Persistent Low Overcast Events in the U.S. Upper Midwest: A Climatological and Case Study Analysis

Abstract

Persistent low overcast conditions, defined as continuous overcast conditions (100% cloud cover) with ceiling heights at or below 2 km for a minimum of 5 days, are found to occur in the cold season in the U.S. upper Midwest on average slightly more often than once every two years. These occurrences are associated with two primary large-scale circulation patterns. Most commonly, the midlatitude westerlies are split across North America, with downstream confluence of the northwesterly polar and the southwesterly subtropical jet streams. A second, less frequent, pattern features an amplified westerly jet across North America, with a correspondingly rapid progression of weakly developed cyclones through the region. In the case of the split flow pattern, composite surface high pressure is established, occasionally disrupted by the emergence from either stream of relatively weak cyclones. These systems act to moisten the affected region at low levels through horizontal transport of moisture and, to a lesser extent, moisture convergence. Subsidence inversions established following the passage of these systems act to slowly erode the depth of the surface-based moist layer but are insufficient in combination with the weak solar radiative input to dissipate the cloud. The properties of the event structure, from the large scale down to that of the cloud layer itself, are stable. Under such conditions, the mechanism that finally removes the cloud is the passage of a relatively well-developed baroclinic wave and its associated forcing (subsidence, dry air advection, moisture divergence). Correspondingly, the difficult act of forecasting the end of such periods requires an accurate assessment of the sufficiency of that forcing to remove the low-level cloud. It is suggested that a relatively simple one-dimensional boundary layer model employed for the time to be critically tested in conjunction with the standard forecast model guidance (forecast vertical motion, profiles of temperature and moisture, Model Output Statistics cloud cover and ceiling) would provide additional information regarding forecast uncertainty.