textproduct: Reno

KEY MESSAGES

* The next impactful storm has arrived, bringing gusty to strong winds with rain and high elevation mountain snow through Wednesday.

* Flooding potential persists with this weak atmospheric river event, however flooding chances remain low.

* Warmer and drier conditions return late this week into the weekend. Although, there are hints of a system this weekend into early next week.

DISCUSSION

Well, our weak AR has arrived right on time. Rain has begun to fall in the Sierra from the Tahoe Basin northward at the time of this AFD. Current snow levels based on current temperature obs from around the area are estimated to be around 7,500-8,500 feet and are expected to rise significantly throughout this morning to around 10,000-10,500 feet.

There's a couple main hazards at play with this storm: rain on snow, and wind. The rain that we're getting with this AR is falling on a fresh, deep Sierra snowpack, which adds to the runoff. This is a prime setup for localized nuisance ponding along waterways and low lying areas. Fortunately, the amount of rain that's forecasted isn't expected to cause major flooding issues. But more on this in the Hydrology section down below.

Total rain amounts for this system through Wednesday afternoon amount to 1.5-2.0" along the Sierra crest, decreasing to 0.5-1.0" in the Tahoe Basin at lake level, becoming very shadowed along the Sierra Front and eastern Sierra in Mono with a 50-60% chance of 0.20" or more. Based on the dynamics of this storm and the direction of the upper level winds, I would expect Reno/Carson/Minden to only see less than 0.05" of rain out of this, though there's always the chance we can see more spillover. Above 10,000 feet, however, the Tahoe Basin southward into Alpine and Mono counties, snow accumulations of up to 6 inches are likely. Higher amounts of 6-12" are focused around the peaks surrounding Tioga Pass.

Now onto the second hazard associated with this storm: wind. The 700mb jet out of the WSW will peak around 70-75kt later this afternoon. So we're talking Sierra ridges gusting 110+ mph. As of right now, Slide Mt. is already at 86 mph and Palisades at 84 mph. One thing I am seeing with this system is an overwhelming signal for downslope winds along the Sierra Front (Washoe Valley, Hwy 395 near Doyle, Carson Valley) and down in Mono County along Hwy 395. I won't get into the nerdy details, but in these wind prone locations, gusts this afternoon between 10am-4pm could peak close to 70 mph. Right now, Washoe Valley has already gusted to 56 mph. And with it being so shadowed later today with very little rain, there is high confidence (>90%) that gusts will exceed 60 mph. But what about the non-wind prone locations? Good question. Other areas like Truckee, Susanville, Fallon, etc have around a 40% chance to exceed 40 mph. In other words, the most likely range is 30-45 mph for most locations.

Once this storm has wrapped up Wednesday, a ridge will move in for the rest of the week and we'll warm up and dry out. Temperatures down in Reno and western NV valleys (and in NE CA) will be pushing mid-upper 60's, if not more than 70 degrees by Friday. Quite the 180 from last week! Winds will still be a bit breezy on Wednesday, but not nearly as strong as today. Otherwise, the rest of the week looks great! Looking a little bit further ahead for all the ski/snowboard enthusiasts out there, there are signs of our next storm arriving this upcoming weekend. Long range models are in pretty good agreement for the positioning and timing, though that may still change between then and now. It looks like we may get a cut-off low moving in off the coast on Sunday, bringing the potential for mountain snow and valley rain. But more details on that later this week.

-Justin

AVIATION

Our next storm has arrived this morning, yielding periods of MVFR to IFR conditions at Sierra terminals with mountain obscurations. Sierra terminals have already started to experience rain showers, and those showers are expected to continue through tonight and into Wednesday afternoon. The highest rain rates for Sierra terminals will be between 22Z-06Z this evening, then lighter, more isolated showers are expected through Wednesday afternoon.

LLWS and mountain wave turbulence will prevail today for terminals in the Sierra, along the Sierra Front, and eastern Sierra. 700mb (FL100) winds will peak around 70-75kt out of the WSW between 16Z- 02Z, but terminals near and in the Sierra will see gusts up to 40kt. For more information regarding turbulence, LLWS, and more, please visit aviationweather.gov.

-Justin

AVALANCHE

The next storm has arrived this morning, bringing more rain and high elevation snow above 9,000-10,000 ft, as well as gusty winds up to 90-110 mph at the highest peaks.

Snow levels come down on Wednesday afternoon to 7.5-9 kft for the Tahoe Basin and for Mono County Wednesday evening, when precipitation chances taper off. However, there may be some lingering/isolated shower chances going into early Thursday.

Liquid amounts continue to fluctuate with this weak AR event. In the latest NBM probabilities, there is still a 15-35% chance of 2+ inches of liquid at the Sierra crest from the Tahoe Basin northward between today and Wednesday evening, with the crest in Mono County having a <10% chance.

Snow totals will be minimal in most areas, with only up to 6" above 9,000 ft from the Tahoe Basin southward. But some locally higher amounts of 6-12" are possible in Mono County across the higher peaks around Tioga Pass.

SLRs will be 2-6:1 above 9-10 kft.

Ridgetop winds decrease tonight after 7 PM to 60-80 mph. Those winds will persist through Wednesday morning and eventually taper off pretty quick by the afternoon.

-Justin

HYDROLOGY

Rain at all but the highest elevations during the weak atmospheric river will lead to rises on rivers and streams throughout the area.

The now expansive low elevation shallow snowpack (below about 5500 feet), will contribute additional runoff as it melts with warm temperatures, rain, and high winds. The deeper and colder high elevation snowpack (above about 7000 feet) is expected to retain most of the rainfall, but warm significantly during the event. How the now significant mid elevations snowpack responds to the rain is an area of greater uncertainty.

The greatest flooding concerns is for mountain urban areas where deep snow large plow berms may block drainage systems and funnel runoff into unexpected areas.

Flooding from rivers and streams is not currently expected, but minor flooding is possible along if rainfall exceeds expectations. Current forecasts show about about a 5% chance of minor flooding along the Susan River near Susanville and the West Fork of the Carson river into Carson Valley, and very low, but non zero chances along the Middle fork of the Feather near Portola.

Elevated flows are expected to continue through the week as warm temperatures will impact the recently ripened snowpack.

The River Forecast Center will be extending hours will add a third evening forecast on Tuesday for river points where flooding is possible. Check for updated river forecasts at: www.cnrfc.noaa.gov

-TB

REV Watches/Warnings/Advisories

NV...Wind Advisory until 7 PM PST this evening NVZ002.

Wind Advisory until 7 PM PST this evening NVZ003.

CA...Wind Advisory until 7 PM PST this evening CAZ072-073.