Heading North

NAM forecast streamlines and
0-30mb AG dew-poiints.

After a travel day on Monday that finished with a storm chase in South Dakota, today looks to feature an even greater severe weather threat across the northern Plains. Both of the Dakotas come into play, but surrounding areas, including portions of the central Plains to the immediate south, could also see a few strong to severe thunderstorms.

Surface analysis Tuesday morning indicated a diffuse area of low pressure across western South Dakota into portions of southwest to south-central North Dakota. Radar and satellite imagery showed abundant cloud-cover and some shower activity to the north of the system, mainly near and north of I-94 in North Dakota. Further east, remnant precipitation from overnight convection continued to advance eastward. In the warm and modestly moist sector, skies were mainly clear, with many stations in South Dakota reporting sunny skies. This is a bit of a change from how many convective setups have started this season, meaning that ample daytime heating could result in strong instability come this afternoon.

There is a fairly broad area with severe potential today. Latest trends and high resolution data would tend to favor three separate areas of focus:

HRRR forecast SBCAPE and wind shear.

-The central target covers much of South Dakota, as low pressure is progged to become somewhat better defined over the central portion of the state this afternoon. This combined with developing strong instability, dew-points already averaging in the lower 60s and somewhere between 30 to 40 knots of bulk shear, should favor at least isolated severe thunderstorms by late in the day. Forcing for ascent will be found near and immediately ahead of the low pressure center.
-The northern target appears to be a result of secondary low pressure area ejecting east from far western Montana into North Dakota later today. At least moderate instability is still expected here, particularly south of morning cloud-cover. Stronger wind fields aloft in this region may work to boost the threat up north, perhaps in the form of isolated tornadoes. With shortwave energy moving from the northern Rockies into North Dakota later today and enhanced wind shear in the vicinity, likely on the order of 40 to 50 knots, isolated to scattered supercell thunderstorms should develop. The HRRR has been fairly consistent with at least one vigorous supercell forming in south-central North Dakota. Aided by stronger wind shear and more sharply backed low-level winds than further south, this scenario seems plausible, if not probable. Some residual outflow from morning precipitation may also become a factor here.
-The southern target should also play off of a lee cyclone, as a subtle area of low pressure moves from eastern Wyoming into the Nebraska panhandle. Although the area further south may be further displaced from stronger forcing aloft, some guidance appears to develop an upper level perturbation in the vorticity field and accompanying modestly stronger mid and upper level winds. Simulated radar forecasts tend to develop thunderstorms near the Wyoming/Nebraska border this afternoon, although these storms may tend to congeal into a broken line segments or an MCS fairly quickly. The southern flank of this action may the be best target for a tornado, especially if there is some low level backing of wind fields there, in the southern Nebraska panhandle into southwestern portions of the state, as the HRRR suggests. What is also interesting about this area is that there are already some faint outflow boundaries in place. On the flip side, moisture return is somewhat less impressive with morning dew-points analyzed in the 50s.

Some red flags:
The central target may have issues with storm initiation. Further north, residual cloud-cover may mitigate the amount of destablization. In general, there is some question to storm coverage and the ability for storms to stay discrete.

A case could be made for any one of these three chase targets today. The decision may boil down to logistics for many chasers. Although I am starting the day west in Rapid City, I have access to any of the targets. Follow me on Twitter @stormchaserQ for more updates as I move out into the field.