r/weather – SPC Issues Day 3 Enhanced

Currently, this has a 15% hatch, and a separate 30% hatch.

SPC AC 071932

Day 3 Convective Outlook AMEND 1 NWS Storm Prediction Center Norman OK 0232 PM CDT Wed Apr 07 2021

Valid 091200Z – 101200Z



…SUMMARY… Scattered severe thunderstorms are forecast on Friday from parts of Oklahoma and Texas eastward through the lower Mississippi Valley. The greatest risk area appears to be the lower Mississippi Valley where large hail to very large hail, severe gusts, and tornadoes are possible.

An enhanced risk was added to portions of northeast Louisiana and central and southern Mississippi. A southern stream mid-level shortwave trough is forecast to amplify during the evening hours on Friday across east Texas and Louisiana. In response, the low-level jet is expected to increase during the evening/early overnight hours across Louisiana and Mississippi. This strengthening low-level jet should increase the threat for damaging winds and tornadoes during the late overnight Friday and early morning hours Saturday across mainly central and southern Mississippi.

…Previous Discussion…

…Synopsis… The southern branch of stronger westerly mid-level flow will reside across the Desert Southwest eastward to the northern Gulf Coast states. Embedded within this belt of flow, a disturbance is forecast to move from the southern High Plains to the lower MS Valley by late evening. Farther north a mid-level trough over WY/CO will move southeastward to the lower MO Valley/Ozark Plateau by Saturday morning. A previously stalled west-east frontal zone will slowly advance northward during the day and be draped from the TN Valley westward to the southern part of the Ozark Plateau. A cold front will push southeast across OK and north and central TX primarily after sunset.

…Parts of OK/TX east through parts of the South… Considerable variability is evident in the latest deterministic model suite with the NAM an outlier compared to the ECMWF showing some recent consistency. Regardless, an area of low pressure is forecast over central TX during the day with a dryline extending southward across central TX. Strong heating over north-central TX with relatively cool 500 mb temperatures (-15 to -16 deg C) will result in 700-500 mb lapse rates in excess of 8 deg C/km. Coupling the lapse rates with 40+ kt effective shear, organized storms including supercells are likely. Very large hail may accompany the stronger updrafts. Additional storms are likely to develop during the evening as a LLJ develops over the lower MS Valley. Upscale growth into one or more bands or MCS/s is possible with the risk for damaging gusts increasing. As the low-level shear strengthens overnight, the risk for a couple of tornadoes may increase.

Farther east across the Gulf Coast states into the southern Appalachians, isolated to widely scattered diurnal storms are possible as the boundary layer destabilizes. Although the focus for storms will probably be nebulous, several clusters of storms seem probable across a large area from the lower MS Valley into the western Carolinas. Ample mid-level flow and more than adequate moisture/buoyancy will aid in storm organization. Damaging gusts and hail are the primary threats with this activity.

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