Frontal analysis in the light of abrupt temperature changes in a shallow valley

The authors examine selected cases of abrupt temperature changes in a shall ow valley in rural Oklahoma and examine their wider associations. All chang es 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 degrees F (11 degrees C) in less than 1 h. Th e pattern of surface potential temperature over the central United Stares i ndicates that these cases represent ideal cold-front passages, although lit tle or no weather accompanies them. Cases of abrupt warming, which are entirely nocturnal, on the other hand, a re associated with weak regional temperature gradients and with strong noct urnal cooling prior to the event under nearly clear skies and with light wi nds. The imputed surface inversion does not occur at a nearby urban locatio n, 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 t hat produce strong cooling at the rural site is the lack of organized surfa ce temperature gradient over Oklahoma on the synoptic scale at the rime of the event at Great Plains Apiaries. The abrupt warming at the rural site ma y be incidental and attributable to local topography. It is urged that in t hese 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 col d advection is evidently considered sufficient evidence for a cold front, d espite the lack of strong surface temperature gradient.