- With few
exceptions, the snow predominant p-type category consists of “all snow”
events for the entire 6 hour forecast period.
92% of the total events were “all snow”
95% of the total events were “mostly
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
- 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.
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.
profiles associated with “all snow”
low layer (<1290) beneath a subfreezing mid layer (<1540)
70% of the soundings in the snow category
- Small melting
layer (1291 – 1303) beneath decisively subfreezing mid layer (<1535)
25% of the soundings
subfreezing layer (<1280) beneath small melting mid layer (1541 – 1545)
5% of the soundings
temperatures in these small melting layers are likely < 1 degree C),
supporting an “all snow’ forecast (an application of top-down
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.*
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.
- 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.
- 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.