textproduct: NWS Flagstaff

SYNOPSIS

Chances for afternoon showers and thunderstorms continue over portions of eastern and central Arizona today Expect less activity Thursday and Friday, then Monsoonal moisture begins to ramp up this weekend and into next week. Dangerous heat is also expected through the period for portions of Northern AZ, including Glen and Marble Canyons and the lower elevations of the Grand Canyon.

DISCUSSION

The ridge that has been driving the influx of moisture into northern AZ has flattened and is retrograding to the west. This will begin to introduce a drier westerly flow aloft, however plenty of moisture still lingers across the area. Today remains a fairly similar setup to yesterday, with scattered to isolated showers and thunderstorms currently firing over eastern AZ. Storms intensity is just a little higher than previous days, though the primary hazards from storms remain gusty outflow winds, lightning, and small hail. Localized downpours will also lead to a low risk for flash flooding over sensitive areas (burn scars).

By Thursday and Friday, the flattened ridge establishes to our west. This will push most of the convective activity to our south under drier northwest flow, though some storms will likely hang around the White Mountains each day. By the weekend, the ridge is forecast to strengthen and rapidly shift to our northeast. This would be a favorable flow pattern for the North American Monsoon. The ensemble mean PW approaches the 90th percentile for this time of year by Monday. The current forecast calls for precipitation chances ramping up on Sunday-Monday with the best coverage of storms along/south of the Mogollon Rim. Stay tuned for further updates on this period!

Heat will also be a concern through the week with above normal July temperatures forecasted each day. The Extreme Heat Warning for the Grand Canyon below 4000 ft has been extended through Sunday, with temperatures in excess of 110 degrees expected each day at Phantom Ranch. An Extreme Heat Warning has also been issued for Marble and Glen Canyons on Saturday and Sunday with temperatures forecasted to reach moderate-extreme HeatRisk.

AVIATION...Wednesday 08/18Z through Thursday 09/18Z

Primarily VFR conditions. Brief MVFR possible in SCT SHRA/TSRA, primarily over southern Apache and Navajo counties during the afternoon. Some ISOLD cells may push as far west as a KFLG/KPAN line by late afternoon. Winds are SW 10-20 kts through 03Z, becoming light and VRB overnight.

OUTLOOK...Thursday 09/18Z through Saturday 11/18Z...Primarily VFR conditions. ISOLD storms possible INVOF the White Mtns each afternoon. Winds are SW-W 10-20 kts each day, light and VRB or terrain during the overnight periods.

FIRE WEATHER...Thursday and Friday

Hot and mainly dry conditions, aside from isolated showers and thunderstorms each afternoon in the White Mountains. Winds west/southwest 10-20 mph each day.

Saturday through Monday...Hot, with increasing chances for showers and thunderstorms each day. Wetting rains possible in any stronger storm. Winds light and variable Saturday, becoming south/southeast 5- 15 mph on Sunday and Monday.

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.