Carson City area weather: Soaking valley rain, high elevation snow ahead through Tuesday
Submitted by Jeff Munson on Thu, 10/21/2021 - 10:27am
UPDATE: The National Weather Service has issued a Flood Watch for portions of western Nevada, including Carson City, Carson Valley, Reno and the greater Lake Tahoe area from Saturday night through Monday morning.
Heavy rainfall from an atmospheric river will move into the region Saturday night and may continue into Monday morning. High snow levels will lower to near 6,000 feet sometime Monday, according to the NWS. Debris flows, ash flows, rock falls, and tree falls are possible, especially on or near burn scars and steep terrain. This includes the Tamarack, Caldor, and Numbers burn scars. Rises on small creeks and streams are expected, equipment in and near river and creek channels should be moved to safety.
Rock falls in canyons and ponding of water on roads will create hazardous driving conditions, especially along Interstate 80 between Truckee and Reno. Localized urban flooding is also possible.
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A number of weather systems lined up is forecast to bring periods of heavy rain to the Carson City area through the weekend and into Tuesday, with western Nevada valleys seeing up to 2 inches, and 2 to 3 feet of snow possible in the higher elevations of the Sierra before its over, according to the National Weather Service.
Thursday night and Friday, the weather service says warm, moist air mass will keep snow levels near and above 8,000 feet with the first storm. Forecasters expect a period of heavy rain in the Sierra from Donner Pass area northward to Plumas and Lassen counties. Rainfall rates may be intense enough to generate debris flows on and near the Dixie, Sheep, Walker, and Beckwourth burn scars.
For Saturday night into Monday, forecasters have Increasing confidence for periods of heavy rain, even in the mountains with possible flood impacts on burn areas, urban flooding in poor drainage spots, and rock slides in steep terrain. Saturday morning may be a good day to clean those drains and gutters ahead of the storm, the weather service advises. Significant wet-dense snow accumulations are possible in the higher elevations of the Sierra.
The following is from the National Weather Service Weather Discussion page as of Thursday morning:
Today through Sunday morning, very active weather week ahead as multiple storms push into the west. Each storm will gradually be a bit stronger and wetter as well, so now is the time to prepare and make sure your drainage areas and gutters are cleaned out of debris.
Early today the moist flow continues into northern California with upslope ascent maintaining light, intermittent showers across the Sierra. Winds will be breezy, but nothing out of the ordinary. There is some significant weather arriving late tonight with storms escalating through the weekend. Let's check it out.
First system: Tonight into Friday:
There will be a decent push of moisture into the Sierra, primarily near and north of Ebbetts Pass, with relatively high snow levels (8500-10000 feet) starting later today. Deep southerly flow will place the best precipitation chances on the western slopes, with not much spillover until late morning Friday.
Higher resolution models still highlight the potential for an NCFR with high hourly rain rates into northeast California and the Sierra. Flash Flood and Debris Flow Watch is in effect for tonight into early Friday for the Dixie, Sheep, Walker, and Beckwourth burn scars. Snow levels will fall behind the cold front on Friday, reaching around 6500-7500 feet by late Friday afternoon, however, most of the precipitation will have passed by that time. Lingering showers may result in pass level travel difficulties from late afternoon into the early evening hours.
Winds will be rather breezy tonight into Friday as well, with valley location gusts 25-25 mph and ridges pushing gusts to 80 mph.
Second system: Saturday/Saturday Night:
Quick hitting and weaker system with the main impacts in the northern Sierra, northeast California, and northern Nevada. Light QPF amounts under a tenth of an inch are possible in the Sierra as far south as Mammoth, but the bulk of the moisture looks to remain north of Tahoe. Typical breeziness, but no big winds. Snow levels look to remain above 7500 feet for the event, so impacts will be limited to higher passes.
The main event: Sunday through Tuesday:
After digging through all of the probabilistic guidance, this event is shaping up to be one for the record books. ECMWF Extreme Forecast Index values are notable for most weather elements during the Sunday-Monday time frame. The shift of tails values are approaching 5 along the eastern Sierra for snow and QPF, which for you stats nerds means that multiple ensemble members are predicting an extreme QPF/snow event. NAEFS return intervals are signaling the significance of this storm as well, with IVT values registering at "outside of the climatology", which means that none of the late October reanalyses were this wet between 1979 and 2009.
Per latest guidance, plan on widespread heavy rain across the region, even into western Nevada as a strong westerly jet will promote excellent spillover. Snow levels will start quite high (8,500-10,000 ft), but are slated to drop towards the end of the event. There is still a bit of variability in the placement of the upper low, and some simulations are showing a bit of a split from the main upper cold core. That being said, most guidance still targets the bulk of the IVT (atmospheric river) at the eastern Sierra and western Nevada.
By Monday night into Tuesday morning, snow levels will eventually fall to near 5,500 feet with foothill locations, especially Virginia City, increasing their risk for accumulating wet snow. If snow levels drop even more, heavy wet snow accumulations would become the primary concern instead of flooding risks.
Let's take a look at some numbers
Liquid totals from this system may easily exceed several inches along the Sierra crest with the potential for up to 2 inches for western Nevada. Liquid totals will taper as we head east of the Sierra Crest, but values in the Basin and Range could be around the 0.50" mark. The chance for liquid totals of 4" or greater will be around 60% for areas in the Tahoe Basin and northward, while areas south of the Tahoe Basin will be around a 35% chance.
As the system evolves and becomes colder, we're talking about feet of heavy, wet snow in the high Sierra. The Tahoe Basin could receive about 2 to 3-plus feet of snow, with similar totals possible for northeastern, California. Sierra cement at its best!
While the relatively dry soils should mitigate widespread flooding risk, burn scars will be of high concern through the weekend, especially as periods of heavy rainfall develop. Medium-high forecast confidence for an ongoing flood and/or debris flow threat for all burn scars in the region during this event.
With the intense precipitation and efficient spillover in place, winds will likely be less of a threat. Upper level winds will be concerning, particularly for aviation and along the highest Sierra ridges, but the intense precipitation rates should limit the stronger wind speeds at the surface. Valley gusts could still exceed 40 mph at times, with 110+ mph across the ridge lines.
See NVRoads.com for the latest road conditions. We will update as more information becomes available.
