textproduct: Reno

KEY MESSAGES

* Heavy rains that could lead to localized flooding, strong to damaging winds, and heavy wet snow in the Sierra above 8000 feet are all in the forecast for the region today going into tomorrow. * Following the beginning week system, a brief lull in precipitation is seen on during the day on Tuesday with gusty ridge winds.

* Another round of stormy conditions could bring significant rain and snow along with enhanced winds late Tuesday through Christmas Day, but confidence in the details remains mixed.

DISCUSSION

Today and Monday: Current radar returns shows rain continuing to fall this morning in northeastern CA and the Tahoe Region with some snow mixed in at the higher elevations in the Sierra. This ongoing system expects bringing a trio of weather concerns to the region for the beginning of the week, so I'll try to break them down as best as I can:

1) Heavy rain that could cause potential flooding: a Flood Watch continues for northeastern CA and the Tahoe region through Monday afternoon. In their latest Excessive Rainfall Outlook, the WPC has recently highlighted a slight risk (at least a 15% chance of excessive rainfall) within the CWA for areas generally along and west of a line from Bieber-Susanville-Truckee-South Lake Tahoe in CA. It's also worth noting that the WPC has an area of moderate risk (at least a 45% chance) near and west of Donner going over the summit with a marginal risk (at least 5% chance) going to the CA state border in northeastern CA and the Tahoe region. Within the slight risk areas, the latest NBM probabilities give a 60-80% chance for a 2-day QPF total of 2+ inches with the moderate risk areas seeing 70-80% chance of 3+ inches due to this system being an atmospheric river event pumping in a generous amount of moisture. For more information on potential impacts this rainfall could have: please see the Hydrology section below. Please use caution if you have to drive through these hazardous conditions today and tomorrow as amongst the potential flooding impacts, visibilities can be reduced due to increased rainfall intensity. Elsewhere, Sierra Front communities can expect light rain chances through tomorrow with showers reaching down into Mono county. The NV Basin and Range see lesser chances for rain (<15%).

2) Heavy, wet snow above 8000 feet in the Sierra: Snow levels are forecast to drop down to around 8-8.5 kft by the late afternoon/evening hours which will turn P-type to snow that continues into Monday. Please see weather.gov/winter for more details including forecast snowfall totals and the Winter Weather Advisory issued for the Greater Lake Tahoe area and Mono County as accumulating snow causing impacts is expected in these portions of the CWA above 8000 feet. SLRs do continue to look to be 5-8:1, so snow will be wetter and slushier. Some snow may also mix into the rain down to areas with elevations around 7000 feet as well, so please keep this in mind if traveling through area as roads will be slick. Increased winds may also cause some blowing snow which could lower visibilities as well.

3) Strong to damaging winds: Forecast guidance is still showing a 700 mb jet over the CWA with winds up to 75 knots which will allow ridgetop gusts of at least 100 mph and valley gusts upwards of 50-65 mph through tonight within the CWA. Please refer to the High Wind Warning, the Wind Advisories and the Lake Wind Advisory for Pyramid Lake for more details on the wind concerns in your area. The latest NBM probabilities for damaging wind gusts (60+ mph) today still look to be around 50% for Reno-Carson City-Minden area down towards Mono County with wind prone areas having around a 50-60% chance of wind gusts of 70+ mph. These strong to damaging winds will bring difficult travel for high profile vehicles, bumpy flights, and recreational concerns amongst other impacts. Please secure your outdoor holiday decorations now if you have not already!

Tuesday: There looks to be a brief break on Tuesday from impactful weather as models show the precipitation tapering off late Monday. While there could still be some lingering precipitation chances in the Sierra on Tuesday, showers look to be lighter in nature with not much QPF expected. If you are targeting a time for necessary holiday travel across the Sierra, Tuesday morning and afternoon currently look to be the best chance (though there could be some lingering impacts from the system at the beginning of the week). One thing to consider though is that ridgetop winds expect to still be a bit gusty with the NBM showing a 50-60% probability of 60+ mph gusts on Tuesday. But by Tuesday evening going into the night, the next impactful weather system begins to move into the region which will close the potential window for "better" travel conditions. Forecast guidance shows western NV staying mostly dry throughout Tuesday.

Wednesday through Friday: Confidence is improving as the Christmas holiday grows closer that an impactful winter weather system will be affecting the region particularly in the Sierra and northeastern CA. As such, a Winter Storm Watch has been issued for these areas starting late Tuesday night through Friday afternoon. The WPC is particularly highlighting the Sierra for potential heavy snow in their medium range hazard forecast. There seems to be good confidence that the snowfall amounts in the aforementioned areas over these three days may be measured in feet instead of inches as there currently is a 60-70% NBM probability for over 1 foot of snow in the Tahoe Basin at lake level by Friday afternoon. Snow levels look to be around 5.5-6.5 kft on Wednesday and reach down to around 5-6 kft by Friday morning. Aside from the wintry precipitation, the system could allow for the Sierra ridgetop winds to gust up to around 100 mph once again. Across the CA/NV border, the Reno-Sparks metro area currently has a 20-30% NBM probability of at least 1 inch of snow by Friday afternoon so the valleys may even have a chance for measurable snow. There is also some potential for effects of the upcoming system to continue into the weekend. Exact details on timing and precipitation amounts with this system are still uncertain, so please watch for forecast updates as travel across the Sierra could potentially be difficult to impossible. -078

AVIATION

An impactful atmospheric river system will bring in rainfall chances today across the region that could be moderate to heavy at times from Alpine County north through Lassen County including in the Greater Lake Tahoe area. As the intensity of this precipitation increases, KTRK/KTVL will see visibilities decrease to IFR/LIFR levels later today. NV TAF sites may occasionally drop to MVFR visibility when rain occurs. Higher elevations in the Sierra may see snow as the P-type today while KTVL could see a rain/snow mix tonight. Area-wide LLWS, surface winds up to around 40-50 kts, and ridgetop winds gusting up to 100 mph are also to be expected today on account of this system which will cause additional aviation impacts. Effects of this wintry system look to continue into tomorrow before tapering off overnight going into Tuesday. -078

HYDROLOGY

Moderate to heavy rains will lead to significant rises on rivers and streams today into Monday most notably from Alpine County north through Lassen County.

Minor flooding of small streams, urban and poor drainage areas and rockfall in steep terrain are possible during any prolonged periods of high intensity rainfall.

While no mainstem river flooding is currently forecast, the Susan River near Susanville and the West Fork of the Carson near Woodfords are currently expected to exceed action stage and minor flooding can not be ruled out in these area. Additionally, the latest forecast shows the Susan River approaching minor flood stage as early as Sunday afternoon. Remain aware of the potential for additional river and stream rises near Christmas Eve in large mountain drainages below about 6000 feet like the Susan River. In these areas additional rain and saturated conditions could lead to fast rises and renewed flooding concerns. In other areas, lowering snow levels will reduce flooding concerns into next week.

Enhanced runoff and sediment transport are likely below recent burn areas, but debris flows are not expected. Watch for additional rises near Christmas Eve and possible flooding concerns especially in drainages with large mountain drainages below about 6,000 feet, like the Susan River for example.

You can find river forecasts updated twice daily at: www.cnrfc.noaa.gov

-Tim/Giralte

REV Watches/Warnings/Advisories

NV...Flood Watch through Monday afternoon NVZ002.

Wind Advisory until 4 AM PST Monday NVZ001.

Winter Storm Watch from Tuesday evening through Friday morning NVZ002.

Wind Advisory until 4 AM PST Monday NVZ002.

Winter Weather Advisory from 4 PM this afternoon to 1 PM PST Monday NVZ002.

High Wind Warning until 4 AM PST Monday NVZ003.

Lake Wind Advisory until 4 AM PST Monday NVZ004.

CA...Flood Watch through Monday afternoon CAZ071-072.

Wind Advisory until 4 AM PST Monday CAZ071.

Winter Storm Watch from Tuesday evening through Friday morning CAZ071>073.

Wind Advisory until 4 AM PST Monday CAZ072.

Winter Weather Advisory from 4 PM this afternoon to 1 PM PST Monday CAZ072-073.

High Wind Warning until 4 AM PST Monday CAZ073.