Chris L Posted January 23, 2012 Share Posted January 23, 2012 What is the highest thickness and lowest thickness it can possibly snow at? Link to comment Share on other sites More sharing options...
usedtobe Posted January 23, 2012 Share Posted January 23, 2012 It depends greatly on the elevation and on the airmass so there is no one highest value that you can have and get snow or lowers value you can get rain. The critical thickness along the west coast is around 518.. I've also seen it near 550 and have it snow at DCA. I don't think any mets use thickness any more. Link to comment Share on other sites More sharing options...
meteorologist Posted January 23, 2012 Share Posted January 23, 2012 http://www.weatheranswer.com/public/Wagner540thickness.pdf Link to comment Share on other sites More sharing options...
superjames1992 Posted January 24, 2012 Share Posted January 24, 2012 This was originally made in Greensboro, NC during a study on precipitation type during ice storms (which we get quite a bit down here due to CAD), but I find that this graph is a decent general rule for most non-maritime environments. Note that this is a just a general rule, but my understanding is that it was used quite often before model soundings came along and it is still used some today by the NWS (at least in RAH). Link to comment Share on other sites More sharing options...
SacrydDreamz Posted February 13, 2012 Share Posted February 13, 2012 It depends greatly on the elevation and on the airmass so there is no one highest value that you can have and get snow or lowers value you can get rain. The critical thickness along the west coast is around 518.. I've also seen it near 550 and have it snow at DCA. I don't think any mets use thickness any more. To elaborate on this further, since thickness is representative of the average temperature within a given column of air one could have a myriad of situations where snow develops. As Wes points out, sometimes you can get rain at 518 and snow at 556. The nomogram posted by Superjames is helpful as it cuts the 1000-500mb column in two (1000-850mb, and 850-700mb), giving a better picture of what is really happening. On the West Coast the marine influence is so strong that it takes a ridiculously cold air-mass to overcome it... hence the critical thickness around 518 (plus or minus a few decameters). It should be noted that the nomogram posted here wouldn't even work in Seattle... the low-level warm air is that strong. In Denver, the critical thickness resides around 552 with snow having occurred well north of this... forget about using the partial thickness method here as the 1000-850mb layer is mostly made up basalt, limestone, and whatever else is under the ground. Link to comment Share on other sites More sharing options...
usedtobe Posted February 14, 2012 Share Posted February 14, 2012 To elaborate on this further, since thickness is representative of the average temperature within a given column of air one could have a myriad of situations where snow develops. As Wes points out, sometimes you can get rain at 518 and snow at 556. The nomogram posted by Superjames is helpful as it cuts the 1000-500mb column in two (1000-850mb, and 850-700mb), giving a better picture of what is really happening. On the West Coast the marine influence is so strong that it takes a ridiculously cold air-mass to overcome it... hence the critical thickness around 518 (plus or minus a few decameters). It should be noted that the nomogram posted here wouldn't even work in Seattle... the low-level warm air is that strong. In Denver, the critical thickness resides around 552 with snow having occurred well north of this... forget about using the partial thickness method here as the 1000-850mb layer is mostly made up basalt, limestone, and whatever else is under the ground. That's why, if you have access to forecast sounding use them. They will work better than thickness or partial thickness techniques. The 1000-500 thickness methods were developed when models have only a few layers so you didn't get a good vertical temperature representation from them. Now you do. Link to comment Share on other sites More sharing options...
SacrydDreamz Posted February 15, 2012 Share Posted February 15, 2012 -* That's why, if you have access to forecast sounding use them. They will work better than thickness or partial thickness techniques. The 1000-500 thickness methods were developed when models have only a few layers so you didn't get a good vertical temperature representation from them. Now you do. Absolutely... Of course, if you see a 534 thickness in Seattle no need to look at a sounding... it's rain. In New York, you may want to pull out the forecast sounding. So in that regard looking at thickness values still has some merit as a first approximation.... as is knowing climo because a forecast sounding is simply that... a model projection. So understanding the climo thickness for snow, storm tracks, etc. are helpful in the medium/long rang (as forecast soundings are prone to subtle, yet critical errors if you accept them as being valid for a given time step). How's retirement treating you, Wes? Link to comment Share on other sites More sharing options...
turtlehurricane Posted February 18, 2012 Share Posted February 18, 2012 As others in this thread have alluded to, just look at the temperature profile and make your decision from there. If it's below 0 °C through the column snow is likely (unless none of the cloud layer is <= -20 °C), if you have a >0 °C layer for ~1000 m you'll get ice pellets at best. Any deeper and you get rain. Link to comment Share on other sites More sharing options...
usedtobe Posted February 19, 2012 Share Posted February 19, 2012 -* Absolutely... Of course, if you see a 534 thickness in Seattle no need to look at a sounding... it's rain. In New York, you may want to pull out the forecast sounding. So in that regard looking at thickness values still has some merit as a first approximation.... as is knowing climo because a forecast sounding is simply that... a model projection. So understanding the climo thickness for snow, storm tracks, etc. are helpful in the medium/long rang (as forecast soundings are prone to subtle, yet critical errors if you accept them as being valid for a given time step). How's retirement treating you, Wes? Pretty good an I still write occasional articles for the Capital Weather Gang website. From Spring thru fall lots of fishing and some golfing and winters like this one, golfing once a week. I do have a pulled/torn abdomin muscle that gets aggravated when I golf or run but do it anyway. Link to comment Share on other sites More sharing options...
Chris L Posted February 20, 2012 Author Share Posted February 20, 2012 Thank you all. Really explains it clearly and I understand now. Link to comment Share on other sites More sharing options...
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