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.
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Revised in 4 passages since the 11:24 AM discussion. See every revision.
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.
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