textproduct: NWS Flagstaff

SYNOPSIS

Moderate monsoon activity will continue through Monday, mainly along the higher terrain during the afternoon and evening hours. Then, as high pressure builds over Arizona, hot weather and a decline in storm activity is expected to develop as the flow of moisture northward into the region diminishes.

DISCUSSION

Upper-air analysis shows the 500mb subtropical high pressure cell centered across central New Mexico, with a northeastward-moving shortwave trough draped across the Yavapai and Coconino County border. This system is actively supporting showers and thunderstorms across western Yavapai and Coconino Counties, while aiding convective initiation along portions of the Mogollon Rim.

Moderate monsoon conditions persist today with storms drifting northeastward on average. The forecast favors high-terrain locations, with a chance for showers and thunderstorms between 30-60% in the mountains. However, mid-level warming is limiting deep atmospheric instability, keeping the threat for severe weather low. Given the presence of slow-moving storms, there is a low to medium risk for flash flooding. This threat is tempered by atmospheric moisture (precipitable water) values ranging from 0.75-0.85 inches north of the Mogollon Rim and 0.85-1.15 inches south of the Rim. Outflow wind gusts between 35-45 mph are possible in the vicinity of storms.

Storm activity is forecast to decline after sunset. High- resolution model guidance indicates overnight showers and thunderstorms may move into the region from the south, impacting areas west of a Payson to Flagstaff to Page line. Similar convective coverage is forecast on Monday, though a surge of moisture northward from the Gulf of California will likely shift the main axis of high storm coverage into Coconino and Yavapai Counties, with more isolated activity expected elsewhere.

By mid-to-late week, guidance generally agrees that the subtropical high pressure circulation will strengthen over Arizona. This will introduce more stable atmospheric conditions and a noticeable downturn in storm coverage, leaving only low- grade monsoon activity through the remainder of the week. Temperatures are forecast to warm during this period, with daytime temperatures approaching 10 degrees above normal across much of the area. Excessive Heat products may become necessary for lower elevation areas, including portions of Yavapai County and Grand Canyon country.

AVIATION...Sunday 23/18Z through Monday 24/18Z.

Mainly VFR conditions. Isolated to scattered SHRA/TSRA starting to develop at 18Z and continuing through at least 03Z. There is the potential that SHRA/TSRA in south-central AZ could produce an outflow boundary that will trigger another round of SHRA/TSRA late tonight or early Monday morning for KPRC-KSEX-KFLG-KINW between 9Z and 18Z. Brief MVFR conditions and gusty/erratic wind possible in heavier SHRA/TSRA. Otherwise, look for south to southwest winds at 5-15 kts, with variable or terrain wind-driven winds overnight.

OUTLOOK...Monday 24/18Z through Wednesday 26/18Z...Expect mainly VFR conditions. Scattered afternoon/evening SHRA/TSRA Monday, with coverage decreasing to isolated by Tuesday. Outside of gusty/erratic outflow driven winds, look for daytime winds mainly southwest to west 5-15 kts. Light and variable winds at night.

FIRE WEATHER...Rest of today through Tuesday

Expect daily chances for showers/thunderstorms, with the greatest coverage across the higher terrain. Activity will ramp up around midday then continue through the evening hours. Drier conditions are possible starting Tuesday, with low-grade monsoonal conditions developing. Outside of gusty and erratic storm driven winds, look for southwest to west winds generally 10 mph or less during the day and variable at night.

Wednesday through Friday...Drier, low-grade monsoonal conditions will likely continue, with activity becoming more limited to the White Mountains. Daytime highs are forecast to become quite hot, reaching up to 10-15 degrees above normal. Look for southwest winds 5-15 mph each afternoon.

FGZ WATCHES/WARNINGS/ADVISORIES

None.