By late afternoon on April 2, a fairly broad risk area for severe storms existed across Oklahoma, eastern Kansas and the Mid/Lower Mississippi Valley, with the main concern of large hail existing in western and northern Oklahoma into southern Kansas (Fig 1). Strong surface heating and increasing low level moisture throughout the day resulted in increasing instability in the region, and with veering wind profiles, the environment supported supercell thunderstorm development. Forcing mechanisms were, however, fairly weak. While a dryline existed in western Oklahoma, a west-east oriented frontal boundary slowly shifted northward through northern Oklahoma.
Figure 1: April 02, 2014 SPC 1630 UTC Day 1 Probabilistic Large Hail outlook.
Between 1900 and 2200 UTC, it appeared more and more likely that convection would initiate along the frontal boundary as an associated cu field showed signs of strengthening per visible imagery. A main forecast concern at that point was when and where significant convection would initiate within that cu field.
One proxy GOES-R product currently being demonstrated to forecasters in the Storm Prediction Center (SPC) is the University of Wisconsin (UW) Cloud Top Cooling (CTC) product. This product measures changes in brightness temperature in GOES infrared imagery that have occurred since the previous GOES scan, displayed as cooling rates. The CTC product may be useful to SPC forecasters by indicating areas of initial/significant convective growth as it’s occurring and by quantifying the vigor of developing convection. It has been shown that CTC rates can be a useful prognostic tool in predicting future radar echo intensification and in increasing lead times for maximum values in several NEXRAD fields. SPC forecasters generally have had a positive attitude toward this product thus far, most notably liking: its non-intrusive display (an overlay on imagery), that it is easy to understand (measures brightness temperature trends in the infrared, displays cooling rates), and that it makes obvious (and quantifies) where in the imagery significant convective growth is occurring, which may otherwise be difficult to see, especially in the absence of visible imagery at night.
On April 2, the CTC product was quiet through 2200 UTC as SPC forecasters watched for signs of initiation. At 2215 UTC, the first CTC rate was indicated along the boundary in far south-central Kansas (Fig 2). Though this convection did not mature further, the rapid cooling was evidence that the cap was likely close to breaking and upscale convective growth might be imminent. By the next GOES scan (2230 UTC) this appeared to be the case, as significant cooling (<-20 C /15 min) was measured for a developing storm further east, which continued to grow rapidly through 2245 UTC when a very strong rate of <-40 C /15 min was measured. This storm had its first severe thunderstorm warning issued at 2249 UTC, and went on to produce severe hail, some of which was significant (>=2” diameter; Fig 3). Over the next hour along the boundary in southern Kansas, the CTC product continued to highlight areas where significant cooling was occurring in the infrared, indicating the initial growth of thunderstorms that would go one to produce severe weather throughout eastern Kansas.
Figure 2: April 02, 2014 2002-0015 UTC GOES-East visible imagery, Cloud Top Cooling rates. Color fill is cooling per 15 minutes.
Figure 3: April 02, 2014 2115-0200 UTC GOES-East infrared imagery, Cloud Top Cooling rates, NWS watches and warnings, 2100-0300 UTC severe weather reports (a=hail, w=wind, t=tornado). Color fill is cooling per 15 minutes.
A SPC forecaster viewing the CTC product during this event mentioned that it was helpful to see when and where significant cooling rates were NOT being measured, evidence that rapid convective growth was not occurring in such areas. It was noted that the product was helpful in highlighting when and where storms finally began to grow rapidly along the boundary, indicating that the cap had been broken, and increasing situational awareness as to which developing storms may become the strongest.
Forecasters understand that the CTC product is a proxy GOES-R product, and that it will benefit from the higher temporal and spatial resolution that will be available with the Advanced Baseline Imager aboard the next generation of GOES satellites.
- Bill Line, SPC/HWT Satellite Liaison