Although the Storm Prediction Center has not outlined it in their graphic (it was mentioned in today's discussion), Friday may hold more potential for a severe weather event than it is currently being portrayed as.
This image, from the Storm Prediction Center's short range ensemble SREF system, shows the forecast for 'Significant Tornado Ingredients' for the evening hours on Friday. While the title of this may seem ominous, I take it to be an indication of if the ingredients needed to produce a tornado are in place. It does *not* mean that a significant tornado will be produced, although areas in the higher regions of this index should be on the lookout for any tornadic activity. In this forecast, we see a large swath of values, beginning with 5 in the dashed line and up to 60 in the center of the swath. Personally (although I haven't used this index in a while), I would advise that those outside of the 15 on the chart above should be relatively safe, while anyone inside the 30 should make sure their emergency plan is at the ready. Between those two benchmarks is iffy as far as how much potential you have- play it safe and keep an eye on weather developments as you go through your day on Friday.
It is true what the SPC SREF is saying in the image above- there is an unusually high value of 60 in the Midwest (of all places). And the bad thing is, that 60 is not unwarranted. Recent model runs of other forecasting systems have told of even a potential tornado outbreak for Friday evening in the Midwest. Current model projections from the GFS have an unusually strong jet stream over the Plains and Midwest for this timeframe, with speeds encroaching on 100 knots. Considering we have a strong Pacific jet backing this synoptic pattern up, it was inevitable that the jet stream would come onshore at some point. On the lower level wind field, the lower level jet stream will be dominating, with wind speeds exceeding 50 knots in the areas most at risk in the SPC SREF graphic at the top of this post. Wind shearing between the surface and mid-level atmosphere was observed to be over 90 knots at one point on the NAM model, although for sanity sake we'll take the GFS projection of maxing out at 70 knots to the west of the most at-risk area in the Midwest. Instability appears to be the holding point for the Storm Prediction Center as far as an upgrade to Moderate Risk for the region goes, but analysis of multiple model runs tells me we should not have a problem with instability. That said, I'm not a forecaster at the SPC and cannot predict if they really will issue a Moderate Risk once instability issues have been resolved.
The Plains will also be at a heightened risk yet again on Friday, with the supercell composite forecast reaching to levels seen at the Moore, OK tornado event. However, it is very much worth noting that the Significant Tornado Ingredients image above does not extend its high values into Oklahoma, meaning the tornado threat for that area could be substantially lower than that of recent days. It is for that reason that this post was mainly dedicated to the Midwest, and its increasing potential for a very active evening on Friday.
Andrew
