Degree Type


Date of Award


Degree Name

Master of Science


Geological and Atmospheric Sciences

First Advisor

Tsing-Chang Chen

Second Advisor

William A. Gallus

Third Advisor

Raymond Arritt


Quantitative precipitation forecasts (QPF) are potentially the most significant challenge of weather forecasting, and have been historically difficult for numerical weather prediction (NWP) models to produce. Given the importance of QPF to weather forecasts, there is a need to improve its accuracy. Understanding and recognizing patterns prone to produce errors in model QPF can help predictions made by weather forecasters.;The low level jet (LLJ) observed over the Great Plains is responsible for supporting warm-season convection by transporting heat and moisture into the Midwest. This study examines the two types of dominant mid and upper-level flow patterns during the warm-season months of May-September over the United States, and how the LLJ interacts with this flow. Near the onset of the North American Monsoon there is a flow regime change that causes the mid and upper-levels of the troposphere to transition from a more frequent trough pattern to an anticyclone over the Midwest. Each of the two different flow regimes exists with one of two different types of LLJs. The LLJ is divided into two types, A-type and C-type. C-type LLJs are characterized by the coupling of the jet with an upper-level trough and are more likely to occur before the onset of the monsoon. A-type LLJs show no coupling with the upper atmosphere and are more likely to occur after the onset of the monsoon. With an anticyclone present, it is not possible for a C-type low-level jet to form over the Midwest, only an A-type is possible. Distinguishing the two types of events and examining them independently led to evidence that NWP models more accurately produce QPF in C-type cases than in A-type cases. Improper simulation of the LLJ, and precipitation dissipation are examined for potential reasons for the inferior accuracy of the A-type cases.



Digital Repository @ Iowa State University,

Copyright Owner

Mark R. Loeffelbein



Proquest ID


OCLC Number




File Format


File Size

53 pages