During the early morning hours of September 7th, 2022, the SPC convective outlook issued at 1200 Z featured two upgraded areas from the previous outlook at 0500 Z (see image gallery below). Today we will focus on the upgrade from ‘Thunderstorm’ to ‘Marginal’ in southeast Texas and southwest Louisiana.
The question is..what caused the upgrade? The SPC Convective Outlook discussion from 0500 Z mentions thunderstorm activity in the area from a weak shortwave trough, but with little shear and poor lapse rates limiting thunderstorm severity potential. The Outlook from 1200 Z reveals that a mesoscale disturbance was moving into this area from the northeast, and modifying the mesoscale environment that would coincide with thunderstorms later in the day.
…Southeast TX and southwest LA…
A minor mid-level impulse evident in water vapor imagery over the Ark-La-Tex should gradually move south-southwest into south TX by this evening. A pocket of relatively cooler mid-level temperatures and steeper lapse rates attendant to this impulse should support a plume of moderate to large MLCAPE of 1500-3000 J/kg. While deep-layer shear will be modest and generally offset west of the greater instability/buoyancy, isolated marginally severe hail and strong to localized severe wind gusts will be possible as scattered thunderstorms occur this afternoon.SPC Day 1 Convective Outlook, 7 September 2022 (12Z)
Water vapory imagery shows the location of the mesoscale disturbance as it moves from southern Missouri into eastern Texas and western Louisiana, and begins to even initiate morning convection.
The SPC Mesoanalysis at 1400 Z reveals increased mid-level lapse rates and ML-CAPE moving southwest into the Marginal hazard area during the morning hours, driven by the mid-level impulse.
One potential failure mode in this scenario would be early morning cloud cover, which would keep the ML-CIN in place and limit thunderstorm development. High clouds from Hurricane Kay in the eastern Pacific are one potential source. In this scenario however, the SPC forecaster used the Nighttime Microphysics RGB to examine the locations of cloud layers before visible satellite imagery became available during the day. The following quote from the SPC forecaster on shift highlights this application.
Deep convection over the Ark-La-Tex. Patchy low clouds to the southwest of this into south-central TX beneath high-level cirrus loosely associated with TC Kay off southern Baja CA. Mainly cloud-free in between across southeast TX to southwest LA indicative of nearly full insolation this morning for the newly added MRGL in the 13Z outlook. Helpful to “see” before visible satellite imagery becomes available.SPC Forecaster Comment