Indeterminate Category

· The “indeterminate” category contains too few cases to specify a 6 hour predominate p-type.

• 9 total cases
• 1 “all snow” case for the 6 hour period
• 2 cases of “snow and sleet” for the 6 hour period
• 1 case of “measurable snow with rain” for the 6 hour period
• 3 cases of “trace snow mixed with rain” for the 6 hour period
• 2 cases of “all rain” for the 6 hour period

Principal forecast issues

· The potential for a significant snowfall event can not be ignored in the indeterminate category since the liquid to snow ratio for these events associated with a northwest flow aloft may well exceed the typical 1:10 values.

Vertical temperature profile characteristics

· Relatively thick above freezing low layers (> 1315) only produced measurable snow when combined with relatively cold (< 1515) sub freezing mid layers (i.e., relatively steep lapse rates).

· Relatively shallow above freezing low layers (<1305) were able to produce measurable snow with relatively less cold sub freezing mid layers (>1525) as well as relatively less steep lapse rates.

Tentative findings

· Data thus far suggests that precipitation events associated with a 1000/700 mb thickness <2830 will support measurable snow provided that there is enough precipitation at a sufficient rate to erode the otherwise relatively shallow (<1305) above freezing low layer.

· In large measure, the occurrence of measurable snow associated with these partial thickness values that characterize relatively steep lapse rates associated with a northwest flow aloft will depend upon whether there is a sufficient amount of precipitation at a sufficient rate to erode the “above freezing” low layer.

Footnote

· The Indeterminate Category is defined on the predominant precipitation type nomogram by the partial thickness values of 1330/1500 – 1303/1500 – 1298/1532 – 1312/1530 – 1312/1537 – 1330/1537