Wintry Mixture Category

• While nearly half of the events produced measurable snow, the degree of predominant p-type variability in this category is larger than any other.

o 47% of the total events produced measurable snow during the 6 hour period

o 20% of the total events were rain mixed with trace snow

o 20% of the total events were freezing rain

o 13% of the total events were rain

o 15 total events

Principal forecast issue

• Differentiating between melting layers vs. near freezing isothermal layers
• Evaluate factors favoring formation of deep near freezing isothermal layers
• Secondary factors (evaporation, melting, precipitation amounts, precipitation rates) are likely primary factors for events in this category
• Potential for development of near freezing deep isothermal layer

Additional details

• The “wintry mixture” predominant p-type category implies that precipitation types within a given six hour interval will most often involve a mixture of wintry precipitation types.
• Indeed, mixtures of rain & snow or freezing rain & snow account for 67% of the events.  All snow, freezing rain only, or just rain alone account for the remaining 33%.  Sleet is not likely and to date (11/15/07) has not yet occurred in this “wintry mixture” p-type category.
• Winter mixtures in forecasts have historically been used to convey a high degree of forecast uncertainty. Here, its use is to convey a different meaning – a persistent winter mixture is to be expected.

Tentative findings

• Many events in this category likely occur with weak thermal advection. Some are characterized by a MSL pattern featuring weak baggy highs and ill defined surface waves beneath an active upper level jet.
• The greatest variability in 6 hour predominant p-types occurs within a range of partial thickness values defined by 1295 – 1310 and 1545 – 1557. This also defines the area most prone to the development of deep near freezing isothermal layers

Quick transition versus persistent wintry mixtures tentative findings

• The forecaster must differentiate between those weather scenarios where a quick transition from one wintry p-type to another is expected versus those where a winter mixture is expected to persist. 

Quick transition wintry mixtures tentative findings

• Quick transitions from one p-type to another are typically associated with defined surface lows and highs whose horizontal thermal advection is the primary factor controlling the predominant p-type. 
• Quick transitions are seen on the predominant p-type nomogram as progressive movement through the wintry mixture category. 

Persistent wintry mixtures tentative findings

• Persistent wintry mixtures are typically associated with ill defined “baggy’ surface highs & “flat’ surface waves, indicating that horizontal thermal advection is absent or very weak.
• This allows for so-called secondary p-type factors to become the primary players for controlling precipitation types. These factors include evaporation, melting, and the latent heat of freezing.
• One of the more frequent weather scenarios associated with persistent wintry mixtures is an upper level jet (typically, the sub-tropical jet) moving across polar or arctic air that is already in place.  The surface wave is ‘flat” while the high is weak and/or no longer located in a favorable position to provide additional low level cold air advection.
• This scenario is reflected on the predominant p-type nomogram as meandering movements within or not far from the wintry mixture category.  These small movements are driven by the evaporation/sublimation of snow in the sub-cloud dry layer. Once the air mass is saturated, the melting of snow or the latent heat of freezing from freezing rain become the principal controlling p-type factors. These factors are one order of magnitude smaller than evaporation.
• The public’s perception of a correct forecast depends less on forecasting a persistent mixture and more so on whether the mixture will result in a measurable snow. This is especially so for the more snow starved regions of the country.
• The amount of precipitation is critical to whether measurable snow will result from the evaporation and sublimation of precipitation into the sub-cloud dry air. The rate of precipitation is vital for whether measurable snow results from melting.
• Due to the degree of difficulty in predicting precipitation amounts & rates, a conservative forecast strategy providing enough lead time that allows for essential preparation by key users such as the Department of Transportation, schools, etc. is in the long run likely the best course of action. 

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Footnote

• The wintry mixture category is defined on the predominant p-type nomogram by the partial thickness values of 1290/1544 – 1312/1538 – 1310/1550 – 1290/1550