textproduct: Reno

KEY MESSAGES

* Mild spring weather continues, then a spring storm brings a notably cooler, wetter, and windier pattern this weekend.

* Isolated thunderstorms are possible for the E.Sierra and far W. Nevada this afternoon, with more scattered thunderstorms and widespread precipitation expected on Friday.

* Winter Storm Watches remain in effect for the Sierra and northeast California for significant snowfall and strong winds with impacts starting as early as Friday evening for the Eastern Sierra.

DISCUSSION

Visible satellite imagery shows cumulus and towering cumulus fields developing across the much of the region this afternoon, except for Pershing and Churchill counties. This activity will lead to showers and isolated to scattered thunderstorms this afternoon, especially over the Sierra, NE CA and far western NV where a 10-35% chance of thunder exists. Storm movement is expected to be towards the north and northeast where the least instability is present. So, the farther into Nevada storms move, the weaker they will become. Activity is expected to decrease after 5-7 PM, once we start loosing daytime heating. The main hazards will be sub-severe with frequent lightning, gusty outflow winds to 40-50 mph, small hail and brief periods of moderate to heavy rain.

Similar activity is expected to continue tomorrow as the upper level low approaches the region. This will expand the area for thunderstorms farther east, now affecting Churchill and Pershing counties in the NV Basin and Range, and keeping similar storm chances of 15-35%. Instability is expected to have better coverage with CAPE values again reaching 200-700 J/kg in the afternoon. Similar hazards compared to today are expected with sub-severe storms. The main change is that snow levels will start to decrease in the evening and overnight to 6500 to 7500 ft. Therefore, expect snow chances to increase starting tomorrow evening, especially for the Sierra crest. The crest around the Tahoe Basin should see around 1-5 inches, and Mono 2-6 inches between Friday afternoon and Saturday morning with isolated higher amounts likely areawide.

The core of this storm system will push through the region on Saturday which is one of the reasons for the Winter Weather Watches. However, it will be followed by a secondary colder system on Sunday (this is the other reason). Storm chances will drop on Saturday afternoon as a colder airmass moves in leading to mostly 10-15% chances for isolated thunderstorms. However, the main hazards will be moderate to heavy snow across the Sierra and NE CA above 6000 ft. There is a good chance for a boom-bust scenario situation based on how unreliable the models have been with this system. Thus, we decided to keep the Watch longer to see more consistency before we choose either warnings or advisories in the eastern slopes of the Sierra. Regardless, people should take precautionary actions now, or have backup plans if you are planning to travel through the Sierra this weekend as impacts are very likely, especially over the western slopes of the Sierra (See Sacramento/Hanford discussions). The issue is how bad those impacts will be on our side, could be just a nuisance or significant. Also this system is expected to take its time to move out of the area with accumulating snow through Monday morning.

Furthermore, snow levels are likely to come down to 4500 ft by Sunday morning which could result in light amounts of snow for valley floors over western NV, especially for the Sierra Front. Although foothill areas above 5000 ft could get a few inches, generally less than 4 inches on Sunday morning.

Winds are going to increase too this weekend with south to southwest winds gusting to 60-80 mph at the crest on Friday, then increasing to 80-100 mph by Saturday, and becoming more from the southwest by Sunday with similar speeds. Winds start to improve by Sunday evening. In the valleys, gusts of 30-40 mph are going to be likely with wind-prone areas potentially reaching 40-50 mph. These numbers will change so keep an eye out for our next forecasts.

Monday through around mid-week, we get a break in between systems before models indicate the potential of another system pushing through the Great Basin. Although there is plenty of uncertainty with that next storm system.

-HC

AVIATION

VFR conditions currently look to prevail through today. However, chances for showers (~20-50%) continue through the night. VCSH is likely at all terminals, but will be monitoring conditions in case a stray SHRA or -TSRA reduces flight restrictions and TAF amendments are needed. Isolated thunderstorm chances look to be ~10-30% through the evening with higher-end chances in NE CA and the Sierra.

The precipitation chances will then increase (~55-95%) and become more widespread across the region going into the weekend. Snow impacts for Sierra terminals are looking more likely by Saturday with generally around 5-10 inches of accumulation possible for KTRK, KTVL, KMMH by Monday morning.

-078/HC

AVALANCHE

Increasing chances for periods of moderate to heavy snowfall in the Sierra are expected this weekend:

* Friday: 70-90% chance of showers with 10-40% chance of thunderstorms in the afternoon along the eastern Sierra. Any snow impacts will be limited to higher elevations above 7.0-7.5 kft through Friday evening with 1-5 inches of snowfall are possible along the Sierra crest with isolated higher amounts. Snow levels will fall below 7 kft by Friday night, while snow ratios are forecast between 8-11:1 in Mono County and vicinity, while Alpine and northward ratios start 4-8:1 in the afternoon then 6-9:1 in the evening. Gusty south to southwest winds of 60-80 mph at ridgetops expected on Friday.

* Saturday-Monday Morning: Snow levels have trended much lower recently with snow levels lowering below 6500 feet by Saturday morning. Snow ratio around 8-12:1 are expected on Saturday before increasing near 12-15:1 by Sunday. While timing remains low confidence at this time, the heaviest snow rates are targeting Saturday evening & overnight. SWE of 2.0-2.50" is possible along the Sierra crest by Monday morning along the Tahoe Basin with 1.5-2.0" in Mono County. Snow totals nearing 3 feet along the Sierra crest in the Tahoe Basin are possible with around 2 feet for the crest in Mono County. Strong and gusty south-southwest winds are expected on Saturday with ridgetop gusts of 80-100 mph expected.

-078/HC

REV Watches/Warnings/Advisories

NV...Winter Storm Watch from Saturday morning through Sunday evening NVZ002.

CA...Winter Storm Watch from Saturday morning through Sunday evening CAZ071-072.

Winter Storm Watch from Friday evening through Sunday evening CAZ073.