textproduct: NWS Flagstaff

SYNOPSIS

Monsoon moisture will remain over the area through the coming week with daily storm chances, along with the risk for heavy rainfall and localized flooding. With the added moisture and cloud cover, daytime temperatures will be below normal.

DISCUSSION

A ridge of high pressure centered to our north, combined with an inverted trough moving from Texas into New Mexico, continues to drive a steady northeasterly steering flow. This pattern has locked a deeply saturated airmass in place across northern Arizona. Precipitable water, a measure of total atmospheric moisture, values range from 1.00 to 1.25 inches along and north of the Mogollon Rim, and 1.25 to 1.75 inches south of the Rim. These moisture parameters are typically associated with active monsoonal conditions and represent a highly favorable environment for efficient, heavy rainfall production.

There is high confidence that thunderstorms developing in this airmass will produce periods of heavy rainfall capable of localized flash flooding. There is also high confidence that convective activity initiating over the Mogollon Rim will propagate southwestward through Yavapai County, and through western into central Coconino County during the afternoon and evening hours. As these storms track southwestward, strong outflow winds will be a secondary hazard; some storms will likely produce localized outflow wind gusts of 30 to 40 mph. Conversely, the risk for large hail is low.

These conditions bring a heightened risk of flash flooding to western and central Coconino County, Yavapai County, and Gila County. A Flash Flood Watch remains in effect for these areas from 11 AM to 11 PM MST today, specifically targeting flood-prone areas such as the Pocket and Dragon Bravo fire scars, Supai, and the slot canyons in northern Coconino County. High-resolution ensemble guidance continues to support this threat, showing neighborhood probabilities near 70% for rainfall rates exceeding 1 inch per hour in the watch area. That said, flash flooding remains possible outside the watch area. Elevated probabilities of precipitation persist overnight into Friday morning as guidance suggests another round of overnight showers and storms developing along outflow boundaries moving into the area from Utah and Colorado.

Friday afternoon, the flash flood risk remains elevated area- wide, but is especially focused over western and central Coconino County and parts of Yavapai County. Forecast ensemble probabilities for 1 inch per hour rainfall exceed 70% once again, with an embedded 10% to 30% probability of extreme rainfall rates reaching 3 inches per hour. Additional Flash Flood Watches will likely be required for this period as well.

Monsoon moisture is forecast to remain firmly in place through the weekend and into next week, supporting daily thunderstorm chances. The steering flow is expected to transition to a more east-southeasterly direction due to subtle positional changes of the northern ridge.

AVIATION...Thursday 16/18Z through Friday 17/18Z

Mostly VFR conditions forecast, but brief MVFR conditions possible in heavier SHRA/TSRA. Widespread activity through 02Z especially near KPRC, becoming isolated to scattered overnight. Gusty/erratic winds likely from storm outflows, with variable winds generally 10 kts or less otherwise.

OUTLOOK...Friday 17/18Z through Sunday 19/18Z...VFR conditions expected through much of the period, but brief MVFR conditions possible in heavier SHRA/TSRA. Greatest coverage for showers/storms will be between 19Z-03Z each day, with isolated to scattered activity forecast during overnight periods. Gusty/erratic outflow winds possible around storms, with variable winds 10 kts or less expected otherwise.

FIRE WEATHER

Likely chances for showers and thunderstorms, with the greatest over the higher terrain in the afternoon. Some activity will last into the overnight hours. Isolated flash flooding and gusty outflow winds are possible with any stronger storm, primarily a concern for recent burn scars. Outside of storm activity, winds south through west 5-15 mph.

Sunday through Tuesday...Daily chances for showers and thunderstorms continue, though decreasing slightly each day. The best chances are over the higher terrain where a widespread wetting rain is expected over this period. Downpours and gusty outflow winds possible with any stronger storm. Outside of storm activity, winds south through west 5-15 mph.

FGZ WATCHES/WARNINGS/ADVISORIES

Flood Watch until 11 PM MST /midnight MDT/ this evening for AZZ004>008-018-037-038.