Friday, April 25, 2014

April 26-30 Severe Weather Outbreak: Saturday Forecast

This forecast is solely for the severe weather on Saturday, April 26th. For the Sunday, Monday and Tuesday outlooks, please look at the bottom of this post for links.

The Storm Prediction Center is highlighting an enhanced severe weather risk over the Southern Plains on Saturday, April 26th. The image above shows a graphical representation of the SPC's thoughts for Saturday's severe threat, with the percentages showing the likelihood of severe weather within 25 miles of any given point. In this graphic, we see that the states of Nebraska, South Dakota, Iowa, Kansas, Oklahoma, Missouri and Texas are in the general 'slight risk' categorical severe weather outlook, indicated by the yellow 15% shading, while south-central Kansas, central Oklahoma and north Texas are in a 30% swath of enhanced severe weather chances. We also see the black hatched denotation, meaning areas within that have a 10% or greater chance of seeing significant severe weather within 25 miles of any point (think extremely large hail, very high winds, and strong tornadoes instead of "tamer" severe weather). Let's go over how this will evolve.

The projected surface analysis on Saturday evening from the Weather Prediction Center shows our strong storm located in northeast Colorado, with a warm front extending through the Central Plains and snaking south into Arkansas and Louisiana. We then identify our cold front way back west in Colorado and New Mexico, but we also see an orange boundary in western Texas, Oklahoma and Kansas. What is that boundary? That is a dryline. In the same sense that warm and cold frontal boundaries separate warm and cold air, a dryline boundary separates dry and moist air, sometimes creating a substantial temperature gradient in the process. Drylines are the source of numerous storms in the Plains, and look to be the source for Saturday's severe weather, as well.

On Saturday evening, we will begin seeing a strong trough (storm system) in the Southwest begin to attain a negative tilt. A negative tilt is visually seen as the isobars trying to 'push' towards the south-east direction, indicating the storm system has reached maturity, and we see that phenomenon occurring in the 500mb vorticity forecast map above. This negative tilt looks to be attained in the late evening hours of Saturday, which brings up some timing issues for this severe weather event on Saturday, which we will discuss more next. The general thing to take away here, though, is that this will be a very strong storm system surging eastward to kick off this multi-day significant severe weather outbreak.

There does look to be some timing issues on Saturday, not just with respect to how fast the storm can acquire a negative tilt, but also with the presence of a capping inversion Saturday evening. Shown above is the Saturday evening forecast for instability, marked in j/kg by contour lines, and stability, shown by the presence of blue shading; the darker the blue shading, the higher the stability. Note that instability means air can rise because the air at the surface is warmer than the air above the surface, and stability means air cannot rise, as the air above the surface is warmer than the air at the surface. If we recall that air can only rise if the surrounding air is colder than the surface, the presence of stability means thunderstorms cannot form. In this forecast image, we see that there is a lot of projected instability over the Southern Plains, over 3000 j/kg in some places, which is a very high amount of instability. In some spots, we see white, which means no capping inversion, but across the remainder of the SPC-outlined severe weather risk, we see a significant capping inversion in place, as the dark shading shows. I'm worried that this cap will be too strong to break through, and the trough will be too slow attaining that negative tilt and moving closer to the Southern Plains leading to a bust in the forecast.

Let's hypothesize for a moment that storms do form on Saturday. A weather model named the WRF-ARW can predict the maximum updraft helicity for the entire 48 hours it forecasts for. In this instance, it means we can see how strong the storms that form rotate and spin from the morning of April 25th to the morning of April 27th, encompassing Saturday's severe weather in the process. We see on this image that the WRF-ARW model projects multiple tornadic storms to develop across the Central and Southern Plains, with two notably strong supercells violently rotating from southwest Oklahoma to the central portion of that state, and another from western to eastern Kansas. So while the environment may not be too favorable right now, any storms that do form and can sustain themselves look to be rather significant.

To summarize:
• A potentially significant severe weather event may occur on Saturday.
• The Central and Southern Plains would be affected.
• Model guidance is forecasting an inhospitable environment for storms, which may greatly hamper the severe weather threat.
• Any storms that do form have the potential to be significant and potentially tornadic.

Other posts pertaining to the April 26-30 Severe Weather Outbreak:

• Sunday Severe Weather Outlook: Click Here
• Monday Severe Weather Outlook: Click Here
• Tuesday Severe Weather Outlook: Click Here (Coming soon)

Links labeled Coming Soon will be out in the next few hours.


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