By: Robert Sandberg/Carson City Weather
Synopsis: A major winter storm will begin to affect the Sierra late early Sunday morning, with the heaviest precipitation Sunday night through Tuesday afternoon. A brief break in the precipitation is expected Wednesday ahead of another weak storm that may bring another round of unsettled wintry weather for Thursday into Friday next week.
The Big Picture: Latest satellite observations show our departing storm from yesterday has moved well east into the midwestern states, while brief ridging has moved over the western US. Meanwhile, a rather cold airmass traversing from Siberia into the Gulf of Alaska currently as can be seen with a large are of cold air convection. A developing trough in the north Pacific is beginning to develop and dig into the northeast Pacific, while pulling in decent moisture as our next major winter storm begins to develop.
A look at the latest upper air analysis upstream shows the overall pattern becoming a bit more amplified over the Pacific. The above noted trough developing over the northern Gulf of Alaska is associated strong 140+ knot jet. The development of this longwave trough is due in part to an upper-level low near the International Date Line, which has been partially responsible for allowing the persistent ridge to retrograde from the western US out into the central Pacific.
Forecast Analysis: The latest forecast analysis continues to indicate that rain and snow will arrive in the northern Sierra early Sunday morning and increase through the day. The heaviest precipitation will arrive as an atmospheric river (AR) moves into the area by early Monday into early Tuesday. For the sake of brevity, the ECMWF IVT and most other models valid for Monday morning show the development of a weak to moderate AR moving into the west coast. This is when the heaviest precipitation and dynamics will arrive along the central west coast to bring heavy rain and snow to the area.
The latest WPC 7-day QPF forecast continues to be rather consistent with 7 to isolated pockets of 10+ inches of liquid precipitation through Friday of next week. For far western Nevada, amounts of 2+ inches are possible especially along the lee side foothills on the west side of Reno and Carson City. The GEFS ensemble mean shows rather strong precipitation anomalies over most of California and much of Nevada especially in the Sierra.
Snowfall Forecast: Latest forecast analysis continues to increase expected snowfall amounts. Currently, the Sierra above 7,000 feet will likely see 6+ feet. Lake Tahoe will likely see mostly all snow, but a brief period of rain/snow mix Sunday evening before changing back to all snow. In all the Lake Tahoe Basin looks to receive 2 – 4 feet of snow by at least Tuesday evening with additional accumulations later next week.
Narrowing things closer to the Reno/Carson City area, latest NBM guidance continue to ramp up precipitation amounts with Reno receiving 2-4 inches of heavy wet snow, while Carson City looks to receive about 4-6 inches, both about a 50% probability. The main issue for the valleys will be snow levels, which currently look to range from 4,500 feet early Sunday to 6,500 feet Sunday evening. Snow levels are currently expected to remain above most valley floors until early Tuesday morning when they will come down to all valley floors and lower through the day into the early evening. A wild card here may be a lower snow level due to heavier precipitation and colder air settling in if winds remain on the lighter side, but that can be quite variable from valley-to-valley location.
Liquid Precipitation Forecast: The same NBM guidance continues to have the Reno area at 1+ inches of liquid through the period, while Carson City is at about 2+ inches. There are some probabilities of much high and lower amounts, so these are middle of the road at about 50%. To give an example currently, Carson City has about a 90-100% chance of receiving around ½ of an inch of liquid precipitation to around a 10% chance or less of 6-7 inches of liquid next week, so there is quite a bit of variability in the forecast. One thing will be certain is the highest amounts will be realized along the foothills with the noted amounts more likely toward the airports on the drier sides of Reno/Carson City.
Looking further out to the next storm for Thursday into Friday, it currently looks like a fast mover and fairly like the system that moved through yesterday, but on the colder side, so most precipitation that fall will remain all snow in western Nevada, so a few inches are currently possible in most valley locations with higher amounts in the Sierra. Looking further out to the week of Christmas, the overall pattern looks drier with cool temperatures. Depending on how much snow remains in the valleys and the inversions, it may be quite cool with areas of low clouds and fog at times. Still, it’s quite a way out so much will probably change.