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The January 25, 2005 Freezing Rain Event over Northeast California and Northwest Nevada
Background
On January 25, an unusual freezing rain event took place over northeast California and western Nevada. Although freezing rain can occur every winter in isolated locations, the widespread freezing rain observed on January 25 was unusual. Areas affected included Mono County north to Lassen County, California and Douglas County, Carson City, and Washoe County, Nevada.
Part of what made this event unusual began with the two heavy snow storms that hit the area in late December and early January. With an unusually widespread and deep snowpack across these regions, the stage was set to develop a shallow cold layer of air in many of the valleys of eastern California and western Nevada. On January 15 th, a strong ridge of high pressure became entrenched over the western U.S. which only helped these shallow cold ‘domes’ in the valleys develop. In fact, this ridge of high pressure helped trap the cold air near the surface and develop the valley fog that was especially stubborn up until the morning of the 24 th.
Now, with the cold air present in the valleys, with warm air above it (a temperature inversion), the stage was set for a possible freezing rain event. Now, while temperature inversions are not that unusual for western Nevada in winter, these strong inversions do not always result in a freezing rain event. The reason is that most incoming storms from the Pacific Ocean bring in strong winds at ridgetop level that can ‘mix out’ the inversion and allow the valleys to warm faster. Typically these wind speeds at the 10,000 foot level are 40 to 50 mph if not stronger in the bigger storms. However, on January 25, the incoming storm was relatively weak with 10,000 foot winds of only 10-15 mph. These weak winds are not strong enough to mix out a strong inversion.
Finally, while weak storms generally do not produce a lot of precipitation, this weak storm system had pulled in subtropical moisture. In addition, with the weak winds aloft, very little shadowing is likely to occur in the valleys east of the Sierra Nevada. Therefore, with all the moisture present, weak systems can produce precipitation across a widespread area. Thus the stage was set for a freezing rain event.
The Event
Beginning around 4 am on the 25 th, weak radar returns were showing up on the Reno radar over Mono County and west of Lake Tahoe. An upper air observation taken from the Reno weather office (Fig. 1a below) showed that the temperature inversion remained, with a 4000 foot deep layer of above freezing temperatures above the surface. However, the 500 feet closest to the surface remained below freezing. Therefore any snow falling through the warm layer would melt, but when it went back into the cold air near the surface, it would not have enough time to refreeze before hitting the surface. Instead, the rain would likely freeze when it hit the surface, especially if ground temperatures were freezing or below. In fact, this continued through the whole event as Figure 1b shows the 1000 am upper air observation still had a warm layer aloft with subfreezing air trapped near the surface.
Figure 1:
The band of freezing rain became better organized and more widespread as it progressed northward (Figures 2, 3, 4, and 5 below), with the freezing rain hitting Douglas County around 500 am, Carson City after 530 am and Reno beginning at 630 am. In eastern California, Mono County had freezing rain around 400 am especially near Crowley and Mono Lakes where temperatures remained below freezing. Freezing rain was reported around Lake Tahoe by 500 am and in Lassen County by 600 am.
Although precipitation amounts were relatively light compared to most winter storms, only a little bit of freezing rain is required to cause travel problems. As little as 0.02” of freezing rain can create a glaze across roadways that will make them extremely slippery. In addition, it is often difficult to see the ‘black ice’ that forms as a result of freezing rain. In fact, numerous traffic accidents were reported across much of eastern California and western Nevada due the freezing rain. The freezing rain also disrupted school schedules as Douglas County schools were delayed one hour with some Lassen County school closed.
Figure 2:
Figure 3:
Figure 4:
Figure 5:
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