textproduct: Reno

KEY MESSAGES

* Mostly dry weather continues into the evening with gusty winds lingering overnight. * Periods of heavy Sierra snow, lower elevation rain and snow, and strong, gusty winds expected tonight through Friday.

* Drier weather returns this weekend and continues into next week, accompanied by chilly temperatures.

DISCUSSION

A mostly dry, albeit windy, day is providing a brief window for pre- Christmas travel in and across the Sierra before our next winter storm arrives tonight. Winds will be the main near term concern as a belt of 40-50 kt 700 mb flow and ample mixing support S/SW gusts of 35-45 mph across lower valleys and 80-100 mph along Sierra ridges. A more concentrated wind concern likely materializes late this afternoon through Wednesday morning across S.Lyon, Mineral, Churchill, and Pershing Co. where southerly gusts increase to 45-55 mph in response to a deepening surface low to our west. Wind prone areas such as US-95 near Walker Lake may even see gusts up to 60 mph. These winds may serve as crosswind driving hazards for those traveling along E-W oriented portions of US-95 and US-50.

The first in a pair of winter storms will begin to impact the region late tonight through most of Christmas Eve. A band of rain and snow overspreads northeast California and the Sierra after 10 PM PST tonight with rain spilling into western Nevada early Wednesday morning. Snow levels begin around 7500-8000' at the onset of precipitation before falling between 6000' and 7000' by midday Wednesday. Additional rainfall around the Tahoe Basin northward will renew flooding concerns until rain switches to snow, especially for urban and poor drainage areas. More details on flooding impacts can be found in the Hydrology section below. Also, don't be surprised to hear a few rumbles of thunder near the Sierra crest on Wednesday!

While we won't get an extended break in between storms, there does appear to be a brief lull in shower activity Wednesday afternoon and evening before the second winter storm arrives early Christmas Day. Periods of heavy snow are slated to begin Christmas morning across the Sierra and continue into Friday, producing significant travel disruptions across mountain passes and at lake level. Lower elevation roadways in Mono County (e.g., US-395) will also see difficult travel conditions due to moderate to heavy snowfall. If heavy snow wasn't enough, strong winds and subsequent blowing snow may drastically reduce visibilities on Christmas. Needless to say, avoid traveling in and across the Sierra after today.

For W NV foothills and valleys, expect periods of moderate to heavy rain until Christmas night when snow levels fall to around 5500'. As such, concerns for snow impacts spread into foothill communities, such as Virginia City, Christmas evening into Friday morning. Snow levels fall near/below valley floors Friday morning, giving the Greater Reno-Carson-Minden area a 20-40% chance of an inch of snow on Friday. Uncertainty is high on how/when snow might reach W Nevada valley areas, but it's certainly something to keep in mind as snow impacts to travel are on the table. Winds will remain gusty each day through Friday, but Thursday is favored to be the windiest day as many areas have a 60+% chance of seeing gusts of 45 mph or greater. So while daily wind impacts to travel and recreation are likely through Friday, Thursday may offer more significant wind impacts.

A drying trend begins after Friday as the Christmas storm finally departs the region, leaving behind very chilly temperatures regionwide over the weekend. Additional winter storms are possible to ring in the new year, but details are to be determined.

-Salas

AVIATION

* VFR conditions this afternoon will be accompanied by gusty surface winds at all terminals. LLWS remains a concern through the forecast period. Wind gusts of 45-50kts are expected to increase to near 55-60 kts this evening ahead of the incoming weather system.

* As early as 06-09z Wednesday we expect showers to arrive to Sierra terminals. By 12z, some showers will spillover into western Nevada valley terminals. Mountain obscurations, moderate turbulence and LLWS with gusty surface winds will all be present on Wednesday. Initial precipitation for Sierra terminals may start as rain or rain/snow mix Wednesday before changing over to all snow for Thursday. Snow will be possible for KRNO/KCXP/KMEV on Thursday, but amounts are expected to be light, with a 5-10% chance for an inch at KRNO and a 10-20% chance for an inch of snow at KCXP and KMEV. HRICH

HYDROLOGY

Flooding concerns will be renewed late Tuesday through Wednesday when the next storm brings additional rain before eventually transitioning to snow. Areas of concern will be urban and poor drainage areas, and creeks and streams from Lake Tahoe through Lassen County. The greatest flooding concern is for rivers and streams draining large watersheds below about 6,500'. This includes many streams in Sierra, Plumas and Lassen counties and the Susan River.

Minor flooding is possible, but not currently forecast, for the Susan River on Christmas Eve. The runoff in the Susan and other similar drainages will be very sensitive to the duration of rain vs. snow, and any delay in the transition to snowfall could easily push the Susan River higher. For example, a simulation with just a 500' increase in the rain/snow line leads to a second second peak on the Susan River approaching moderate flood stage on Christmas Day. No other main-stem rivers are forecast to flood, but slow rises and high flows are expected on the Pit River below Canby, and Middle Fork of the Feather near Portola.

Enhanced runoff and sediment transport including rock fall are possible within and below recent burn areas. Post-Fire debris flows are not expected.

The River Forecast Center will be extending hours this week and issuing additional forecasts for rivers where flooding is expected. Check for updated river forecasts at: www.cnrfc.noaa.gov

-Bardsley

REV Watches/Warnings/Advisories

NV...Winter Storm Warning from 10 PM this evening to 10 AM PST Friday NVZ002.

Lake Wind Advisory until 10 PM PST this evening NVZ002.

Wind Advisory until 7 AM PST Wednesday NVZ001-004.

CA...Flood Watch from 10 PM PST this evening through Wednesday evening CAZ071.

Winter Storm Warning from 10 PM this evening to 10 AM PST Friday CAZ071>073.

Lake Wind Advisory until 10 PM PST this evening CAZ072.