textproduct: NWS Flagstaff

SYNOPSIS

Dangerously hot temperatures remain at the lower elevations of the Grand Canyon and the Page area through Sunday. Scattered showers and thunderstorms are expected over the White Mountains today, with low chances for storms expanding westward to include the Mogollon Rim and Yavapai County on Saturday. A surge of monsoon moisture beginning Sunday is forecast to cause increasing thunderstorm coverage and the risk for flash flooding by early next week.

DISCUSSION

High pressure currently centered off the coast of southern California will migrate to the northeast over the next few days. This repositioning will establish a favorable pattern to transport deep monsoon moisture into northern Arizona. For today, the atmosphere remains too stable to support storm activity across most of the region, with the exception of the White Mountains where there is a 20-40% chance of afternoon showers and thunderstorms.

By Saturday, the northeastward migration of the high will bring a slight increase in monsoonal moisture. A 20-40% chance of storms continues for the White Mountains, while a low 10-20% chance expands westward to include the higher terrain along the Mogollon Rim and the mountains of Yavapai County. Lighting and outflow wind gusts to 40 mph are the primary hazards for Saturday's storms. On Sunday, confidence is high that the low-level wind flow will initiate a more robust moisture surge northward from the Gulf of California. This will result in increasing instability, with the best coverage and chances for storms (30-60%) forecast along and south of the Mogollon Rim. The strongest storms will be capable of producing outflow wind gusts in excess of 40 mph, though the overall threat for severe weather remains low.

An active monsoonal pattern will become established across the region next work week. Model guidance is in excellent agreement that high pressure will anchor near the Four Corners region, resulting in widespread scattered shower and thunderstorm activity through the week. Precipitable water values will remain near 1.0 inch or above area-wide, fueling slow-moving storms capable of producing heavy rainfall. Consequently, there is a moderate risk for flash flooding over sensitive areas and recent burn scars.

Daytime temperatures will continue to run above normal through the weekend. Dangerously hot temperatures are forecast for elevations below 4000 feet in the Grand Canyon today through Sunday, where high temperatures at Phantom Ranch are forecast to range from 111-116 degrees. Dangerous heat will expand to Glen and Marble Canyons, including Page, on Saturday and Sunday. By next week, the combination of extensive cloud cover and persistent thunderstorm activity will nudge daytime temperatures downward, effectively reducing the probability of extreme heat across the lower elevations.

AVIATION...Friday 10/12Z through Saturday 11/12Z

Mainly VFR conditions through the period, although MVFR visibilities are possible due to isolated/scattered SHRA/TSRA in the vicinity of KSOW/KJTC between 18Z to 03Z. Surface winds will remain light through the morning hours. Southwest winds 5-15 kts with gusts 15-25 kts develop between 15Z-03Z, becoming light afterward.

OUTLOOK...Saturday 11/12Z through Monday 13/12Z...Periods of MVFR visibilities possible due to isolated/scattered SHRA/TSRA near and south of a KSEZ-KSOW-KJTC line on Saturday, expanding to near and south of a KGCN-KFLG-KSJN line on Sunday. Gusty and erratic winds are likely in the vicinity of TSRA. Winds W/SW 10-20 kts on Saturday, becoming S/SW on Sunday.

FIRE WEATHER...Today and Saturday

Hot and mainly dry conditions prevail. Storm development remains limited to the White Mountains today (20-40%), with low chances (10-20%) expanding to include the Mogollon Rim and portions of Yavapai County on Saturday. West to southwest ridge winds today with afternoon gusts 10-20 mph. On Saturday, winds are more light and terrain-driven, but outflow wind gusts to 40 mph are possible in the vicinity of storms. Minimum afternoon RH 5-20%.

Sunday through Tuesday...A gulf moisture surge focuses storms along/south of the Mogollon Rim Sunday, with a threat of powerful outflow wind gusts in excess of 40 mph. Wetting rain chances increase Monday and Tuesday as storms turn wet and slow-moving, transitioning hazards to a flash flood threat. Surface winds remain light and terrain-driven. Minimum afternoon RH 10-25% Sunday, increasing to 15-30% Monday and Tuesday.

FGZ WATCHES/WARNINGS/ADVISORIES

Extreme Heat Warning from 11 AM MST /noon MDT/ Saturday to 8 PM MST /9 PM MDT/ Sunday for AZZ005.

Extreme Heat Warning until 8 PM MST Sunday for AZZ006.