textproduct: Reno

KEY MESSAGES

* Strong valley inversions and air stagnation remain into Wednesday.

* Quick moving storm brings rain showers with high elevation snow late Wednesday into New Year's Day, with a risk of freezing rain in colder valleys.

* Stronger storm brings heavy mountain snow, strong ridge winds, and rain showers to valleys Friday evening into Saturday and possibly Sunday. Some flooding concerns for the Susan and Feather Rivers

DISCUSSION

* Rather impressive inversions today will remain in place through tonight and Wednesday morning with continued light winds. Example: Noon temps holding near 10F in the Long Valley Caldera near Mammoth meanwhile it's near 40F at the summit of Mammoth Mountain.

* First of a pair of systems will lift north late Wednesday through New Year's Day. Moisture transport (IVT) is southerly which typically is not great for efficient spillover east of the Sierra crest. That being said, most models including RRFS bring light to moderate rain showers with high elevation (8-9000'+) snow to our area. With antecedent inversions, the setup is decent for pockets of freezing rain Wednesday evening into Thursday morning in the Eastern Sierra and Lassen County (e.g. 20-40% chances of FZRA precip type at Bridgeport and Mammoth Airport in NBM). Deterministic NAM3 and RRFS show freezing rain soundings in these areas too.

* Second wave is more impactful for travel, Friday evening into Saturday with more traditional frontal trajectory and lower rain- snow lines (7500' lowering to 5500'). RRFS showing a NCFR heading into CA by sundown Friday which would move into the Sierra Friday evening with rapid drops in rain-snow lines, strong winds, and heavy snow rates for travelers on a busy evening. Winter Storm Watches issued as a result. NBM guidance shows continued risk for heavy snow showers for the mountains Saturday into Sunday in the cold and unstable post-frontal airmass, but confidence is more mixed on Sunday where some deterministic models show a pronounced dry period.

* For the W Nevada & NE California valleys, periods of light to moderate rain are expected Friday night into Saturday. With some snow cover and wet soils, we may see runoff rather quickly so expect to see rises on creeks and rivers. Mainstem river flooding concerns are focused on the Susan River and Feather, where the RFC has upward trajectories into action stages late Friday into Saturday. Winds do not look overly strong in the valleys, with NBM showing only 20-30% odds of 40 MPH gusts, but the Sierra ridges will be rather brisk with S/SW 700mb flow 40-50 knots.

* Additional storms possible early next week but large spread and noisiness in the NBM guidance keeps confidence low in terms of potential amounts and impacts.

-Chris

AVIATION

* VFR conditions with valley haze due to inversions expected the rest of today into tonight. FZFG possible (20% chance) at TRK, Mono Lake, and other valleys but probabilities are lower than previous nights due to increasing cloud cover ahead of next storm system.

* Showers develop from south to north tomorrow evening into tomorrow night. Mainly MVFR conditions due to sub-optimal southerly flow pattern for efficient spillover east of the Sierra crest. Rain-snow lines high enough that most TAF sites would be rain showers, except MMH and SVE where snow (20% chance) or even light freezing rain (20%) is possible. Additional rain showers continue through New Year's Day but mainly of the MVFR variety.

* Winds are not a major issue through Wednesday. Thursday sees an increase in 700mb flow and ridge winds ahead of a deepening trough off-shore. Mountain waves and some wind shear is possible New Year's Day.

-Chris

AVALANCHE

* Strong inversions with mountain top temps into the 40s and low 50s today will persist into tonight. Tomorrow the inversion intensity should gradually wane as temps aloft cool ahead of incoming storm, though limited winds could counter that effect.

* Quick moving and warm system for Wednesday late afternoon through Thursday brings mainly rain to the mountains, with snow lines 8000- 9000' for the most part. NBM 50th percentile precip amounts 0.5 to 1.0" with SLR's below 6:1.

* Heavier precip and more snowfall expected Friday evening through Saturday with lower rain-snow lines (7000' lowering to 5500'). SWE pickups generally 1 to 1.75" at the 50th percentile of the NBM guidance with SLRs on the wet side, 8-11:1 on average. Heavy snow showers in a cold, unstable airmass will continue Saturday and possibly Sunday.

* Mountain winds with first storm are not too significant, but become stronger with the second one Friday into Saturday. 700mb flow projected S/SW at 40-60 knots which can translate to ridge gusts in the 70-90 MPH range.

-Chris

REV Watches/Warnings/Advisories

NV...Winter Storm Watch from Friday afternoon through Sunday morning NVZ002.

CA...Winter Storm Watch from Friday afternoon through Sunday morning CAZ072-073.