All Snow
or Snow with Sleet Category
- Consists
of measurable snow events. Most frequently, snow is mixed with sleet or
falls as “all snow” during the 6 hour forecast period. Less frequently,
snow is mixed with freezing rain
- 100% of the total events produced
measurable snow.
- 38% of the measurable snow
events mixed with sleet.
- 38% fell as “all snow’.
- 24% were snow mixed with
freezing rain
- 16 total events
Principal forecast issues
- Differentiate
snow/sleet events from “all snow” events & resolve impact on liquid
equivalent to frozen ratios
- Processes
leading to “all snow events”
o
Insufficient
melting layer – a
maximum temperature in the melting layer <1 C supports a
forecast of snow. Evaluate soundings.
o
Increased
precipitation rates can
erode the small melting layer. The appearance of a near freezing isothermal
segment in the soundings signals cooling from melting. Evaluate soundings.
o
Significant
thermal advection can
erode or increase the small melting layer. Evaluate trends in the partial
thickness values for significant thermal advection.
o
If the inland
low associated with a Miller “B” or “A/B” cyclogenesis pattern occludes, then
warm advection ahead of the inland low will end and likely lead to the erosion
of the small melting layer
- Processes
leading to snow/sleet mix events
o
Small melting
layer – Snow/sleet
mix is favored if the max temperature in the melting layer is > 1 C and <
3 C. Evaluate soundings.
Vertical temperature
profiles associated with the snow/sleet predominant p-type category
- Small subfreezing
cold nose (1280s) beneath a very small above freezing warm nose (1540-45)
- Small
subfreezing cold nose (1280s) with a near freezing isothermal segment
(1545-1555)
- Large
subfreezing cold nose (<1280) beneath a small to moderate warm nose
(> 1545 & < 1560).
Tentative Findings
- Miller
“B” cyclones and cold air damming - Snow/sleet mix events are often associated with
Miller “B” or “A/B” cyclogenesis and cold air damming, producing
“corridors of predominant p-types” across central North Carolina.
- Liquid
equivalent to frozen ratios – Given the greater compaction of sleet relative to snow,
empirical data suggests a 1:4-6 ratios for snow/sleet mix events.
Footnote
- The
snow/sleet category is defined on the predominant p-type nomogram by the
partial thickness values of 1260/1545 – 1278/1542 – 1290/1537 – 1290/1550
– 1282/1550 – 1260/1562