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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:
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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. |
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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. |
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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. |
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MUCAPE: Most-unstable CAPE across
ILN's CWA was low and did not exceed 500 J/kg. This instability
originated from near 800 hPa. |
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0-3 km Helicity: Helicity values
were high, ranging from 200-500 m2/s2. |
Sounding:
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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:
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0.5° Radar Loop: 1.5-hour loop
shows quick initiation, rapid storm movement (~50-kt) and the miniature nature
of supercells. |
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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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www.nws.noaa.gov
