# Presentation: 369404

## Composite Analyses of Midwestern Convective Snow Events

**Angela Oravetz****1****, Patrick Market****1****, David Gaede****2****, Evan Bookbinder****2****, and Anthony Lupo****1**

- 1 Department of Atmospheric Science
-   University of Missouri - Columbia

- 2 National Weather Service Office
-   Springfield, Missouri

*Presented at the NWA Annual Meeting – 24 October 2002*

## Support

- University of Missouri – Columbia
- Graduate Research Board

## Introduction

- Analyses of 99 thundersnow (TS) events
- All associated w/ a transient mid-latitude cyclone
- 1961-1990
- Market et al. (2003)
- Northeast and northwest quadrants of surface cyclones exhibit the bulk of thundersnow occurrence

## TS Distribution w/ Transient Midlatitude Cyclone

***L***

## Motivation

- On the synoptic and meso- scales, why are these regions preferred for thundersnow?
- Assumption 1:  thundersnow results from convection
- Assumption 2:  convection results from the release of instability

## Hypothesis

- Northeast of SFC cyclone
**North of warm front**
- Anticyclonic shear more likely
**Release of conditional symmetric instability (CSI) in the presence of ****F**
**(Young cyclone?)**
- Northwest of SFC cyclone
**Strong cyclonic shear**
**Release of potential instability (PI) within a trowal**
**(Occluded cyclone?)**

## Method

- Assemble composite fields of TS events Northeast and Northwest of SFC cyclone
**9x9 grid with a grid spacing of 190.5 km.**
- Composite cases processed
**19 cases of TS NE of cyclone**
**28 cases of TS NW of cyclone**

## Results - Surface

- Northeast (TS NE of Low center)
- 19 cases
- Mean central pressure of 999.0 mb
- S.D.
- Only 2 cyclones (11%) clearly occluded
- Northwest (TS NW of Low center)
- 28 cases
- Mean central pressure of 998.0 mb
- S.D.
- 7 cyclones (25%) clearly occluded

## Thundersnow Northeast of SFC Cyclone

## NE - 300 mb Heights & Isotachs

## NE - 300 mb Temp. Advection

- 10-1 K hr-1

## NE - 500 mb Heights & Temp.

## NE - 500 mb Heights & Abs. Vort.

**X**

## NE - 700 mb e

## NE - 850 mb Heights & Isotachs

***L***

## NE - 850 mb Heights & Temp.

***L***

## NE - 850 mb RH & Moist. Conv.

## NE – Surface Example

## NE – Instability

- Examined cases with TS at 00Z or 12Z
- N=5
- 4 of 5 systems open wave (1 occluded)
_*NO*_ potential instability
- Weak conditional symmetric stability (CSS)
- Median value of equivalent potential vorticity
- 	(EPV) = 0.1*10-6(m2 K s-1 kg-1)
- Median level elevation = 650 mb

## NE - Summary

- TS occurs ahead of 300 mb trough
- Jet core SE of event
- Temp. advect. max S of event
- TS beneath 500 mb ridge axis
- Vort. max (16) SW of TS event
- TS in zonal 700 mb e pattern
- TS in 850 mb southerly flow
- Nose of LLJ
- Warm air advection
- Moisture convergence

## Thundersnow Northwest of SFC Cyclone

## NW - 300 mb Heights & Isotachs

## NW - 300 mb Temp. Advection

- 10-1 K hr-1

## NW - 500 mb Heights & Temp.

## NW - 500 mb Heights & Abs. Vort.

**X**

## NW - 700 mb e

## NW - 850 mb Heights & Isotachs

***L***

## NW - 850 mb Heights & Temp.

***L***

## NW - 850 mb RH & Moist. Conv.

## NW – Surface Example

## NW - Instability

- Examined cases of TS at 00Z and 12 Z
- N = 12
- 11 of 12 systems open wave (1 occluded)
- Potential Instability in 6 of 12 (50%)
- Median level of base of unstable layer = 700 mb
- Weak CSS in remaining 6
- No potential instability in these profiles
- Median value of EPV = 0.2 *10-6 (m2 K s-1 kg-1)

## NW - Summary

- TS occurs ahead of 300 mb trough
- Jet streak SE of event (more meridional)
- Temp. advect. max over event
- TS ahead of negatively tilted 500 mb trough
- Vort. max (19) SSW of TS
- TS along 700 mb e ridge
- TS NW of 850 mb Low
- Warm air advection diminished
- Moisture convergence persists
- TS resides in RH max (85%)

## Conclusions

**Surface analysis alone is inadequate for locating TS**
- Not all TS events NW of a cyclone center are associated with occluded cyclones
- Not all TS events NE of a cyclone center are associated with nascent cyclones
**Upper air analyses suggest TS NW of cyclone *****does***** occur with more mature cyclones**

## Conclusions

**Preliminary stability analysis**
**Northeast of a cyclone**
- The occurrence of CSI in the presence of frontogenesis more prevalent
**Northwest of a cyclone**
- The occurrence of CSI in the presence of frontogenesis occurs ~50% of the time
- Dynamics here?
- The occurrence of PI occurs ~50% of the time
- Presence of a trowal airstream?

## Questions, Comments, Criticism?

- _[marketp@missouri.edu](mailto:marketp@missouri.edu)_
- _[amo598@mizzou.](mailto:amo598@mizzou.edu)__[edu](mailto:amo598@mizzou.edu)_
- _[http://weather.missouri.edu/](http://weather.missouri.edu/)_