Saturday, May 18, 2019

Potentially Significant Severe Weather Event Monday

A potentially significant severe weather event is being highlighted by the Storm Prediction Center (SPC) for Monday, with further threats emerging on Tuesday. Click on any image to enlarge. Check out the post on this weekend's severe threat by clicking here.

Monday, May 20th
The Storm Prediction Center's categorical (left) and probabilistic (right) outlook for severe weather on Monday.
Source: Storm Prediction Center
The Storm Prediction Center has issued a Moderate Risk (level 4 out of 5) of severe weather for the western half of Oklahoma (not including the panhandle), north-central Texas into the Texas panhandle, and extreme south-central Kansas for Monday, with an Enhanced Risk (level 3 out of 5) surrounding that area through much of west Texas, the majority of Oklahoma and southern Kansas. On a probabilistic level, viewing the probability of severe weather within 25 miles of any given point, the SPC has placed a 45% chance of severe weather in the same areas first mentioned, thereby green-lighting the issuance of a Moderate Risk. Across both the Moderate and Enhanced risk areas, as well as a small part of the Slight risk region in western Texas and the Oklahoma panhandle, a hatched area is outlined, which delineates a 10% or greater probability of significant severe weather within 25 miles of any given point.

The threat for Monday looks to be twofold over the course of the day, with the first threat appearing early in the morning.

Forecasted 250 millibar wind speeds and heights at 4am central Monday.
Source: TwisterData
By 4 am central time on Monday, weather models expect a strong upper-level low to move into the Southwest, heading up a robust extension of the Pacific jet stream that will include a jet streak exceeding 140 knots rounding the base of the low. It is this jet streak that may help fire off a substantial severe weather event later in the day. For the time being, though, notable divergence in the southern Plains should provide for an early-morning threat of strong to severe storms.

Forecasted 250 millibar wind speeds and heights at 7am central Monday.
Source: TwisterData
A little later in the morning, model guidance sees a reservoir of moisture readily available across the primary risk area, with dewpoints in excess of 60 degrees across Oklahoma and Texas. Further, a dryline is already evident in extreme western Texas and eastern New Mexico, which will provide the focus for thunderstorm development later on in the day.

Forecasted radar reflectivity at 6am central Monday.
Source: College of DuPage
By the time the early morning commute on Monday is beginning, strong to severe thunderstorms are projected to be firing off the northern part of the dryline in the Texas and Oklahoma panhandles into southwest Kansas. Some scattered development is possible further south, but is not expected to be as widespread as areas further north by Amarillo. With limited instability and a material inversion expected to be in place at this point in time, thunderstorms are not expected to be significant, though with the upper air dynamics forecasted to be in place, severe weather cannot be ruled out.


By the afternoon hours, the potential for a significant severe weather event becomes more tangible.

Forecasted 250 millibar wind speeds and heights at 7pm central Monday.
Source: TwisterData
As briefly noted at the start of this post, the jet streak that had been rounding the base of the low in the morning is expected to curve around to the east side of the trough by the evening, seemingly in an effort to maximize the severe weather threat. Once again, upper-level divergence is forecasted to be present across Texas and Oklahoma, encouraging the development of thunderstorms in those areas.

Further, note that the positioning of the trough has moved from looking as if it's tilting towards the bottom-left of the picture in the morning to now almost pointing straight down. This portrays a maturing and strengthening trough, an additional factor suggesting substantial severe weather is a possibility.

Forecasted instability (CAPE) at 7pm central Monday.
Source: TwisterData
Also in the evening, instability is expected to have both moved northward to the Oklahoma/Kansas border and increased quite a bit throughout the day. Indeed, convective available potential energy (CAPE) on the order of 3,000 to 4,000+ joules per kilogram (j/kg) of CAPE is forecasted across western Texas and much of Oklahoma, even into western and southern Arkansas. For reference, a general rule of thumb holds that severe thunderstorms can function in only 2,000 j/kg of CAPE. The area will also be uncapped by the evening hours, removing any barriers to potentially explosive thunderstorm formation.

This lack of a cap also presents a potential fly in the ointment for severe weather potential, however. Model guidance projects the cap to be eroded by 1pm, and instability building to more than 2,000 j/kg by that same time. This presents an opportunity for storms to fire earlier in the day, potentially corrupting a more substantial severe threat that could evolve in the evening hours. I find such a development possible, particularly if the early morning storms are able to lay down outflow boundaries that could then provide a lifting mechanism for those potential mid-day storms. Such an outcome must be monitored closely, as it could materially change the forecast.

Forecasted surface-500 millibar wind shear at 4pm central Monday.
Source: TwisterData
If thunderstorms are still able to fire in the late afternoon and evening hours, the atmosphere appears primed for at least a decent severe weather event. Combined with abundant moisture, instability and a  lifting mechanism by way of the dryline and any possible outflow boundaries from earlier storms, wind shear is expected to be sufficient for the development of rotating thunderstorms in western Texas, much of Oklahoma and especially in Kansas, which will be in close proximity to the surface low in eastern Colorado.

More details will need to be ironed out in coming model runs today and tomorrow, but at this point in time it appears Monday evening will hold the potential for a significant severe weather outbreak in portions of Texas, Oklahoma and Kansas.

Tuesday, May 21st
Forecasted severe weather threat for Tuesday, May 21st.
Source: Storm Prediction Center
On Tuesday, the severe weather threat is expected to shift to the north and east, seemingly to a less-intense degree than Monday. The progression of the trough to the east will bring Missouri, much of Arkansas, southwest Illinois, extreme southeast Iowa and northeast Texas under the gun for potential severe weather. At this time, a significant severe weather event is not anticipated, but this may change depending on the evolution of Monday's severe weather episode.

A potentially significant severe weather event is forecasted for Monday across portions of the southern Plains, with all modes of severe weather (including tornadoes, which could be strong) possible. The threat area will then shift eastward for Tuesday and likely decrease in intensity relative to the day before.


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.