textproduct: NWS Flagstaff

SYNOPSIS

Active monsoon pattern will continue through Monday, with flash flooding being a primary concern. Drier and slightly warmer conditions then set in but the middle of the week, before precipitation chances potentially return to close out the week.

DISCUSSION...This afternoon

Another round of showers and thunderstorms are expected across central and northern Arizona today. Storms have already initiated this morning across central Arizona, though activity remains fairly mundane. Coverage is expected to increase gradually through the rest of the afternoon, especially north and east of the Mogollon Rim.

Latest RAP guidance has slightly lower MLCAPE values today, but remain around 500 J/Kg across the Four Corners, increasing along a northeast to southwest gradient, to around 1500 J/Kg across Yavapai County. 0-6 km wind shear looks to remain around 20-30 kts from the trough over the west, however this looks to be decreasing through the day. Therefore, a couple strong to severe thunderstorms look to remain a possibility this afternoon, with damaging winds and large hail the main threats.

PWATs look slightly lower this morning based on the latest RAP analysis, however, guidance forecasts these values to increase through the afternoon. PWATs are expected to range from around an inch across the northeast to around 1.5 inches south of the Mogollon Rim. This will lead to very high rainfall rates with any stronger storm, which combined with already saturated soils will lead to an enhanced flash flood potential today.

Tonight...Much of the hi-res guidance continues to forecast a complex of storms to move northward out of the Lower Deserts later this evening and overnight. As the MCS moves northward, widespread thunderstorms look to continue through the overnight across much of northern Arizona. 12Z HREF PMM guidance has a 30-50% chance of exceeding 1" in 3 hours with this system across much of Yavapai and Gila counties overnight, and potentially expending a bit further northward into Coconino. The HREF also highlights a 10% probability for exceeding 3" in 3 hours.

The widespread nature of the coverage and the already saturated soils in some areas will only further enhance the risk. Therefore, an elevated risk of flash flooding looks to remain into the overnight hours across much of central and potions of northern Arizona. A Flash Flood Watch is in effect starting this afternoon and lasting through Monday night.

Monday...One last day of active monsoonal conditions looks to continue into Monday. Coverage looks to increase as a shortwave moves through the area from the west, adding upper-level dynamics to aid in precipitation development. However, some uncertainty does remain given the potential for debris clouds from overnight convection. This could potentially result in delayed convective initiation and/or reduced coverage. Nevertheless, the flash flood threat remains, along with potentially a couple strong to severe thunderstorms.

Tuesday through Saturday...A fast-moving trough looks to move through Great Basin near the middle of next week. This looks to initially suppress monsoonal moisture further south, with much of the GEFS, EPS, and CMCE member showing a sharp decline in PWATs for the middle of next week. Operational guidance seems to have trended further westward with Hurricane Karina, thereby limiting the northward push of the sub-tropical moisture surge along Baja California.

However, with another tropical system potentially developing behind Karina and troughing remaining over the west, an increase in moisture later in the week remains a possibility. GEFS, EPS, and CMCE members remain split on continuing with the downward trend, or increasing PWATs potentially as early as Thursday/Friday. Therefore, uncertainty remains high near the end of the forecast package as the exact timing and track of the both the trough and potential tropical cyclone are crucial.

The operational guidance does have heights tick upwards slightly, so a slight warming trend seems likely, but overall we look to remain seasonable to slightly cooler than normal temperature wise into next week.

AVIATION...Sunday 30/18Z through Monday 31/18Z

Mainly VFR conditions outside of localized MVFR/IFR in heavier precipitation. Scattered to numerous SHRA/TSRA expected to develop through the afternoon and continue at times through Monday. Gusty/erratic winds likely near storms, with daytime W-SW winds 10-20 kts otherwise.

OUTLOOK...Monday 31/18Z through Wednesday 02/18Z...Localized MVFR/IFR conditions may continue through around 06Z Tuesday, due to areas of SHRA/TSRA. Afterwards, expect improving conditions with only isolated -SHRA/-TSRA expected on Tuesday. Gusty/erratic winds likely near storms, with SW winds 10-20 kts during the afternoon hours and becoming light and variable overnight.

FIRE WEATHER...This afternoon through Tuesday

Active monsoonal weather continues through Monday evening, including the overnight hours. Storms will be capable of producing hail, heavy rain, frequent lightning, and gusty/erratic winds. A Flash Flood Watch is in effect for all of central/northern AZ this afternoon through Monday night. A cooling trend is expected, leaving daytime highs 5- 10 degrees below normal by Monday. Outside of storm driven outflows, look for southwest winds 10-20 mph this afternoon, then 5-15 mph Monday and Tuesday.

Wednesday through Friday...Precipitation chances decline through the middle of the week, with only isolated storms across the higher terrain through Thursday. An increase in storm activity is possible by Friday. Daytime highs will be close to seasonal norms. Look for southwest winds 10-20 mph each afternoon, becoming light and variable overnight.

FGZ WATCHES/WARNINGS/ADVISORIES

Flood Watch through Monday evening for AZZ004>018-037>040.