David Rowe
National Weather Service Forecast Office
Sacramento, California
INTRODUCTION
Widespread, heavy snow over the higher terrain of northern California is normally associated with strong frontal systems approaching the region from the eastern Pacific Ocean. These frontal systems tap deep moisture, however, heavy snow with embedded convective elements does occur in less moist air masses (Tardy 2002), especially in an unstable post-frontal environment. The heavy snowfall event of April 2nd, 2003 exemplified this type of event in that it occurred in a relatively low precipitable water environment, but this limitation was able to be overcome by strong forcing from a mid-tropospheric jet, strong cold advection, steep lower-level lapse rates, and favorable orography.
EVENT SYNOPSIS
Synoptic scale analysis at 12Z on April 2, 2003 indicated a highly amplified upper-air pattern was present over the western United States. A deep trough was present at 500 mb along the west coast, extending from a 521 dm closed low near Vancouver Island southward to near Point Conception on the central California coast (Fig 1). This model analysis and water vapor satellite imagery (Fig. 2) showed several strong vorticity centers embedded within the northwest flow on the backside of the trough, poised to move into northern California.
The airmass over northern California early on the morning of April 2nd was post-frontal in nature, as satellite imagery indicated the main cold frontal cloud band had moved into southern California and weakened. IR imagery indicated plenty of open-cellular cumulus over the eastern Pacific Ocean just offshore of northern California. Morning upper-air soundings over the region at 12Z did not indicate impressive moisture as observed precipitable water was only 0.29 inches at Medford, OR (MFR) (Fig. 3) and 0.52 inches at Oakland, CA (OAK) (Fig 4). However, the morning soundings did indicate significant instability was present in the lower-troposphere (for a winter-type event) across the region, with 0-3 km lapse rates ranging from ~ 6.7 C/km at OAK to ~ 8.1 C/km at MFR, and observed convective available potential energy (CAPE) was 321 J/kg at MFR (Fig 3) and 478 J/kg at OAK (Fig 4).
By 18Z, water vapor imagery was showing a strong cyclonically-curved jet at 700 millibars (50 knots in the core) beginning to push onto the west coast just to the south of the San Francisco Bay Area. In the exit region of this jet, strong upward vertical motion on the order of 7-9 microbars/sec in the 850-500 mb layer was forecast by the ETA over a large region of northern California, stretching from the northern Sierra Nevada northward into the extreme southern Cascade Range. In addition, the ETA forecast soundings indicated that most unstable CAPE (MUCAPE) was in the range of 250-500 J/kg across this same region between 18Z and 03Z (Fig 5). Shortly after 18Z, radar began to show a rapid increase in coverage of showers and thunderstorms over much of northern California. The showers and thunderstorms continued through the afternoon across the entire region, and heavy snow showers persisted over the west slopes of the northern Sierra Nevada well into the evening hours.
VERIFICATION
Significant precipitation occurred over the higher terrain of northern California from the afternoon through the evening of April 2nd. Gage data indicated that about an average of 0.50 to 1.00 inch of liquid equivalent precipitation fell across the Feather River basin (in the Sierra Nevada northeast of Sacramento), while the American River basin (in the Sierra Nevada east of Sacramento) saw about 0.75 to 1.25 inches of liquid equivalent. The higher elevations of the northern Sierra Nevada saw some impressive snowfall totals, especially for so late in the season. The Sugar Bowl Ski Area near Donner Pass received 11-15 inches of snow, while the Kirkwood Ski Resort to the south (just to the west of Carson Pass) reported 16-24 inches.
In addition, one thunderstorm over the northern Sierra Nevada foothills did manage to reach severe criteria - dropping 1 inch diameter hail in Shingletown (~ 25 miles east of Redding).
CONCLUSIONS
Moisture is just one of the parameters that must be investigated when evaluating a winter weather event's potential to generate heavy snowfall. Many cases have been documented where less than ideal moisture values were overcome, whether through strong dynamic forcing, instability, favorable cloud microphysics, or orography (Tardy 2002) resulting in heavy snowfall. The GOES sounder offers forecasters an additional tool to the more traditional satellite and radar data, and can be used to follow trends in instability in near real-time (Schrab 2000). This can provide valuable information in the forecast decision process as to whether convection will enhance a snowfall event.
REFERENCES
Schrab, Kevin, 2000: Using GOES DPI on AWIPS to Track Instability on Convective Winter Day (Feb. 14, 2000). National Weather Service Western Region TA-Lite 2000-01.
Tardy, Alexander, 2002: Significant Snow in the Sierra Nevada from Maritime Polar Origins. National Weather Service Western Region Technical Attachment No.02-04.
Tardy, Alexander, 2002: Thundersnow in the Sierra Nevada. National Weather Service Western Region Technical Attachment No. 02-13.
