nearly half of the events produced measurable snow, the degree of
predominant p-type variability in this category is larger than any other.
47% of the total events produced
measurable snow during the 6 hour period
20% of the total events were rain
mixed with trace snow
20% of the total events were freezing
13% of the total events were rain
15 total events
Principal forecast issue
between melting layers vs. near freezing isothermal layers
factors favoring formation of deep near freezing isothermal layers
factors (evaporation, melting, precipitation amounts, precipitation rates)
are likely primary factors for events in this category
for development of near freezing deep isothermal layer
“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.
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.
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.
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.
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
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
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.
transitions are seen on the predominant p-type nomogram as progressive
movement through the wintry mixture category.
mixtures tentative findings
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.
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.
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.
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
- 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