The Impact of Low-Level Moisture Errors on Model Forecasts of an MCS Observed during PECAN

This article investigates forecast errors of the environment near an elevated mesoscale convective system (MCS) in Iowa on 24-25 June 2015 during the Plains Elevated Convection at Night (PECAN) field campaign. The eastern flank of this MCS produced an outflow boundary (OFB) and propagated southeastward along this OFB as a squall line.  The western flank of the MCS remained quasi stationary at a location approximately 100 km north of the system's OFB and produced localized flooding.  A total of 16 radiosondes were launched near the MCS's eastern flank and 4 near the MCS's western flank during this event.

Convective available potential energy (CAPE) increased and convective inhibition (CIN) decreased substantially in observations during the 4 hours prior to the arrival of the squall line.  In contrast, the model analyses and forecasts substantially under-predicted CAPE and over-predicted CIN owing to their under representation of moisture.  Numerical simulations that placed the MCS at varying distances too far to the northeast were analyzed.  MCS displacement error was strongly correlated with models' under-representation of low-level moisture and their associated over representation of convective inhibition.  The over-predicted CIN in models resulted in northeastward moving air parcels requiring unrealistically long residence times and northeastward travel in regions of gradual meso-alpha scale lift before these parcels initiated convection.  These results suggest that erroneous MCS predictions by NWP models may sometimes result from poorly analyzed low-level moisture fields.