Nearcasting for low-topped convection over the Southeast

Yesterday the GOES-R support desks also looked at convection. The early runs of the Nearcast model showed the dry air dipping into the middle of the country and the instability associated with the cold front edging into the Southeast U.S. The 1700 UTC Nearcast from the 0800 UTC run (Figure 1) indicated two distinct boundaries of instability (dashed yellow lines).

20140812 1715 UTC Nearcast theta-e difference from the 0800 UTC run

The instability values in both areas wasn't particularly impressive, but still enough to conclude that 1) convection was possible once the front worked its way into the region and 2) any convection that did form would likely be non-severe and probably low-topped.

Now take a look at the GOES-R CI at 1715 UTC (Figure 2). The dash lines indicate the instability noted on the early morning run of the Nearcast.

20140812 1715 UTC GOES-R CI

Note that convection had started firing almost exactly on the areas of Nearcast instability in the 0800 UTC run. The only exception was over NW Georgia, and this was likely due to an area of fog that didn't dissipate until after 1600 UTC.

Also note the abundance of yellow CI signals. In previous CI images, these ~50% probabilities extended all the way into the Ohio Valley, but by 1715 UTC convection had started firing there, and subsequently the algorithm had already shut off. However, lower probabilities did still extend southeast. These probabilities weren't particularly impressive. Why? Because the CI is designed to provide probabilities for robust convection. So it follows that with the Nearcast showing less instability, the probabilities of initiation to 30dBZ would also be lower.

The CTC at 1730 UTC also supported this (Figure 3). There were a few signals along both lines, but they remained fairly weak at -10K/15 minutes or less.

20140812 1730 UTC Cloud Top Cooling

Was it correctly assumed that the lower Nearcast instabilities would result in lower-topped convection? In this case yes. The GOES-R cloud altitudes below were highlighted to show tops at 18,000 feet and above. Note that at 1715 UTC, some aren't even reaching those levels.

20140812 1715 UTC cloud altitudes