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Snowfall / Liquid dilemna


NorthShoreWx

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I was going to put this in the blizzard obs thread, but since the questions about technique are more important to me than embedding a couple more minor facts in an obs thread I am posting here.

 

I am looking for some insight into why I may have measured what I did.  Here are the facts:

 

  • I measured final snow depth for this one.  I used a snow board but did not wipe it during the event.  I suppose by rights I could have wiped it once (I would have chosen midnight, but we only had 5" at that point and I doubt it made much difference).
  • I placed the board after we already had a couple of inches, but the old hard pack from Saturdays storm made a very firm base.  I actually took most of my measurements over that as I only placed one board.
  • I measured 15".  This was subjective due to the drifting.  I could have called it 14" and slept fairly easily, but decided 15" was the better call. Most of my measurements were less than 15", 14" being the most common.
  • I do not use the driveway for measurements, but poked around for yucks and it ranged from 19.5" next to the garage (a drift) to 8" towards the end, but before the big rampup to the plowed bank,
  • I took a core sample off of the hardpack (excluding the old hard snow) yesterday and it melted to 1.70".
  • I took a core off of the board today (the depth remaining on the board at the time of the sample was 10".  This melted down to 1.57"
  • I used a standard COCORAHS 4" gage to take the core.

 

So, to my knowledge I did everything to a reasonable standard, yet I have the following measurements which seem inconsistent:

 

  • Ratios less than 10:1 in a storm with supposedly high ratios.  ISP (10 miles south) had closer to 15:1
  • Anywhere from 25 - 40% less snowfall than other reports from my town, depending on who you ask.  This doesn't actually concern me as much as I believe the others are just eyeballing the drift next to the house and/or taking as gospel what they heard on TV.

So for the pros (Ray Martin where art thou?) while I am as always mildly concerned about whether I can improve on the snowfall measurements, I am majorly interested in discussing what could have gone wrong with the SWE process.  Literally, the snow on the board accumulated (apparently) at an 8:1 ratio.

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I was going to put this in the blizzard obs thread, but since the questions about technique are more important to me than embedding a couple more minor facts in an obs thread I am posting here.

 

I am looking for some insight into why I may have measured what I did.  Here are the facts:

 

  • I measured final snow depth for this one.  I used a snow board but did not wipe it during the event.  I suppose by rights I could have wiped it once (I would have chosen midnight, but we only had 5" at that point and I doubt it made much difference).
  • I placed the board after we already had a couple of inches, but the old hard pack from Saturdays storm made a very firm base.  I actually took most of my measurements over that as I only placed one board.
  • I measured 15".  This was subjective due to the drifting.  I could have called it 14" and slept fairly easily, but decided 15" was the better call. Most of my measurements were less than 15", 14" being the most common.
  • I do not use the driveway for measurements, but poked around for yucks and it ranged from 19.5" next to the garage (a drift) to 8" towards the end, but before the big rampup to the plowed bank,
  • I took a core sample off of the hardpack (excluding the old hard snow) yesterday and it melted to 1.70".
  • I took a core off of the board today (the depth remaining on the board at the time of the sample was 10".  This melted down to 1.57"
  • I used a standard COCORAHS 4" gage to take the core.

 

So, to my knowledge I did everything to a reasonable standard, yet I have the following measurements which seem inconsistent:

 

  • Ratios less than 10:1 in a storm with supposedly high ratios.  ISP (10 miles south) had closer to 15:1
  • Anywhere from 25 - 40% less snowfall than other reports from my town, depending on who you ask.  This doesn't actually concern me as much as I believe the others are just eyeballing the drift next to the house and/or taking as gospel what they heard on TV.

So for the pros (Ray Martin where art thou?) while I am as always mildly concerned about whether I can improve on the snowfall measurements, I am majorly interested in discussing what could have gone wrong with the SWE process.  Literally, the snow on the board accumulated (apparently) at an 8:1 ratio.

during the blizzard of 96 my core samples had more water eq than what I melted from my gauge...I measured 23" in the gauge and it had a water eq about 1.75"...The core sample of 20" on the ground yielded 2.30" or so...I'm going on memory here...

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I often think that LE are often under reported and that if we used core samples we would find that ratios are often lower than what we think...snowcatch in a gage is influenced by wind and heated rain gages(of which I have one)will lose precip due to evaporation from the heat...How is the LE determined at asos sites?...don't think they use heated gages.

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during the blizzard of 96 my core samples had more water eq than what I melted from my gauge...I measured 23" in the gauge and it had a water eq about 1.75"...The core sample of 20" on the ground yielded 2.30" or so...I'm going on memory here...

I used a 4" clear plastic gauge...every 6 hours I would exchange it with a empty one...I measured the snow with a ruler...One side had more than the other so I averaged it...I think my measurement's  were pretty accurate...I melted the snow each time and recorded what I got...The core sample was after the snow stopped and compacted...I got a better ratio from the gauge...

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I measured final snow depth for this one.  I used a snow board but did not wipe it during the event.  I suppose by rights I could have wiped it once (I would have chosen midnight, but we only had 5" at that point and I doubt it made much difference).

 

It might've made a difference given the compaction which can occur from blowing and drifting. 

 

  • I placed the board after we already had a couple of inches, but the old hard pack from Saturdays storm made a very firm base.  I actually took most of my measurements over that as I only placed one board.
  • I measured 15".  This was subjective due to the drifting.  I could have called it 14" and slept fairly easily, but decided 15" was the better call. Most of my measurements were less than 15", 14" being the most common.
  • I do not use the driveway for measurements, but poked around for yucks and it ranged from 19.5" next to the garage (a drift) to 8" towards the end, but before the big rampup to the plowed bank,
  • I took a core sample off of the hardpack (excluding the old hard snow) yesterday and it melted to 1.70".
  • I took a core off of the board today (the depth remaining on the board at the time of the sample was 10".  This melted down to 1.57"
  • I used a standard COCORAHS 4" gage to take the core.

 

So, to my knowledge I did everything to a reasonable standard, yet I have the following measurements which seem inconsistent:

 

  • Ratios less than 10:1 in a storm with supposedly high ratios.  ISP (10 miles south) had closer to 15:1

 

The ISP ratio on the 26th is almost certainly way off.  Probably pure ASOS measurement with no manual cross-check.  The pure ASOS measurement on the 27th was actually only 0.60", but was obviously cross-checked manually which resulted in the reported 1.44".  So, don't worry about that.

 

Anywhere from 25 - 40% less snowfall than other reports from my town, depending on who you ask.  This doesn't actually concern me as much as I believe the others are just eyeballing the drift next to the house and/or taking as gospel what they heard on TV.

 

This I can't help you too much with, other than giving ideas about how that happened.  You might have people using the 6-hour method, quite possibly doing it wrong (that method screams of potential errors from snow drifting onto the board if you leave it in any sort of depression in the snow).  Maybe the 6-hour method without errors still resulted in significant differences.  People also might have just measured in driftier areas, not realizing it.  I have long suspected that around structures during big storms, snow may be inflated as the snow blows off the surrounding buildings.  Anyway, that's a few ideas.

 

So for the pros (Ray Martin where art thou?) while I am as always mildly concerned about whether I can improve on the snowfall measurements, I am majorly interested in discussing what could have gone wrong with the SWE process.  Literally, the snow on the board accumulated (apparently) at an 8:1 ratio.

 

The ratio is a bit low, but...  with crazy winds (I notice ISP gusted to 50) and lots of blowing and drifting, the snow is going to lose ratio as the flakes are shredded.  Snow has big ratios because the crystals have a lot of branches and there's a lot of air spaces, but blowing/drifting kills those spaces as the flakes are shattered. 

 

It would help to see 360 panoramas from your snow measurement spot to diagnose any factors which might influence your measurements.

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I often think that LE are often under reported and that if we used core samples we would find that ratios are often lower than what we think...snowcatch in a gage is influenced by wind and heated rain gages(of which I have one)will lose precip due to evaporation from the heat...How is the LE determined at asos sites?...don't think they use heated gages.

 

I think you are quite right, ratios are often reported higher than reality.  Climate ASOS sites are *supposed* have a manual cross-check, but that doesn't necessarily always occur.  The 1/27 precip from ISP was a manual cross check which was more than double what ASOS reported, while the 1/26 total was apparently pure ASOS and certainly too low. 

 

ASOS uses a mixture of heated tipping buckets and heated weighing gauges, depending on whether the NWS funded the gauge or the FAA did.  The NWS-funded weighing gauges are supposed to do better than the FAA-funded tipping buckets when it comes to snow, but wind can still play real havoc since most ASOS gauges are extremely exposed to wind (being out in a wide open airport). 

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NYC ASOS tends to be very good for liquid equivalent in snow events, I think it recorded .83 for this storm and they had 9.8 inches. I would assume the reason it may be so accurate is that it's surround by trees, so little wind influence.

It is by far the most trust-worthy ASOS precip gauge I have ever seen.  Too bad the wind readings suck.  Oh well.

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