Forecasters: Mike (NWS) and Morris Weisman (NCAR)

The STEPS region was covered by fog this morning, with surface winds generally SE and surface dewpoints ranging between 50 and 56 F. Slight cooling aloft along with the enhanced surface moisture and expected surface heating once the fog burns off has increased convective potential significantly from yesterday, especially in the northern portions of the STEPS region. Convection is expected to trigger along the Cheyenne ridge by early-to-mid afternoon, and propagate within range of STEPS observational teams by late afternoon or early evening. With eta model progs of -6 to -7 LI's by evening in NE Colorado and Nebraska, and continued strong vertical wind shear, there is a significant chance of supercells developing in this northern region.

A moderately strong cold front is expected to propagate SE across the region by Thursday morning, reducing convective potential on Thursday, but SE return flow by Friday may increase convective potential again by late Friday.

Weather situation for today suggests late afternoon convection to occur in SE Wyoming and NE panhandle. We are hopeful that we will get some back builders into the northern portion of our observational network. We will have the mobile mesonetwork and NSSL balloon deploy to Wray at 2 PM and wait for further guidance. We will have the T-28 standby starting at 2 PM. Radars will start ops at 12:30 to 1 PM. We will practice coordinated dual-Doppler scanning early this afternoon. MGLASS will take 18Z soundings at Goodland and Wray. Will likely take 21 Z soundings at same locations to monitor erosion of the cap. We will consider working convection to the north and east of our prime network if it is clear that nothing will fire in the prime domain. We will consider operating the T-28 in this extended region as well, provided we can work out ways to provide them guidance.

[Radar images]

Ops period: 2200 UTC 31 May to 0330 UTC 1 June

Successful operations were conducted. Highlights included sampling by the mobile mesonet and S-pol of a well defined boundary that later triggered convection at the extreme northern fringes of the observational network. This feature was located near the leading edge of a large anvil from a thunderstorm to the south that is described next. CHILL observed this southern storm (from roughly 2330 UTC to 0130 UTC, 1 June). During this time the storm produced a fair amount of lightning, including positive CG's from the leading anvil. It was a rain producer as no evidence of surface hail was seen by CHILL (ranges from CHILL were nominally 80-90 km). NSSL launched an EFM into the anvil and the flight was successful. Electric fields to 60 kV/m were observed. This storm was also penetrated by the T-28 (two passes), but the convection was weakening during the mission. Mainly rimed aggregates were observed near the base of the extensive anvil. It was clear that this storm was entraining dry environmental air along the leading edge. This entrainment killed the storm, and drove an extrememly strong downdraft and associated microburst outflow. Wind gusts of 70 kts were observed at the ITR site, with blowing dust.

We turned our attention to developing convection to the NE of S-pol from 0130 to 0245 UTC, 1 June. This complex eventually produced a storm near McCook that had characteristics of an LP supercell (high Z, low Zdr, and small Ldr), indicative of dry growth hail. This storm, although at the fringes of the radar coverage, nonetheless represents a great case (radar and LMA only). The CG lightning switched from negative to nearly all positive (CG's) around 0230 UTC. After this event, S-pol and CHILL did coordinated scanning on another strong storm to the south (0245 UTC to 0330 UTC). Again the entrainment of mid level air from the front side was obvious in the CHILL Doppler data. This entrainment forced an early demise to this event, and again drove a strong surface microburst.

Debrief comments/discussion:

It was apparent that control of the Hailstone intercept van should be placed under direction of the Ops Director and CHILL Radar Scientist for maximum efficiency for intercepting hail and intense rain cores. This will be done from now on.

It was also clear that we need to think of distributed observations at times, say using one research radar and the mobile mesonet to work developing convection (or even a boundary, or collision of boundaries, etc), while the other research radar focusses on the storm at hand, say being worked by the T-28 and EFM platforms. The Goodland WSR-88D can provide dual-Doppler with either CHILL or S-pol when the other research radar is operating in a single Doppler mode (provided the right geometry is available). This should be kept in mind for future operations. The mesonet is a pre-storm platform (although not exclusively) while the other platforms focus on mature convection.