A Diagnostic Analysis of Significant Cold-Season Elevated ThunderstormsErin E. Snavely1,2, Scott M. Rochette3, and Charles E. Graves1

1Cooperative Institute for Precipitation Systems, Department of Earth and Atmospheric Sciences, Saint Louis University2NOAA/NWSFO Lake Charles, LA

3Department of the Earth Sciences, SUNY College at Brockport

Methodology

Conclusions

• There is a large, progressive trough at 250 hPa associated with a strengthening jet streak to the northeast of the convection.

• Convection is associated with the right entrance region of the upper-level jet streak, as opposed to the exit region, in an area of 250-hPa divergence.

• The buildup of moisture over a large layer from 1000-500 hPa is relatively rapid.

• A tongue of moisture is present and points to the region of heaviest rainfall.

• 850 hPa moisture convergence and θθθθe advection are nearly coincident, forming an elongated axis. Storm initiation occurs in the gradient along this axis.

• A low-level jet is present at 850 hPa, strengthened with isallobaric forcing associated with the upper-level jet.

• Direct thermal circulations from frontogenesis and the upper-level jet’s ageostrophiccirculation couple to enhance upward vertical motions.

Lower Troposphere Composites

Middle/Upper Troposphere Composites

Introduction

• An initial search was performed using the River Forecast Center 10 km Quantitative Precipitation Estimate MOSAIC maps (see below right).

• Daily surface maps from the Hydrometeorological Prediction Center (HPC) were then examined to determine if the conditions for elevated thunderstorms were met on each of the high precipitation days. This yielded 29 cases.

• A Meteorological Aviation Report (METAR) search was performed and regional radars were examined for evidence of convection . Cases were discarded if no convection was present or if it was determined the precipitation was in the form of snow.

• Finally, synoptic patterns were examined using the North American Regional Reanalysis (NARR) dataset displayed in plan-view using the General Meteorological Package (GEMPAK).

• This yielded 15 cases with a similar synoptic pattern, elevated convection, and 1” or greater liquid precipitation over a 24 hour period.

• Locations of convective initiation were then qualitatively determined for each case by examining the regional radars in the GEMPAK Analysis and Rendering Program (GARP) and determining the latitude/longitude when cells reached >30 dBz. This location is represented with a star on each composite plot.

• A 117 x 81 grid with 32 km spacing was then extracted from the NARR dataset centered on the initiation points.

• The 15 cases were then averaged with a locally written compositing program utilizing the GEMPAK software.

• Cold-season elevated thunderstorms associated with heavy rainfall have the potential to cause severe damage through flash flooding and hail and pose a significant forecast problem to forecasters in the central United States.

• Previous studies of elevated thunderstorms include:

� Maddox et al. (1979)4 described three types of synoptic patterns associated with flash flooding. The second, which he termed frontal type, is associated with elevated convection.

� Colman (1990)5 defined criteria for elevated thunderstorms:

�Observation must lie on the cold side of an analyzed front that shows a clear contrast in T, Td, and wind.

�The station’s T, Td, and wind must be qualitatively similar to the immediate surrounding values.

�The surface air on the warm side of the front must have a higher θevalue than the air on the cold side.

• Moore et al. (2003)6 used compositing software to study parameters important for warm season elevated convection with heavy rainfall.

Moore et al. (2003)

• The aim of this study is to determine parameters important for forecasting cold season (1 November – 28 February) elevated thunderstorms associated with 1” or greater rainfall amount in 24 hours.

• Composites of parameters at the time of initiation of convection and six hours prior to the initiation have been computed for 15 cases from the years 2000-2006.

• MUCAPE values are relatively small, although composites do show some destabilization of the atmosphere.

• Convective instability results from folding of isentropes, and is released with coupling of the thermal circulations.

• Future efforts for this project include principal component analysis of the parameters to determine the correlations between the parameters found in this study and their biases.

• Another extension of this study is to investigate other synoptic patterns to compare/contrast the patterns of the parameters with those found in this study.

4Maddox, R. A., C. F. Chappell, and L. R. Hoxit, 1979. Synoptic and meso-α scale aspects of flash flood events. Bull. Amer. Meteor. Soc., 60, 115-123.

5Colman, B. R., 1990. Thunderstorms above frontal surfaces in environments without positive CAPE. Part I: A climatology. Mon. Wea. Rev., 118, 1103-1121.

6Moore, J. T., F. H. Glass, C. E. Graves, S. M. Rochette, and M. J. Singer, 2003. The environment of warm-season elevated thunderstorms associated with heavy rainfall over the central United States. Wea. Forecasting, 18, 861-878.

Composite analysis of 850-hPa mixing ratio (orange, g kg-1) and moisture transport vectors (gray) at t = -6 h (left) and t = 0 h (right).

Composite analysis of 950-hPa heights (black, gpm) and temperature (purple, °C)at t = -6 h (left) and t = 0 h (right).

Composite analysis of 850-hPa heights (black, gpm), isotachs (blue, shaded > 15 m s-1), wind barbs (gray, m s-1), and θe advection (orange, K h

-1) at t = -6 h (left) and t = 0 h (right).

Composite analysis of 850-hPa isotachs (dark blue, shaded > 15 m s-1), wind barbs (gray, m s-1),and moisture convergence (yellow, 10-1 g kg-1 h-1) at t = -6 h (left) and t = 0 h (right).

Composite analysis of MUCAPE (brown, J kg-1) and MUCIN (green, shaded < -20 J kg-1)at t = -6 h (left) and t = 0 h (right).

Composite analysis of 850-hPa heights (black, gpm) and frontogenesis (purple, 10-1 K [100 km]-1 [3 h]-1 ) at t = -6 h (left) and t = 0 h (right).

Cross Section Composites

Cross section through the composite initiation point showing isotachs (green, m s-1), omega (blue, µbar s-1), ageostrophic circulation (blue arrows), θe (black, K) and frontogenesis (red, 10

-1 K [100 km]-1[3 h]-1)at t = -6 h (top) and t = 0 h (bottom).

Composite analysis of 250-hPa heights (black, gpm), isotachs (blue, shaded > 50 m s-1), divergence (red, 10-5 s-1), and ageostrophic wind barbs (gray, kt) at t = -6 h (left) and t = 0 h (right).

Composite analysis of 500-hPa heights (black, gpm), wind barbs (gray), and absolute vorticity (red, shaded > 12 x 10-5 s-1 ) at t = -6 h (left) and t = 0 h (right).