Saturday, May 18, 2019

Multi-Day Severe Weather Outbreak Continues This Weekend

A multi-day severe weather event that began at the end of this last workweek will continue through the weekend and into the start of next workweek. This post will address the Saturday and Sunday threats, with a forthcoming post addressing the Monday and Tuesday threats. Click on any image to enlarge it.

Today: Saturday, May 18
Severe weather outlook for Saturday, May 18.
Source: Storm Prediction Center
For today (Saturday), the Storm Prediction Center has outlined a rather large area of severe weather risk, spread from the Gulf Coast across Texas and Louisiana up through the Plains and Midwest, tapering off in the Great Lakes and Upper Midwest regions. Out of five levels of severe weather risk, the SPC has assigned an Enhanced Risk (level 3/5) of severe weather to northeast Texas, northwest Louisiana, much of Arkansas and southeast Oklahoma. This means that thunderstorms (not necessarily severe) should be expected, and severe thunderstorms are relatively likely, with some potentially posing a threat for significant severe weather (i.e. very large hail, significant damaging winds, and even tornadoes).

Latest surface analysis as of 10:30am central time on May 18.
Source: Weather Prediction Center
A look at frontal positions and surface observations as of this typing reveals a low pressure system in southeastern South Dakota, with a strength of 998 millibars. As a general rule of thumb, low pressure systems below 1000 millibars are considered relatively strong, and this is no exception. A stationary front is seen draped across the Midwest, a boundary which has been the focus for thunderstorm development over the last couple of days already. To the south, another frontal boundary is identified through Nebraska, Kansas and Oklahoma, which is then attached to a second low pressure system in the Texas Panhandle.

We can glean a few focal points for today's severe weather from this chart. First, the Weather Prediction Center (WPC) has identified an outflow boundary positioned in the northern half of Texas, west of Dallas. You may notice how this lines up well with the Enhanced risk of severe weather, and this is no mistake. It seems plausible that, with the movement of the dryline eastward, there will be more than enough forcing to ignite thunderstorms, particularly in the absence of a substantial cap. Indeed, this has proven to be the case, with strong to severe thunderstorms ongoing along that corridor in Texas.

Current (as of this posting) radar view.
Source: College of DuPage
The highest severe weather threat for today appears to rely on the continued eastward progression and consolidation of the aforementioned dryline. Model guidance has this dryline diffusing somewhat during the evening hours over central Texas, which may diminish some of the impetus for severe storms, but with the main event already ongoing this does not seem to be a significant concern.

Additional severe weather is possible north of the Enhanced Risk area, although with a lower reservoir of instability and relatively weaker forcing for storms, the risk is understandably downgraded to Slight for areas in the Plains and Midwest. It should be cautioned that model guidance does have thunderstorms re-developing in northwest Oklahoma by the late afternoon hours, although SPC guidance suggests this will hinge on the development of the atmosphere in the wake of the ongoing storm complex. As such, residents in western Oklahoma and Kansas should keep updated throughout the day as the risk of severe weather may evolve differently than discussed here.

Tomorrow: Sunday, May 19
The threat of severe weather continues into Sunday.
Severe weather outlook for Sunday, May 19.
Source: Storm Prediction Center
The highest level of severe weather highlighted by the Storm Prediction Center for Sunday is a Slight Risk, a level 2 out of 5 on the agency's risk scale. While this is lower than the Enhanced Risk, notable severe weather events have occurred in Slight Risk areas multiple times before. As such, this threat should not be completely brushed aside.

Forecasted precipitable water values for 1pm central, Sunday.
Source: TwisterData
By Sunday afternoon, the system of frontal boundaries and primary low pressure system will move to the east and north, with the low pressure system ending up in the Wisconsin-Michigan vicinity by the afternoon hours. Precipitable water values in excess of 1.0" are expected across the Midwest and Ohio Valley, with a narrow corridor of 1.5"+ values expected ahead of the cold front. For reference, it can begin to feel 'muggy' when precipitable water values exceed 1.0", and it is generally noticeably humid when values exceed 1.5". This suggests not only a muggy day ahead on Sunday for some, but also a readily available area of moisture for thunderstorm development.

Forecasted instability (CAPE) values for 1pm central, Sunday.
Source: TwisterData
At the same time as the above image, a wide swath of elevated instability is seen stretching from southeast Texas and Louisiana north into slivers of western Kentucky, southern Illinois and southeast Missouri. Instability as shown here, defined as Convective Available Potential Energy (CAPE), is generally conducive for severe thunderstorms when exceeding 2,000 joules per kilogram (j/kg).

You may be wondering why there isn't a higher severe weather risk over southern Texas and Louisiana given the high instability forecasted, and that would be not only a very good question but a good way to consider how there are more factors than just instability required for severe thunderstorms to develop. Indeed, in my Severe & Unusual Weather course I took at the University of Oklahoma for my final semester, the teacher identified four necessary ingredients for strong thunderstorms: Instability, a lifting mechanism, moisture, and wind shear.

Looking over model guidance for southern Texas, we certainly have enough instability present, as the above graphic shows. The image above that one shows there will also be enough moisture, with precipitable water values exceeding 1.5". It's the wind shear and lifting mechanism features where the severe risk for southern Texas and Louisiana runs into problems.

Forecasted surface-500 millibar wind shear ("bulk shear") values for 1pm central, Sunday.
Source: TwisterData
There are a number of ways to identify wind shear, and each one has its own merits. For example, there is value in examining wind shear at the lower levels of the troposphere, while there is also value  in checking out wind shear across the majority of the troposphere. For this situation, we'll look at wind shear from the surface to the 500 millibar level. In the above graphic, we see a swath of 30+ knot wind shear stretching from Texas and Oklahoma into the Ohio Valley and Great Lakes regions.

In my opinion, wind shear becomes abundant using the surface-500mb range when exceeding 50 knots, as this should be enough wind shear to separate the updraft from the downdraft and prevent any thunderstorms from popping up, sticking around for less than an hour, and then collapsing because there isn't enough wind shear for the updraft to be tilted away from the downdraft. We don't see this benchmark met in southeast Texas or Louisiana- indeed, values there struggle to exceed 30 knots.This suggests it will be difficult for thunderstorms to sustain themselves and become severe for a prolonged period of time. That's not to say they can't become severe at all, but we would want quite a bit more wind shear to believe that the environment near the Gulf Coast is conducive for severe weather on a larger scale.

Forecasted 500 millibar wind speed & height values for 1pm central, Sunday.
Source: TwisterData
We also don't see a prominent mechanism to force the air at the surface higher and both initiate & sustain severe thunderstorms. The above image shows the 500 millibar geopotential height field, as well as wind speeds. This is the main level of the atmosphere meteorologists use to look for ridges and troughs, areas of relatively-calm and relatively-active weather, respectively. Ridges are identified by seeing the height contours (solid black lines) push northward- a great example of this is seen in the Southeast U.S. into the Atlantic Ocean. Troughs are identified by seeing height contours push southward, usually increasing wind speeds near the base of the trough as well. Great examples of troughs are seen here on the West Coast and Upper Midwest.

As just mentioned, Sunday afternoon will see a trough pushing through the Upper Midwest, driving that low pressure system over the same area we discussed a little earlier. However, note the relative lack of any trough along the Gulf Coast. This absence of an explicit forcing mechanism in that area suggests it may be difficult to get storms to push up and sustain themselves, especially with a lack of wind shear. It is likely that having only two of those four 'main' ingredients for severe weather is reasoning behind the lack of a higher severe weather threat outlined in Texas and Louisiana. The presence of those four factors is also the reason why a higher risk *is* outlined further to the north. The absence of stronger instability and an environment with more moisture are likely driving forces as to why there isn't an Enhanced Risk of severe weather in the Ohio Valley and Midwest regions.

Both today (Saturday) and tomorrow look to present opportunities for severe weather before this low pressure system moves out of the picture and another, potentially more intense severe weather event commences on Monday. A forthcoming post will address this new storm system.


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