textproduct: NWS Flagstaff

SYNOPSIS

Northern Arizona will remain in an active weather pattern through the week. Scattered showers and a few thunderstorms and forecast for today, with activity becoming more isolated on Tuesday. High chances for showers and thunderstorms are forecast begin Wednesday morning, continuing Thursday as a strong weather system approaches. The main concerns are flooding and flash flooding, especially across eastern Arizona. Additionally, wind speeds will increase, with well below normal temperatures forecast by Thursday or Friday.

DISCUSSION

Current satellite and upper-air analysis show a shortwave trough over northern Arizona. This feature is interacting with modest low-level moisture (surface dew points between 45 and 55 degrees F) to produce scattered showers, although a few isolated thunderstorms are possible this afternoon as well. Atmospheric instability is limited (CAPE of 100-500 J/kg), but 0-6km bulk shear is elevated (25-35 kts) due to the passing shortwave trough. The best combination of instability and shear is expected along and east of the I-17, resulting in the highest potential for showers and storms there. Today's storms will be capable of producing outflow winds between 25-35 mph with isolated pockets of rainfall between 0.5-1.0 inches. While the probability is low, a severe storm or two capable of producing 1 inch hail cannot be ruled out.

On Tuesday, a low pressure system is forecast to dive southward along the West Coast. This will sandwich northern Arizona between the deepening West Coast trough and broad high pressure over the central United States. As this happens, only minor changes to the sensible weather are expected. High temperatures are forecast to increase by a couple of degrees with afternoon winds becoming more breezy. High-resolution model guidance does indicates a low potential for some isolated shower, or thunderstorm activity, but stable conditions due to the presence of an inversion above the boundary layer should keep activity subdued.

On Wednesday and Thursday, the West Coast low shifts eastward toward northern Arizona allowing deep subtropical moisture to push into the area from the south. Some of this incoming moisture will be assocaited with what is currently Tropical Storm Trudy centered south of the Baja Peninsula. Model forecasts shows Precipitable water (a measure of atmospheric moisture) ranging from 0.70 to 1.0 inches. This level of moisture is exceedingly rare for mid-October, with values above the 98th percentile. Due to this, flooding/flash Flooding will be the primary hazards. At this point, the heaviest rainfall is expected along and south of the Mogollon Rim, where the forecast calls for a broad area of storm total rainfall of 1.0-3.0 inches. Further north over the Navajo Nation, storm total rainfall of 1.0-2.0 inches is forecast. The severe weather potential is more difficult to assess, as thick cloud cover may limit instability. That said, forcing from the passing low and strong wind shear (50+ kts) suggest at least a low potential for severe, rotating storms. Otherwise, southerly wind speeds will increase during this time, strongest on Thursday. However, the probability of wind speeds exceeding wind advisory thresholds (>40 mph) is low.

On Friday, the forecast calls for decreasing chances for showers and thunderstorms through the day, and well below normal temperatures. Forecast confidence in the evolution of the overall weather pattern heading into the weekend remains low.

AVIATION...Tuesday 20/06Z through Wednesday 21/06Z

Mainly VFR conditions. Isolated sprinkles or light showers possible overnight in eastern Arizona. Isolated showers and thunderstorms possible over the White Mountains after 18Z Tuesday. Showers and thunderstorms begin moving up from the south between 03Z-06Z Wednesday. Winds light and variable or terrain driven overnight, S-SW 5-15kts gusts to 20kts daytime Tuesday. Light and variable or terrain driven Tuesday night.

OUTLOOK...Wednesday 21/06Z through Friday 23/06Z...Mainly VFR conditions, periods of MVFR/IFR in heavier showers and storms. Showers and thunderstorms move in from the south Tuesday night, becoming numerous all day Wednesday and Thursday. Winds SE-S 10-20kts with gusts to 30kts Wednesday, S-SW 10-20kts with gusts to 30kts Thursday.

FIRE WEATHER...This afternoon through Wednesday

Cool conditions continue. Scattered showers and thunderstorms will continue through this evening, then become isolated on Tuesday. Activity will increase in coverage late Tuesday and become widespread on Wednesday. Some storms may produce very heavy rainfall leading to flooding or flash flooding on Wednesday. South winds 5-15 mph through this evening, increasing to 10-20 mph on Tuesday, then 15-25 mph with gusts 30-40 mph on Wednesday.

Thursday through Saturday...Cool temperatures continue. Widespread showers and thunderstorms likely on Thursday, then becoming isolated for Friday and Saturday. Some storms may produce very heavy rainfall leading to flooding or flash flooding on Thursday. South winds 15-25 mph with gusts 30-40 mph on Thursday, becoming south through southwest 10-20 mph with gusts 25-30 mph on Friday, then light northerly winds 5-15 mph on Saturday.

FGZ WATCHES/WARNINGS/ADVISORIES

None.