Area Forecast Discussion

UPDATE

Flood Watch Expansion: The Flood Watch was expanded east-northeast into the western Nevada Basin and Range. Thunderstorms are showing a tendency to develop repeatedly over the same areas, with weak steering flow supporting slow-moving or nearly stationary cells. The anomalously moist air mass will allow storms to produce efficient rainfall rates, and additional development is expected through this evening. Rapid runoff could still affect normally dry creek beds, low-water crossings, rural roads, and areas near steep terrain.

-Johnston

PREVIOUS DISCUSSION

/issued 1240 PM PDT Tue Jul 21 2026/

KEY MESSAGES...

* Thunderstorm chances peak today as storms develop in Mono County and spread north, with flash flooding the primary concern along with lightning, gusty outflow winds, and small hail.

* Storm coverage decreases Wednesday, mainly favoring Mono and Mineral counties before shifting northeast, while winds increase Thursday and Friday across northeast California and western Nevada.

* Monsoonal moisture shifts east late this week, bringing drier weather Friday through the weekend with typical late July warmth.

DISCUSSION...

Thunderstorm and Flash Flood Potential Today:

* Convection has initiated along the Eastern Sierra, where dual-pol radar precipitation estimates indicate 1.00-1.50 inch per hour rainfall rates in the past couple hours near Twin Lakes. These initial storms are already demonstrating efficient rainfall production, with locally higher rates possible as instability increases this afternoon. Convection will expand northward through the afternoon and evening, with the greatest flash-flood threat where cells anchor to terrain, backbuild, or repeatedly cross the same drainage.

* An anomalously moist monsoonal air mass remains over the region. The 12Z REV sounding measured 1.21 inches of precipitable water, with forecast values of 1.0-1.35 inches representing roughly 175- 300% of normal. Forecast soundings contain 800-1100 J/kg of MUCAPE, 300-600 J/kg of MLCAPE, midlevel RH of 75-90%, and freezing levels near 16000 ft MSL. Lower cloud bases and warm- cloud depths around 2-2.5 km will support relatively efficient rainfall processes. Higher cloud bases and drier subcloud layers across the hotter Basin and Range valleys may favor a greater outflow-wind component.

* A weak midlevel shortwave crossing Nevada will augment daytime heating, terrain convergence, and subsequent outflow-boundary interactions. Corfidi upshear vectors of only 5-10 kt favor slow north to northeast storm movement, with the slowest motions over Mono County. Terrain anchoring and redevelopment could therefore extend rainfall duration even where individual cells move more quickly.

* HREF guidance for the heaviest cores shows a 40% probability of exceeding 0.50 inch in one hour, a 35% probability of exceeding 0.75 inch in one hour, a 25% probability of exceeding 1.00 inch in one hour, and a 20% probability of exceeding 1.50 inches in one hour. These are probabilities for the strongest cores rather than the entire watch area, but support localized rainfall rates of 0.75-1.50 inches per hour. Training thunderstorms could produce localized storm totals exceeding 2 inches.

* The HREF median remains considerably lighter and more spatially limited than its maximum, reinforcing that flooding will be localized rather than areawide and dependent on where the strongest cores and any training develop. Stronger storms may also produce outflow gusts of 40-50 mph, with an isolated gust near 60 mph possible across the Basin and Range.

Thunderstorm Coverage Decreasing Through Late Week:

* Monsoonal moisture shifts east Wednesday, with PWAT falling to 0.50-0.80 inch. The best shower and thunderstorm chances remain across Mono and adjacent Mineral counties (20-40%), with more isolated chances (15-25%) near Reno-Sparks, eastern Pershing and Churchill counties, and the Oregon border. Faster storm motions and lower moisture will reduce the flooding threat, although localized heavy rain remains possible.

* Thursday storm chances contract into Mono, Mineral, and the eastern Basin and Range (10-20%), with less than a 10% chance farther west. Afternoon gusts increase to 25-30 mph.

Drier Weather Friday through the Weekend with Typical late-July Warmth:

* Most areas will remain dry Friday through the weekend as monsoonal moisture shifts east. A stray storm remains possible across far eastern Mono or Mineral counties (10-15%), but NBM thunder guidance appears too aggressive compared with the PoP and QPF signals.

* Western Nevada valleys will reach the 90s, with the hottest lower elevations near 100 degrees, while Sierra communities remain in the 80s. Afternoon wind gusts could exceed 30 mph, especially Friday, which may bring elevated fire weather concerns. For more information, please see our Fire Weather Planning Forecast at weather.gov/reno.

-Johnston

AVIATION...

* VFR conditions will prevail, but showers and thunderstorms will spread north from Mono County through 03Z Wed. Direct impacts are most likely at KMMH from 20-00Z, with a 30% chance at KCXP/KMEV from 21-02Z. KTRK/KTVL should primarily see showers. Stronger cells may produce brief MVFR/IFR conditions, mountain obscuration, and erratic wind gusts up to 40 kt. Lower storm chances return Wednesday, followed by drier weather Thursday.

* RNO: VFR with northwest winds near 10 kt after 22Z. VCTS is forecast from 22-03Z, including a 30% chance of direct impacts and gusts near 25 kt. Showers may linger through 06Z.

-Johnston

HYDROLOGY...

* A Flood Watch remains in effect through 11 PM for the Tahoe Basin, Reno-Carson City-Minden corridor, Mono County, and Mineral and southern Lyon counties. WPC maintains a Slight Risk (at least 15%) for excessive rainfall across Mono, Mineral, and southern Lyon counties, with a Marginal Risk (at least 5%) elsewhere along and south of US-50.

* The primary flash flood corridor extends from Mono County through the Walker, Smith, and Mason valleys, north into Carson Valley, the eastern Tahoe slopes, Virginia Range, and Reno-Sparks. MRMS 24- hour QPE shows widespread wetting rainfall, with localized 0.50- 1.00 inch amounts. Storms near Twin Lakes have already produced 1.00-1.50 inches of rainfall in one hour according to radar estimates. Additional development or training could quickly exceed one-hour flash flood guidance of 0.75-0.85 inch. Rapid runoff and debris flows are possible near steep terrain, burn scars, dry channels, and other flood-prone areas. Creeks and streams may rise quickly, but no mainstem river flooding is expected.

* With the flash flooding threat, conditions can become extremely dangerous quickly. If you encounter a flooded roadway, remember: Turn around, don't drown!

-Johnston/Bardsley

REV Watches/Warnings/Advisories

NV...Flood Watch until 11 PM PDT this evening NVZ001>004.

CA...Flood Watch until 11 PM PDT this evening CAZ072-073.