textproduct: NWS Flagstaff

SYNOPSIS

Low-grade monsoon conditions as well as hot daytime temperatures are expected to continue today before strong high pressure system near Arizona shifts eastward. Look for, isolated to scattered showers and thunderstorms to continue over the higher terrain each afternoon and evening. On Saturday deeper moisture will start to return, bringing back moderate monsoon activity with increasing showers and thunderstorms heading into early next week.

DISCUSSION

The strong high pressure system over the Arizona/New Mexico border will continue daytime temperatures in the mid 80 to lower 90s in the mountains to 105 to 110 at elevations below 4000 feet in our forecast area today before shifting eastward. Due to the hot temperatures an Extreme Heat Warning remains in effect for the lower portions of the Grand Canyon and Yavapai County through 8 PM MST/ 9PM MDT Today. Enough moisture will remain for the daily development during the afternoon, mainly along the higher elevation, with some showers and thunderstorms potentially lingering into the evening hours in northern Navajo and Apache counties and near the White mountains.

Starting Saturday and lasting into early next week...The high pressure center shifts eastward in response to a trough moving over the western states. The trough will remain just far enough north and west to allow monsoon moisture from over the Gulf of California to stream northward across Arizona. Look for the daily influence of daytime heating on thunderstorm development but with the addition of passing shortwave disturbances interacting with this moisture (classic transition season stuff). Shower and thunderstorms coverage will become scattered to numerous with some level of overnight activity continuing. We're confident that the pattern will contain decent vertical wind shear with the potential for organized and severe thunderstorm development. In addition, about 10-20% percent of model ensemble members are pointing toward a deeper tropical moisture tap, perhaps being pulled off a dying tropical system over the Pacific. If the deeper moisture tap develops then a serious flash flood threat could develop. However, the timing of various players in the flash flood scenario remains uncertain and thus our confidence remains low.

Later next week...Looks like monsoon moisture lingers and unsettled conditions remain with at least low-grade monsoon activity continuing.

AVIATION...Friday 28/12Z through Saturday 29/12Z

Mainly VFR conditions, brief MVFR possible in storms. Scattered showers/storms begin around 18Z in the higher terrain today. Showers and storms could last through 06Z far north-northwestern Arizona, east- southeast of KPGA and near the White Mountains near KJTC, then relatively quiet conditions are expected through 12Z Saturday. SW-W 5-15kts this afternoon. Light and variable or terrain driven overnight. Gusty and erratic winds possible near storms with the potential for gusts up to 40 kt.

OUTLOOK...Saturday 29/12Z through Monday 31/12Z...VFR conditions with MVFR to IFR possible in heavier storms. Scattered to numerous showers/storms Saturday afternoon across all of northern Arizona with some of these storms lasting overnight Saturday into Sunday morning. Another round of scattered to numerous showers/storms on Sunday with most of northern Arizona seeing coverage, with scattered showers/storms potentially lingering overnight Sunday. Winds SW-W 10-20kts daytime, light and variable or terrain driven overnight. Gusty and erratic winds near storms.

FIRE WEATHER...Today through Saturday

Isolated to scattered showers and thunderstorms are expected this afternoon and into the early evening hours, then humidity and storm coverage increasing for Saturday. Storms may persist through the overnight hours Saturday night. Outside of gusty and erratic storm driven winds, expect southwest to west winds around 10- 15 mph during the day Friday and reaching up to 20 mph during the daytime Saturday. Variable winds at night.

Sunday through Tuesday...Increasing moisture with scattered to numerous showers and thunderstorms, possibly extending through the evening and overnight hours. The increase in showers and thunderstorms will cause high temperatures to cool to slightly below normal. Outside of storms, expect southwest to west winds around 10- 15 mph each afternoon, becoming variable at night.

FGZ WATCHES/WARNINGS/ADVISORIES

Extreme Heat Warning until 8 PM MST this evening for AZZ006-037.