textproduct: NWS Flagstaff

SYNOPSIS

Dry and seasonable weather is expected today before moisture returns Monday, bringing increasing chances for showers and thunderstorms. More widespread precipitation is expected Tuesday, along with breezy southerly winds. Uncertainty increases Wednesday onward, but daily shower and thunderstorm chances remain in the forecast through the work week.

DISCUSSION

Northern Arizona is currently under the influence of a short-wave ridge induced by a West Coast low centered off the coast of north/central California. Water vapor satellite imagery shows that very dry low and mid-level air has overtaken the region. Preciptable Water (PW), a measure of atmospheric moisture, has dropped to between 0.25-0.50 inches. For comparison, PW values yesterday morning ranged from 0.35-0.75 inches. Model soundings indicate the atmosphere is too dry and stable for shower or thunderstorm development today, and the forecast calls for dry weather across the area. Otherwise, expect increasing high cloud cover associated with the West Coast low, light surface winds and near-normal high temperatures.

On Monday, model forecasts are in good agreement that the West Coast low will begin to influence northern Arizona's weather as it moves inland across central California. High resolution model guidance has become increasingly confident that the area will see a return of shower and thunderstorm development beginning as early as Monday afternoon, especially in the northwest around Grand Canyon Country. The current forecast calls for a 30-50% chance for showers and thunderstorms in the northwest, with a 10-25% chance along the Mogollon Rim and White Mountains.

On Tuesday, the low lifts to the northwest. As it does so, increasing atmospheric moisture and forcing associated with the low will bring more widespread shower and thunderstorm development, with the best potential for precipitation occurring from the Mogollon Rim northward. In addition, southerly wind speeds are forecast to increase, but are expected to remain below Wind Advisory thresholds, with gusts near 30 mph forecast over higher terrain. According to the high-resolution model guidance, chances for rain rates exceeding 0.5 inches in 3 hours are minimal, indicating a low risk of flash flooding on Monday and Tuesday.

Model spread increases significantly beginning Wednesday, indicating decreasing confidence in the evolution of the longer- term weather pattern late next week. This is due to uncertainty regarding interactions between a deep Gulf of Alaska low, the potential development of a closed low somewhere off the coast of northern Baja, and activity in the tropical eastern Pacific. It is difficult to say exactly how these features will interact, so the current forecast maintains chances for showers and thunderstorms each day through the remainder of the work week.

AVIATION...Sunday 20/18Z through Monday 21/18Z

VFR conditions. Winds are SW-W 5-15 kts through 02Z, then light and VRB or terrain driven overnight.

OUTLOOK...Monday 21/18Z through Wednesday 23/18Z...Primarily VFR conditions. ISOLD-SCT SHRA/-TSRA returns Monday evening with the best coverage across northern Coconino county. SCT-NMRS SHRA/-TSRA across Navajo and Apache counties on Tuesday, ISOLD elsewhere. Winds are S-SW 10-15 kts gusting to 20 kts Monday, increasing to 10-20 kts gusting to 25 kts on Tuesday.

FIRE WEATHER...Monday and Tuesday

Showers and thunderstorms returning late Monday. Best chances are across northwest AZ on Monday evening/night, then across eastern AZ on Tuesday. Winds are south through southwest 10-15 mph on Monday, increasing to 10-20 mph gusting to 25 mph on Tuesday.

Wednesday through Friday...Chances for showers and thunderstorms continue each day, declining slightly on Friday. The best chances will be seen across eastern Arizona each day. Winds are generally light each day, trending southeast through east on Wednesday and Thursday then southwest on Friday.

FGZ WATCHES/WARNINGS/ADVISORIES

None.