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Mini-Supercell/Marginal Hail Case
November 5, 2007

Overview: A quick-moving cold front moved across the ILN CWA during the afternoon of 11/5/07.  A cool late-autumn airmass was in place ahead of the front (temperatures in the upper 50s and lower 60s), which inhibited any surface-based instability.  However, very weak elevated instability existed above the stable boundary layer (MUCAPE <500 J/Kg lifted from 800 hPa).  In addition, shear ahead of the front was high, with 0-6 km shear around 60 kt, and 0-3 km helicities ranging from 250-500 m2/s2.  Because of the very limited instability, neither the local office nor the Storm Prediction Center anticipated any strong or severe thunderstorms.

As the front entered ILN's CWA, numerous thunderstorms developed and quickly took the form of elevated mini-supercells.  The cores of the storms were small and were generally confined to below 15,000 feet, and none of them extended above the -20 °C level.  However, many of the storms exhibited weak midlevel rotation, and almost all storms with rotation ended up producing severe hail.  Because of this, rotation was used as a warning decision factor during the event.

It is shown here that the high 0-3 km helicity favored midlevel mesocyclones, which led to significantly stronger dynamically-induced updrafts.  The air mass in place was cool and freezing levels were low, so these meso-induced updrafts grew within the hail-growth zone of -10 to -30
°C, and large hail was the result.  Because of the low freezing levels, hail was able to make it to the surface without much melting. 




Synoptic Setting:

Upper levels: A broad 300-hPa trough was deepening over the Great Lakes as a 120-kt jet max rounded the base.  A large area of upper divergence can be seen in the vicinity of the jet streak across much of Indiana and western Ohio, supportive of deep-layer lifting ahead of the front.
850 hPa: The trough at 850-hPa was deeper than at upper levels as it moved into the Ohio Valley.  A 40-50 kt jet existed ahead of the trough.  A weak cold front is evident in the wind and height fields, which at 20 UTC was located near the I-71 corridor.  Despite being on the tail end of the jet max where divergence would be expected, convergence along the front was sufficient to initiate convection.
Surface: The surface cold front lagged the 850-hPa front, and at 21 UTC was moving into northwestern parts of the CWA.  Surface temperatures were only around 60 degrees, and the highest dewpoints were confined to the lower Ohio Valley.  Dewpoints in ILN's CWA were only in the upper 40s.  This led to a stable surface layer. 
MUCAPE: Most-unstable CAPE across ILN's CWA was low and did not exceed 500 J/kg.  This instability originated from near 800 hPa.
0-3 km Helicity: Helicity values were high, ranging from 200-500 m2/s2.


Sounding:

The 20Z RUC sounding for Columbus shows an elevated unstable layer around 800 hPa, but the instability is weak with only 400 J/Kg of MUCAPE. However, note that MUCAPE is low partly because the buoyant layer is relatively shallow (about 15,000 feet deep).  At the most unstable point, the updraft temperature is about 3-4 °C warmer than the ambient temperature which suggests that the normalized CAPE (NCAPE, defined as the total CAPE divided by the depth of the Free Convective Layer ) is weak to moderate. In low CAPE situations such as this, NCAPE may be a better indicator of mean buoyancy, since going by value of CAPE alone may underestimate the potential strength of updrafts.  The NCAPE on this sounding was 8 cm s-2, which is toward the lower end of NCAPE values associated with severe weather (although there does not exist a formal study correlating NCAPE to severe weather).

Also of interest is the high 0-3 km helicity, greater than 400 m2 s-2.  The 0-6 km shear is also high at around 60-kt.  In addition, the WBZ level is only around 7 kft, and the maximum hail growth zone (-10 to -30 °C) is concentrated between 12 and 22 kft.


Radar Signatures:

0.5° Radar Loop: 1.5-hour loop shows quick initiation, rapid storm movement (~50-kt) and the miniature nature of supercells.

Storm 1: Brown County

Storm 2: Pickaway County Storm 3: Warren County

0.5° Reflectivity

3.1° SRM (11.4 kft)

0.5° Reflectivity

1.8° SRM (8.9 kft)

0.5° Reflectivity

8.0° SRM (15.3 kft)

These 3 supercells were among the storms which produced severe hail that afternoon. All of them exhibited a midlevel mesocyclone, ranging from 8-17 kft.  Most of them were broad and weak, though the Warren County storm exhibited a deeper and tighter mesocyclone, along with a weak echo region.   This storm produced the largest hail of the day (1.25" in Morrow).


Vertical Cross Section of Storm 3: The precipitation core in this storm is small both vertically and horizontally.  Maximum reflectivity values are around 65 dBz, but these are located below the freezing level.  The highest cores aloft are between 55 and 60 dBz which extend slightly above the -10 °C level (not marked), and the 45-dBz values extend to about 22 kft which is well above the -20 °C level. An ill-defined weak echo region up to 16 kft may also be present, but is difficult to tell for sure in this image.  The echo tops around 27 kft correspond well with the equilibrium level of 26.5 kft in the Bufkit sounding. Note that the highest reflectivities and possible WER both top out around 16-17 kft...near the level of the midlevel mesocyclone shown in storm 3 above.  This is also around the level that CAPE is its "fattest", which suggests the updraft is likely strongest near this level. 



 

Discussion: Even though the total value of MUCAPE is very low, the normalized CAPE would be more significant and suggests that potent short updrafts were possible. With high 0-3 km helicity, any updrafts that formed were able to tilt strong vorticity within the environment and create midlevel vertical vortices.  It has been documented that such midlevel mesocyclones create dynamic low pressure centers.  The stronger the midlevel rotation, the deeper the low.  This low then induces an upward-directed PGF beneath the meso, which enhances and widens the updraft below it (see figure below).

  

Studies indicate that in environments of weak CAPE (<1000 J/kg), this dynamically-driven updraft can meet or exceed the strength of an updraft in a moderate or strongly unstable air mass (2500+ J/kg). This appears to have been the case on November 5. The rotation in the Warren County storm was focused between 9,500 ft and 17,000 feet AGL, maximized at 16,000 feet. Looking at the highest reflectivities and the (weak) WER in the radar cross section, it appears the strongest updraft was indeed maximized near and below the level of midlevel rotation. Looking at the CMH sounding, a strong updraft near and below 16 kft puts it into the max hail growth region of -10 to -30
°C. In this case, the max hail growth region was much lower than usual (WBZ was around 7k feet) so the mesocyclones and the resulting dynamically-enhanced updrafts were maximized where hail was favored. Note that the Warren County storm had the strongest mesocyclone and produced the largest hail of the day, illustrating the direct relationship between midlevel vortex strength and the strength of the induced updraft.  The low freezing level also prevented hailstones from melting significantly before they reached the ground.  Even though large hail is not typically associated with WBZ as low as 7k feet, this is because strong updrafts are not normally found in environments this cold.  A case such as this in which strong updrafts are favored will have an even greater hail threat with WBZ heights this low.

In summary, this case illustrates two important points in low CAPE situations.  First, always look at forecast soundings to see if CAPE is tall and skinny or short and fat. The latter case may still be capable of strong but short updrafts which makes the low value of total CAPE deceiving.  Second, look at the low level shear to determine if mesocyclones are favored (high 0-3 km helicity and favorable 0-6 km shear). In such situations, it is suggested to analyze the forecast sounding keeping in mind that enhanced updrafts are likely near and below a mesocyclone (8k-17k feet, or where CAPE is fattest). If these enhanced updrafts would fall in a hail-favored region (as in this case), anticipate an enhanced hail potential if WBZ heights are favorable. If the enhanced updrafts are close to the ground in a low LCL, low CIN, strong 1-km shear environment, be on the lookout for an enhanced tornado potential.

Dan Hawblitzel
NWS Wilmington, Ohio


 



Storm Reports:

PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE WILMINGTON OH
549 PM EST MON NOV 05 2007

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0156 PM     HAIL             HAMILTON                39.39N 84.56W 
11/05/2007  E0.88 INCH       BUTLER             OH   TRAINED SPOTTER 

0215 PM     HAIL             MORROW                  39.35N 84.13W 
11/05/2007  E1.25 INCH       WARREN             OH   PUBLIC          

0222 PM     HAIL             2 N WILMINGTON          39.47N 83.83W 
11/05/2007  M0.50 INCH       CLINTON            OH   NWS EMPLOYEE    

0229 PM     HAIL             ERLANGER                39.01N 84.59W 
11/05/2007  E0.75 INCH       KENTON             KY   TRAINED SPOTTER 

0238 PM     HAIL             WILMINGTON              39.44N 83.83W 
11/05/2007  M0.75 INCH       CLINTON            OH   NWS EMPLOYEE    

0241 PM     HAIL             ALEXANDRIA              38.96N 84.38W 
11/05/2007  E0.88 INCH       CAMPBELL           KY   EMERGENCY MNGR  

            MANY LOCATIONS IN THE COUNTY.

0245 PM     HAIL             NEW RICHMOND            38.96N 84.28W 
11/05/2007  M1.00 INCH       CLERMONT           OH   TRAINED SPOTTER 

0245 PM     HAIL             BETHEL                  38.96N 84.08W 
11/05/2007  E0.75 INCH       CLERMONT           OH   PUBLIC          

0250 PM     HAIL             INDEPENDENCE            38.95N 84.55W 
11/05/2007  E0.75 INCH       KENTON             KY   PUBLIC          

0250 PM     HAIL             REYNOLDSBURG            39.96N 82.80W 
11/05/2007  E0.75 INCH       FRANKLIN           OH   PUBLIC          

0257 PM     HAIL             BETHEL                  38.96N 84.08W 
11/05/2007  M1.00 INCH       CLERMONT           OH   TRAINED SPOTTER 

0259 PM     HAIL             GROVE CITY              39.87N 83.07W 
11/05/2007  E0.75 INCH       FRANKLIN           OH   PUBLIC          

0300 PM     HAIL             NEW HOPE                38.96N 83.91W 
11/05/2007  E0.75 INCH       BROWN              OH   LAW ENFORCEMENT 

0305 PM     HAIL             CANAL WINCHESTER        39.85N 82.82W 
11/05/2007  E0.88 INCH       FRANKLIN           OH   PUBLIC          

0310 PM     HAIL             NEWARK                  40.07N 82.42W 
11/05/2007  E0.25 INCH       LICKING            OH   PUBLIC          

0310 PM     HAIL             PICKERINGTON            39.89N 82.77W 
11/05/2007  E0.75 INCH       FAIRFIELD          OH   PUBLIC          

0312 PM     HAIL             4 S NEWARK              40.01N 82.42W 
11/05/2007  E0.75 INCH       LICKING            OH   PUBLIC          

0315 PM     HAIL             WINCHESTER              38.94N 83.65W 
11/05/2007  E1.00 INCH       ADAMS              OH   LAW ENFORCEMENT 

0315 PM     HAIL             CIRCLEVILLE             39.60N 82.94W 
11/05/2007  E0.75 INCH       PICKAWAY           OH   PUBLIC          

0315 PM     HAIL             CIRCLEVILLE             39.60N 82.94W 
11/05/2007  E0.75 INCH       PICKAWAY           OH   PUBLIC          

0315 PM     HAIL             CIRCLEVILLE             39.60N 82.94W 
11/05/2007  E1.00 INCH       PICKAWAY           OH   PUBLIC          

0315 PM     HAIL             BUCKEYE LAKE            39.94N 82.48W 
11/05/2007  E0.75 INCH       LICKING            OH   PUBLIC          

0315 PM     HAIL             CANAL WINCHESTER        39.85N 82.82W 
11/05/2007  E1.00 INCH       FRANKLIN           OH   PUBLIC          

0320 PM     HAIL             LANCASTER               39.72N 82.60W 
11/05/2007  E0.88 INCH       FAIRFIELD          OH   PUBLIC          

0320 PM     HAIL             PICKERINGTON            39.89N 82.77W 
11/05/2007  E1.25 INCH       FAIRFIELD          OH   PUBLIC          

0324 PM     HAIL             2 E BUENA VISTA         39.55N 82.64W
11/05/2007  E0.75 INCH       HOCKING            OH   PUBLIC             

$$


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