The Storm Prediction Center (SPC) has issued a Slight Risk today for two separate areas across the Great Plains and the Midwest. This forecast will focus on the Slight Risk area over the High Plains.
Associated with this Slight Risk area is a 5% Tornado Threat Ring.
The SPC is basing this threat on a complex set of synoptic scale features. We have an omega pattern over the country, but it is not a blocking omega at this time. Instead, a trough over the Great Basin will eject into eastern Colorado by this evening. As this trough moves east, a dryline will sharpen across the High Plains, and it will interact with a set of lee-side lows east of the Rocky Mountains.
Synoptically speaking, the 300 mb NAM chart (from Unisys) shows that the omega pattern circles an upper level low, as described by the SPC. Notice the nose of the jetstreak poking into western Kansas by 0Z.
The 500 mb NAM chart shows some moderate Positive Vorticity Advection (PVA) into northwestern Kansas and southwestern Nebraska by 0Z. The vorticity isn’t really strong, but the winds blow directly across the gradient. This will boost convection in this area.
The 700 mb NAM chart shows a pocket of rapidly-rising air over northeastern Colorado by 0Z. This is over my old buddies, Phillips, Sedgewick and Yuma counties.
The 850 mb NAM chart shows that thermal advection is not very strong. However, there is a large plume of warm air poking into the Slight Risk area.
The 12Z upper air sounding from Denver, CO, shows Convective Available Potential Energy (CAPE) already this morning (627 J/kg). There was plenty of moisture at the boundary layer as well, with low dewpoint depressions near the surface, and 0.57 inches of precipitable water in the column this morning. There was a moderate capping inversion around 750 mb, and the 0-3 km average lapse rate was 6.1 C/km.
The deep-layer shear was 65 kts, and the low-level shear was 8 kts. The shear is a mix of speed and directional changes, with speed changes being the dominant factor. The deep-layer shear will support rotating storms.
The 12Z upper air sounding from North Platte, NE, shows less moisture in the boundary layer, but more moisture aloft. The precipitable water was 0.83 inches, (higher than Denver), though most of the moisture is above 600 mb. There was no CAPE present in the column this morning. There was a strong thermal inversion above 850 mb and the 0-3 km average laspe rate was 4.5 C/km.
The deep-layer shear was 50 kts, and the low-level shear was 19 kts. Even though the deep-layer shear is lower in this region, it is enough to support rotating storms. The low-level shear is enough to support tornadoes as well.
TwisterData.com shows that the RAP is predicting a well-defined dryline through the High Plains by 21Z. This will be an important boundary for storm formation.
TwisterData.com shows that the RAP is predicting a strong pocket of CAPE to develop near the CO/KS/NE tri-state area by 0Z.
Currently, the surface observations (from the SPC Mesoscale Analysis Map) shows the moderate dewpoints this morning, and the radar overlay shows a few early-morning showers.
The Supercell Parameter shows very little potential for the current time, but within the next six hours (according to the RAP), the Supercell Parameter will climb to 8 in northeastern Colorado. I expect that it will continue to climb throughout the afternoon.
The visible satellite imagery shows a few clouds associated with the morning convection, but otherwise, severe clear in much of the Slight Risk area.
The enhanced infrared satellite imagery shows that the morning convection is not very strong, and has relatively low, warm tops. It is also not very wide-spread.
The water vapor satellite imagery shows the plume of warm air pushing north into the threat area. Mostly, it has already arrived, but I really like that you can see it on the water vapor imagery.
Overall, I expect some strong storms today. with the low-level moisture in this air mass, I do expect a few classic supercells. Earlier this week, I thought the moisture would not be as strong, and I expected more high-based, hailer-type cells, but that threat has shifted.
If I were chasing today, I would probably target some place like Yuma, CO. This is always a great place for storms (I’ve caught three tornadoes in this area, and a bunch of other very good storms), however the road network is tricky. It is a good place to get hailed on, and lose a windshield, too, as there are not many escape routes.
Thank you for reading my forecast.
The upper air soundings and mesoscale analysis plots are from the Storm Prediction Center website.
The forecasted upper air soundings are from TwisterData.com.
The surface observation and upper level charts are from Unisys Weather.
The satellite data is from <a href="http://weather.msfc.nasa.gov/GOES/"