February 24-25, 2003: Red River Thunder-Snow Event Author: Pam Heinselman, CIMMS/OU Contributors: Terry Schuur and Kevin Scharfenberg, CIMMS/OU Last Updated: 1 April 2003

Forecast/Weather:

At 13:30 PM (local), there was a band of very light snow in northern Oklahoma and a region of convection developing along the Red River, which was moving into southern Oklahoma. Temperatures along the Red River were around 20F. The system along the Red River moved into S and SE Oklahoma and intensified. Several locations in S/SE Oklahoma had 5-8" of snow. Kevin notes several regions of low RhoHV and very high ZDR that appeared to be collocated with transitions from sleet to large aggregates (Figs. 2, 3; he called spotters in the late afternoon). Sounding data from 18Z showed a shallow cold layer near the ground with an above-zero layer aloft. Apparently large aggregates were forming in this warm layer and then falling to the ground through the shallow cold layer. These features appeared to be located W of higher reflectivity areas (Figs. 1-3). Kevin said some thunder was reported in southern Oklahoma, so some cells appeared to be convective.

Also, in late afternoon (about 4PM local), there was an echo with low RhoHV and very high ZDRs that formed south of Norman (Figs. 2,3).

Radar Operations:

Start time: 19:35 2/24/03 UTC (13:35 2/24/03 local time)
Stop time: 13:50 2/25/03 UTC (07:50 2/25/03 local time)

Approximately 10 min of down time (data was not being shipped) from approximately 20:30 UTC until approximately 20:40 UTC on 2/24/03 to adjust noise levels.

Radar/Software Status:

Hydrometeor Classification Algorithm: Cold season
Radar: Good

Operational Delivery:

Good interaction with the forecasters. Kevin described several features (as indicated in Forecast/Weather Section) associated with the transition from sleet to snow. Forecasters said they partially used the observation of the lack of these features (high ZDR, low RhoHV) later in the day to upgrade to a heavy snow warning.

Snapshots:

Figure 1: Radar reflectivity at 21:41 UTC on 24 February 2003.	Figure 2: Pocket of high differential reflectivity values at 21:41 UTC, collocated with a pocket of low correlation coefficient, indicates transition from sleet to snow (e.g., large aggregates forming in a warm layer).
Figure 3: Pocket of low RhoHV, collocated with high ZDR at 21:41 UTC, indicates transition from sleet to snow (e.g., large aggregates forming in a warm layer).	Figure 4: The hydrometeor classification algorithm identifies a mix of wet and dry snow at 21:41 UTC.