Frontal Analysis in the Light of Abrupt Temperature Changes in a Shallow Valley

Abstract

The authors examine selected cases of abrupt temperature changes in a shallow valley in rural Oklahoma and examine their wider associations. All changes in the valley, whether rise or fall, are accompanied by a northerly wind shift at or shortly before the time of the abrupt change. Cases of cooling show a drop of as much as 20°F (11°C) in less than 1 h. The pattern of surface potential temperature over the central United States indicates that these cases represent ideal cold-front passages, although little or no weather accompanies them. Cases of abrupt warming, which are entirely nocturnal, on the other hand, are associated with weak regional temperature gradients and with strong nocturnal cooling prior to the event under nearly clear skies and with light winds. The imputed surface inversion does not occur at a nearby urban location, and the breakdown is an important factor in the temperature rise at the rural site. The important distinction between these cases and the cold-front passages that produce strong cooling at the rural site is the lack of organized surface temperature gradient over Oklahoma on the synoptic scale at the time of the event at Great Plains Apiaries. The abrupt warming at the rural site may be incidental and attributable to local topography. It is urged that in these cases the wind shift be denoted a “trof” rather than a cold front. In present practice the shift to a northerly wind with rising pressure and cold advection is evidently considered sufficient evidence for a cold front, despite the lack of strong surface temperature gradient.