View Full Version : Snowfall Rates

Bill O
11-16-2007, 03:10 PM
Does anyone know how the summit distinguishes between SN, SN+, and SN-?

Or snow, heavy snow, and light snow.

I noticed that there was 11.1" in the can at today's noon observation, yet the summit was recording light snow for the six hour period.

When I was up there this was always a continuing debate, as visibility was the limiting factor.

Mike D
11-16-2007, 04:38 PM
Funny how 100+ MPH winds can distort your sense of snow "fall".

Pete S
11-16-2007, 11:05 PM
Good question Bill,
If I remember correctly, in the METAR manual , precip rates are determined by how the precip limts visibility. Many times during precip, the summit is in the fog, so how the precip is affecting the visibility is very subjective--- and relative. Jim or the current staff might have a better definition, but I seem to rember just trying to be consistant each storm with -/ /+ ,and guaging them from how similar "fall rates" affected the visibility during the few storms without fog.......

Not much of an answer, but some general direction

11-16-2007, 11:28 PM
Intensity of now, snow pellets, snow grains, drizzle or freezing drizzle are based on prevailing visibility. Visibility > (greater than) 1/2 miles is considered light, visibility > (greater than) 1/4 mile but < or = (less than or equal to) 1/2 mile is moderate, and visibility < or = (less than or equal to) 1/4 mile is heavy. Those are the only criteria that the FMH-1 give (the federal guidelines). When the guidelines were set up, they were set up for airports primarily not the summit of Mount Washington. So what we teach observers is to try envision what the visibility would be if fog and blowing snow were not occurring and use their best judgment. The best use is accumulation rates on top of the visibility reduction. After working there a while, you get a feel as to what the three intensities look like and we can loosely use radar to aid in our decisions. For instance, if a giant red cell of precipitation is directly overhead according to the dopplar radar, odds are we have heavy snow occurring. I know this sounds like an inaccurate method but having worked at an airport, I know that it can be even more inaccurate at unmanned stations that work completely automated. So I guess the leg up we have is the humanistic educated guess.

Also, we can report light snow at the time of the ob when the previous 45 minutes were heavy snow that dumped 10+ inches. The thing to keep in mind is that the observations are two minute averaged snap shots of the weather taken once an hour. So a lot could have occurred leading up to that observation.

Bill O
11-17-2007, 07:50 AM
Thanks for the explanations Ryan and Pete. I thought it was a visibility thing. Its good to know the observers take an active role to determine the snowfall rate. Taking into account radar data and wind speeds.

Bill O
11-17-2007, 07:55 AM
Another note.

What happened to the 13+ inches I saw in the metar? This morning's report says the summit only got 9.4". Did you make some adjustments in the check?

11-17-2007, 04:17 PM
Another note.

What happened to the 13+ inches I saw in the metar? This morning's report says the summit only got 9.4". Did you make some adjustments in the check?

I will try to work through this, I am currently in the valley so I am just going to use what I can find online to answer this. On the current F-6 (our monthly summary), it states that we 0.1" of snow on the 15th and 9.4" on the 16th. These are the official totals. I called up to the summit to confirm the 13+ inches you are adding up from the METAR's. What happened is yesterdays 11.1" measured at 1752 UTC was erroneous due to blowing wind. This was not caught until later so it could not be corrected.

Why some of you may ask? METAR's can only be corrected up to 15 minutes after the time they are transmitted due to programming language at NWS and NCDC. If the current crew had caught the error in that time they could have sent out a COR METAR and had the right information available but this was not the case. So when the blowing snow was factored out, only about 4" was to believe to have fallen instead of the 11.1". This is the first winter for a few of the observers so they were unaware of blowing snow so when Kyle noticed such a high number, he had to review it to get a more accurate number. So 9.4" came from 0.1" from midnight to the 1 am measurement, 2" at the 7 am measurement, 4" at the 1 pm measurement, 2.6" at the 7 pm measurement, and 0.7" at the "midnight" measurement.

Bill O
11-17-2007, 05:23 PM

Although possible, the 11.1" did seem high.

11-28-2007, 04:24 PM
Sorry if this is a basic question. But what do you measure the snowfall in, a large tube/cylinder? How do you distinguish between wind blown snow and actual snowfall?