Area Forecast Discussion

SYNOPSIS

Dry and mostly precipitation-free conditions will continue through Sunday. Typical fall weather will occur, with seasonable high temperatures and colder overnight lows. Shower and thunderstorm chances begin to increase Monday, with more widespread activity expected Tuesday as moisture returns to northern Arizona. Unsettled weather is then expected to continue through much of the remaining week.

DISCUSSION

A 500 mb longwave trough is slowly moving eastward across northern Arizona. This system is pulling dry air into the region. Satellite measurements of the atmospheric moisture (PW), show values between 0.35-0.45 inches over the High Country, with 0.45-0.75 inches within the Little Colorado River Valley and the lower deserts south of the Mogollon Rim. For comparison, PW was significantly higher Thursday afternoon, with 0.75 to 1.25 inches observed across the area.

This dry air, combined with a weak mid-level inversion around 560mb, will suppress thunderstorm activity this afternoon. This will leave almost all of northern Arizona precipitation free, except along the eastern Mogollon Rim and White Mountains (Heber- Overgaard eastward), where the forecast calls for a 10-20% chance of showers and thunderstorms there.

On Sunday, drier and more stable conditions will suppress shower and thunderstorm activity area-wide. A few pockets of freezing temperatures may develop in sheltered valleys above 6500 feet.

On Monday, an approaching West Coast low begins to influence northern Arizona weather. Dry and stable conditions continue, but model guidance suggests a return of isolated showers and perhaps a thunderstorm or two are possible northwest of Flagstaff. The current forecast calls for a 10-20% chance of showers there.

By Tuesday, the low moves far enough inland to transport moisture into the area for an uptick in shower and thunderstorm activity across much of northern Arizona. At this point, the main focus areas for storms are over eastern Arizona and Grand Canyon Country. In addition, southerly wind speeds are forecast to increase, likely remaining below Wind Advisory level thresholds, with gusts near 30 mph forecast over higher terrain.

On Wednesday, model guidance is in agreement that the low will exit the area, but the development of another West Coast trough combined with surges of tropical moisture keeps chances for showers and thunderstorms in the forecast for the remainder of the week.

AVIATION...Saturday 19/18Z through Sunday 20/18Z

Primarily VFR conditions. Very ISOLD -SHRA/-TSRA across southern Navajo and Apache counties through 02Z. Winds are SW 10-20 kts through 02Z, light and VRB overnight.

OUTLOOK...Sunday 20/18Z through Tuesday 22/18Z...Primarily VFR conditions. Winds are S-SW 10-15 kts each afternoon, light and VRB each night. ISOLD-SCT -SHRA/-TSRA may return Monday evening/night. .

FIRE WEATHER...Sunday and Monday

Staying mostly dry through the period. Dry air will lead to much colder lows each morning. Isolated showers and thunderstorms may return across northwest AZ on Monday night. Southwesterly winds 5 to 15 mph each day.

Tuesday through Thursday...Chances for showers and thunderstorms continue each day, with the best chances across eastern AZ. South through southwest winds 10 to 20 mph with gusts up to 30 mph on Tuesday, becoming light and variable on Wednesday and Thursday.

FGZ WATCHES / WARNINGS / ADVISORIES

None.