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.


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