Precip Type Forecasting – The Top Down Method

Top down approach uses soundings to forecast precip type.

Four Steps:1. Is there ice in the cloud?2. Consider dry layers.3. Consider warm layers.4. Consider near surface conditions.

Is There Ice in the Cloud?Start at top of sounding

Find first cloud layerDetermine temperature at this cloud layer

Coldest temperature

Minimum Temperature Is ice present?

-4C No ice

-10C 60% chance

-12C 70% chance

-15C 90% chance

-20C There is ice

Cloud layer

Cloud Layer

Consider the Dry LayersWill the precip particle evaporate?

Will temperature cool to the wet bulb temperature? If so, the layer will become saturated.

Is there a seeder feeder situation?2 cloud layers, one with ice Ice does not evaporate and falls into lower cloudWhole cloud layer eventually contains ice

Seeder Feeder

Seeder

Feeder

Evaporates

Probably does not evaporate

Dry Layer

Consider the Warm Layers If there is no warm layer, ice continues to fall

If there is, must determine if ice will melt

Keep wet bulb temp in mind Layer will cool if precip is falling

If ice is falling:Max Temp of Warm Layer

Result

< -1C Ice does not melt

1C to 3C Ice partially melts (mix falling now)

>3C Ice completely melts

Ice completely melts

Warm Layer

Ice does not melt

Warm layer

Lastly, Look Near the Surface

Does precip re-freeze, continue to melt, freeze on contact, etc.

If ice is falling:

Condition Result/Precip Type

Wet bulb < 0C Snow

Wet bulb > 0C Rain with some wet snow

If mix is falling:

Condition Result/Precip Type

Wet bulb < 0C Mix refreezes --> Sleet

Wet bulb > 0C Rain

If super cooled water is falling:

Condition Result/Precip Type

Min near surface temp > -10C & surface wet bulb < 0C

Freezing rain

Min near surface temp > -10C & surface wet bulb > 0C

Rain

Min near surface temp < -10C & surface wet bulb < 1C

Snow/sleet mix

Min near surface temp < -10C & surface wet bulb > 1C

Rain

Basically, look at depth and temp of near surface cold layer and surface wet bulb temperature.

There are always exceptions!

Example freezing rain sounding

Example snow sounding