textproduct: NWS Flagstaff

SYNOPSIS

Low-grade monsoonal conditions, due to a strong high pressure system over Arizona, will continue hot daytime temperatures through at least Friday. Isolated to scattered showers and thunderstorms will develop along the higher terrain each afternoon and evening. Moisture will start to return this weekend, bringing back moderate monsoon activity and increasing the coverage of showers and thunderstorms.

DISCUSSION

A strong high pressure system will remain centered over or near Arizona through Friday and continue daytime temperatures in the mid 80 to lower 90s in the mountains to 105 to 110 at elevations below 4000 feet in the forecast area. Due to the hot temperatures at lower elevations an Extreme Heat Warning is in effect for the lower portions of the Grand Canyon as well as Yavapai county through Friday. Enough moisture will remain for the daily development of isolated to scattered showers and thunderstorms during the afternoon and evening hours, mainly at higher elevations. The risk for flooding or hail is expected to remain limited, but gusts up to 40 mph will be possible in or near thunderstorms.

Starting Saturday, the high pressure center will shift eastward in response to a longwave trough and embedded disturbances moving over the western states. The trough will remain just far enough north and west to allow monsoon moisture to stream northward across Arizona this weekend. 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), leading to an increase in shower and thunderstorm coverage, potentially becoming scattered to numerous. Uncertainty is still relatively high, but the pattern could contain a decent amount of vertical wind shear which could lead to more organized thunderstorms with an elevated potential to become severe. In addition, about 10-20% percent of model ensemble members are pointing toward a deeper moisture tap. If the deeper moisture tap develops cooler trough related temperatures aloft, organized vertical wind shear and passing enhancements to upward motion by the weak disturbances could lead to a quite active weekend, with potential risks including flash flooding, hail, and potentially severe wind gusts. Around Tuesday or Wednesday, the longwave trough is expected to push further eastward and closer to Arizona, with long range ensembles showing a potential drying trend as drier air from the west is pushed towards Arizona.

AVIATION...Thursday 27/18Z through Friday 28/18Z

VFR conditions, brief MVFR possible in storms. Isolated to scattered showers/storms through around 03Z, highest coverage along/north of a KPRC-KSJN line. SW-W 5-15 kts daytime winds, light/variable or terrain driver overnight. Gusty and erratic winds possible near storms, potentially reaching up to 40 kts.

OUTLOOK...Friday 28/18Z through Sunday 30/18Z...Mainly VFR conditions, brief MVFR possible near storms. Scattered afternoon showers/storms over the higher terrain Friday with some evening and nocturnal storms also possible. Storms become more numerous and widespread on Saturday with most of the area seeing a chance of storms throughout both the day and nighttime hours. Daytime winds SW-W 10-15 kts, nighttime winds light and variable or terrain driven. Gusty and erratic winds possible near storms.

FIRE WEATHER...Today through Friday

Hotter and drier conditions, with max daytime temperatures will be 5-10 degrees above normal. Isolated showers and thunderstorms along the higher terrain Thursday afternoon, and isolated to scattered Friday afternoon. Outside of gusty and erratic storm driven winds, expect southwest to west winds around 5-15 mph during the day, becoming variable at night. Minimum RH will range from 10-15% for the lower deserts to 30-35% in the White Mountains and San Francisco Peaks.

Saturday through Monday...Scattered showers and thunderstorms each day, especially across the higher terrain during the afternoon and early evening hours. Shower and thunderstorm coverage may become numerous Saturday and Saturday due increase moisture and passing disturbances. The increase in showers and thunderstorms will cause high temperatures to cool closer to normal. Outside of storms, expect southwest to west winds around 5-15 mph each afternoon, becoming variable at night.

FGZ WATCHES/WARNINGS/ADVISORIES

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