textproduct: Reno

KEY MESSAGES

* Gusty winds are forecast today along with more shower chances which increase going into the night.

* While strong winds return tomorrow, heavier rains are likely early Sunday through Monday that could lead to localized flooding concerns.

* Another storm or two could bring significant rain and snow Tuesday into Christmas Day, but confidence in the details remains mixed.

DISCUSSION

Today: Forecast guidance shows a generally westerly flow aloft through most of the day with the occasional shortwave disturbance over CA. This pattern changes going into the night as a PacNW trough is seen developing that will give the upper air flow a southwesterly component. As a result, gusty winds can be expected today with the Sierra ridges seeing a 40-60% NBM probability of max wind gusts of 50+ mph out of the west-southwest. As for precipitation chances for today, the models give the Sierra a 40-60% chance for a light shower through the afternoon with Sierra Front areas having a 20-40% chance. P-type looks to be mostly rain as forecast snow levels will be around 8-8.5 kft around the Tahoe Region with the Eastern Sierra having coming in with 9-9.5 kft early today. But precipitation chances expect to increase this evening and going through the night with coverage spreading more into western NV as well. Rain intensity does look to pick up as well with rainfall rates looking to begin to surpass 0.1 inches per hour at the crest by around 6-7 PM and continue to increase going into the night. With overnight snow levels looking to be 9-10 kft, only the highest peaks may see a few inches of measurable snowfall. Roads may be slick and visibility could decrease as rainfall intensifies, so please use caution if traveling tonight.

Sunday and Monday: With the Pacific trough continuing to develop, this pattern aloft allows precipitation chances to continue. When looking at the CW3E's GEFS AR landfall tool, the pattern also favors atmospheric river moisture pumping into the CWA especially in northeastern CA and the Tahoe region. The latest blended guidance continues to show a 48-hr QPF total through Monday afternoon between 4.5-5.5 inches from Tahoe northward along the Sierra. Lower Sierra valleys and northeastern CA still have a 70-80% chance of at least 2.5 inches of rainfall during this time. As such, the Flood Watch continues for the Tahoe Basin northward up through Lassen County for Sunday through Monday. More details on potential impacts are available in the hydrology section. Elsewhere, there looks to be a 50-60% chance of at least 1.5 inches along the Sierra Front and the Eastern Sierra. Spillover may be limited during this time except for northern Washoe County where they could see as much as around 1-1.2 inches of QPF. As QPF amounts taper off going eastward, the rest of western NV looks to see generally under 1 inch of QPF possible during this period. Snow levels are forecast to be 8.5-9.5 kft at the Tahoe region going south through Mono County on Sunday and then drop to around 7.5-8.5 kft by Monday afternoon. Northeastern CA may see slightly lower snow levels of 7.5-8 kft on Sunday that drop to around 7-7.5 kft by Monday morning. Another concern appears to be some enhanced winds causing aviation and marine impacts on Sunday into early Monday with a cold front passage. While Lake Wind Advisories have been issued for Lake Tahoe and Pyramid Lake, we will be monitoring model guidance in case winds trend upward towards Wind Advisory criteria. Sierra ridge winds on Sunday could be similar to those experienced today if not a tad faster with the RRFS showing 700 mb winds around 70 knots.

Tuesday and beyond: Models are showing precipitation chances persisting as the strengthening Pacific trough allows for some cooler storms to move through the area going through the Christmas holiday. Snow levels look to drop far enough where there may be daily travel impacts over the Sierra passes. Not enough confidence just yet to issue winter weather products particularly in the Sierra, but please keep an eye on the forecast especially if you plan to travel for the holiday. The Reno-Tahoe International Airport currently has 25% NBM probability of seeing at least 0.1 inches of snow on Christmas Day which repeats on December 26th. Will continue to monitor this going forward, but please be prepared for travel delays and other impacts this week. -078

AVIATION

The forecast calls for LLWS at all area TAF sites today going into tomorrow. KRNO/KCXP/KMEV see VFR conditions throughout the day with mostly dry conditions ahead of rain chances beginning in the late afternoon/evening that continue into tomorrow. Sierra rain chances persist this morning before tapering off a bit in the afternoon. KTRK/KTVL have MVFR ceilings in the forecast this morning ahead of sub-VFR visibilities this evening when rain chances increase going into the night. VCSH for KMMH expects to continue into the afternoon with precipitation chances returning overnight after a brief break in the evening. Enhanced winds return to the forecast on Sunday ahead of an approaching cold front with precipitation chances continuing through at least Monday. -078

HYDROLOGY

Moderate to heavy rains will lead to significant rises on rivers and streams Sunday 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. 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

REV Watches/Warnings/Advisories

NV...Flood Watch from late tonight through Monday afternoon NVZ002.

Lake Wind Advisory from 10 PM this evening to 4 AM PST Monday NVZ002.

Lake Wind Advisory from 4 AM Sunday to 4 AM PST Monday NVZ004.

CA...Flood Watch from late tonight through Monday afternoon CAZ071- 072.

Lake Wind Advisory from 10 PM this evening to 4 AM PST Monday CAZ072.