The Case of the Unwitnessed Hailstorm

By: 24hourprof , 1:57 PM GMT on August 09, 2013

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On Saturday morning, August 3, 2013, a high-precipitation supercell (HP supercell; see 12Z mosaic of composite reflectivity below; larger image) developed near the Nebraska-South Dakota border, likely producing very large hail that did not appear on the display of SPC storm reports (even though hail might have exceeded four inches in diameter).


The 12Z mosaic of composite reflectivity on August 3, 2013. At the time, a mesoscale convective system was affecting parts of South Dakota and Nebraska, with a high-precipitation supercell producing large hail near the state border. Larger image. Courtesy of Penn State.

That's because the large hail was not observed in this sparsely populated area of the country. I'll have more to say about how I arrived at this estimate of hail size in just a moment, but I just have to get this off my chest...records for hail size, rainfall, snowfall, etc. are probably not really records in the grand scheme of weather. Extreme weather happens all the time, and, when it occurs, there's not always instruments or weather observers around to measure or document the "event."

Why did I refer to this storm as a high-precipitation supercell? Yes, the answer to this question is a bit of a digression, but I want all my readers "on the same page." There's a spectrum of supercells, ranging from a low-precipitation (LP) supercell, to a classic (CL) supercell, to a high-precipitation (HP) supercell. I don't want to get into all the gory details in this blog, but it stands to reason that it rains hardest in the general vicinity of the rotating updraft (mesoscyclone) inside an HP supercell (this assertion is not exactly rocket science, wouldn't you agree?). In turn, the more abundant rain associated with an HP supercell readily gets wrapped into the mesocyclone's circulation, tending to "camouflage" visual clues that a mesocyclone is present (one of the reasons forecasters look at images of storm-relative velocities). Without an obvious hook echo on reflectivity images (example of a supercell with a hook echo on radar reflectivity), the overall appearance of HP supercells often resemble kidney-beans.


The 12Z NAM model analysis of 850-mb streamlines on August 3, 2013, indicates upslope flow over parts of western Nebraska and western South Dakota. The upslope flow was part of the anticyclonic circulation associated with a high centered along the border between North Dakota and South Dakota (the blue "H" marks the center of an 850-mb high). Larger image. Courtesy of Penn State.

With my caveat about weather records and my digression about the spectrum of supercells out of the way, let's get back to the "unwitnessed hailstorm" on August 3. For starters, the supercell developed behind a cold front (12Z surface analysis) as winds around a ridge of high pressure forced relatively moist air up the sloping terrain. To see the upslope flow of air, check out the 12Z NAM model analyses of 850-mb streamlines (above; larger image) and compare it to the pocket of upward motion at 700 mb (below; larger image). Note that the pocket of upward motion at 700 mb at 12Z coincides with the mesoscale convective system (12Z mosaic of composite reflectivity) that included the supercell.


The 12Z NAM model analysis of 700-mb vertical velocities (in microbars per second) on August 3, 2013. Negative values indicate upward motion. Larger image. Courtesy of Penn State.

By the way, folks that insist on explaining the development of severe thunderstorms as a result of the "clashing of air masses" should really start to question the wisdom of this overly simple phrase...the supercell formed back in the cool air (far to the north of the stalled cold front; revisit the 12Z surface analysis; 12Z mosaic of composite reflectivity and the 12Z NAM model analysis of surface temperatures). And to readers who insist that all severe thunderstorms develop in concert with a 500-mb trough, I note that this HP supercell was initiated below a 500-mb ridge of high pressure (here's the NAM model analysis of 500-mb heights at 12Z on August 3, 2013). There, I feel much better now!


The 1216Z image of base reflectivity (Level III data) from the radar at North Platte, NE (KLNX...lower right) on August 3, 2013. At the time, an HP supercell was producing large hail. Larger image. Courtesy of NOAA.

To get a sense of the HP supercell's structure, check out, above (larger image), the 1215Z image of base reflectivity (Level III data) from the radar at North Platte, NE (KLNX). For reference, 1216Z is 8:16 A.M. CDT, and the storm was roughly 64 nautical miles from the radar site at this time. Yes, I can see the resemblance to a kidney bean. On closer inspection, note the values of 75-80 dBZ that presumably mark the hail core of the supercell. Here's the corresponding image of storm-relative velocities...I annotated the velocity couplet to confirm the storm's rotating updraft (the altitude of the couplet detected by the Doppler radar at North Platte was roughly 6000 feet at this time).

Okay, I'm getting close to where the rubber meets the road. There's no doubt that such high vales of reflectivity indicate hail (nothing new here). But how am I estimating the diameters of hailstones in excess of four inches? Good question! Below is the 1214Z analysis of hail size (larger image) derived from the National Severe Storms Laboratory's (NSSL)
Warning Decision Support System – Integrated Information (WDSS-II)).


The 1214Z WDSS-II estimates of hail size associated with the HP supercell in northwest Nebraska on August 3, 2013. At the time, WDSS-II indicated hail as large as four inches in diameter, perhaps larger. Larger image. Courtesy of NSSL via SPC.

In a nutshell, WDSS-II integrates data from a variety of sources...radars, satellites, models, observations, etc. in an effort to help weather forecasters analyze, diagnose, and predict severe weather (read more). For the specific case of large hail, algorithms estimate hail size, incorporating maximum vertical reflectivity (composite reflectivity), the height of the wet-bulb-zero (wet-bulb temperature), the height of the maximum reflectivity, vertically integrated liquid (VIL), and several other data sources. Perhaps I can write a series of short blogs on each topic in the future (example).

At any rate, the 1214Z WDSS-II analysis on August 3 indicated hailstones with diameters greater than or equal to four inches. Of course, we'll never know for sure because reliable reports are difficult to come up in such sparsely populated regions, but, had the supercell been near a more heavily populated area on the morning of August 3, reports of large hail would likely have been listed on SPC's Storm Reports Web page. Such cases are quite sobering because it emphasizes my point that some of the state, national and world weather records are likely not records at all. Indeed, many extreme weather events simply fall through the cracks in our observational network.

Lee

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16. WunderAlertBot (Admin)
3:27 PM GMT on August 12, 2013
24hourprof has created a new entry.
15. 24hourprof
10:16 AM GMT on August 11, 2013
Quoting 13. weatherhistorian:
Great blog Lee!

What do you think, given our current knowledge of atmospheric physics, would be the maximum weight/size a hailstone could possibly become by the time it reaches the surface?


Thanks a million, Chris.

Man, that's a tough one. There's so much complexity to how hailstones grow. Allow me a little time to think about this. It's a great question, but a very difficult answer, assuming there is one. :-)

Best,

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
14. 24hourprof
10:14 AM GMT on August 11, 2013
Quoting 12. taistelutipu:
Thanks Lee and George for your answers regarding the updraft speed needed for extremely large hailstones.

The link to the July 23rd extreme hail event was very informative with all the graphics. I'm trying to get CAPE data for the extreme hail events in Germany to see what a punch thunderstorms pack nowadays on the other side of the world.

94 m/sec is 210 mph. Wow, that's insane but I guess that would be possible only under 'perfect laboratory conditions' so the 160-180 mph seems a good guess to me. Still insane, as that's the equivalent wind speed of a cat 5 hurricane within these thunderstorms - vertically, that is.


Agreed, my friend.

Did you see the monster, Typhoon Utor, approaching the Philippines?
Member Since: October 24, 2012 Posts: 91 Comments: 803
13. Christopher C. Burt , Weather Historian
12:16 AM GMT on August 11, 2013
Great blog Lee!

What do you think, given our current knowledge of atmospheric physics, would be the maximum weight/size a hailstone could possibly become by the time it reaches the surface?
Member Since: February 15, 2006 Posts: 312 Comments: 293
12. taistelutipu
8:42 PM GMT on August 10, 2013
Thanks Lee and George for your answers regarding the updraft speed needed for extremely large hailstones.

The link to the July 23rd extreme hail event was very informative with all the graphics. I'm trying to get CAPE data for the extreme hail events in Germany to see what a punch thunderstorms pack nowadays on the other side of the world.

94 m/sec is 210 mph. Wow, that's insane but I guess that would be possible only under 'perfect laboratory conditions' so the 160-180 mph seems a good guess to me. Still insane, as that's the equivalent wind speed of a cat 5 hurricane within these thunderstorms - vertically, that is.
Member Since: August 20, 2007 Posts: 12 Comments: 640
11. 24hourprof
6:51 PM GMT on August 10, 2013
Quoting 10. georgevandenberghe:


It may be simplistic but a CAPE of 6000j/kg implies potential for an updraft speed over 100m/sec

(E=1/2mV**2) 6000=1/2V**2 12000=V**2 V=sqrt(12000).
6000j/kg is near the highest CAPE I've ever seen.

Comment from severe weather experts solicited.


The CAPE in the Vivian event was 4500j/kg yielding
an updraft speed of 94m/sec.



George,

As it turns out, CAPE is not always a good predictor of updraft strength because, as updraft speeds increase, conditions start to depart from parcel theory (which is the basis for your calculation).

Nonetheless, you need honkin' updrafts to suspend giant hailstones.
Member Since: October 24, 2012 Posts: 91 Comments: 803
10. georgevandenberghe
4:16 PM GMT on August 10, 2013
Quoting 9. 24hourprof:


P.S. I found this link about the Vivian hailstone. If you scroll up, you'll see

"Initial estimates indicate that the updraft strength in the Vivian hail storm likely ranged from 160-180 mph!"

So my earlier answer to you (updrafts well over 100 mph) was reasonable.


It may be simplistic but a CAPE of 6000j/kg implies potential for an updraft speed over 100m/sec

(E=1/2mV**2) 6000=1/2V**2 12000=V**2 V=sqrt(12000).
6000j/kg is near the highest CAPE I've ever seen.

Comment from severe weather experts solicited.


The CAPE in the Vivian event was 4500j/kg yielding
an updraft speed of 94m/sec.

Member Since: February 1, 2012 Posts: 19 Comments: 1938
9. 24hourprof
10:58 AM GMT on August 10, 2013
Quoting 3. taistelutipu:
Thanks for reporting on the large hail. I've been worrying about severe hail events lately. My parents live near the area in Germany where large hail fell, as shown in the link provided by barbamz. I've also seen a couple of videos of the hail. It destroyed cars, metal blinds, solar panels, crops, greenhouses, and even shattered rooftiles. As densely populated as Germany is, hail of that size does an incredible amount of damage.

Fortunately, this is very rare (in fact, I've never seen hail that large in this part of the world) but I fear it may become more common as the intensity of thunderstorms in Europe increases and supercells can form.

Btw, In one of the articles on wetteronline.de, they said that the largest hail stone on record was found in 2010 in South Dakota and it measured 20 cm in diameter, i.e. 8 inches and weighed 875g or 1.93 lbs. I wonder if even larger hail could theoretically form if the updrafts are strong enough. How strong would they have to be though?


P.S. I found this link about the Vivian hailstone. If you scroll up, you'll see

"Initial estimates indicate that the updraft strength in the Vivian hail storm likely ranged from 160-180 mph!"

So my earlier answer to you (updrafts well over 100 mph) was reasonable.
Member Since: October 24, 2012 Posts: 91 Comments: 803
8. 24hourprof
10:45 AM GMT on August 10, 2013
Quoting 7. Weatherfox:
Good read Lee & thanks. I did a local NWS office search of storm reports and there actually was some hail reports albeit not extreme. Us weather affectionados are all over the place and even live in the less populated areas. There were Spotters, the Public and a AWOS report :).


PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE NORTH PLATTE NE
1107 AM CDT SAT AUG 03 2013



0635 AM TSTM WND GST 6 N GORDON 42.89N 102.20W
08/03/2013 E60.00 MPH SHERIDAN NE CO-OP OBSERVER

DIME TO NICKEL SIZE HAIL ALSO OCCURRED AT REPORT
LOCATION.


0720 AM TSTM WND DMG 10 S MERRIMAN 42.78N 101.70W
08/03/2013 CHERRY NE PUBLIC

WIND DRIVEN HAIL KNOCKED WINDOWS OUT AND DAMAGED PART OF
A PASTURE. SIZE OF HAIL IS UNKNOWN. TIME IS ESTIMATED BY
RADAR.

0920 AM TSTM WND GST 1 W THEDFORD 41.98N 100.59W
08/03/2013 E60.00 MPH THOMAS NE TRAINED SPOTTER

PEA TO MARBLE SIZE HAIL WAS ALSO REPORTED.

0933 AM TSTM WND GST THEDFORD 41.98N 100.57W
08/03/2013 M47.00 MPH THOMAS NE AWOS


SNIVELY




Note that there were no reports around the time (8:15 A.M.) that the HP supercell produced large hail near the Nebraska border (and after the supercell transitioned to a squall line...the topic of my next blog).

So, as far as I know, there were no official reports of hail approaching four inches in diameter around 8 AM in northwest Nebraska.

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
7. Weatherfox
12:10 AM GMT on August 10, 2013
Good read Lee & thanks. I did a local NWS office search of storm reports and there actually was some hail reports albeit not extreme. Us weather affectionados are all over the place and even live in the less populated areas. There were Spotters, the Public and a AWOS report :).


PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE NORTH PLATTE NE
1107 AM CDT SAT AUG 03 2013



0635 AM TSTM WND GST 6 N GORDON 42.89N 102.20W
08/03/2013 E60.00 MPH SHERIDAN NE CO-OP OBSERVER

DIME TO NICKEL SIZE HAIL ALSO OCCURRED AT REPORT
LOCATION.


0720 AM TSTM WND DMG 10 S MERRIMAN 42.78N 101.70W
08/03/2013 CHERRY NE PUBLIC

WIND DRIVEN HAIL KNOCKED WINDOWS OUT AND DAMAGED PART OF
A PASTURE. SIZE OF HAIL IS UNKNOWN. TIME IS ESTIMATED BY
RADAR.

0920 AM TSTM WND GST 1 W THEDFORD 41.98N 100.59W
08/03/2013 E60.00 MPH THOMAS NE TRAINED SPOTTER

PEA TO MARBLE SIZE HAIL WAS ALSO REPORTED.

0933 AM TSTM WND GST THEDFORD 41.98N 100.57W
08/03/2013 M47.00 MPH THOMAS NE AWOS


SNIVELY

Member Since: September 9, 2005 Posts: 0 Comments: 1
6. 24hourprof
11:28 PM GMT on August 09, 2013
Quoting 3. taistelutipu:
Thanks for reporting on the large hail. I've been worrying about severe hail events lately. My parents live near the area in Germany where large hail fell, as shown in the link provided by barbamz. I've also seen a couple of videos of the hail. It destroyed cars, metal blinds, solar panels, crops, greenhouses, and even shattered rooftiles. As densely populated as Germany is, hail of that size does an incredible amount of damage.

Fortunately, this is very rare (in fact, I've never seen hail that large in this part of the world) but I fear it may become more common as the intensity of thunderstorms in Europe increases and supercells can form.

Btw, In one of the articles on wetteronline.de, they said that the largest hail stone on record was found in 2010 in South Dakota and it measured 20 cm in diameter, i.e. 8 inches and weighed 875g or 1.93 lbs. I wonder if even larger hail could theoretically form if the updrafts are strong enough. How strong would they have to be though?


Probably well over 100 miles an hour.
Member Since: October 24, 2012 Posts: 91 Comments: 803
5. 24hourprof
11:24 PM GMT on August 09, 2013
Quoting 4. Astrometeor:


You mean hail, instead of fail right?

Otherwise, good catch on the storm there, how do you find these cells? Too much time on your hands?

-Nathan


Good catch on "fail," Nathan. Thanks! I corrected the typo.

I'm a faithful reader of SPC's stuff. They do a great job. They were talking about supercells on Saturday, and my good friend, Steve Corfidi, a lead forecaster at SPC, pointed out the 4" hail.

It's good to have connections in high places.

Best,

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
4. Astrometeor
11:03 PM GMT on August 09, 2013
Quoting 24hourprof:


That's because the large fail was not observed in this sparsely populated area of the country.


You mean hail, instead of fail right?

Otherwise, good catch on the storm there, how do you find these cells? Too much time on your hands?

-Nathan
Member Since: July 2, 2012 Posts: 101 Comments: 10425
3. taistelutipu
9:59 PM GMT on August 09, 2013
Thanks for reporting on the large hail. I've been worrying about severe hail events lately. My parents live near the area in Germany where large hail fell, as shown in the link provided by barbamz. I've also seen a couple of videos of the hail. It destroyed cars, metal blinds, solar panels, crops, greenhouses, and even shattered rooftiles. As densely populated as Germany is, hail of that size does an incredible amount of damage.

Fortunately, this is very rare (in fact, I've never seen hail that large in this part of the world) but I fear it may become more common as the intensity of thunderstorms in Europe increases and supercells can form.

Btw, In one of the articles on wetteronline.de, they said that the largest hail stone on record was found in 2010 in South Dakota and it measured 20 cm in diameter, i.e. 8 inches and weighed 875g or 1.93 lbs. I wonder if even larger hail could theoretically form if the updrafts are strong enough. How strong would they have to be though?
Member Since: August 20, 2007 Posts: 12 Comments: 640
2. 24hourprof
2:26 PM GMT on August 09, 2013
Quoting 1. barbamz:
Lee, thank you! I'm glad that you have obviously recovered and are out in full force again to give us some in depth explanations of weather phenomena :)
Hail has been on topic in Germany some days ago, too, for a reason:


Swabia, August 6

(German source which tries to explain the development of (very large) hail with a graphic).
As most of Germany is densely populated chances are low that such a severe hail event would stay undetected.


Many thanks.

Wow!!! That's quite a photograph!

And I agree wit you about going undetected in Germany.

Best,

Lee
Member Since: October 24, 2012 Posts: 91 Comments: 803
1. barbamz
2:24 PM GMT on August 09, 2013
Lee, thank you! I'm glad that you have obviously recovered and are out in full force again to give us some in depth explanations of weather phenomena :)
Hail has been on topic in Germany some days ago, too, for a reason:


Swabia, August 6

(German source which tries to explain the development of (very large) hail with a graphic).
As most of Germany is densely populated chances are low that such a severe hail event would stay undetected.
Member Since: October 25, 2008 Posts: 58 Comments: 6251

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

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