or Snow with Sleet Category
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
- 38% of the measurable snow
events mixed with sleet.
- 38% fell as “all snow’.
- 24% were snow mixed with
- 16 total events
Principal forecast issues
snow/sleet events from “all snow” events & resolve impact on liquid
equivalent to frozen ratios
leading to “all snow events”
melting layer – a
maximum temperature in the melting layer <1 C supports a
forecast of snow. Evaluate soundings.
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.
thermal advection can
erode or increase the small melting layer. Evaluate trends in the partial
thickness values for significant thermal advection.
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
leading to snow/sleet mix events
layer – Snow/sleet
mix is favored if the max temperature in the melting layer is > 1 C and <
3 C. Evaluate soundings.
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)
subfreezing cold nose (1280s) with a near freezing isothermal segment
subfreezing cold nose (<1280) beneath a small to moderate warm nose
(> 1545 & < 1560).
“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.
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.
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