Atmospheric River to bring Heavy Rain Late Tonight through Thursday Evening

Synopsis: Heavy rain in the valleys and heavy snow in the higher elevations of the Sierra tonight through Thursday evening.   A colder but weaker storm is expected for Friday night through Sunday. Southwest winds will be gusty at times from 15-35mph.

So far with the rain from yesterday through early this afternoon about .25 to .50 of an inch of has fallen around the Carson City area, with the greatest amounts closer to the Carson Range.

The latest IR satellite image continues to show significant amounts of subtropical moisture streaming into the area at this time. A very cold and dynamic low developing in the eastern Gulf of Alaska is beginning to slowly shift south along the west coast of North America. Meanwhile, precipitable water is on the increase with the atmospheric river (high PWAT plume) showing values from 1 to 1.5+ inches moving into central and southern California. This plume will phase with the cold low in the Gulf of Alaska to bring heavy rains to the area beginning late this evening lasting through much of the day on Thursday before tapering off in the late evening.

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The current precipitation forecast over the next 4 days continues to trend wetter in the forecast simulations. The 12Z ECMWF and latest 18Z GFS shows amounts of 5-7 inches, with some localized heavier amounts along the Sierra crest, with good spillover into western Nevada. Amounts for the local Carson City area continue to look impressive with 1+ inch with some locally higher amounts to near 1.5 inches toward the foothills. For the Reno area, total precipitation looks to be around 0.50 inches with greater amounts toward the foothills. This looks reasonable compared to the SREF plume model (not shown) which has about 0.50 inches in Reno over the same period. The ECMWF looks a bit overdone at this time, but it commonly does not show as much shadowing in the lee of the Sierra. The WPC forecast is closer to the GFS with total precipitation through the period and the NAM (not shown) has too much shadowing in the lee of the Sierra and does not look reasonable given the amount of moisture present. As for snowfall over the Sierra, the snow levels through the early portion of the event will range from 7,500-8,500 feet before lowering Thursday afternoon to 6,000 feet. As a result of the heavy rains and high snow levels, some rises on area streams and main stem rivers are expected, no flooding is currently forecast.

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After the rain winds down Thursday night, expect Friday to be mostly dry in the valleys with a few snow showers in the Sierra. Another cold but weaker storm system arrives Friday night into Saturday with another small disturbance moving into the area Saturday night into Sunday. Snow levels will start out near or just above the valley floor then lower with accumulating snow possible. While it’s a bit to early discuss snowfall amounts, it looks like another 6-12 inches will fall over the Sierra and Carson Range and perhaps 1-2 inches in the Reno/Carson City area. I’ll fine tune the potential for snowfall as we get closer to this weekend. Since we are getting later into March, accumulating snow will be difficult in the lower elevations with the higher sun angle and daytime temperatures in the low to mid 40s. If the brunt of the snow occurs in the overnight hours, accumulations are more likely at all elevations.

Robert Sandberg

Graphics are courtesy of NOAA WPC and WeatherBell Analytics.

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