Flash Flooding Issues Remain Possible from Slow-Moving Cluster of Storms

Mesoscale Precipitation Discussion: #0659

Summary…A slow-moving cluster of storms currently over northeast Mississippi and northwest Alabama will produce localized heavy rainfall, which could lead to flash flooding. The cluster of storms may build to the west and north later this morning, into additional portions of northern Mississippi and southern Tennessee.

Discussion…GOES-16 IR satellite channels showed a convective cluster that appeared to be nearly stationary over the past two hours (0930-1130Z), with gradually cooling and expanding cloud tops. KHTX and KGWX radars show that this is actually comprised of several convective clusters that have been slowly lifting north (with the deep layer mean flow) and exhibiting clear periods of strengthening, followed by decay. Despite the lack of a strong cold pool, new convective clusters have been growing on the upshear flank of existing convection, near the nose of broad 15-20 knot inflow. This has created a short period of backbuilding, which makes sense conceptually given the upstream location of a regional instability maximum over N MS. The convection is situated in an environment with deep tropical moisture (PW over 2.2 inches) and narrow, thin CAPE profiles. This is supportive of efficient, warm rain processes and rainfall rates exceeding 2 inches per hour. Indeed, MRMS appears to be applying a tropical convective classification to these clusters and has shown rainfall rates up to around 3 inches per hour recently. This will be supportive of localized rainfall maxima in excess of 5 inches, and the potential for some flash flooding during the morning hours.

The evolution beyond 13Z is somewhat uncertain as hi-res models do not seem to be representing the ongoing convection particularly well. The overall synoptic set up is not expected to change drastically. Broad 15-20 knot inflow should continue over MS and W TN, with an upstream instability maximum, which would suggest that backbuilding could be maintained as the cluster may slowly lift northward given the deep layer mean flow. Additionally, the convection should continue to be situated in the right entrance region of a 50-knot upper-level jet streak, so the upper-level dynamics will also favor large scale vertical motion. Nevertheless, we are headed into the typical diurnal convective minimum, and it is not uncommon for such convective clusters to gradually diminish either, particularly when they lack substantial organization as this convection has thus far. Trends will continue to be monitored, but the threat of localized flash flooding should continue to persist for at least another couple hours.

Category: Alabama's Weather, ALL POSTS, Severe Weather