Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 1:57 PM GMT on August 09, 2013
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.
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