New NWS snow measurement guide

[famartin]

Yup, certainly does. Consistency is key when it comes to observations, or the data is more or less useless. IMO, the measuring technique that best reflects a snowfall's impact on society is the measuring technique that should be implemented. Makes no sense that one measuring method works for the NWS, but not for all spotters/co-ops as a whole. This really isn't complicated stuff...anybody with half a brain can properly utilize the 6hr method.

 

For me personally, as a record keeper, I like to be on the conservative side when it comes to measuring snow, but even the 24 hr method seems to have little value in conveying the impact of a snowstorm to the public. After all, it's not really the first rate NWS observer sites that you see in snowfall reports in the media...it's from spotters/co-ops and other everyday folk that make up the vast majority of reported snowfall totals.

 

Really?  No one cares about what Central Park got anymore?

[masomenos]

Really?  No one cares about what Central Park got anymore?

In my book the first order sites are the ones that count, but when you watch the local news after a storm it's usually reports from the public that dominate the reported totals.

[famartin]

In my book the first order sites are the ones that count, but when you watch the local news after a storm it's usually reports from the public that dominate the reported totals.

Naturally.  They fill a lot more screen space and air time

[Amped]

This makes it easier on forecasters when there are higher amounts. They can predict 18"-24" instead of 20"-30".  Weenies will just have to live with the fact that it takes more snow to get from 10-20" than it does to get from 0"-10". Most will survive but a few may perish, or find some other excuse to inflate totals.

[masomenos]

Naturally.  They fill a lot more screen space and air time

It's awful. Then again, it's just human nature to make things seem more dramatic than the reality of the situation. Like my grandfather the maritime pilot who told me stories of 40 ft waves in the Raritan.

[Absolute Humidity]

Oh good, maybe I won't see 20+ totals all around me when I measure 14"

Then perhaps a better metric would be to measure the end totals of each multiple instance of accumulating snow within said 24 hr period regardless of hours. If it snows 1.8" over a two hour period, then mixes with RN for an hour and a half, reducing the total to .6" but then, for the next 2.5 hours it snows heavily and adds an additional 3.5" Do I say it only snowed 4.1" in that 6 hr period when without the 6 hour rule, my actual accumulation was really 5.3"?

Personally, be it 6hrs or 24hrs, I would report the 4.1". Seems fake otherwise.

[famartin]

Then perhaps a better metric would be to measure the end totals of each multiple instance of accumulating snow within said 24 hr period regardless of hours. If it snows 1.8" over a two hour period, then mixes with RN for an hour and a half, reducing the total to .6" but then, for the next 2.5 hours it snows heavily and adds an additional 3.5" Do I say it only snowed 4.1" in that 6 hr period when without the 6 hour rule, my actual accumulation was really 5.3"?

Personally, be it 6hrs or 24hrs, I would report the 4.1". Seems fake otherwise.

What about the scenario I posted:  2" falls, then melts completely, then another 2" fall.  What do you call that?

[Amped]

Then perhaps a better metric would be to measure the end totals of each multiple instance of accumulating snow within said 24 hr period regardless of hours. If it snows 1.8" over a two hour period, then mixes with RN for an hour and a half, reducing the total to .6" but then, for the next 2.5 hours it snows heavily and adds an additional 3.5" Do I say it only snowed 4.1" in that 6 hr period when without the 6 hour rule, my actual accumulation was really 5.3"?

Personally, be it 6hrs or 24hrs, I would report the 4.1". Seems fake otherwise.

No I'm not complaining. This is how I wanted to measure all along. It's the fairest system other than going to liquid equivalents, which would over-rate sleet events.  Some events like 2/10/10 you just aren't going to know,  it's not an exact science.

[Absolute Humidity]

What about the scenario I posted: 2" falls, then melts completely, then another 2" fall. What do you call that?

If this occurred in the 6hr board cleaning time constraint then it would be, as always was 2". That's my point. Whether this occurs in 6 hrs or 24, 4" still fell. If we're going to measure total accumulation periods, it seems more logical to measure by accumulated instances rather than be confined by time.

[Absolute Humidity]

No I'm not complaining. This is how I wanted to measure all along. It's the fairest system other than going to liquid equivalents, which would over-rate sleet events.  Some events like 2/10/10 you just aren't going to know,  it's not an exact science.

Same here and exactly. I was only illustrating how time constraints, be it 6 or 24 hours can impact one's real occurrence of actual snow accumulation, if one chooses to measure that way.

[famartin]

If this occurred in the 6hr board cleaning time constraint then it would be, as always was 2". That's my point. Whether this occurs in 6 hrs or 24, 4" still fell. If we're going to measure total accumulation periods, it seems more logical to measure by accumulated instances rather than be confined by time.

Under the new rules, if you get 2", it melts, and then 2" more falls within the 24-hour time period, it now only counts as 2".  You don't add these like you would have in the past. 

[Absolute Humidity]

Under the new rules, if you get 2", it melts, and then 2" more falls within the 24-hour time period, it now only counts as 2".  You don't add these like you would have in the past.

Of course there's this:

Measuring snowfall once every 6 hours

YOUR SERVICING NWS OFFICE WILL LET YOU KNOW IF YOU ARE REQUIRED TO TAKE 6-HOURLY MEASUREMENTS. OTHERWISE ONLY TAKE 24-HOUR MEASUREMENTS.

So those who like clearing a board every 6 may seek permission from their local WFO?

My head hurts.

[famartin]

Of course there's this:

Measuring snowfall once every 6 hours

YOUR SERVICING NWS OFFICE WILL LET YOU KNOW IF YOU ARE REQUIRED TO TAKE 6-HOURLY MEASUREMENTS. OTHERWISE ONLY TAKE 24-HOUR MEASUREMENTS.

So those who like clearing a board every 6 may seek permission from their local WFO?

My head hurts.

 

This pretty much only will apply to observers providing snowfall observations for airport climate summaries.  I.E., EWR, PHL, LGA, BDR, ACY... you get the idea.  Some of those are handled by observers on-site(EWR, LGA); the rest are handled by observers located near the airport grounds. 

 

In the "business", we refer to such sites as "Local Climatological Data" (LCD) sites, because their data is published here:

http://www.ncdc.noaa.gov/IPS/lcd/lcd.html

 

Observers on-site are referred to as Contract Weather Observers, which also do ASOS augmentation (i.e. putting in high clouds, sleet, etc into the observations).

Observers located off-site but nearby, specifically for snowfall, are known as Snow-Paid Observers.  Their primary task is doing what ASOS can't for the climate record:  Snowfall, snow depth, liquid equivalent of snow.

[Absolute Humidity]

This pretty much only will apply to observers providing snowfall observations for airport climate summaries.  I.E., EWR, PHL, LGA, BDR, ACY... you get the idea.  Some of those are handled by observers on-site(EWR, LGA); the rest are handled by observers located near the airport grounds. 

 

In the "business", we refer to such sites as "Local Climatological Data" (LCD) sites, because their data is published here:

http://www.ncdc.noaa.gov/IPS/lcd/lcd.html

 

Observers on-site are referred to as Contract Weather Observers, which also do ASOS augmentation (i.e. putting in high clouds, sleet, etc into the observations).

Observers located off-site but nearby, specifically for snowfall, are known as Snow-Paid Observers.  Their primary task is doing what ASOS can't for the climate record:  Snowfall, snow depth, liquid equivalent of snow.

Thanks for clearing that up.

[Stebo]

What about the scenario I posted:  2" falls, then melts completely, then another 2" fall.  What do you call that?

 

I was telling my boss about this, and I used this exact scenario, he is floored that have made this change and he has been in weather since the 60s.

[famartin]

Here's a copy of the old snow guidelines, in case anyone is interested:

 

10-23-96, replaces form WS TA B-0-26; 9-79

The following procedures were developed from previous National Weather Service procedures and input from a broad array of expertise from climatologists, snow specialists, weather observers, and data users. Some of the materials have been extracted from "The Snow Booklet" by Nolan J. Doesken and Arthur Judson, CSU, 1996).

It is essential for all observers to understand the importance of taking standard measurements in the prescribed consistent manner. Inconsistent observing and reporting methods result in incompatible data which can result in profoundly incorrect differences between stations and observers.

Each season before the first snows come: Review these instructions for measuring snow. It is easy to forget what needs to be measured, especially in those parts of the country where snow falls infrequently.

At the beginning of each snowfall/freezing season, remove the funnel and inner measuring tube of the eight-inch manual rain gauge to expose the 8-inch diameter overflow can so that it can more accurately catch frozen precipitation.

Put your snowboard(s) out and mark their location with a flag or some other indicator so they can be found after a new snowfall. They should be located in the vicinity of your station in an open location (not under trees, obstructions, or on the north side of structures in the shadows).

Check your gauge to make sure there are no leaks. If there are leaks, take appropriate action.

Once your equipment has been readied for winter you are prepared for taking snowfall measurements.

Observers should determine three values when reporting solid precipitation. They are:

1. Measure and record the snowfall ( snow, ice pellets ) since the previous snowfall observation,
2. Determine the depth of snow on the ground at the normal observation time,
3. Measure and record the water equivalent of snowfall since the previous day's observation.

Measure and record the greatest amount of snowfall that has accumulated on your snowboard (wooden deck or ground if board is not available) since the previous snowfall observation. This measurement should be taken minimally once-a-day but can be taken up to four times a day, (every 6 hours) and should reflect the greatest accumulation of new snow observed (in inches and tenths, for example, 3.9 inches) since the last snowfall observation.

Snowfall amounts can be measured hourly or at any interval as long as the snow measurement board is NOT cleared more frequently than once every 6 hours.

If you are not available to watch snow accumulation at all times of the day and night, use your best estimate, based on a measurement of snowfall at the scheduled time of observation along with knowledge of what took place during the past 24 hours. If you are not present to witness the greatest snow accumulation, input may be obtained from other people who were near the station during the snow event. If your observation is not based on a measurement, record in your remarks that the "snow amount based on estimate".

Remember, you want to report the greatest accumulation since the last observation. If snowfall occurred several times during the period, and each snowfall melted either completely or in part before the next snowfall, record the total of the greatest snowdepths of each event and enter in your remarks "snowfall melted during the OBS period". For example, three separate snow squalls affect your station during your 24-hour reporting day, say 3.0, 2.2, and 1.5 inches. The snow from each event melts off before the next accumulation and no snow is on the ground at your scheduled time of observation. The total snowfall for that reporting 24-hour day is the sum of the three separate snow squalls, 6.7 inches, even though the snow depth on your board at observation time was zero. Snow often melts as it lands. If snow continually melts as it lands, and the accumulation never reaches 0.1 inches on your measuring surface, snowfall should be recorded as a trace (T) and record in your remarks that the "snow melted as it landed".

It is essential to measure snowfall (and snow depth) in locations where the effects of blowing and drifting are minimized. Finding a good location where snow accumulates uniformly simplifies all other aspects of the observation and reduces the numerous opportunities for error. In open areas where windblown snow cannot be avoided, several measurements may often be necessary to obtain an average depth and they should not include the largest drifts. In heavily forested locations, try and find an exposed clearing in the trees. Measurements beneath trees are inaccurate since large amounts of snow can accumulate on trees and never reach the ground.

If your daily schedule permits, you may wish to make a snowfall observation every 6-hours, beginning with your regularly scheduled time of observation. This is the procedure followed by National Weather Service Forecast Offices. Follow the same rules for a once-a-day observation, but the snow accumulation reported will be the greatest for the previous six hours instead of 24 hours. If you take your observations at this frequency, make sure that you clear your snowboard (or other measuring surface) no more than once every 6 hours. Record the frequency of observations during the day in the comments section of your report. Never sum more than four, six-hourly observations to determine your 24-hour snowfall total. If you use more than four observations, it would falsely increase snowfall totals.

Freezing rain (glaze ice) should never be reported as snowfall. This precipitation type is liquid precipitation and should be reported as such.

1. Determine the total depth of snow, ice pellets, or ice on the ground. This observation is taken once-a-day at the scheduled time of observation with a measuring stick. It is taken by measuring the total depth of snow on exposed ground at a permanently-mounted snow stake or by taking the average of several depth readings at or near the normal point of observation with a measuring stick. When using a measuring stick, make sure the stick is pushed vertically into the snow until the bottom of the stick rests on the ground. Do not mistake an ice layer or crusted snow as "ground". The measurement should reflect the average depth of snow, ice pellets, and glaze ice on the ground at your usual measurement site (not disturbed by human activities). Measurements from rooftops, paved areas, and the like should not be made. Note: Hail accumulation is not entered with snow and ice pellets. Hail accumulation is entered in the “/remarks/” section with the amount and diameter (inches and tenths) of the stones. 
   
   Report snow depth to the nearest whole inch, rounding up when one-half inch increments are reached (example 0.4 inches gets reported as a trace (T), 3.5 inches gets reported as 4 inches). Frequently, in hilly or mountainous terrain, you will be faced with the situation where no snow is observed on south-facing slopes while snow, possibly deep, remains in shaded or north-facing areas. Under these circumstances, you should use good judgement to visually average and then measure snow depths in exposed areas within several hundred yards surrounding the weather station.
   
   For example, if half the exposed ground is bare and half is covered with six inches of snow, the snow depth should be entered as the average of the two readings, or three inches. When in your judgement, less than 50 percent of the exposed ground is covered by snow, even though the covered areas have a significant depth, the snow depth should be recorded as a trace (T). When no snow or ice is on the ground in exposed areas (snow may be present in surrounding forested or otherwise protected areas), record a "0".
   
   When strong winds have blown the snow, take several measurements where the snow was least affected by drifting and average them. If most exposed areas are either blown free of snow while others have drifts, again try to combine visual averaging with measurements to make your estimate.
2. Measuring the water equivalent of snowfall since the previous day's observation. This measurement is taken once-a-day at your specified time of observation. Melt the contents of your gauge (by bringing it inside your home or adding a measured amount of warm water) and then pour the liquid into the funnel and smaller inner measuring tube and measure the amount to the nearest .01 inch (use NWS provided measuring stick) just as you use for measuring rainfall. Do not measure the melted precipitation directly in the large 8-inch outer cylinder. Make sure the inner measuring tube can't fall over when pouring the liquid back into it. If the melted water equivalent (including any added warm water) exceeds two inches and cannot fit into the measuring tube all at one time, then empty the full measuring tube and pour the remaining liquid from the large 8-inch outer cylinder into the emptied measuring tube. Then, add and record the water equivalent of the multiple measurements.
   
   If you added warm water to the gauge to melt the snow, make sure you accurately measure the amount of warm water added before pouring it into the gauge. Then, when you take your liquid measurement, subtract the amount of warm water added from the total liquid measurement to get your final liquid water equivalent of the snowfall.
   
   As winds increase, gauges collect less and less of the precipitation that actually falls. Generally speaking, the stronger the wind and the drier the snow, the less is captured in the gauge. If you notice that less snow is in the gauge than accumulated on the ground, you should first empty any existing snow from inside the 8-inch cylinder, then use it to take a snow sample, sometimes referred to as "take a core" or "cut a biscuit" from your snow board with the 8-inch overflow can. Melt the biscuit of snow, pour the liquid into the small measuring tube to measure the water equivalent.

[uncle W]

when I tried to measure the blizzard of 1996 my rain gauge was in my back yard between my house and a garage...strong winds blew most of the snow that was on my roof and some of it could have gone into the gauge...with that said every six hours I'd emty the guage but measure it first...I the accumulated snow would be at a 45 degree angle inside...I measured the middle part...My totals were 23.0" out of 1.75" of precip...Then I took a core sample of 20" of untouched snow that was on the ground and the water equlvalent was 2.40"...I made a note of both measurements in my log...

[famartin]

when I tried to measure the blizzard of 1996 my rain gauge was in my back yard between my house and a garage...strong winds blew most of the snow that was on my roof and some of it could have gone into the gauge...with that said every six hours I'd emty the guage but measure it first...I the accumulated snow would be at a 45 degree angle inside...I measured the middle part...My totals were 23.0" out of 1.75" of precip...Then I took a core sample of 20" of untouched snow that was on the ground and the water equlvalent was 2.40"...I made a note of both measurements in my log...

The most likely outcome in windy situations is under-catch in a rain gauge, so the core sample was probably closer to being accurate.  Assuming, of course, you didn't have excess snow from the roof in the core sample.

[CT Rain]

I'll continue to do 6 hour measurements. Not sure why you would want a disconnect between coops/cocorahs sites and first order/airport sites. How ridiculous.

[rgwp96]

good because to the avg person they only care whats on the ground anyway

[uncle W]

good because to the avg person they only care whats on the ground anyway

this is true...If you look at storm obs from the past for NYC almost all the big snows had the same amount of snow depth without any compacting...I assume it was measured after the snow stopped...one example is January 12-14 1964...12.5" of snow fell and a 13" snow depth was recorded for two days after...all the 1960 major storms were like that...

December 3-4, 1957...8.0" of snow fell...8" snow depth...snow started with no snow on the ground...

February 15-16 1958...7.9" fell...8" snow depth...no snow on the ground at the start...

December 21-22 1959...13.7" fell...14" snow depth...no snow on the ground when it started...

March 3-4 1960...14.5" fell...15" snow depth when ended...no snow on the ground before it started...

December 11-12 1960...15.3" fell...15" snow depth................"...................."........................."

I can go on but I'm tired...When did it change?...even February 1983 had 17.6" with 1-2" on the ground and the snow depth when it ended was 19"...

[michsnowfreak]

I was told that it was felt that the old guidelines gave too much weight to small snowfalls like that, and the new ones would reduce their effect.

 

Oh. My. Goodness. LOL.

 

First of all...what is their excuse as to why official climate sites STILL measure and clear every 6 hours and others do NOT? My god as if they didnt want to screw with an areas climo anymore. In places where it snows often (esp lake belts) you would have ridiculous discrepancies from the first order stations versus the coops (and again, this is IF anyone changes their practices, which I doubt most will..I know I wont).

 

But the small snowfall thing cracks me up. Think about it this way....you get a small snowfall, say, anything 3" or less. they are worried about the weight something like that carries. (We get a LOT of small snowfalls in the Great Lakes). For any individual small snowfall, how much margin for error can there be ANYWAY? Not much. NOW....look at the biggest snowstorms. They are usually accompanied by wind, which causes drifting, and even the most skilled observers can have a lot of margin for error in measuring them. So, say a coop lives in a big city with a lot of roof blowoff (think, I dont know, Brooklyn)....they can be inflating (not on purpose) the actual snowfall by INCHES....and it would take how many dozen small snowfalls to get this same "inflation"?????!!!!!

[michsnowfreak]

Then perhaps a better metric would be to measure the end totals of each multiple instance of accumulating snow within said 24 hr period regardless of hours. If it snows 1.8" over a two hour period, then mixes with RN for an hour and a half, reducing the total to .6" but then, for the next 2.5 hours it snows heavily and adds an additional 3.5" Do I say it only snowed 4.1" in that 6 hr period when without the 6 hour rule, my actual accumulation was really 5.3"?

Personally, be it 6hrs or 24hrs, I would report the 4.1". Seems fake otherwise.

It snowed 5.3" and your depth is 4". Since weather records STARTED there has been a difference between snowFALL and snowDEPTH. Why they want the unofficial observers to disregard this is beyond me.

 

If you had 1.8" of snow then it turned to rain but never turned back to snow....you would still report that 1.8".

If you had started as rain (no 1.8) then turned over to snow and had 3.5" of snow, you would report 3.5".

So if it snows 1.8"....then rains....then snows 3.5".....it has still snowed 5.3".

[michsnowfreak]

I was telling my boss about this, and I used this exact scenario, he is floored that have made this change and he has been in weather since the 60s.

I know right? But...obviously that does not apply to DTW. It would apply to Dearborn and Grosse Pointe though, I cant imagine how one could possibly lowball anymore than they already do

[michsnowfreak]

I'll continue to do 6 hour measurements. Not sure why you would want a disconnect between coops/cocorahs sites and first order/airport sites. How ridiculous.

+1000000

[famartin]

 

Oh. My. Goodness. LOL.

 

First of all...what is their excuse as to why official climate sites STILL measure and clear every 6 hours and others do NOT?

 

No one spoke to me on that particular point, however I would imagine its because they felt that most COOPs use the new methods and probably used them for most of the COOP site's existence, while first-order sites have theoretically always used the 6-hour method.

[famartin]

this is true...If you look at storm obs from the past for NYC almost all the big snows had the same amount of snow depth without any compacting...I assume it was measured after the snow stopped...one example is January 12-14 1964...12.5" of snow fell and a 13" snow depth was recorded for two days after...all the 1960 major storms were like that...

December 3-4, 1957...8.0" of snow fell...8" snow depth...snow started with no snow on the ground...

February 15-16 1958...7.9" fell...8" snow depth...no snow on the ground at the start...

December 21-22 1959...13.7" fell...14" snow depth...no snow on the ground when it started...

March 3-4 1960...14.5" fell...15" snow depth when ended...no snow on the ground before it started...

December 11-12 1960...15.3" fell...15" snow depth................"...................."........................."

I can go on but I'm tired...When did it change?...even February 1983 had 17.6" with 1-2" on the ground and the snow depth when it ended was 19"...

 

Here's some recent observations:

 

2009-12-20

NYC had 10.9", snow depth 10.

LGA had 8.8", snow depth 9.

EWR had 11.2", snow depth 8.

JFK had 14.5", snow depth 14.

 

2010-02-10

NYC had 10", snow depth 10.

LGA had 10.4", snow depth 10.

EWR had 13.2", snow depth 11.

JFK had 11.1", snow depth 10.

 

2010-02-25

NYC had 20.9", snow depth 21.

LGA had 12", snow depth 10.

EWR had 14.9", snow depth 12.

JFK had 11.4", snow depth 10.

 

2010-12-26

NYC had 20", snow depth 20.

LGA had 13.4", snow depth 13.

EWR had 24.2", snow depth 24.

JFK had 15.6", snow depth 16.

 

2011-01-26

NYC had 19", snow depth went from 4 to 23.

LGA had 17.3", snow depth went from 3 to 18.

EWR had 18.9", snow depth went from 4 to 22.

JFK had 10.3", snow depth went from 2 to 12.

 

2013-02-08

NYC had 11.4", snow depth 10.

LGA had 12.1", snow depth 10.

EWR had 10", snow depth 8.

JFK had 6.4", snow depth 6.

 

Note that the discrepancies are not large; many (such as on 2013-02-08) can be pinned down to mixing/changeover.

[N. OF PIKE]

In 10-15 years it seems to me this could be convenient AMMO for anyone touting an alarmist perspective of AGW could now point to NOAA climotalogic observations and the recent decrease in totals over a xyz period.

This is not something i'm saying definitively bc i'm not privvy to the in's and outs of politics at NOAA, but just for discussions sake, whenever politics enters the picture and a formula is changed (albeit the way inflation is now calculated or GDP (lol, at new technique) it is done with a purpose (not being accuracy). Food for thought.

[famartin]

COOP observer instructions from 1955:

"3040. Depth Markers (Snow Boards). Depth markers are used as a means of identifying the surface
of snow or ice that has been covered by more recent snowfall (sect: 3141). They are used as an aid in
obtaining representative samples of frozen precipitation at times when the catch in the gage is considered
erroneous (as in windy weather or with wet snow), and they are used to provide a reference level in the
measurement of the depth of new snowfall and ice. Markers may be made of thin metal, boards, or
other light-weight material that will not sink into the snow, but not so light that they are likely to be
blown away in strong winds. They should be large enough to allow the cutting of more than one snow
sample from each marker—at least 16 inches by 16 inches is recommended. After each observation,
the markers should be removed from the snow, cleaned and placed on top of the snow in a representative
location. They should be pushed into tho snow until the tops of the markers are flush with the snow
surface. Because of evaporation, or drifting, daily adjustment of the markers may be required between
snowfalls. Light colored and reflective material, such as polished metal, or material covered with white
cloth or plastic, tends to minimize the nonrepresentative melting of snow near the markers.

3141. Where the gage is not equipped with a wind shield, and when the ground is snow covered, it is
necessary to place depth markers (snow boards) on the surface of the snow to provide for representative
sampling of subsequent snowfall in the event that the catch in the gage is not considered representative.
Markers should be exposed where the depth of snowfall is usually representative of the average depth
over the area surrounding the gage and is least affected by drifting. Where the depth is frequently
nonuniform, several markers should be used, if practicable, to facilitate obtaining a representative
measurement. A remark should be included on the observation form when the gage catch differs significantly
from the water equivalent of samples cut from the snow surface. Differences of more than
about 5% are considered significant. The larger amount should be recorded as the snowfall for the
observation period, and the lesser amount should be included as a remark, along with an indication of
whether it is from the gage or snow samples.

 

3530. Depth of Frozen Precipitation Occurring Between Observations. This is the depth measurement
of snow or hail that has fallen since the previous observation. Make this type of measurement
to the nearest 0.1 (one-tenth) inch. Thrust the measuring slick (metal rod or other depth indicator)
vertically into the snow until the end rests on a depth marker, or on the crust of old snow that formed
the surface at the time of the preceding observation. The measurement should be made where the
depth is least affected by drifting. When a snowfall occurring between observations is melting on the
ground, measure the greatest depth on the ground during the snowfall, if practicable, at the time it is
noted; otherwise estimate this depth. Whenever the old surface (as it existed at the time of the preceding
observation) cannot be located with certainty, the snowfall depth between observations can be
determined approximately by subtracting the depth on the ground at the preceding observation from
the present depth as obtained in accordance with Ц 3520."

[famartin]

Here are instructions for aviation observers from 1953:

4230. Snowfall Within Specified Periods -- These measurements are most
conveniently made on a surface that has been cleared of previous snowfall.
If such a spot is not available, measure the total depth of snow and subtract
the depth previously measured.  If the previous snowfall has crusted, the
new fall may be measured by permitting the end of the measuring stick to
rest on the crust.  If different falls of snow are mixed by drifting, measure
the total depth of snow and subtract the previously measured depth.  The
remainder is the approximate depth of the new fall, which will be adjusted
if necessary to correct for melting, evaporation and run-off.  If several snow
showers occur between observations and each melts before the following
one occurs, the total snowfall for the period will be the sum of the maximum
depths (measured or estimated) for each occurrence.

4250.  Twenty-Four Hour Snowfall (Col. 69) -- Enter to inches and tenths
the total amount (unmelted) of frozen precipitation that has fallen during the
twenty-four hours ending at midnight.  Determine this value as specified in
4230.  Entries for hail will be followed by an asterisk and "*Hail" entered into
column 90.  Enter "T" for trace, and "0" if none has fallen.  When precipitation
melts as it falls, enter "T" with a note "Melted as it fell" in column 90.