textproduct: Reno

KEY MESSAGES

* Increased southwest breezes will elevate fire weather and recreation concerns this afternoon, while smoke from ongoing fires persists across northeast CA and western NV.

* Isolated showers and thunderstorms return south of Highway 50 this afternoon, with greater coverage Tuesday and more widespread, wetter storms expected Wednesday and Thursday.

* Heat will ease slightly but linger through Tuesday across west central NV, followed by substantially cooler temperatures for the remainder of the week.

DISCUSSION

Webcams around midday show small, generally shallow smoke plumes from the Bug, Fred Mountain, and Ironwood fires, while the newly reported Stallion Fire about 3 miles northwest of Pah Rah Mountain continues to produce intermittent bursts of smoke. Regional smoke from Oregon also remains across northeast CA and northwest NV. Mixing heights are forecast to reach 12-15 kft AGL this afternoon with west-southwest transport winds, providing excellent ventilation and some improvement in valley smoke and haze. Local plumes will be carried mainly east-northeast, while smoke may settle back into valleys tonight as mixing collapses. The same west-southwest flow will produce gusts of 30-35 mph, which combined with critically low RH will create a 1-4 hour window of elevated to locally critical fire-weather conditions, especially across northeast CA, northwest NV, and near the western NV fires. Please see the Fire Weather section for more information. Isolated thunderstorms remain possible over Mono, Mineral, and southern Lyon counties (15-20%), with lightning and HREF-supported outflows of 40-50 mph capable of reaching well beyond rain cores. One last hot afternoon is expected, with some lower western NV valleys reaching 100-102 degrees.

The Four Corners ridge will continue to flatten as a compact low develops near the CA coast and drifts inland Tuesday through Thursday. Increasing southerly flow ahead of this feature will draw deeper moisture northward, with PWATs rising to 0.6-0.9 inch and locally approaching 1 inch Wednesday and Thursday. Thunderstorm chances increase to 25-50% south of I-80 Tuesday, although a deep and dry subcloud layer will limit wetting rainfall from many storms. Lightning and new fire starts will therefore remain the primary concerns, along with HREF-supported outflows of 45-55 mph over Mineral County and the adjacent Basin and Range. As the low approaches Wednesday and Thursday, storm chances increase to 45-65% south of I-80 and 25-45% north of I-80. Storms will become progressively wetter, shifting the concern toward locally heavy rainfall and flash flooding near burn scars, steep terrain, and poor- drainage areas. Proximity of the low may also support showers and isolated lightning outside the typical afternoon-evening period. Highs fall into the 80s across most lower elevations and the 70s for Sierra communities by Thursday, roughly 15-20 degrees cooler than today.

From Friday through next Monday, synoptic ascent and mid-level cooling associated with the initial disturbance shifts east and storm coverage should trend down from its midweek peak. A complete dry-out is not evident, however, as ensemble guidance maintains a broad western trough, below-normal heights, and residual moisture over the region. A second Pacific disturbance may approach the CA coast late in the weekend or early Monday, maintaining low-end daily thunderstorm chances and potentially producing another increase in coverage depending on its eventual track. The greatest chances should favor areas east of the Sierra and across the Basin and Range. Gradient winds remain weaker than today, so fire-weather concerns will become more localized and tied primarily to lightning and convective outflows. Temperatures rebound modestly after Thursday but remain near to slightly below mid-August averages, with lower western NV valleys generally in the mid 80s to near 90 and Sierra communities in the 70s. Confidence is moderate in the overall cooler and unsettled pattern, but lower regarding the daily placement and timing of storms this weekend into Monday.

-Johnston

AVIATION

VFR conditions will prevail outside convection and localized smoke impacts through Tuesday. Southwest to west winds strengthen this afternoon, with gusts of 20-25 kt at most terminals before easing between 02-04Z and becoming light overnight. Isolated thunderstorms remain focused south of the main western NV terminals today, with a 15-20% chance at KMMH and VCTS favored from roughly 22-03Z. Any cell could produce abrupt outflows of 35-45 kt and wind- direction changes well beyond the precipitation core. Tuesday thunder probabilities rise to around 25% at KMMH and 15% at KRNO- KCXP-KMEV, while remaining below 10% at KTRK-KTVL. Tuesday storms may produce outflows of 40-50 kt. More frequent convection and greater terminal impacts are expected Wednesday and Thursday, with activity potentially extending beyond the usual afternoon and evening period.

Smoke from local fires is producing FU north through northeast of KRNO, while smoke from Oregon continues across northeast CA and northwest NV. P6SM surface visibility should prevail at the main terminals, although slantwise visibility and terrain obscuration may be reduced near and downwind of the smoke plumes. Deep afternoon mixing and west-southwest transport winds should improve surface conditions, followed by renewed smoke and haze overnight and Tuesday morning as winds and mixing diminish.

-Johnston

FIRE WEATHER

* Today and Tonight: Gusty west/southwest winds of 25-35 mph and low RH will create a few hours of critical fire weather conditions this afternoon across northeast CA and northwest NV. This will directly impact ongoing wildfires in Washoe and Douglas counties. Poor overnight recoveries (20-40%) tonight may keep fires active overnight tonight.

* Tuesday: An enhanced zephyr breeze returns in the afternoon though winds won't be as strong as Monday (today). There will be a 30-50% chance of showers and thunderstorms over Mono, Douglas, S Lyon, and Mineral counties, which will carry a threat of dry thunderstorms and outflow winds of 40+ mph.

* Wednesday Through Friday: A major pattern change arrives mid- week, bringing cooler temperatures and higher afternoon RH. Increased monsoonal moisture will move in to the region, supporting daily chances for showers and thunderstorms over parts of western NV and the eastern Sierra. With these storms comes the threats of heavy rainfall and potential flash flooding, isolated dry t-storms north of I-80 including the Reno-Sparks area, and gusty outflow winds.

-Justin

REV Watches/Warnings/Advisories

NV...None. CA...None.