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TitleCase Study of an Anomalous, Long-lived Convective Snowstorm
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Public Abstract
Rebecca Ebert, ID #789272
M.S.
Atmospheric Science
Case Study of An Anomalous, Long-Lived Convective Snowstorm
Advisor: Dr. Patrick Market

—————————————
Graduation Term: Summer 2004

On 23 March 1966, thundersnow was reported over a period of 9 hours (non-
consecutive) at Eau Claire, Wisconsin. This event constitutes the longest period
of thundersnow at a single station from a surface observation dataset of 226 sta-
tions spanning the years 1961-1990. For 30 years researchers have studied the at-
mospheric link between routine snowstorms and those with convection. We can
assume that the ingredients are dynamic in nature and that thundersnow occurs
around a surface cyclone, or near a lake area in association with surface-based in-
stability, or in mountainous terrain.

In this study, the dynamic characteristics of a long-lived convective snowstorm
are examined. Using objective analysis of rawinsonde data and model simulation
output from the Workstation-Eta (WS-Eta), we have determined that the thermo-
dynamic characteristics of the thundersnow event did not change with the evolu-
tion of the cyclone. For the duration of the event Eau Claire was north-northeast
of the surface cyclone, with ample moisture, and forcing for ascent. Equivalent
potential vorticity (EPV) and conditional symmetric instability (CSI) were present
in cross-section analysis. The �e pattern at 700 mb indicates a trough of warm air
aloft (TROWAL) upstream at 0000 UTC and then coinciding with Eau Claire at
1200 UTC. Elevated convective (potential) instability fails to develop.

The WS-Eta run provided an excellent meso-� simulation of the storm. The
WS-Eta output was almost identical to the subjective surface analysis as well as
the 48-hr precipitation field. This snow event resulted largely from the prolonged
presence of frontogenesis in the presence of weak symmetric stability. That the
event should remain convective after 0300 UTC when measurable SCAPE is not
present is not altogether clear, even in the presence of a fine-scale model simula-
tion.

The case presented here resulted from strong frontogenetical forcing in the
presence of weak conditional symmetric stability northeast of a surface cyclone.
This scenario was created and maintained by the presence of a TROWAL airstream
over the EAU region for an extended period.

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EAU

Figure 4.13: A cross-section from Kenora, Ontario (YQK) to Lacon, Illinois (C75)
from 0000 UTC 23 March 1966. Dashed lines depict Mg (every 5 kg m s�1) and
solid contours depict �e (every 2 K). The shaded region denotes where EPV is less
than 1 � 10�7K m2 kg�1 s�1. The black line represents the location of Eau Claire;
notice how this line intersects the shaded area representing negative EPV.

the Mg will ”lay down” or become less vertical then �e. When this occurs a negative
area of equivalent potential vorticity (EPV) is present. It is assumed that slantwise

convection will dominate in the environment if �e is not doubling over with height.
The shaded area in the figure (4.13) represents negative EPV; when negative EPV

is present conditional symmetric instability (CSI) is also possible.

In order to better diagnose the area of instability, a vertical profile through the

objectively analyzed radiosonde data was prepared for Eau Claire, Wisconsin. The

first profile taken at Eau Claire was of EPV (Fig. 4.14); in this profile EPV ap-

proaches zero and becomes slightly negative between 800-700 mb. This analysis

shows that moist potential vorticity is present at Eau Claire three hours before the

first report of thundersnow occurred. The presence of EPV around zero in a po-

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Page 49

tentially stable environment suggests the presence of weak symmetric instability

in that layer. The next profile for Eau Claire is that of frontogenesis (Fig. 4.15),

showing the vertical location where compression/stretching of the horizontal �
gradient is most likely to occur. In this profile, the frontogenesis maximum is lo-

cated between 825-750 mb which resides beneath the area of negative EPV. This

arrangement places the best frontogenesis ahead of the frontal zone, with the best

forcing for ascent to the south and farther aloft of Eau Claire, in the presences of

the negative EPV region.

The last profile that was taken at Eau Claire is that of �e, (Fig. 4.16). In this
vertical profile notice how �e almost doubles over with height. This indicates a
moist neutral environment, and that the thundersnow development in the next

three hours likely will be slantwise in nature. If the �e profile had doubled over
with height this would lead us to believe that upright convection was the dominant

cause of the thundersnow.

4.2 1200 UTC 23 March 1966

In the previous section a synopsis of the conditions three hours before the first

thundersnow report was analyzed. In this section, conditions twelve hours later,

when the last report of thundersnow was observed will be analyzed.

4.2.1 Synopsis

In the 1200 UTC 23 March 1966 surface analysis (Fig. 4.17), the cyclone has tracked

to the northeast and matured into an occluded cyclone. This evolution now places

Eau Claire north-northwest of the surface cyclone. The warm front extends through

southern Michigan, and the cold front is positioned south and through eastern Illi-

nois into Kentucky and Tennessee. The occluded portion of the cyclone extends

from extreme northern Illinois into southern Wisconsin. The central pressure has

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