PERSISTENT LOW OVERCAST EVENTS IN THE US UPPER MIDWEST - A CLIMATOLOGICAL AND CASE-STUDY ANALYSIS

Persistent low overcast conditions, defined as continuous overcast con ditions (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 circul ation patterns. Most commonly, the midlatitude westerlies are split ac ross North America, with downstream confluence of the northwesterly po lar and the southwesterly subtropical jet streams. A second, less freq uent, 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 em ergence from either stream of relatively weak cyclones. These systems act to moisten the affected region at low levels through horizontal tr ansport of moisture and, to a lesser extent, moisture convergence. Sub sidence 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 di ssipate the cloud. The properties of the event structure, from the lar ge scale down to that of the cloud layer itself, are stable. Under suc h conditions, the mechanism that finally removes the cloud is the pass age of a relatively well-developed baroclinic wave and its associated forcing (subsidence, dry air advection, moisture divergence). Correspo ndingly, the difficult act of forecasting the end of such periods requ ires an accurate assessment of the sufficiency of that forcing to remo ve the low-level cloud. It is suggested that a relatively simple one-d imensional boundary layer model employed for the time to be critically tested in conjunction with the standard forecast model guidance (fore cast vertical motion, profiles of temperature and moisture, Model Outp ut Statistics cloud cover and ceiling) would provide additional inform ation regarding forecast uncertainty.