I. Factors to consider when making a quantitative precipitation forecast

Always remember that precipitation is difficult to forecast and there is always great deal of uncertainty in all quantitative precipitation forecasts.

Understand what the end "user" (e.g., hydrological models, skill scoring, etc) is concerned about.

Local climatology: consider time of year and location; Influence of local terrain and orographic features.

Synoptic Forcing (e.g., PVA, temperature advection, q-vectors, etc...)

Moisture availability

Convective potential (CAPE, CIN, etc...don't focus only on surface parcels).

Mesoscale terrain or orography interaction with the synoptic scale flow.

Model QPF: Understand weaknesses and biases (particularly important in the west).

Consensus generally works best.

II. Precipitation types

Remember that freezing = 0 C, but water will not freeze at temperatures between -40 C and 0 C unless it comes in comes in contact with activated ice nuclei

Rain: Liquid precipitation with drops larger than .5 mm; smaller drops can qualify as rain if they are widely separated. Occurs when the surface layer and/or layers aloft are warm enough and deep enough to melt frozen precipitation particles (e.g., snow) before they reach the ground.

Snow: Occurs when no sub-cloud layer is warm and deep enough to melt the snow before reaching the ground.

Freezing Rain: Occurs when a layer aloft is warm and deep enough to melt frozen precipitation particles but the below freezing surface layer is not cold or deep enough that the precipitation refreezes before reaching the ground.

Sleet: Occurs when a layer aloft is warm and deep enough to melt frozen precipitation particles and the surface layer is cold and deep enough to refreeze the liquid precipitation before it reaches the ground.

Drizzle: Precipitation composed exclusively of small drops (generally < .5 mm). Frequently produced by "warm" or supercooled clouds that are not cold enough for significant ice nucleation to occur.

Freezing Drizzle: Drizzle that freezes on contact with the ground.

Grauple: A rimed precipitation particle that no longer exhibits the shape or character of the initial collector particle. Typically shaped like a "Mercury capsule."

Hail: A heavily rimed precipitation particle in the form of a small ball or piece of ice.

III. Forecasting Precipitation Type

Critical Thickness: At many stations, a critical 1000-500 mb thickness is used as a first guess for rain vs. snow. At low-elevation, continental locations in the eastern united states, 540 dm is generally used as the critical thickness. Lower thicknesses are used at coastal locations.

Critical 850/700 mb temperatures: Similar to above, except 850 or 700 mb temperatures are used. 0 C at 850 mb is used at many lower elevation, continential locations, -8 C 700 mb is what's used at SLC airport.

Do not rely on simplistic critical thicknesses and temperatures. They provide no information on the vertical thermal structure of the atmosphere and therefore can be problematic.

It is CRUCIAL to examine observed and model soundings when forecasting precipitation type. Use such soundings to determine what precipitation type(s) a station will observed.

Remember the importance of evaporative cooling and other diabatic effects.

Adjust for local effects (e.g. cold air damming east of the Cascade Mountains is not handled by the AVN, NGM, or 48 km ETA).

Updated May 27, 1997