Meteo 361: e-portfolio #1

Sunday June 17^th of 2007 was a busy afternoon and evening in the eastern Wyoming, western South Dakota and Nebraska panhandle region. The SPC issued Mesoscale Discussion #1178 for that area due to developing thunderstorms that were expected to intensify over the next few hours. As a result of these storms the SPC also issued two Severe Thunderstorm Warnings within the Mesoscale Discussion area, numbers 408 and 409. During the following hours there were numerous reports of High Winds (in excess of 65 knots) and Hail (in excess of 2”) as a result of these storms. While the severe weather that day traveled as far to the northeast as the northwest corner of Wisconsin, I will be focusing on the cause and effect of Mesoscale Discussion #1178.

The Low Pressure area depicted in Mesoscale Discussion #1178 closely resembles the Low and its accompanying fronts shown on the HPC’s 1800z Surface Analysis, released at 1925z. The discussion was released at 2053z so this makes sense. The HPC’s Surface Analysis shows a pair of Lows, one in southwest North Dakota with a Warm front stretching across to the Chicago area and a Cold Front draping down into the second Low Pressure in central Wyoming. A Cold Front drapes from this Low to just north of Las Vegas, Nevada. We can also see a Lee Trough associated with this pair of Lows. The Trough is shown on the 18z Surface Analysis just east of the Nebraska and Wyoming border and dropping into eastern Colorado.

So why did the folks at the SPC issue Mesoscale Discussion #1178? Let’s take a look at the discussion and reasoning.

“Thunderstorms are increasing over northeastern Wyoming in a moist unstable environment behind the Cold Front in response to increasing dynamic forcing associated with a strong upper trough moving eastward across the northern Rockies.” You can see the upper level trough on both the 500mb (highlighted in red) and the 300mb charts from the SPC archives. Taking a closer look at the 300mb chart you will see divergence (yellow highlights inside the black box) at the 300mb level over the Mesoscale Discussion area due in part to a Diffluent pattern (outlined by the black box) where the streamlines spread wider as they flow from the southwest, over central Wyoming to the northeast to the US and Canada border.

“Additionally, high based storms are developing along the dryline over the Nebraska panhandle within a deep mixed boundary layer.” Taking a look at the 18z Surface Dewpoint (in degrees Fahrenheit, from Plymouth State Weather Center), we can see why the folks at the SPC drew the Dryline where they did on the Mesoscale Discussion. We can also see a bulge in the Dryline right at the Nebraska and South Dakota border. The bulge helps enhance the low level convergence that in turn helps the dryline initiate deep, moist convection. Looking at the RADAR (courtesy of University Corporation for Atmospheric Research or UCAR) from 21z you can already see some of the storms starting to form. We can also see the low level (850mb) convergence and high level (250mb) divergence on this SPC 850-250 mb Differential Divergence image. If we sneak a peek at the 21z RUC analysis from the NASA Langley Cloud and Radiation Research web page, we can see the deep mixed layer that they are talking about for Chardon (KCDR) which is located in the northwest corner of Nebraska’s panhandle. Here is a map from the Plymouth State Weather Center that you can refer to for locations of some of these examples: Midwest city IDs.

“Thunderstorms are expected to develop eastward and northeastward into parts of western and central South Dakota and north central Nebraska during the next few hours and watches will be issued soon.” They did in fact follow up with two watches, numbers 408 and 409. These two watches covered a large portion of the Mesoscale Discussion area and were very well thought out as a number of the severe weather reports for Hail and High Winds came from within the watch boundaries, according to the SPC Storm Reports for 06/17/07.

“Mesoscale objective analysis indicate that Cap is weakening over the Nebraska panhandle northeastward into the western half of South Dakota, suggesting the potential for additional thunderstorm development is increasing.” You can see a good example of the eroding Cap from the 21z RUC analysis (also from NASA) for Pierre (KPIR), which is in central South Dakota, ahead of the storms at the time of the Mesoscale Discussion. You can also see this on the SPC 21z Surface Based CAPE/CIN chart. If you look closely you can see an area of CIN less than 25 J/kg that matches the Mesoscale Discussion area. Also, as you follow that area northeast you move across contours representing CAPE of 2000 and 3000 J/kg getting very close to an area of CAPE in excess of 4000 J/kg which is extreme.

“Low level convergence along a Cold Front moving southeastward across western South Dakota and a southwestward extending dryline into Nebraska coupled with strong heating in the warm sector and increasing dynamic forcing aloft should promote initiation of new storms during the next 1-2 hours.” You can see the strong heating on this Surface Temperature snapshot from the Plymouth State Weather Center. It shows the intense diurnal heating of the plains at work in the warm zone preceeding the front. Looking at the RADAR (again courtesy of UCAR) for the discussion area for the next few hours (22z, 23z and 00z) shows us the storms are indeed continuing to grow.

“Strong deep layer shear is also present over Wyoming and western and north central South Dakota which will enhance storm organization and intensity.” Looking at the SPC's 850-500 Wind Crossover image we can see the Low Level (850mb) Jet denoted by the red wind barbs moving moisture out of the Gulf and almost due north into northern Nebraska where they undercut the 500mb Jet Stream denoted by the blue wind barbs.

“As storms move into more favorable instability, especially over South Dakota, the threat for severe storms will increase.” This will be the end of my discussion as the storms moved on and resulted in another Mesoscale Discussion (#1180) that followed the storms to the northeast, as well as another Severe Thunderstorm Watch (#411)