Saint Johns, AZ Weather

Synopsis

A period of very active weather continues, with a Severe Thunderstorm Watch is in effect until 10 AM MST today for all of northern Arizona. The primary hazards include large hail, damaging winds, flooding, and even a few tornadoes. The severe weather threat shifts toward eastern Arizona later today, with drier conditions and only low chances for showers and thunderstorms over the higher terrain of eastern Arizona on Friday. Over the weekend, the forecast calls for below -normal temperatures and an uptick in shower and thunderstorm activity Saturday, perhaps persisting into Sunday.

Discussion

Current satellite and upper-air analysis show an unusually deep low-pressure system off the southern California coast. This system is pulling a rich subtropical moisture plume into the state, yielding record amounts of atmospheric moisture for this time of year. As a result, active weather conditions continue across northern Arizona tonight, with scattered thunderstorms capable of producing heavy rain, along with severe hail and powerful downdraft winds. In addition, radar indicates the presence of rotating supercells due to 0-6 km deep-layer bulk shear in excess of 50 kts.

The West Coast low will continue to pump warm, moist low level air into the state as it moves inland and lifts to the northeast today. Unstable conditions (CAPE of 500-1500 J/kg) will combine with 40-55 kts of deep-layer bulk shear to produce additional rounds of severe, supercell thunderstorms. These storms will be capable of producing hail between 1 and 2 inches in diameter, damaging winds in excess of 58 mph, and even a few tornados. The SPC has reissued a Severe Thunderstorm Watch for northern Arizona, which is in effect until 10 AM MST. In addition, heavy rain will accompany these fast moving storms. So, flash flooding and flooding are concerns in areas that receive multiple rounds of convective activity. Soils are saturated across northern Arizona, and flash flooding is already occurring across portions of the Navajo Nation within Moenkopi Wash and Chinle Creek. A Flood Watch continues through 11 PM MST tonight. The passing low is forecast to push a cold front through the region from later this afternoon through Friday morning, shifting the risk for severe weather and flash flooding eastward as it does so. Behind the front, drier and more stable conditions will inhibit storm formation.

On Friday, dry conditions return, with only low chances for showers and thunderstorms (10-25%) forecast over the higher terrain of eastern Arizona. That said, a closed low is forecast to develop over northern Arizona this weekend bringing well below- normal temperatures and an uptick in shower and thunderstorm activity Saturday, perhaps persisting into Sunday. These storms will develop in an environment characterized by low shear, low instability and much less atmospheric moisture, resulting in a low severe weather and flash flood threat. Quiet weather is forecast to start next workweek.

Aviation

Friday 23/00Z through Saturday 24/00Z...Overnight, SHRA/TSRA activity will continue through 07Z, gradually shifting eastward through the period. Large hail and isolated tornadoes will still be possible. After 07Z, look for partly to mostly cloudy skies. Expect frequent MVFR/IFR conditions through 12Z with much improved conditions after that point. Winds generally light from the southwest at 5-15 kts.

OUTLOOK...Saturday 24/00Z through Monday 26/00Z...Mainly VFR conditions. Friday will see isolated SHRA/TSRA over the higher terrain of eastern Arizona. Chances for SHRA/TSRA increase again Saturday into Sunday. Winds south to southwest 10-15 kts Friday, light and variable on Saturday, then becoming west to northwest 5-15 kts on Sunday.

Fire Weather

Today and Friday...Active weather conditions continue today. Today's storms may produce very heavy rainfall leading to flooding or flash flooding. In addition, stronger storms may produce severe winds, large hail, and even a tornado or two. Outside of storms, expect south to southwest winds 10-20 mph with gusts 30-40 mph. Mainly dry conditions and lighter winds are forecast on Friday, with only low chances for showers and thunderstorms (10-25%) forecast over the higher terrain of eastern Arizona.

Saturday through Monday...A weak low pressure system is forecast to bring an uptick in shower and thunderstorm activity on Saturday, with the potential for isolated activity persisting on Sunday. Dry weather is forecast to start next workweek. Expect light northerly winds 5-15 mph on Saturday and Sunday, becoming more northeast to east Monday but remaining light.