All Snow Category

• With few exceptions, the snow predominant p-type category consists of “all snow” events for the entire 6 hour forecast period.

o 92% of the total events were “all snow”

o 95% of the total events were “mostly snow”

o 40 total events

Principal forecast issue

• The few non snow events were due to unfavorable cloud micro physics. In-cloud temperatures > -10 C generally will not contain sufficient ice to support frozen precipitation (evaluate soundings for cloud microphysics issues)
• In central North Carolina, the above scenario is most often observed near the end of a snow event as dry air aloft advects into the area on the back side of an exiting low pressure system. This accounts for many snow events ending as freezing drizzle.  

• Infrequently, a passing upper level disturbance lifts warm cloud temperatures to values colder than -10 C, producing a period of frozen precipitation from a freezing precipitation event.

Vertical temperature profiles associated with “all snow”

• Subfreezing low  layer  (<1290)   beneath a subfreezing mid layer  (<1540)

• characterized 70% of the soundings in the snow category

• Small melting layer (1291 – 1303)  beneath decisively subfreezing mid layer  (<1535)

o characterized 25% of the soundings

• Decisively subfreezing layer  (<1280)  beneath small melting mid layer (1541 – 1545)
• characterized 5% of the soundings

• Maximum temperatures in these small melting layers are likely < 1 degree C), supporting an “all snow’ forecast (an application of top-down thinking).   

Tentative findings

• Heavier snows – a good percentage of Raleigh North Carolina’s heavier snows (> 6”) likely reside in the 1275 – 1290 and 1530 – 1535 range*.
• Cyclones tracking near the North Carolina coast are likely associated with a good percentage of these heavier snow events.*
• Liquid equivalent to snow ratio – snowfalls in the above partial thickness range likely have liquid to snow ratios equal to or a little higher than the area’s climatological values of 1:10.  
• Cold air cyclogenesis - one of the coldest, significant snowfalls in the TREND data base is defined by 1263 & 1486 and 1249 & 1449 (23 January 2003). It was due to cold air cyclogenesis well behind the initial cold front and is infrequent in Central North Carolina.

Footnotes

• The snow category is defined on the predominant p-type nomogram by the partial thickness values of 1303/1500 – 1300/1520 – 1298/1532 – 1288/1538 – 1278/1542 – 1260/1545
• *These largely subjective findings are also supported by local studies on heavy snowfall conducted by Alan Huffman, a graduate studies student of Dr. Gary Lackmann at North Carolina State University.