Icing - Measurable Sleet with Freezing Rain Category

· Consists chiefly of measurable sleet events, most often mixed w/freezing rain during the 6 hour forecast period.  

· 86% of the total events produced measurable sleet.

o 86% of the measurable sleet events mixed w/freezing rain

o 7% mixed w/snow

o 7% were all sleet

o 14% of the total events were all snow

o 14 total events

Due to the close proximity of the Atlantic and the Gulf of Mexico, an established inversion with a significant melting layer is prone to be maintained. This in part may explain the tendency for measurable sleet in Central North Carolina to be associated with freezing rain

Principal forecast issues

• Liquid equivalent to sleet ratio
• Erosion of a sleet producing melting layer, supporting snow at the expense of sleet
• If due to melting, evaluate soundings for presence of a near freezing isothermal layer. More likely to occur when the thickness values in the 850/700 mb layer are in the 1550-57 range.
• If due to cold air advection, note trends in the 850/700 mb partial thickness values. 
• A max temperature in the melting layer < 1 C supports snow while temperatures > 1 C and < 3 C support a snow/sleet mix to sleet event. Check soundings.

• Anticipating strong low level thermal advection.
• TREND data base shows several events where the partial thickness values were only marginally in support of measurable sleet. These events trended quickly toward colder low level thickness values likely due to cold air damming. 
• Anticipating prolonged sleet events
• Influence of elevated convection – ice seeding aloft
• Influence of in-cloud temperatures colder than -10 C in the 1000/850 mb layer
• Influence of an occluding inland low (Miller “B” cyclogenesis) - limiting warm air advection

Vertical temperature profiles associated with the measurable sleet w/freezing rain category

• Prominent subfreezing cold nose  (<1280)  beneath a prominent melting warm nose  ( > 1560)

o characterized 25% of the soundings

• Small subfreezing low level cold nose  (~1290)  beneath a prominent melting warm nose  ( >1560)

o 33% of the soundings

o these events were trending toward colder low level partials during the 6 hour forecast period likely due to cold air damming.

• Potential for mid level near freezing isothermal segments (850/700 mb thickness values in the 1550s)

o 21% of the soundings

o 850/700 mb thickness values in the 1550s in this predominant p-type category can be associated with near freezing isothermal segments

o when near freezing isothermal, then any melting layer aloft is very small and would lead to snow falling at the expense of sleet.

Prolonged Sleet - Tentative Findings

• Liquid equivalent to sleet ratios - a limited number of “largely to all sleet” cases suggest a liquid equivalent to sleet ratio of 1:2-3 inches.
• The prolonged sleet storm of Feb 16-17 1987 (6.5” RAH, 2” GSO) was a rare event.
• The sleet may have been prolonged due in part to a max cold nose temperature colder than – 10 C in the 1000/850 mb layer, providing an ongoing ice seeding mechanism. 
• The rare prolonged sleet event was also associated with elevated convection, producing ice seeding through a deep layer aloft.
• Partial thickness values <1275 in the 1000/850 mb layer, minimum temperatures colder than -10 C in this layer and/or the presence of elevated convection alerts the forecaster to the potential of prolonged sleet   
• Miller “B” – Rare prolonged sleet events can be associated with Miller “B” cyclogenesis where the inland surface low is without warm advection due to occlusion and there is also strong cold air advection.

Footnotes

• The  “measurable sleet with freezing rain” category is defined on the predominant p-type nomogram by the partial thickness values of 1260/1562 – 1282/1550 – 1293/1550 – 1290/1580

• Events where evaporative cooling in dry layers aloft produce a brief period of sleet that reaches the ground with surface temperatures well above freezing do not occur in this predominant p-type quadrant. Furthermore, since TREND is designed to specify predominant p-type based upon potential impact, it does not include nuisance (i.e. no impact) sleet events in its data base.