Unprecedented Radar Reflectivities and Snowfall Rates

By: 24hourprof , 2:57 PM GMT on February 19, 2013

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I have one last observation to make about NEMO before I move on, and it's a lulu. There were some 50+ dBZ reflectivities in Connecticut the evening of February 8, and, in my 40 years as a forecaster, I've never observed higher reflectivities associated with snow. For confirmation, check out the 0218Z base reflectivity from the radar at New York City / Upton, NY (KOKX), on February 8 below (larger image). Courtesy of NOAA.


The 0218Z image of base reflectivity from the radar at New York City / Upton, NY (KOKX) on February 9, 2013 (the evening of February 8). Larger image. The 50+ dBZ reflectivity of snow is the highest I've observed in my 40 years of forecasting.

Of course, I've seen high reflectivities associated with bright bands (wet snow near the top of a melting layer; stay tuned), so I'm referring exclusively to high reflectivities associated with snow that reaches the ground. Before I discuss Nemo and the storm features that led to such high reflectivities over Connecticut on the evening of February 8, 2013, I think a slight digression on bright banding is in order (I've heard too many erroneous explanations for relatively high reflectivity in wintry situations that never even consider bright banding as a possible cause). For the record, bright banding was discovered during military operations in World War II, and there were papers written in the 1940s that explained the underpinning science (example 1946 paper).

Here's how I explain bright banding. As falling snowflakes reach the top of the melting layer, where the temperature is 0 degrees Celsius, they began to melt (again, 0 degrees Celsius = 32 degrees Fahrenheit is NOT the "freezing mark"). Melting snowflakes soon become covered with a film of meltwater, and they look like large raindrops to the radar. Thus, the base reflectivity abruptly (and dramatically) increases. In rather quick fashion, however, snowflakes melt completely, shriveling in size as raindrops take shape. Since radar is very sensitive to particle size, reflectivity decreases rapidly once the melting process is complete. Moreover, raindrops now quickly accelerate earthward (raindrops fall faster than snowflakes). This rather abrupt acceleration after the water-covered snowflakes melt completely into raindrops decreases the number of radar targets just below the melting level (like the flow of traffic quickly opening up after cars accelerate away from a crowded toll booth on a super-highway). The combined effect of decreasing both the size and number of drops causes radar reflectivity to decrease just below the melting layer, creating a band of higher reflectivity (a bright band) above the ground.

To summarize my discussion on bright banding, check out this nifty flash animation of an idealized bright band, copyrighted by Penn State's online Certificate of Achievement in Weather Forecasting.

During the evening of February 8, 2013, the tops of the melting layer lay south of New England (check out the 02Z Rapid-Refresh model analysis of the height of the melting level, in meters. Note the bulge in the contours toward eastern Massachusetts, a sign of warmer air pressing northward on the eastern flank of the low-pressure system (02Z Rapid-Refresh model analysis of MSL isobars). Just to be sure, I grabbed the 02Z Rapid-Refresh model skew-T at New Haven, Connecticut, which shows the entire temperature sounding (in red) to the left of (lower than) 0 degrees Celsius, which I highlighted in yellow. So snowflakes were the dominant hydrometeor indicated by the 50+ dBZ shown on the 0218Z images of base reflectivity (near the top of this blog) and composite reflectivity.


The 0218Z image of Hydrometeor Classification from the radar at New York City / Upton, NY, on February 9, 2013. Note the red and dark pink, which indicate hail and graupel, respectively. To be fair, the Hydrometeor Classification algorithm identified some unknown precipitation in southern Connecticut (purple). Larger image. Courtesy of NOAA.

Reflectivity is much more sensitive to particle size than number, so I can only deduce that wet snow, which is more likely to form large aggregates (agglomerations) than dry snow, generated such large reflectivities. Like I said before, I don't think I've ever observed 50+ dBZ associated with snow. On the other hand, I've never heard of snowfall rates as high as 8 inches per hour that were sustained for two hours. The rarity of the reported snowfall rates seemed to fit the rarity of the radar data for this event. I think the radar reflectivities for snow were so large that the hydrometeor classification algorithm, a utility of dual-polarization radars, simply failed in this case. Indeed, the hydrometeor classification (HC) algorithm "concluded" that the heaviest snow had to be hail or graupel (ice). For confirmation, check out the 0218Z HC product (above; larger image) from the dual-polarization radar at KOKX). Presumably, the red (hail) and dark pink (graupel) shown on the 0218Z image of hydrometeor classification were erroneously selected because there must be a red flag in the HC algorithm that automatically eliminates snowflakes as the possible dominant hydrometeor when radar reflectivities are 40 to 50 dBZ or higher. In effect, these very high radar reflectivities likely "overruled" other dual-pol metrics (such as differential radar reflectivity) that probably indicated otherwise.

A quick look at the synoptic set-up during the evening of February 8, 2013, starts with the 02Z Rapid-Refresh model analysis of 850-mb heights, which showed a large gradient in 850-mb heights off the New England Coast. In response, a robust low-level jet stream, with wind speeds exceeding 80 knots (see the 02Z Rapid-Refresh model analysis of 850-mb isotachs and streamlines below; larger image), rapidly transported Atlantic moisture inland, setting the stage for unprecedented radar reflectivities and snowfall rates over Connecticut and other parts of New England.


The 02Z Rapid-Refresh model analysis of 850-mb isotachs (color-filled in knots) and 850-mb streamlines. Larger image. Courtesy of Penn State.

For me, this snowstorm will make my lifetime top-ten list. And I'm pretty old.

Lee

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60. WunderAlertBot (Admin)
24hourprof has created a new entry.
Quoting skubaaruba:
I live in West Haven, Connecticut. I have never witnessed it snowing and hailing at rate at which it was coming down in my life. I went out and cleaned up my walkway and driveway at about 8pm. When I finished one side of the task it was like I did nothing when I went back to the starting point. This took about 15 minutes. What was falling was not sleet but pea sized hail. By the next morning the snow that I had piled up through out the day on Friday was totally gone. I could not open my doors. When I pushed open the front door all the snow fell into the living-room. My day of shoveling started by scooping out the living-room and filling up the sink. The snow at the front door was close to being 3.5 feet high. I took me two-days to do what took me a half-an-hour on Friday afternoon.


For what it's worth, I was at PSU in February 1979 and missed the DC Presidents day storm. However my parents also described not being able to open the front door at the end of that storm. When they did our
20 pound poodle bounded out into it and... disappeared. After a few seconds she stood on hind legs and could just get her head above the surface. She was about thirty inches tall when she did this.

My house in suburban MD has a porch and snow piling up against doors is not an issue.
Member Since: February 1, 2012 Posts: 19 Comments: 2226
The bottom line for the extremely high reflectivities that I discussed in my blog is still in play because the riming of snowflakes in places where freezing fog was reported likely took place below the radar beam.

Thanks to everybody for this great discussion!
Member Since: October 24, 2012 Posts: 97 Comments: 837
Quoting skubaaruba:
I live in West Haven, Connecticut. I have never witnessed it snowing and hailing at rate at which it was coming down in my life. I went out and cleaned up my walkway and driveway at about 8pm. When I finished one side of the task it was like I did nothing when I went back to the starting point. This took about 15 minutes. What was falling was not sleet but pea sized hail. By the next morning the snow that I had piled up through out the day on Friday was totally gone. I could not open my doors. When I pushed open the front door all the snow fell into the living-room. My day of shoveling started by scooping out the living-room and filling up the sink. The snow at the front door was close to being 3.5 feet high. I took me two-days to do what took me a half-an-hour on Friday afternoon.


Wow! Thanks for sharing.

Your distinction between sleet and hail (probably graupel) is consistent with snowflakes becoming rimed as they pass through freezing fog (which was widely reported with the snow).

So I believe your distinction is right on target. Thanks. (and thanks to George for offering this explanation).
Member Since: October 24, 2012 Posts: 97 Comments: 837
I live in West Haven, Connecticut. I have never witnessed it snowing and hailing at rate at which it was coming down in my life. I went out and cleaned up my walkway and driveway at about 8pm. When I finished one side of the task it was like I did nothing when I went back to the starting point. This took about 15 minutes. What was falling was not sleet but pea sized hail. By the next morning the snow that I had piled up through out the day on Friday was totally gone. I could not open my doors. When I pushed open the front door all the snow fell into the living-room. My day of shoveling started by scooping out the living-room and filling up the sink. The snow at the front door was close to being 3.5 feet high. I took me two-days to do what took me a half-an-hour on Friday afternoon.
Member Since: Posts: Comments:
Quoting wxchaser97:

I have heard thundersnow multiple times up here in SE MI, the most recent in a snowstorm on February 20, 2011. It is pretty cool to be in plus the snowfall rates are cool. Speaking of snowfall rates, it is amazing to see some of these intense confirmed snowfall rates from these storms in the past few weeks.
Thanks for the blog professor!


You're quite welcome.

I taught for 42 years. My students all called me Lee. Please feel free to call me by my first name. The title "professor" gets in the way, I think, of discussing weather one on one. I've never been one for titles. But I appreciate your polite approach.

Best,

Lee
Member Since: October 24, 2012 Posts: 97 Comments: 837
Quoting TropicalAnalystwx13:
I've seen higher (snow) radar reflectivities in the past two weeks than I ever have before. Nemo had values on the order of 55 dBZ with snowfall rates near 8"/hr and this morning I saw Winter Storm Q producing values near 45 dBZ with 5-7"/hr snowfall rates.

Thanks for the blog.


It was nuts, wasn't it? Oh how I wish I could have been there.

Many thanks for contributing, and I look forward to future discussions with you.

Best,

Lee
Member Since: October 24, 2012 Posts: 97 Comments: 837
Quoting Astrometeor:


Congrats, I am interested in PSU since I have family up there.

Lee, pleasure to read your blog, I have read it a couple of times before, but first time ever to comment. Good luck and congrats on becoming a featured blogger!


Many thanks. Much appreciated. I look forward to interacting with you and other WU readers.

Best,

Lee
Member Since: October 24, 2012 Posts: 97 Comments: 837
Quoting weatherh98:
thanks to RTL for recommending this, very educational... I'm more of a tropics person myself, but Ishould probably start working on winter weather.

Thanks professor, I'll definitley stop in again. Welcome!


Many thanks. I'm sure I'll also comment on the Tropics every now and then. I'm always fascinated by the Tropics, and I look forward to contributing!

Again, you're very kind to welcome me. Much appreciated.
Member Since: October 24, 2012 Posts: 97 Comments: 837
51. 24hourprof
12:22 PM GMT on February 22, 2013
Quoting WxGeekVA:
I love this blog Professor. I had similar thoughts on the setup, and the reasons for the high reflectivity of the snow that night.

Also, as a side note, I was just accepted to Penn State, and I will be there in the fall to study meteorology. It's been my goal for several years now, and I'm extremely excited about going! Go Nittany Lions!


Congratulations! You'll just love Penn State. I still come around and I'll be giving weather briefings once in a while (we have all-comers weather briefings every Thursday during the semester).

Welcome!
Member Since: October 24, 2012 Posts: 97 Comments: 837
50. 24hourprof
12:20 PM GMT on February 22, 2013
Quoting etxwx:
Hello Professor...ummm...*Lee*. Just heard your interview on NPR - well done and very interesting. We don't get much snow is this part of Texas...plenty of regular thunder though. I heard thunder in a snowstorm one time when we were visiting in Michigan. It's quite startling!

Looking forward to more of your blog entries. Glad you are part of the WU community.


:-)

Many thanks!!!!!!
Member Since: October 24, 2012 Posts: 97 Comments: 837
49. wxchaser97
4:18 AM GMT on February 22, 2013
Quoting etxwx:
Hello Professor...ummm...*Lee*. Just heard your interview on NPR - well done and very interesting. We don't get much snow is this part of Texas...plenty of regular thunder though. I heard thunder in a snowstorm one time when we were visiting in Michigan. It's quite startling!

Looking forward to more of your blog entries. Glad you are part of the WU community.

I have heard thundersnow multiple times up here in SE MI, the most recent in a snowstorm on February 20, 2011. It is pretty cool to be in plus the snowfall rates are cool. Speaking of snowfall rates, it is amazing to see some of these intense confirmed snowfall rates from these storms in the past few weeks.
Thanks for the blog professor!
Member Since: March 16, 2012 Posts: 127 Comments: 7972
48. TropicalAnalystwx13
2:47 AM GMT on February 22, 2013
I've seen higher (snow) radar reflectivities in the past two weeks than I ever have before. Nemo had values on the order of 55 dBZ with snowfall rates near 8"/hr and this morning I saw Winter Storm Q producing values near 45 dBZ with 5-7"/hr snowfall rates.

Thanks for the blog.
Member Since: July 6, 2010 Posts: 113 Comments: 32862
47. Astrometeor
2:42 AM GMT on February 22, 2013
Quoting WxGeekVA:
I love this blog Professor. I had similar thoughts on the setup, and the reasons for the high reflectivity of the snow that night.

Also, as a side note, I was just accepted to Penn State, and I will be there in the fall to study meteorology. It's been my goal for several years now, and I'm extremely excited about going! Go Nittany Lions!


Congrats, I am interested in PSU since I have family up there.

Lee, pleasure to read your blog, I have read it a couple of times before, but first time ever to comment. Good luck and congrats on becoming a featured blogger!
Member Since: July 2, 2012 Posts: 101 Comments: 10500
46. Angela Fritz , Atmospheric Scientist (Admin)
2:36 AM GMT on February 22, 2013
Quoting WxGeekVA:
I love this blog Professor. I had similar thoughts on the setup, and the reasons for the high reflectivity of the snow that night.

Also, as a side note, I was just accepted to Penn State, and I will be there in the fall to study meteorology. It's been my goal for several years now, and I'm extremely excited about going! Go Nittany Lions!


Congratulations!
45. weatherh98
2:35 AM GMT on February 22, 2013
thanks to RTL for recommending this, very educational... I'm more of a tropics person myself, but Ishould probably start working on winter weather.

Thanks professor, I'll definitley stop in again. Welcome!
Member Since: June 17, 2011 Posts: 11 Comments: 6539
44. WxGeekVA
2:26 AM GMT on February 22, 2013
I love this blog Professor. I had similar thoughts on the setup, and the reasons for the high reflectivity of the snow that night.

Also, as a side note, I was just accepted to Penn State, and I will be there in the fall to study meteorology. It's been my goal for several years now, and I'm extremely excited about going! Go Nittany Lions!
Member Since: September 3, 2011 Posts: 13 Comments: 3477
43. etxwx
11:44 PM GMT on February 21, 2013
Hello Professor...ummm...*Lee*. Just heard your interview on NPR - well done and very interesting. We don't get much snow is this part of Texas...plenty of regular thunder though. I heard thunder in a snowstorm one time when we were visiting in Michigan. It's quite startling!

Looking forward to more of your blog entries. Glad you are part of the WU community.
Member Since: September 4, 2011 Posts: 0 Comments: 1529
42. 24hourprof
11:04 PM GMT on February 21, 2013
Quoting JNCali:
Good afternoon Prof Lee.. I just noticed the 'shout-out' Dr. Masters gave you on his blog. I recommended your blog highly as I have been gleaning so much from your articles as well as the great interaction you provide with the commentors.. Get ready for an influx of visitors... and have fun with your interview on NPR!


Many thanks for your kind words.

Please call me Lee...Professor sounds too formal. I think folks will learn from me and I'll learn from readers. So I really appreciate your gesture of respect, but it's okay to call me Lee.

Best,

Lee

P.S. George has already taught me, and Gaara's ground truth during Nemo really sparked a great dialogue. So, yes, I'm learning from you guys. Thanks!
Member Since: October 24, 2012 Posts: 97 Comments: 837
41. 24hourprof
10:49 PM GMT on February 21, 2013
Quoting Picatso:
Hi Lee,

I read your blog, even if I don't comment on it. I don't have a physics or meteorology background so sometimes when I read I have to take off and study some of the terminology so I can better understand the point you are making.

This is good for me as I enjoy all aspects of learning new things. However, it does mean that I'm still a few blogs back.

Thank you for the posts. They are fun to read and learn from.



Many, many thanks.

Don't be afraid to ask questions. I'll be happy to answer them.

Best,

Lee
Member Since: October 24, 2012 Posts: 97 Comments: 837
40. JNCali
10:02 PM GMT on February 21, 2013
Good afternoon Prof Lee.. I just noticed the 'shout-out' Dr. Masters gave you on his blog. I recommended your blog highly as I have been gleaning so much from your articles as well as the great interaction you provide with the commentors.. Get ready for an influx of visitors... and have fun with your interview on NPR!
Member Since: September 9, 2010 Posts: 5 Comments: 1034
39. Picatso
10:01 PM GMT on February 21, 2013
Hi Lee,

I read your blog, even if I don't comment on it. I don't have a physics or meteorology background so sometimes when I read I have to take off and study some of the terminology so I can better understand the point you are making.

This is good for me as I enjoy all aspects of learning new things. However, it does mean that I'm still a few blogs back.

Thank you for the posts. They are fun to read and learn from.

Member Since: June 12, 2011 Posts: 0 Comments: 70
38. 24hourprof
9:49 PM GMT on February 21, 2013
Quoting RTLSNK:


Hello Lee,

My name is Chris, I'm one of the Moderators for the
website. Don't let the numbers get you down, this is
what we call the off season. Three quarters of the
bloggers take the Winter season off but will be back
for Hurricane season.

Once they see a new featured blog and stop in for a
visit they will stick around.

Welcome to Wunderground by the way.


Very kind of you to encourage me. Thank you.

I already feel like I know several of my faithful readers. Just a great group of people who, like me, love to learn and talk about weather.

Again, thank you for being so kind and hospitable. I really enjoy what I'm doing, and I hope I'm making a difference. I am very grateful for this opportunity.
Member Since: October 24, 2012 Posts: 97 Comments: 837
37. RTLSNK
9:40 PM GMT on February 21, 2013
Quoting 24hourprof:


Many thanks for encouraging me. I sometimes get down that folks don't post more and I thought about easing up on the science. But I respect my readers enough to give them my best shot. Thanks again.


Hello Lee,

My name is Chris, I'm one of the Moderators for the
website. Don't let the numbers get you down, this is
what we call the off season. Three quarters of the
bloggers take the Winter season off but will be back
for Hurricane season.

Once they see a new featured blog and stop in for a
visit they will stick around.

Welcome to Wunderground by the way.
Member Since: September 3, 2008 Posts: 0 Comments: 22189
36. 24hourprof
8:52 PM GMT on February 21, 2013
Quoting DelWeather:


Nice! I get it now. Again, the whole time thing keeps me guessing, but you are just doing an instantaneous analysis here... a snapshot, as it were. Very good.


George and Mark,

I think I'm all in on George's explanation. Obviously, freezing fog and snow can co-exist, although it's probably fairly rare. Snowflakes (wet or not) falling through freezing fog (water droplets at temperatures below 0 degrees Celsius) would collect some of them. This might result in strange-looking snowflakes. Perhaps they appeared as ice to Gaara???
Member Since: October 24, 2012 Posts: 97 Comments: 837
35. 24hourprof
8:40 PM GMT on February 21, 2013
Hey everybody,

I was writing a blog about thunder-freezing rain this morning (I didn't finish it) when Jeff Masters asked me if I wanted to discuss thunder-snow on National Public Radio.

I was excited. My interview airs this afternoon or this evening on NPR.

I'll try to finish my blog tomorrow!!!!!
Member Since: October 24, 2012 Posts: 97 Comments: 837
34. 24hourprof
8:29 PM GMT on February 21, 2013
Quoting DelWeather:


It sure does! This is more thorough than the source I found on the internet.

Mark


Great, Mark. Thanks for your enthusiasm. It's catchy!!!!
Member Since: October 24, 2012 Posts: 97 Comments: 837
33. DelWeather
3:17 PM GMT on February 21, 2013
Quoting 24hourprof:


Hope this helps.

Lee


It sure does! This is more thorough than the source I found on the internet.

Mark
Member Since: October 9, 2012 Posts: 0 Comments: 52
32. DelWeather
3:04 PM GMT on February 21, 2013
Quoting georgevandenberghe:



The 7% is actually 6.3%. The reason why only a small fraction of supercooled water freezes immediately is because in freezing it releases latent heat of fusion, warming the rest to the melting point. The latent heat of fusion of water is 333 joules/gram and the specific heat of water is 4.186 joules/gram/kelvin. The fraction 4.186*5/333 defines the amount that will freeze before the temperature is raised to the melting point.

Contact with colder air will eventually freeze the rest but this is the amount that will freeze immediately in supercooled water once freezing starts around an ice nucleus. (A snowflake or ice crystal is an
excellent ice nucleus)


Nice! I get it now. Again, the whole time thing keeps me guessing, but you are just doing an instantaneous analysis here... a snapshot, as it were. Very good.
Member Since: October 9, 2012 Posts: 0 Comments: 52
31. 24hourprof
1:53 PM GMT on February 21, 2013
Quoting georgevandenberghe:



The 7% is actually 6.3%. The reason why only a small fraction of supercooled water freezes immediately is because in freezing it releases latent heat of fusion, warming the rest to the melting point. The latent heat of fusion of water is 333 joules/gram and the specific heat of water is 4.186 joules/gram/kelvin. The fraction 4.186*5/333 defines the amount that will freeze before the temperature is raised to the melting point.

Contact with colder air will eventually freeze the rest but this is the amount that will freeze immediately in supercooled water once freezing starts around an ice nucleus. (A snowflake or ice crystal is an
excellent ice nucleus)


Once again, thank you, George. Well done.
Member Since: October 24, 2012 Posts: 97 Comments: 837
30. 24hourprof
1:51 PM GMT on February 21, 2013
Quoting DelWeather:


I have a question or two about the meteogram. I've been trying to find things on the internet about how to read these and I understand most of the abbreviations. I am guessing that the middle graph with visibility data (VSBY) on the right vertical axis also contains information about cloud cover (the vertical axis on the left is in feet?), but what do CHC1 and CHC2 stand for? I'm guessing altitude of the cloud base... but why two of them?

Then there's T6NF and T6CF, which are, I'm guessing, temperatures in degrees F. But what temperatures? And P06I... something about precipitation?

Thanks!


Here's a general description for you. T6NF, for example, is the miNimum temperature for the preceding six-hour period (from one main synoptic time to the next). In degrees Fahrenheit (F).

Hope this helps.

Lee
Member Since: October 24, 2012 Posts: 97 Comments: 837
29. georgevandenberghe
1:48 PM GMT on February 21, 2013
Quoting DelWeather:


George, I can see how this could work from a physical standpoint. I'm not sure where the 7% number comes from, though. Could you fill us in?

The temperature and dew point are nearly the same up to 700 mbar, so the air is pretty saturated with water molecules through that zone. The condensational warming could keep the condensed water on flakes in the liquid state for awhile, I would guess. How long is another question (all these energy-related questions get far more complex once you fold time evolution into them). But still, your handwaving stays below relativistic speeds, in my humble opinion. Gives food for thought.

Here's a question for everyone: why does a drop of water have a higher radar reflectivity than a similar sized snowflake? Is it just density related? Or is there a solid vs. liquid part to the answer?

Next week I start my experimental weather unit with my sophomore chemistry kids... gas laws, phase changes and graph reading with weather as the hook. I'm not sure if I've bitten off more than I can chew! At least swim season will be over soon and I can stop driving youngsters all over the countryside.




The 7% is actually 6.3%. The reason why only a small fraction of supercooled water freezes immediately is because in freezing it releases latent heat of fusion, warming the rest to the melting point. The latent heat of fusion of water is 333 joules/gram and the specific heat of water is 4.186 joules/gram/kelvin. The fraction 4.186*5/333 defines the amount that will freeze before the temperature is raised to the melting point.

Contact with colder air will eventually freeze the rest but this is the amount that will freeze immediately in supercooled water once freezing starts around an ice nucleus. (A snowflake or ice crystal is an
excellent ice nucleus)
Member Since: February 1, 2012 Posts: 19 Comments: 2226
28. DelWeather
1:30 AM GMT on February 21, 2013
Quoting 24hourprof:


Here is the meteogram at Bridgeport...all snow, at least on the hourly observations.





I have a question or two about the meteogram. I've been trying to find things on the internet about how to read these and I understand most of the abbreviations. I am guessing that the middle graph with visibility data (VSBY) on the right vertical axis also contains information about cloud cover (the vertical axis on the left is in feet?), but what do CHC1 and CHC2 stand for? I'm guessing altitude of the cloud base... but why two of them?

Then there's T6NF and T6CF, which are, I'm guessing, temperatures in degrees F. But what temperatures? And P06I... something about precipitation?

Thanks!
Member Since: October 9, 2012 Posts: 0 Comments: 52
27. DelWeather
1:15 AM GMT on February 21, 2013
Quoting georgevandenberghe:



However I still have some waviness left in my hands. Here goes
some free waving.

With condensation ruled out by logic, the other mechanisim I can think of is simple accretion of supercooled water onto a snowflake falling through a supercooled cloud with high liquid content. This process is fast but again I've never seen it in the literature in the context of snowstorms. It is, I believe the dominant term in hail growth rates.
Latent heat of fusion would prevent most of the accreted water from freezing until accretion stopped. In the case of a sensible supercooled temperature of -5C, only
about 7% would freeze.





George, I can see how this could work from a physical standpoint. I'm not sure where the 7% number comes from, though. Could you fill us in?

The temperature and dew point are nearly the same up to 700 mbar, so the air is pretty saturated with water molecules through that zone. The condensational warming could keep the condensed water on flakes in the liquid state for awhile, I would guess. How long is another question (all these energy-related questions get far more complex once you fold time evolution into them). But still, your handwaving stays below relativistic speeds, in my humble opinion. Gives food for thought.

Here's a question for everyone: why does a drop of water have a higher radar reflectivity than a similar sized snowflake? Is it just density related? Or is there a solid vs. liquid part to the answer?

Next week I start my experimental weather unit with my sophomore chemistry kids... gas laws, phase changes and graph reading with weather as the hook. I'm not sure if I've bitten off more than I can chew! At least swim season will be over soon and I can stop driving youngsters all over the countryside.

Member Since: October 9, 2012 Posts: 0 Comments: 52
26. 24hourprof
7:39 PM GMT on February 20, 2013
Quoting georgevandenberghe:



However I still have some waviness left in my hands. Here goes
some free waving.

With condensation ruled out by logic, the other mechanisim I can think of is simple accretion of supercooled water onto a snowflake falling through a supercooled cloud with high liquid content. This process is fast but again I've never seen it in the literature in the context of snowstorms. It is, I believe the dominant term in hail growth rates.
Latent heat of fusion would prevent most of the accreted water from freezing until accretion stopped. In the case of a sensible supercooled temperature of -5C, only
about 7% would freeze.





Interesting, George. Very interesting. Thanks.
Member Since: October 24, 2012 Posts: 97 Comments: 837
25. DelWeather
6:43 PM GMT on February 20, 2013
Quoting georgevandenberghe:



Where is Mercersburg? Is it in VA or PA.

The late April 1978 storm was an elevation event. Lower elevations got rain (flooding amounts of it). Roanoke VA got no snow but it's 900 feet lower than Blacksburg. I suspect it was confined to southwest VA and southeast WV


Mercersburg is just a bit north of the PA-MD line. Not too far west of Hagerstown. I guess that storm must have stayed south of us, but I do remember some flooding that spring. Mostly witnessed the high streams via bicycle, since swim season was over and poor adults weren't driving my young butt all over the countryside anymore.
Member Since: October 9, 2012 Posts: 0 Comments: 52
24. georgevandenberghe
3:56 PM GMT on February 20, 2013
Quoting georgevandenberghe:


RETRACT BELOW!! I just can't see how this could happen unless dew points are above freezing which I doubt!

I have a possible explanation for ice pellets and graupel but it is speculative, I've not seen it in the literature and it is way outside of my knowledge of cloud physics.

The sounding was below freezing but not below the temperature where cloud water, especially abundant in this case, would supercool. Usually, snow (or ice crystals) falling through such layers scavenges the water through evaporation from the water and condensation onto the ice (dewpoint > frost point). And usually latent heat of condensation is not sufficient to melt it and it remains snow.
However if there is enough water and condensation rates are rapid, the snow may partially melt through latent heat of condensation and pelletize into graupel. Melting all the way to a drop that then freezes into an ice pellet is a stretch. However I've also observed ice pellets in intense snow events where I thought the whole sounding was below freezing (the Feb 1983 snowstorm in Princeton for example) and wondered how they could form.

I repeat this is speculative and is very possibly wrong.




However I still have some waviness left in my hands. Here goes
some free waving.

With condensation ruled out by logic, the other mechanisim I can think of is simple accretion of supercooled water onto a snowflake falling through a supercooled cloud with high liquid content. This process is fast but again I've never seen it in the literature in the context of snowstorms. It is, I believe the dominant term in hail growth rates.
Latent heat of fusion would prevent most of the accreted water from freezing until accretion stopped. In the case of a sensible supercooled temperature of -5C, only
about 7% would freeze.



Member Since: February 1, 2012 Posts: 19 Comments: 2226
23. georgevandenberghe
3:25 PM GMT on February 20, 2013

RETRACT BELOW!! I just can't see how this could happen unless dew points are above freezing which I doubt!

I have a possible explanation for ice pellets and graupel but it is speculative, I've not seen it in the literature and it is way outside of my knowledge of cloud physics.

The sounding was below freezing but not below the temperature where cloud water, especially abundant in this case, would supercool. Usually, snow (or ice crystals) falling through such layers scavenges the water through evaporation from the water and condensation onto the ice (dewpoint > frost point). And usually latent heat of condensation is not sufficient to melt it and it remains snow.
However if there is enough water and condensation rates are rapid, the snow may partially melt through latent heat of condensation and pelletize into graupel. Melting all the way to a drop that then freezes into an ice pellet is a stretch. However I've also observed ice pellets in intense snow events where I thought the whole sounding was below freezing (the Feb 1983 snowstorm in Princeton for example) and wondered how they could form.

I repeat this is speculative and is very possibly wrong.
Member Since: February 1, 2012 Posts: 19 Comments: 2226
22. 24hourprof
1:25 PM GMT on February 20, 2013
I looked more closely at the METARS at KBDR:

KBDR 090152Z AUTO 01018G23KT 1 1/4SM -SN BR BKN014 OVC018 M02/M02 A2957 RMK AO2 PK WND 36031/0108 LTG DSNT NE UPB14E28SNE14B28 SLP012 P0002 T10221022 $
KBDR 090128Z AUTO 02014G25KT 2SM -SN BR FEW007 BKN012 OVC023 M02/M02 A2958 RMK AO2 PK WND 36031/0108 UPB14E28SNE14B28 P0000 PNO $

I note that lightning was reported along with "UP" (unknown precipitation), so, yes, I believe that there was some hail mixed with snow for a while during the evening hours along the Connecticut Coast.

Did you folks hear any thunder?
Member Since: October 24, 2012 Posts: 97 Comments: 837
21. 24hourprof
1:06 PM GMT on February 20, 2013
Quoting DocNDswamp:
Another informative post Lee, thanks!
I viewed WU's radar that night, amazed at the incredibly intense bands over CT into mid-Long Island.

As little Winter frzn precip have here, what infrequent "bright banding" events I've seen usually involves patchy bands or bursts of sleet / graupel embedded within light rain shield. Apart from tstm hail of course, when I'll view the VIL intensity...

Good comment exchange here, great having seasoned observers / "ground truth" to offer details verses instrumental observations. Does make me curious if the automated stations' METARS, not necessarily in error but simply reporting a preponderance of snow while mix sleet / graupel was also embedded?
Cheers!


It's a good question...and yes, I am very pleased with the discussion here. Very informative and interesting.

To be honest, I miss the good old days with the human SA reports instead of automated ASOS. I remember standing by the teletype in the PSU weather station with Stu Ostro as we anticipated the latest human observations.

I don't have a good answer for you. When I look at the skew-Ts, I just don't see any melting, so the idea of sleet is a stretch for me. Of course, a model analysis is imperfect, so I recognize that I'm not looking at gospel.


The 02Z Rapid-Refresh model skew-T indicates a lack of a melting layer at New Haven, CT.

I want to really thank you folks for a stimulating discussion. I just love this stuff. Much appreciated.
Member Since: October 24, 2012 Posts: 97 Comments: 837
20. 24hourprof
12:32 PM GMT on February 20, 2013
Quoting Gaara:
Oh, wait, I found the timestamps on the Facebook chat. AP is me (Woodmont) and SK is my friend (Stratford). Looks like the sleet was pretty much entirely out of the picture by 10PM. There were still some solid waves of it in between the 2049 and 2140 comments. I'll blame the beers for saying 11PM earlier!

----

1940EST
AP: Am I hearing sleet? I think it's just the wind.
SK: Def look at dual pol, it's flying towards HVN

1944EST
AP: i prefer kbox to kokx for classification and even it shows the mix

1954EST
SK:Just came in
SK: Like 80/20 SN/IP
SK: Got average of about 10" from several spots
SK: And sounding like more IP by the min

2010EST
AP: Yup.
AP: Sleet confirmed here.
AP: About half and half, the bottle caps are still showing, but they are not long for this world.
SK: Ugh well it'll flip quickly enough hopeuflly

2041EST
AP: half and half here now.. big flakes and some near-graupel

2043EST
AP: Absurdly loud sleet on the east side of the house.
SK: Same here
SK: Huge

2049EST
AP: Someone turned the sleet dial off just like that.
AP: Big flakes pounding, every once in a while we will get a wave of sleet.

2052EST
SK: It's gonna be gone soon but we lost like 3-4 inches at least

[roughly an hour of us talking about how we got "screwed" out of 3-4 inches of snow removed]

2140EST
AP: Sleet disappearing.
SK: Yeah all snow, this band is the real deal

2158EST
AP: Awesome that some of it was showing as hail
SK: Yeah it was weird some people said graupel but it was huge sleet and like hail just from I'd guess huge convection going on
AP: Lotsa lift
SK: Yep never saw/heard it like that tho..historic on its own!

2202EST
Haven't heard any either maybe we'll get a gravity wave at some point like the nutty one on the E side of the band
SK: Notice BDR crapped out
SK: Overcast, light snow, light snow, overcast, overcast, mist
AP: haha
SK: ASOS suck ass

[Then we talk about how Taunton is lazy with their snow maps not having fine gradients like KOKX for a bit]

2219EST
SK: OMG whiteout right now
SK: Worst of the night easy
SK: Can't see the house across the street right now
AP: Yeah same the houses in woodmont are close together



These time stamps make a lot of sense to me. Thanks again.
Member Since: October 24, 2012 Posts: 97 Comments: 837
19. 24hourprof
12:31 PM GMT on February 20, 2013
Quoting Gaara:


Unfortunately, facebook doesn't timestamp conversations, I don't have time tonight to check my IRC log timestamps, and I am way too lazy to check EXIF data (preparing for a vacation to Vermont/NH), but I do have my posts from Zach's blog. Some are paraphrased, some are direct quotes:

2130GMT: Large flakes.

2248GMT: 2-3"/hr est. Smaller flakes, much higher density.

2318GMT: put beers in snow w/ 3" left on neck.

0034GMT: 1" left on neck

0109GMT: 1/4" to go on bottle neck, "just a dab of sleet"

0141GMT: visual estimate:

0146GMT:

0242GMT: Sleet almost gone:

0401GMT: Sleet had been gone for a while:


Now that's some detailed obs! Thanks.
Member Since: October 24, 2012 Posts: 97 Comments: 837
18. 24hourprof
12:30 PM GMT on February 20, 2013
Quoting sucmyballs:


Estimate size of 2-4mm range. There was lots of change over during that time, I was outside for about two plus hours between 8 - 10:30 pm est.


There's nothing like having folks in the field taking these types of observations. Thank you for your informative input. Much appreciated.

Lee
Member Since: October 24, 2012 Posts: 97 Comments: 837
17. Gaara
4:22 AM GMT on February 20, 2013
Oh, wait, I found the timestamps on the Facebook chat. AP is me (Woodmont) and SK is my friend (Stratford). Looks like the sleet was pretty much entirely out of the picture by 10PM. There were still some solid waves of it in between the 2049 and 2140 comments. I'll blame the beers for saying 11PM earlier!

----

1940EST
AP: Am I hearing sleet? I think it's just the wind.
SK: Def look at dual pol, it's flying towards HVN

1944EST
AP: i prefer kbox to kokx for classification and even it shows the mix

1954EST
SK:Just came in
SK: Like 80/20 SN/IP
SK: Got average of about 10" from several spots
SK: And sounding like more IP by the min

2010EST
AP: Yup.
AP: Sleet confirmed here.
AP: About half and half, the bottle caps are still showing, but they are not long for this world.
SK: Ugh well it'll flip quickly enough hopeuflly

2041EST
AP: half and half here now.. big flakes and some near-graupel

2043EST
AP: Absurdly loud sleet on the east side of the house.
SK: Same here
SK: Huge

2049EST
AP: Someone turned the sleet dial off just like that.
AP: Big flakes pounding, every once in a while we will get a wave of sleet.

2052EST
SK: It's gonna be gone soon but we lost like 3-4 inches at least

[roughly an hour of us talking about how we got "screwed" out of 3-4 inches of snow removed]

2140EST
AP: Sleet disappearing.
SK: Yeah all snow, this band is the real deal

2158EST
AP: Awesome that some of it was showing as hail
SK: Yeah it was weird some people said graupel but it was huge sleet and like hail just from I'd guess huge convection going on
AP: Lotsa lift
SK: Yep never saw/heard it like that tho..historic on its own!

2202EST
Haven't heard any either maybe we'll get a gravity wave at some point like the nutty one on the E side of the band
SK: Notice BDR crapped out
SK: Overcast, light snow, light snow, overcast, overcast, mist
AP: haha
SK: ASOS suck ass

[Then we talk about how Taunton is lazy with their snow maps not having fine gradients like KOKX for a bit]

2219EST
SK: OMG whiteout right now
SK: Worst of the night easy
SK: Can't see the house across the street right now
AP: Yeah same the houses in woodmont are close together

Member Since: December 27, 2010 Posts: 0 Comments: 401
16. Gaara
4:03 AM GMT on February 20, 2013
Quoting 24hourprof:


Got it. Sounds convincing to me. Appreciate the update. I'm wondering how widespread this was. Do you remember the time you went out to grab a beer?


Unfortunately, facebook doesn't timestamp conversations, I don't have time tonight to check my IRC log timestamps, and I am way too lazy to check EXIF data (preparing for a vacation to Vermont/NH), but I do have my posts from Zach's blog. Some are paraphrased, some are direct quotes:

2130GMT: Large flakes.

2248GMT: 2-3"/hr est. Smaller flakes, much higher density.

2318GMT: put beers in snow w/ 3" left on neck.

0034GMT: 1" left on neck

0109GMT: 1/4" to go on bottle neck, "just a dab of sleet"

0141GMT: visual estimate:
Quoting Gaara:
"50/50 sleet/snow"


0146GMT:
Quoting Gaara:
"sleet is trending down, flakes are larger"


0242GMT: Sleet almost gone:
Quoting Gaara:
Ladies and gentlemen, we have a foot in Milford, CT. Dual-pol indicates the sleet is eroding (echoes on the standard products are diminishing as a result) and the 'taka-taka-tak' against our east-facing windows is now more of a 'ssss.'


0401GMT: Sleet had been gone for a while:
Quoting Gaara:

Some of the pellets here were pretty monstrous here.. I didn't get out calipers or anything but I'd guess around 1/8".. When the ESE/E winds kicked up it was LOUD against the windows.

Up to almost 17" here.
Member Since: December 27, 2010 Posts: 0 Comments: 401
14. DocNDswamp
10:14 PM GMT on February 19, 2013
Another informative post Lee, thanks!
I viewed WU's radar that night, amazed at the incredibly intense bands over CT into mid-Long Island.

As little Winter frzn precip have here, what infrequent "bright banding" events I've seen usually involves patchy bands or bursts of sleet / graupel embedded within light rain shield. Apart from tstm hail of course, when I'll view the VIL intensity...

Good comment exchange here, great having seasoned observers / "ground truth" to offer details verses instrumental observations. Does make me curious if the automated stations' METARS, not necessarily in error but simply reporting a preponderance of snow while mix sleet / graupel was also embedded?
Cheers!
Member Since: September 21, 2005 Posts: 94 Comments: 4803
13. 24hourprof
10:06 PM GMT on February 19, 2013
Quoting Gaara:


Oh, I didn't just hear it! I went out in it to grab a beer from the porch and was pelted in the hand by the ice, as well. KBDR is about 9 linear miles (KBDR is technically located in Stratford, but is owned/operated by Bridgeport) from my location that night. I have a met hobbyist friend in central Stratford who reported a sleet/graupel/near-hail mix as well. Same thing in West Haven on White street (roughly .5 miles from the sound).


Got it. Sounds convincing to me. Appreciate the update. I'm wondering how widespread this was. Do you remember the time you went out to grab a beer?
Member Since: October 24, 2012 Posts: 97 Comments: 837
12. 24hourprof
10:03 PM GMT on February 19, 2013
Quoting sucmyballs:
Hi Lee,
Thanks for the post.
I was just north of Upton at the time of the Skew-T for New Haven (2z) clearing the drieway. I can confirm that there was small 'hail' and wetsnow falling at that time. i guess i would call it graupel. it was wet thoug, my clothing was wet from the stuff. Up to that point, we had only received a few inches of the stuff. It wasn't until later sometime between 11pm and 5am est. that the heaviest snow fell.


Interesting. Thanks for the ob. Can you remember how small the ice was?
Member Since: October 24, 2012 Posts: 97 Comments: 837
10. Gaara
9:04 PM GMT on February 19, 2013
Quoting 24hourprof:I note that there was freezing fog reported with the snow, but no sleet at all. So I'm not sure what you were hearing. All I know is that it was pretty much an all snow event, at least according to the weather observations I've seen.


Oh, I didn't just hear it! I went out in it to grab a beer from the porch and was pelted in the hand by the ice, as well. KBDR is about 9 linear miles (KBDR is technically located in Stratford, but is owned/operated by Bridgeport) from my location that night. I have a met hobbyist friend in central Stratford who reported a sleet/graupel/near-hail mix as well. Same thing in West Haven on White street (roughly .5 miles from the sound).
Member Since: December 27, 2010 Posts: 0 Comments: 401

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About 24hourprof

Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.