April 30 Severe Threat
Tornadoes and large to very large hail will be possible Tuesday afternoon and evening over parts of Oklahoma and Texas, while another focus area for tornadoes may evolve in southern Illinois and surrounding areas.
Radar and surface observations Monday evening show that a nearly stationary frontal boundary is draped from West to Northwest Texas, northeastward to a position near the I-44 corridor across Oklahoma. South of the front, particularly from southeastern Oklahoma into much of Texas, dew-points are in the mid to upper 60s, except for the lower 60s in Southwest Texas. To the north, drizzle, patchy fog and considerable cloudiness have kept cooler, more stable air in place from the Texas Panhandle into western and northern Oklahoma.
As of midnight, very little in the way of convective activity has been noted across the southern Plains, as isolated convection west of Midland, TX has struggled to sustain itself. A few showers are moving north across southern Oklahoma, in association with warm air advection near the frontal boundary, which may make some slight northward progress as an effective warm front overnight.
Predicting the convective evolution for Tuesday has been a challenge, but it appears as if computer models and near-term trends are converging toward a solution. It appears that some showers and thunderstorms may develop through the early morning hours from West Texas into western/central Oklahoma, but that most of this activity should lift northeastward through the rest of the morning. Instead of considerable convective overturning or even a mesoscale convective system (MCS) evolving early in the day, it appears that convection will remain limited, prior to peak heating.
The end result is that a moderately to potentially strongly unstable environment may be in place across a large swath of Oklahoma and North Texas by early afternoon Tuesday. To the north and east, warm sector destabilization is also expected from southern and central Missouri into southern Illinois, beneath a lifting warm front. Seasonably strong surface heating is also anticipated closer to a dryline across the Low Rolling Plains of Texas into parts of West Texas.
Along with this instability, enhanced wind fields associated with a shortwave ejecting from the Four Corners region toward the central Plains will overspread much of the region. The combination of moderate to strong instability and favorable deep layer shear will set the stage for the potential for severe thunderstorms over a broad area from the Plains into the Midwest Tuesday afternoon and evening.
Oklahoma and immediately surrounding areas:
Any morning precipitation should tend to lift northward and weaken through the day, but any residual outflow boundaries will be very important with respect to warm sector storm development during the afternoon. Forcing may be maximized near a quasi-stationary front in the vicinity of the I-44 corridor in Oklahoma, northeastward into southeastern Kansas, but discrete or at least semi-discrete storm development remains possible from North Texas into southern and eastern Oklahoma. Should any robust convection develop in this region, an increasingly strong low-level jet in conjunction with an upper level speed max, all severe hazards will be possible, including large to very large hail, damaging winds and tornadoes.
Convection allowing models have strayed away from the idea of a midday/early afternoon MCS and favor relatively discrete storm development in the warm sector (North Texas to southern/eastern Oklahoma and possibly western Arkansas), potentially as early as midday. This storm mode, when coupled with an already strong low-level jet, i.e. 30-40 knots by mid-afternoon and 40-60 knots by early evening, suggests that the environment could be favorable for tornadoes, potentially strong (EF-2+) given the degree of instability, low LCL heights, strong low-level wind fields, backed surface winds and rich boundary layer moisture. Storms in this regime will tend to move northeastward and may merge into clusters by late afternoon, prior to interaction with the main frontal boundary during the evening.
Elsewhere, from Northwest Texas into western, central and northern Oklahoma, additional semi-discrete to possibly discrete storms will likely develop by early the mid-afternoon. While low level wind fields may be less strong and less sharply backed than areas farther east, favorable deep layer shear and somewhat steeper mid-level lapse rates may work to offset this. As a result, large to very large hail will also be possible with these storms, as well as damaging winds. Any tornado threat would maximize closer to late afternoon and early evening, if any storm(s) can remain at least somewhat isolated. This is questionable, given deep layer shear vectors nearly parallel to the frontal boundary. Nonetheless, a few tornadoes will be possible in this area, especially if any storms form south of the frontal boundary, and/or in proximity to remnant outflow from early day convection.
Missouri into southern Illinois vicinity:
Another focused area for severe thunderstorm development will be in vicinity of a northward-lifting warm front. Storms could initiate as early as late morning from the general vicinity of St. Louis into southern Illinois and possibly southern Indiana. High-resolution guidance suggests that this area will quickly destabilize during the day, due to little if any early day convection. As storms initiate, large hail will be possible due to substantial buoyancy and tornadoes will be possible with any storms that form near or interact with the warm front. This threat may persist through much of the afternoon as the warm front lifts toward central Illinois.
Analog data from prior April/early May events with similar atmospheric conditions in place and a warm front over Missouri/Illinois suggest an elevated threat of tornadoes. This combined with recent trends increases confidence in the potential for, if not likelihood of, tornadoes in this area.
To the south and west, across the Ozarks vicinity, convection may be delayed until late in the afternoon. By this time, open warm sector convection originating in eastern Oklahoma/western Arkansas may cross over into Missouri and pose a risk for severe weather.
West Texas/Low Rolling Plains vicinity:
While this region will be displaced to the southwest of stronger large scale forcing and more favorable wind fields, daytime heating and erosion of convective inhibition may result in isolated storm development in vicinity of a dryline by late afternoon. While buoyancy may be moderate to large, relatively weak low to mid-level winds suggest that the main storm hazard will probably be hail, which could be very large. Storm longevity is questionable, but if any storms can persist far enough east and/or into early evening, as they encounter a modestly strengthening low-level jet, there may be some conditional tornado potential before nocturnal cooling results in a gradually stabilizing boundary layer.
