Development and propagation of a narrow cold frontal rain band in the Sierra Nevada of Northern California

On January 4, 2008, the Northern Sierra Nevada and Northern Nevada region experienced a significant precipitation event with the passage of a surface level cold front and upper level trough. Both the Tahoe region and Truckee Meadows area received rain which turned to heavy snow over the day. Although the forecast by the National Weather Service predicted a change from rain to snow on this day, it missed the timing of the precipitation change by nearly six hours. The precipitation change from rain to snow during this storm was likely due to a narrow cold frontal rain band (NCFR) which formed in the left exit region of an upper level jet streak. This talk will explore the dynamics of this event as it moved across Northern California and into Northwestern Nevada using both observational and model generated data to determine what phenomena occurred at both the synoptic and meso-scales.

The Weather Research and Forecasting Model (WRF) was employed to enhance the observational data of the NCFR event. A number of sensitivity studies were run to determine how to best capture the observed features. The WRF was only able to correctly diagnose the frontal wind shift late in the day when it was initialized approximately three hours before the arrival of the NCFR at Truckee. This result showed the model's lack of ability to capture the mesoscale dynamics of the event when initialized earlier, the difficulty of forecasting the event, and the model's sensitivity to initial conditions.

Both observations and WRF model results support the conclusion that ahead of the cold front a NCFR developed in conjunction with a subsynoptic low pressure center that brought intense precipitation to a very narrow region on the upslope of the Sierra Nevada. Additionally, the change from rain to snow was slowed by warm air advection due to a strong, narrow southerly low-level jet which transported a latent heating-fortified warm air surge from near Mammoth Lakes and Bishop, California, keeping the melting level high and preventing formation of snow. As the winds turned from out of the south to southwesterly effectively cutting off the source of warm air, both the NOAA profiler observations and the model show a change from rain to snow and the progression of the cold front into the Truckee and Reno areas.