Week 2 Summary: 5-9 May 2008

Overview: Monday – Thursday

Week 2 of the EWP was relatively active with probabilistic warning activities occurring on Monday, Tuesday and Thursday while both CASA and PAR evaluations occurred on Wednesday. This week featured visiting forecasters Bill Rasch (NWS Billings), and Craig Shoemaker (NWS Tucson). A visiting meteorologist from Environment Canada, Bryan Tugwood also spent the entire week with the EWP team. Other meteorologists from Environment Canada watched EWP experiments including Ria Alsen and David Schmidt. Cynthia Whittier, from WDTB, also participated in the EWP. Greg Stumpf and Kristen Kuhlman served as ProbWarn scientists, Jim LaDue served as EWP coordinator. CASA scientists included Don Rude and Jerry Brotzge.

Monday evening featured a supercell with large hail near GCK that merged with a line to the east and became an outflow dominated multicell event. The hail threat decreased as largescale wind threat increased. This team consisting of Bill, Craig, Cynthia and Brian focused on hail threats given their unfamiliarity with the probwarn software. The challenge of the day was getting used to new software and then dealing with threat areas as isolated multi- and supercells coalesced into a larger complex. Due to the visitor’s unfamiliarity with the software, Greg and Jim decided that all visitors group together as a team with one of the probwarn scientists operated WDSS-II. An instance of D2D proved very helpful for storm interpretation.

Tuesday evening featured a probwarn exercise in west Texas. Monday’s big team was split in two with Bill and Kristen taking Midland’s CWA while Craig and Bryan tackled the Lubbock CWA.

The southern team concentrated on hail and tornado threat areas for mainly isolated convective modes including one supercell southeast of MAF and a splitting storm in southeast NM. The splitting storm provided some challenge to the team as to how to handle the threat areas as the split occurred. They decided to keep the original threat area with the right mover and issue a new one for the left moving component. But the sequence in which they decided to do the edits resulted in a period when the left mover was not covered by a threat area.

The northern team focused on hail and tornado threat areas for a relatively complicated cluster of small multicells in which of them merged together. Bryan decided to issue much larger threat areas than the southern team to group three areas of relatively higher storm density. His reasoning for such grouping was that the cells appeared to pulse in ways that could not be anticipated.

Wednesday evening’s severe storm threat included central Oklahoma and so PAR and CASA activities were scheduled. An upper-level low with an accompanying surface low tracked across Oklahoma City. The CASA network featured numerous outflow dominated small multicell line segments featuring strong downbursts. Bryan and Craig worked with the CASA scientists while Kristen worked with Bill on the PAR. The CASA team enjoyed observing two radars, Lawton and Rush Springs, get direct hits from 60kt and greater downbursts. To the north of these outflow dominated multicells, a boundary with strong vertical vorticity was tracked by the PAR sector. Kristen routinely updated the PAR sector in order to keep up with these storms as they rapidly crossed azimuths toward OKC. At least two TVS circulations were monitored by the PAR as they produced tornadoes near Lake Overholser and then up in Edmund. Meanwhile, the north edge of the outflow dominant multicells in the CASA network also generated significant vertical vorticity from which a TVS and a tornado subsequently formed just south of the National Weather Center. The tornado was visible from the center but only the TDWR could adequately track the TVS.

Thursday evening’s threat did not include the PAR and CASA domains and so the probabilistic warning activity was the sole experiment. The question was whether to set up the experiment in VA or NC where line and isolated supercell convection was forecast to occur or whether to focus on the supercell threat in western Kansas? This time, the SHAVE experiment was active and the probabilistic warning team wanted to coincide with them in order to get enhanced report density. Since the SHAVE team already picked a supercell west of Garden City, KS by the time the probabilistic warning activity ensued, we chose the same supercell. Now instead of two teams tracking multiple threats in two geographic areas, the two teams picked the same area and split up operations by threat type. Cynthia and Craig covered the hail threat while Bill and Bryan grappled with the tornado threat.

There was a leading, HP supercell that had an almost certain large hail probability and an uncertain tornado threat. This supercell spawned numerous, shortlived mesocyclones that became embedded in rain during their maximum intensities. This behavior provided a challenge to the tornado warning team in designing their threat areas. Many chasers were providing live streaming video coverage through severe studios website but despite this plethora of visual ground truth data, there was always the possibility of a rain obscured tornado. Therefore, the tornado warning team issued probabilities that just exceeded their legacy warning threshold of 50%. A later squall line started to exhibit strong vertical vorticity and so the team decided a large, low probability threat area was the best solution unless a specific vortex intensified.

The following is a highlight of observations regarding each of the three experiments over the week:

PAR discussion:

  • The PAR allowed circulation features to be more easily tracked through the motion of reflectivity patterns. The velocity static images did not appear as clean as the 88D but the quick updates allowed for easier tracking of vorticies.
  • Bill appreciated the opportunity to react more quickly as a function of more rapidly updating data, especially with the vortices in OKC.
  • Bill was thrown off by the interlaced 0.5 deg scan. A recent fix puts this interlaced scan into a different product.
  • Both Bill and Craig appreciated the rapid scanning for midlevel downburst precursor signatures. They thought there could be an addition 3-4 minutes lead time.
  • They thought an additional interlaced mid-level user selectable scan would provide more benefit for downburst precusor signatures.
  • Bill thought he didn’t have enough time to do a full volumetric analysis. Would there need to be some automated products to assist?

CASA discussion:

  • Bill believes the adaptive scanning appeared very intelligent during archive and playback cases.
  • Bill thinks the CASA network is very good for fire weather issues when quickly changing winds were occurring.
  • Craig found potential value for the Tucson CWA such as dust storms, non mesocyclonic tornadoes, even dust devils. He thinks a CASA network would be a valuable compliment to 88Ds in the west to monitor boundary layer action.
  • Bill wanted to see high elevation tops in CASA and he wondered if the 88D data could be overlaid.
  • Jerry wondered if a 3-D grid of CASA and 88D data would solve Bill’s wish using w2merger? However such a merger would be a challenge given the vastly different resolutions of the two datasets.
  • The RHIs were a hit when they were in the right place. Many times they weren’t in the right place according to Bill and Craig.
  • CASA volumes were easier to keep up with according to Bill. However he thought he lost track of the big picture owing to rapid paced data.
  • 3dVAR 2-D wind analysis was useful to forecasters though it was a bit smoothed. They want this display in WDSS-II and plans are for that to occur.

Probwarn discussion:

  • Bill thought he had a better handle of location of threat area than he did for assigning probabilities. However he thought he also had a good handle assigning probabilities.
  • Assigning hail probabilities was easier than that of tornado.
  • The team appreciated high resolution reports from CASA, especially live chaser video. However they also realized that unreported events could occur inside of this dense observation network.
  • They felt that if the same storm as the Thursday supercell approached a city, their legacy warning probability threshold would go down. But on the other hand, they felt that the denser reporting network would raise confidence, partially negating the first consideration.
  • The size of threat areas increased as the workload increased in order to group multiple cells in close proximity. It also increased if the uncertainty of where the threat materialized increased (reported from GFK scenario).
  • The expiration timing of swaths is uncertain. They tended to stick with traditional validation times.

General issues with both PAR/CASA:

Basic Logistics

  • Craig appreciated the day 1 arrangement of a large team then easing to smaller teams the next day as his experience with software and probwarn decision making improved.
  • Bill appreciated the WDSS-II training on day 1 though the ambient noise level was distracting. Do we shut the garage door or move training to the Dev Lab? Bill also liked the one group training.
  • Bryan and Craig like the 1-9 pm shift. They were fatigued at the end of the shift, especially on Wednesday.
  • Bryan thought surveys were redundant in that his conclusions were the same.

Jim LaDue (EWP Weekly Coordinator, 5-9 May)

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