What's "freezing at the surface" mean?

I've often wondered about the term too, so I sent an e-mail to the NOAA forecasters when I first read the thread. Here's the reply:

> Hi - do you mind if I post a snipped portion of your reply?

" ... That would be fine.

<<names/e-mail address snipped>>

Tony,

The snow and freezing levels in our forecast are in respect to sea level. "...at the surface" is a reference to a snow or freezing level at or below sea level.

Thanks for your question. I hope this helps. Thanks for using our website.

and this...

I read the linked string *after* first replying to you. The term is indeed rather vague and ambiguous. Therefore, I e-mailed the entire forecast staff to see what their interpretation is. I'll get back to you in about a week once everyone's had a chance to reply. At the very least, "freezing level at the surface" tells me that the temperature above the surface is going to be sub-freezing.

When asking the question I think you need to put it into context as "freezing level at the surface" when used for a Mountain Pass forecast, or maybe cut-n-paste an actual example forecast that they could then explain.

I think bscott pretty much answered the question though.....

Actually, that 6000-ft freezing level is a critical factor in determining if there will be freezing rain or snow in the passes. If the freezing level is at the surface in the passes but above 6000 ft elsewhere, and precipitation is forecast, then there will be freezing rain in the passes. If it is below 6000 ft under the same circumstances, then it will snow in the passes.

Hope this helps.

Bryan Scott-retired weatherman.

In my experience this assessment is not right even a majority of the time. Other critical factors in the passes are the strength of the surface gradients and the depth of the cold air coming from Eastern Washington. This leads to a great variety in weather in the passes...and freezing rain in my 6 years of experience in the passes isn't very often what happens (in the scenerio described above). Stevens, for example will often get pure snow when the freezing level is much higher in other places. Often as a new system approaches and/or old one exits and the winds shift back to a westerly...then the precip changes sometimes to freezing, somes just to rain.

Other times freezing rain is experienced at high elevations and light fluffy is experienced down low. I've skied frequently into a rain crust and freezing rain above 6000 only to find light and dry powder below 5000 in the same region.

So my advice: check the webcams and telemetry from nwac. If the east winds are strong and cold, it could be quite warm on the west side and still snow in the passes.

Thank you, Savegonder, for clarifying my statement about freezing rain. I was referring to the possibility of freezing rain prior to a frontal passage when the airflow is still easterly through the passes. After a frontal passage, the cold easterly flow is shut off and warmer air begins to flow through the passes from the west. This can change the snow or freezing rain to entirely rain as you stated.

However, Savegonder, you are wrong. My assessment above about the freezing level is correct the majority of the time. If the freezing level is much above 6000 ft it is very unlikely that the precipitation at pass level (Stevens or Snoqualmie) will remain snow for very long. The one exception occurs if the snow rate is very heavy, on the order of one inch/hour or greater.

Savegonder, you claim that you have skied frequently in a rain crust and freezing rain above 6000 only to find light and dry powder below 5000 in the same region. What you likely encountered was a situation, which occurred with warm air overlying cold air. At levels where the air was above freezing the snow melted. Lower down, the snow remained cold and dry. Later, the snow at higher elevation refroze forming your ”raincrust”. Crust up high, soft snow down low. But there could not have been just rain or freezing rain falling when you encountered this situation. Otherwise there would have been an ice layer forming at lower elevations.

You claim to have encountered freezing rain at high elevations and light fluffy down low. Maybe the snow still felt light and fluffy, but the stuff falling on it was anything but light and fluffy. It would not be long before the snow character changed. It is physically impossible for raindrops to fall into colder air and form snowflakes. When raindrops fall into freezing air they either freeze into ice pellets or contact the surface and immediately freeze (freezing rain). If the cold layer is thick enough and humid enough, the ice pellets can rime and grow quite large. These ice pellets are often called sleet or graupel. But they would never be mistaken for snowflakes, let alone “powder”.

Bryan Scott