Lefse and lenticulars: A Montana Thanksgiving mountain wave event

The week of 21-25 November 2011 was characterized by persistent strong westerly winds along the Rocky Mountain Front Range of north-central Montana. While gusts exceeding 50 knots (25 m/s) were measured on each day, we focus here on the 24th due to the presence of a complicated mountain wave - frontal interaction scenario. This study uses both operational observations and a 4-km WRF simulation covering the entire week of events. The major synoptic event on Thanksgiving was the passage of a surface cold front, which could be identified by both a shift from strong westerlies to notherly or northeasterly flow and wave clouds which propagated south and southeastward along the leading edge of the cold front. As these features passed to the south, short-wavelength trapped lee wave clouds developed in the cold air behind the front extending at least 100 km out onto the plains. While this sequence of events made westerly winds generally weaker on the 24th than the previous days, strong westerlies became reestablished again by the next day.

The 81-87 hour WRF forecast performs remarkably well, especially in terms of capturing the observed surface flow pattern during the frontal passage. East-west vertical cross-sections of isentropes and streamlines across the mountains and frontal surface reveal that as the front approaches the topography, the wave pattern resembles that of the classic atmospheric rotor pattern. More detailed analysis of the model results will be performed to determine to what degree the flow field is determined by the topography and what part is played by the frontal boundary.