Credit for this guide goes to the Raleigh office of the National Weather Service. This is the best example I've seen on how to read the P-Type nomogram. Mouse over the different areas on the nomogram to detailed information about that section.

Below the Nomogram image, is more detailed insight as to how the P-Type Nomogram works. These forecast images are no longer being produced to my knowledge, but the page is still on the Raleigh website, so we'll see this winter.

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• With few exceptions, consists of “all snow” events for the entire 6 hour forecast period.
• 92% of the total events were “all snow”
• 95% were mostly snow

Principal Forecast Issue

• Cloud microphysics (“in-cloud” temperature > -10 C) - sole reason for a non snow event when the entire vertical temperature profile was below freezing (evaluate soundings)

Tentative findings

• Partial thickness values greater than 1530 (850/700 mb) combined with 1291-1303 (1000/850 mb) represent a decisively subfreezing layer above a surface based shallow melting layer. These melting layers likely have maximum temperatures less than 1 C, supporting snow.
• Heavier snows - a good percentage of heavier snows (> 6”) likely reside in the 1275 – 1290 and 1530 – 1535 range.
• Cyclones tracking near the North Carolina coast are likely associated with these heavier snow events.
• Liquid equivalent to snow ratio in the above partial thickness range likely have liquid to snow ratios equal to or a little higher than the area’s climatological values of 1:10.

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• Consists of measurable snow events. Most often, 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 measurable snow.
• 38% of the measurable snow events were mixed with sleet.
• 38% fell as “all snow’.
• 24% were snow mixed with freezing rain.
• 16 total events

Principal Forecast Issue

• Differentiate snow/sleet mix from all snow events, resolving impact on liquid equivalent to frozen ratios
• Maximum temperature in the melting layer less than 1 C supports all snow
• 1 – 3 C max temperature supports snow/sleet mixture to

Tentative findings

• Liquid equivalent to frozen ratios – Given the greater compaction of sleet relative to snow, empirical evidence suggests a 1:4-6 ratio for snow/sleet combination events.
• Miller “B” cyclones and cold air damming - Snow/sleet mix events are often associated with Miller “B” or “A/B” cyclogenesis and cold air damming.
• Freezing rain mixing with snow in this category is most likely due to a significant 6 hour warming trend in both the 1000/850 and 850/700 layers.

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• 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.
• 14% of the total events were all snow.
• 86% of the measurable sleet events mixed w/freezing rain.
• 14 total events

Principal Forecast Issue

• Erosion of a sleet producing melting layer, supporting snow at the expense of sleet (evaluate configuration of surface high)
• Liquid equivalent to sleet ratio
• Prolonged sleet

Tentative findings

• Liquid equivalent to sleet ratios ~ 1:2-3 inches
• Significant measurable sleet events likely associated with Miller “B” or “A/B” cyclogenesis and cold air damming
• Prolonged sleet events
• elevated convection – ice seeding aloft
• in-cloud temperatures less than -10 C in the 1000/850 layer – an atypically located ice seeding mechanis
• The frequent occurrence of measurable sleet with freezing rain in central North Carolina is likely due to the transport of moist warm air from the nearby Atlantic and the Gulf of Mexico and/or weakening cold air damming.

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• Largely consists of freezing rain & often mixed with a trace of frozen precipitation during the 6 hour period. Infrequent exception is significant snow associated with a deep near freezing isothermal layer.
• 70% of the total events were predominantly freezing rain
• 15% of the total events were predominantly snow
• 15% of the total events were rain w/trace snow
• 26 total events

Principal Forecast Issue

• Surface wet bulb temperatures define southern & eastern extent of icing area
• High potential for significant icing
• Freezing rain as a self limited process vs. sustained low level cold air advection
• Evaluate factors favoring the formation of a deep near freezing isothermal layer:
  • Weak thermal advection (baggy highs, flat lows)
  • Northwest quadrant of mature cyclone
  • Moderate to heavy precipitation rates & surface temperatures within several degrees of 0 C (precipitation banding)
  • Near snow/rain boundary on the snow side

Tentative Findings

• In this predominant p-type category, deep near freezing isothermal layers are most likely to be associated with 850/700 mb partial thickness values from 1550-1557.

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• With few exceptions, consists of freezing rain or rain for the 6 hour period.
• 57% of the total events were freezing rain.
• 40% of the total events were rain.
• 3% of the total events were measurable snow w/rain.
• 30 total events

Principal Forecast Issue

• Use surface wet bulb temperatures to differentiate freezing rain vs. rain
• Nuisance vs. significant icing event
• Freezing rain as a self limiting process vs. sustained low level cold air advection (evaluate configuration of surface high)

Tentative Findings

• Freezing rain events in this category will typically be accompanied by temperatures close to freezing, limiting the impact of icing. Exceptions:
  • Significant icing at tree top level due to colder temperatures just above ground
  • Preexisting very cold ground

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• With very few exceptions, this category consists of all rain events for the entire 6 hour period.
• 95% of the total events were rain
  5% of the total events were freezing rain or rain mixed with trace frozen
• 75 total events

Principal Forecast Issue

• The 1000/850 mb layer is too thick to reliably capture the rare occurrence of a slowly modifying and exceptionally shallow surface based subfreezing arctic airmass, supporting a significant freezing rain. Evaluate the surface wet bulb temperatures.

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• Snow/rain mixtures are likely in this category with most events resulting in measurable snow during the 6 hour period. Infrequently, freezing rain or all snow occurs.
• 66% of the total events produced measurable snow during the 6 hour period
• 65% of the measurable snow events occurred with rain
• 35% of the measurable snow events were all snow
• 17% of the total events were rain mixed with trace snow
• 17% of the total events were freezing rain
• 17 total events

Principal Forecast Issue

• Differentiating snow/rain mixtures with measurable snow vs. those with trace snow
• Potential for caa from intensifying cold air damming
• Potential for evaporative cooling
• Accounting for the infrequent “all snow” events
• Potential for near freezing isothermal layer from melting
• Accounting for infrequent freezing rain events
• Potential for in-cloud temperatures > -10 C
• Potential for waa from fast approaching surface waves
• Evaluate soundings, low level wet bulb temperatures, trends in partial thickness values, cloud microphysics, patterns of cyclogenesis and surface high configurations

Tentative findings

• The vertical temperature profiles most typically found in this category are surface based melting layers of varying degrees.
• Measurable snow events in this category likely result from low level cooling associated with cold air damming and/or evaporative cooling and less so from melting.
• Freezing rain events in this category may be due to in-cloud temperatures > -10 C

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• While 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 rain
• 13% of the total events were rain
• 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

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 – 1558. This also defines the area most prone to the development of deep near freezing isothermal layers

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• The “indeterminate” category contains too few cases to specify a 6 hour predominate p-type.

Principal Forecast Issue

• The vertical temperature profile associated with the indeterminate category is characterized by a very steep lapse rate.
• In central North Carolina, very steep lapse rates are commonly dominated by a west to northwest flow pattern in the low to mid layers.
• Precipitation events with very steep lapse rates are infrequent in central North Carolina due to:
  • limited moisture found in the west to northwest flow pattern
  • down slope winds prevail on the lee side of the Appalachian Mountains