Factors Influencing P-type that TREND Does Not Directly Account For
TREND does not account for cloud microphysics issues
If in-cloud temperatures are -10 C or colder, then cloud supports ice and frozen precipitation is possible.
If in-cloud temperatures are warmer than -10 C, then cloud likely does not contain enough ice to support frozen precipitation.
Diabatic Processes (evaporative cooling)
TREND indirectly accounts for evaporative cooling
Though typically a secondary factor in determining p-types, evaporation becomes a primary factor when horizontal thermal advection is weak.
Horizontal thermal advection is typically weak when the MSL pressure pattern is ill defined and characterized by weak baggy highs and flat surface waves.
Use soundings to evaluate the potential for evaporative cooling. The deeper and drier the sounding, the greater the potential for cooling from evaporation.
Many of central North Carolina’s snow events begin as rain. The evaporation of rain or the sublimation of snow into dry air can cool an existing melting layer to freezing or below.
As evaporation and sublimation is taking place, the cooling realized can be as much as 5-10 degrees F within an hour.
In order for evaporation to be an effective cooling mechanism, there must be enough precipitation to realize the cooling potential. In the drier air masses, the first 0.10 -0.25 inches of precipitation will likely evaporate before precipitation reaches the ground.
When there is a sub cloud dry layer, low level wet bulb temperatures are an excellent means to fine tune the p-type forecast. Refer to the forecast soundings in BUFKIT and turn on the Tw profile.
Once the air mass is saturated, the wet bulb temperatures are no longer an effective means for anticipating p-types. Instead use the ambient temperature.
Prior to the onset of precipitation with a sub cloud dry layer, use the surface wet bulb temperatures to determine how far south and east the p-type threat area will extend. Also, use the wet bulb temperature profile in BUUFKIT to determine if the frozen or freezing precipitation is supported at the surface.
Polar air masses are typically very dry through deep layers.
These air masses are often characterized by vertical temperature profiles that approach an isothermal lapse rate through a significant layer. In the absence of significant horizontal thermal advection, these air masses are predisposed to the development of near freezing isothermal layers.
Diabatic Processes (melting)
The TREND technique indirectly accounts for melting.
Melting is a process that can cool the temperature to freezing.
The effectiveness of melting to lower a temperature to freezing is dependent upon the precipitation rate as well as the depth of the melting layer.
An empirical rule of thumb states that a 100 mb melting layer requires 0.2 inches of precipitation to erode. To determine the amount of precipitation needed to erode an existing melting layer use the equation below:
(Depth of melting layer) (melting layer mean wet bulb temperature)
where the precipitation is in inches; the layer is in mb, and the wet bulb temperature is in degree C
The deeper the melting layer, the less cooling realized from a given precipitation rate.
As the precipitation rates increases (decreases), melting increases (decreases) and the snow level is lowered (raised). This explains why there can be relatively frequent changes between rain and snow as the precipitation rate change.
Relative to evaporation, melting is a less effective means for cooling. Cooling from melting is typically an order of magnitude smaller than cooling from evaporation.
Though typically a secondary factor for determining p-types, melting becomes a more important factor when horizontal thermal advection is weak.
In a mature cyclone, horizontal thermal advection is weak since the atmosphere is now tending toward a barotropic state. For those situations where there is a weak melting layer preventing snow from reaching the ground, the northwest quadrant of a mature cyclone can be a favorable location where melting via increased precipitation rates can lower the snow level to the ground.
The combination of weak highs and lows at the surface (i.e., weak horizontal thermal advection) and an active jet aloft is another scenario where increased snow rates can erode a marginal melting layer. This situation results in the infrequent pattern of snow islands embedded in a cold rain.
In the long term, precipitation rates are hard to anticipate. In the short term for updates and nowcasts, you can use the WSR 88D to monitor precipitation rates and also the reflectivity products looking for evidence of the melting “bright band”.
Locations on the cold side near the snow/rain boundary are prone to enhanced snow fall associated with an ageostropic circulation restoring thermal balance to differential heating along the snow/rain boundary.
Isothermal near freezing layers developed in response to melting and can be identified in the soundings only when other physical processes impacting the vertical temperature profile are weak.
Latent heat of freezing
TREND does not account for the latent heat of freezing
In the absence of sustained cold air advection, freezing rain is a self limiting process
As temperatures near 30 degrees F, the ratio between liquid and ice accumulation is no longer 1:1. Once temperatures are 32 F, (point at which water freezes and ice melts), there is no longer any significant ice accumulation.
Without a source of sustained low level cold air advection, temperatures increase to 32 F as supercool (< 32F) water droplets freeze.
Warmth of ground and road surfaces
TREND does not account for surface temperatures
Warm soil temperatures can melt snow as it falls, limiting accumulations.
Use 4 inch soil temperatures (refer to agricultural observational network) to determine if soil temperatures are warm enough to limit snow fall accumulations.
Monitor DOT and Airport Authority sites providing pavement temperatures to determine if surfaces are cold enough to support the accumulation of snow and ice.
Be alert for those situations where surface temperatures are above freezing while tree top temperatures are cold enough to support the accumulation of ice.
Recall that freezing rain is a “self limiting” process. If there is no source for sustained cold air advection, then the latent heat of freezing associated with freezing rain will gradually increase temperatures to the freezing. Once temperatures reach 32 degrees, ice no longer accumulates.
Atypical means for clouds to become cold enough to support ice
Seeder – Feeder…Ice supporting cold cloud (-10 C or colder) moves over lower cloud composed of super cool droplets (warmer -10 C). Ice falls into and seeds lower cloud. Lower cloud must be within 5,000 ft of upper cloud or ice sublimates. Capable of supporting frozen precipitation for short periods of time.
Elevated Convection…If elevated convection accompanies a warm cloud precipitation event, then convection seeds ice into the cloud. Capable of producing a “surprise” period of thunder snow or thunder sleet.
Sub freezing surface based cold layer has a cold nose colder than -10 C beneath a warm melting layer. This cold nose can introduce ice into a cloud making snow and/or sleet possible.
Vertical Motion…..A passing disturbance moves across an area of otherwise warm clouds consisting of super cool droplets. The enhanced vertical velocities lift and cool the cloud to temperatures cold enough to introduce ice. Process is capable of producing frozen precipitation from an otherwise non frozen event.