KF model soundings suggesting analogs of 2+” hail cases. SPC’s significant hail paramenter on its mesoanalysis page also centered a threat in this area.

As I write this, we were not far enough west with our initiation and the severe threat is continuing a bit north of the area we anticipated. Tomorrow’s review ought to be quite enlightening, assuming we can keep our brains focused on the review and not on Wednesday’s expected event!

— Bruce E, forecasting on the West team today

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

Another “messy” day.

Another day with ongoing storms to mess-up the afternoon convective environment. Attention was focused over the southeastern and mid-Atlantic states. While the observations led to plenty of forecast uncertainty, today was notable in that it was the first day of the Spring Experiment where the different 0000 UTC models showed some consistency…but of course just because they were consistent doesn’t mean they’ll produce good forecasts.

One of the more intriguing forecasts was from the 4-km WRFNMM which developed several discrete, rotating storms along a warm front located in northern North Carolina. The other models also produced storms in this region, but they were smaller, more numerous, and not nearly as organized. The behavior of the NMM can probably be attributed to its surface wind field, which predicted a more easterly wind component north of the warm front (compared to the other models), thus, enhancing shear along the boundary. As of this posting, there are numerous storms in that area, but I do not believe any are supercellular.

Once again, the 1200 UTC model runs were out to lunch and didn’t provide any helpful guidance, probably due to ongoing convection influencing initialization. I don’t think either forecasting team used the 1200 UTC model runs as guidance for the final forecast products.

It seems like we’re just waiting for a day without morning convection. Perhaps then, the 1200 UTC models will have a better handle on the situation and can provide useful guidance.

Craig S