textproduct: Reno

KEY MESSAGES

* Thunderstorms with heavy rain, hail, and strong outflow winds will expand northward today and Thursday. This will increase the threat of flash flooding and new lightning fires. * Cooler than average temperatures arrive Thursday and continue through the weekend.

* Thunderstorm chances decrease Friday with mainly dry conditions expected through the weekend.

DISCUSSION

The combined effects of a low pressure trough off the CA coast and a high pressure ridge near the Four Corners region will increase moisture into the Sierra and western NV the next couple of days. The added forcing provided by this low will also bring increased coverage of showers and thunderstorms, expanding northward to the Reno area and closer to the Bug and Stallion fires this afternoon. Across the rest of western NV, the northward extent of the storm threat reaches the I-80 corridor through this evening. Slower moving storms with heavy rain capable of producing flash flooding and increased lightning will be the primary threats, but stronger storms could produce outflow gusts to 40 mph.

For late tonight through Thursday morning, much of the latest guidance supports showers and embedded thunderstorms forming across parts of far western and west central NV. This activity then could extend into northwest NV, the Tahoe basin and far northeast CA around daybreak and continue through late Thursday morning, bringing an increased threat of lightning with limited rainfall, as these areas are expected to be on the fringes of the deeper moisture.

Then for Thursday afternoon-evening, the trough off the CA coast ejects northward with stronger forcing allowing for a quicker development of stronger storms near or just after midday. Mixed layer CAPE values increase to 500-1000 J/kg in areas where cloud cover from the morning showers/storms thins out. This greater instability would support a few severe storms producing wind gusts of 60+ mph and hail to near 1" diameter (or accumulations of smaller hail). Storms look to form by early afternoon near the US-395 corridor from northeast CA to Mono County, then spread to the north and east across much of western NV through the remainder of Thursday afternoon. Even though faster storm motions are likely compared to today, heavy rain capable of producing flash flooding remains a threat, especially in areas where cells redevelop or persist for a longer time span.

For both days, some areas of western NV may see thicker cloud cover form in the afternoon and limit the instability needed for thunderstorms, but with abundant moisture in place, wetting rainfall (at least 0.10") in the form of shower bands would occur.

By Friday, thunderstorm activity diminishes and shifts to west central NV as a drier southwest flow reaches the Sierra. Overall, this weekend into early next week is favoring dry conditions, with a less than 10% chance for showers near the Sierra and the Oregon border. Temperatures hold below mid-August averages from Friday through early next week, with highs mainly in the 80s for lower elevations and 70s for Sierra communities. MJD

AVIATION

Showers and thunderstorms may affect the western NV terminals (15-20% chance) southward to KMMH (25-30% chance) this afternoon mainly between 21-03Z. While the greater risk for persistent heavy rain stays east of these terminals, a well placed storm could bring rain sufficient for MVFR CIGS/VIS, outflow gusts of 30+ kt and lightning affecting these terminal sites. For KTVL/KTRK, the risk for storms affecting these sites is less than 10% through this evening.

Overnight into Thursday morning, all main terminals have a 15-20% chance for showers and isolated lightning from elevated convection (instability aloft rather than surface-based).

Thursday afternoon could see stronger storms with more hazardous aviation impacts near the western NV terminals and KMMH mainly between 19-23Z, before the bulk of storm activity pushes to the northeast.

Smoke from the Bug and Stallion fires north of RNO will lower visibilities near the fire sites with potential impacts for KRTS (Reno-Stead), but prevailing west winds should keep VFR conditions for KRNO. The only exception is a small 5-10% chance of a stronger t-storm outflow forming near these large fires and pushing a larger area of smoke southward toward the Reno vicinity. MJD

FIRE WEATHER

Moisture continues to advance northward across the eastern Sierra and western NV today with more storms producing wetting rainfall. However, the northern edge of these storms including near the I-80 corridor and the Bug and Stallion fire sites may be a mix of dry and wet storm types, with a risk of new fire starts from isolated lightning strikes outside the rain cores, along with outflow wind gusts to near 40 mph.

Areas from the Tahoe basin northward to Lassen and northern Washoe counties will be on the edge of the moisture increase, with an increased risk of dry lightning strikes as soon as early Thursday morning. By Thursday afternoon-early evening, a mix of wet and dry storms with faster storm motions is anticipated. With lower humidity hanging on longer and plenty of dried out vegetation from the recent heat event, a Fire Weather Watch remains in place through much of Thursday for these areas. MJD

REV Watches/Warnings/Advisories

NV...Fire Weather Watch from late Wednesday night through Thursday evening NVZ458.

Flood Watch from Wednesday afternoon through Thursday evening NVZ001-003.

CA...Fire Weather Watch from late Wednesday night through Thursday evening CAZ270>272-278.

Flood Watch from Wednesday afternoon through Thursday evening CAZ073.