Forecasters: Bruce Entwistle (NWS) and Morris Weisman (NCAR/MMM)
Today:
A convective system developed over southeastern Wyoming overnight, decaying over western Nebraska this morning and producing an outflow boundary which is now situated from Limon to Goodland to McCook. Surface dew points are quite high north of this boundary, ranging from the low-to-mid 50s along the front range and northeast Colorado to low 70s in the Imperial-Benkleman-McCook Nebraska region. Surface flow in this region is currently from the NE at 10-15 kts. An MCS has also developed in north-central Nebraska, and an outflow boundary is beginning to propagate south- and southwestward from this area as well. This boundary is expected to be the focus for convective triggering this afternoon, with forecast CAPEs north of the boundary in southwestern Nebraska and northern Kansas reaching 2000-4000 j/kg. Shear values may be just sufficient to support supercellular activity within this airmass. South and west of this boundary, conditions are expected to be hot and dry, with little chance for significant convective activity.
Days 2 and 3:
For Thursday and Friday, hot and dry conditions are expected, with little chance for significant convective activity.
| VARIABLE | Day 1: 1-4 pm | Day 1: 4-9 pm | Day 2 | Day 3 |
|---|---|---|---|---|
| Convective Potential |
2 | 2 | 1 | 1 |
| Storm Type | 2 | 2 | 1 | 1 |
| Triggering | 1 | 1 | 1 | 1 |
| Intuition | 1 | -1 | -1 | |
| Total | 5 | 6 | 2 | 2 |
| SYSTEM | STATUS |
|---|---|
| Ops Center | Operational. |
| CHILL | Operational. |
| S-Pol | Operational. |
| LMA | Operational. |
| MGLASS | Operational. There are about 14 sondes left. |
| T-28 | Engine failure, no longer available to the project. |
| Storm ballooning | Operational. There are 6 EFMs available. |
| Mobile mesonet | Recalled to Norman, no longer available to the project. |
| Storm chase van | Operational as the primary precipitation verification vehicle, but no one is available to staff it. |
| YRFS | Operational. |
| NWS | Operational. |
Radars will be on standby at noon. Electrical ballooning units will be updated at noon for possible 1:00 pm departure to Benkleman NE. MGLASS will be taken at 18-19 UTC at Goodland and Benkleman.
Benkelman storms: Since the 18Z soundings at both Benkelman NE and Goodland KS showed about 100-mbs of moisture, and the winds north of Benkelman were strong easterlies from a nocturnal MCS outflow, we decided to deploy the electrical ballooning unit on to Benkelman from Wheeler KS. At 2227Z we shifted the radar focus on cells near 38 deg and 65 km from CHILL and began coordinated scans. The LMA reported significant numbers of intra-cloud flashes, and the NLDN showed +CGs in the the northeastern cell of two in this area. (Note: The LMA was experiencing significant interference on channel 3.) We had a positive Zdr column (big drops being carried up in the updraft), while NSSL5 reported very big drops at the ground in Benkelman.
By 2309Z, we had a strong, classic supercell in Nebraska and a weaker cell in northwestern Kansas near St. Francis area. This storm southwest of the supercell began to exhibit classic LP characteristics such as a very erect cloud, with very little precipitation out the base as reported by Morris Weisman who had accompanied the ballooning unit.
At 2315Z, the Goodland WFO issued a severe storm warning and at 2333 issued a tornado warning (based on radar). At 2342Z NSSL5 reported a well-defined wall cloud and we had a definite hook echo on the CHILL radar. Tops of the supercell storm were now to 17.5 km above CHILL. S-Pol reported about 20 mm of rain from the LP storm (it was also producing -CGs as seen on the NLDN), and the supercell storm produced about 60 mm of rain (and +CGs) in a one-hour period. The accumulated rain from the LP (visually verified) was contaminated some by the earlier, weaker cells in this same area before Morris' report.
NSSL5 launched 3 EFMs, two of which where in the reflectivity notch on the southeast side of the main reflectivity core. These two almost certainly went into the updraft region, though this is yet to be confirmed. Additionally, the 21Z environmental sounding was taken between the two storms and about 4 mi south of Benkelman. An 18Z sounding had also been taken at Benkelman.
Other NWS reports included: one inch hail (5:50-6:00pm MDT) 3 mi east of Max NE in Dundee Cnty, funnel (6:00pm) 5 mi south of Stratton in Hitchcock Cnty and golfball-sized hail 4 mi west of Stratton, and golfball-sized hail (6:40pm) 6.5 mi southeast of Stratton.
The northeastern supercell storm formed on the moist-air side of the boundary near the NE-KS border, while the LP storm formed on the drier-air side.
Sydney storm: By 0214Z, we abandoned the Benkelman storms since the last balloon was on its way down and turned our attention to a severe supercell that had passed near Sydney NE where it produced funnel clouds and some tornadoes. This storm had taken a marked right turn from its original eastward track and was projected to pass southestward near S-Pol and continue on toward Goodland if it survived. This storm had 65 dBZ and strong rotational characteristics as determined by both research radars. S-Pol was closest to the storm and had a well-defined tornadic signature. This storm also produced golfball-, baseball-, and grapefruit-sized hail in the area of Haxton-Holyoke CO. The largest sizes have not been confirmed. By 0330Z the storm was clearly weakening and beginning to turn back eastward. We secured operations at 0423Z when the storm was no longer a threat.
Although the Benkelman storms were 120-140 km from CHILL and 80-120 km from S-Pol, they were well covered in coordinated scans by both radars. S-Pol may have been scanning the Benkelman storms (especially the LP storm) and/or their predecessor(s) when they were much closer, perhaps 20-60 km. Both research radars also scanned the Sydney storm but it was aligned mostly along and north of the SPol-CHILL baseline.
The LMA channel 3 interference did clear up between the Benkelman and Sydney storms.