textproduct: NWS Flagstaff

SYNOPSIS

Numerous to widespread showers and thunderstorms are expected each day through Monday, including overnight activity. With deep monsoonal moisture in place, heavy rainfall will be possible with any stronger stronger storm, potentially leading to flash flooding. Precipitation chances then begin to decrease near the middle of next week, along with gradually warming temperatures.

DISCUSSION...Today

Overnight convection across far northern Arizona is giving way to clearing skies across the remainder of the region. Given this trend, daytime heating looks to be able to maximize destabilization over the area later today. 14Z RAP guidance shows a large area of 1000-2000 J/Kg of MLCAPE forecast from areas Coconino and Yavapai counties. At the same time, the approaching trough from the west will increase vertical wind shear across the area, especially to the north. 0-6 km shear values of around 20-30 kts look probable across western and far northern Arizona. As a result, strong to severe thunderstorms are expected to develop this afternoon across northern Arizona, with the best chances being over Coconino and Yavapai counties. The main threat with any severe storm will be large hail, however some damaging wind gusts will be possible over the lower elevations.

Deep monsoonal moisture is also in place across the region, with PWATs around an inch in the Four Corners region, increasing to around 1.5 inches south of the Mogollon Rim. Therefore, flash flooding will remain a possibility this afternoon (in addition to the severe threat). The only limiting factor for flash flooding looks to be 15-25 kt storm motions, though the likelihood of training storms will likely negate this factor.

Tonight...Following today's convection across southern Arizona, an MCS looks to move northward out of the Lower Deserts into northern Arizona late this evening. While the severe threat will diminish with the loss of daytime heating, the flash flood potential will begin to ramp up. Latest U of A WRF guidance shows the complex moving up through the Phoenix metro into Yavapai and Gila counties after 8-9 PM MST, then lifting northeast as it begins to decay through the rest of the overnight hours.

12Z HREF PMM of >= 1" of rain in 3 hours signals towards the greatest risk areas for flash flooding overnight being across Yavapai and Gila counties. For this reason, a Flash Flood Watch is in effect through the overnight hours.

Sunday and Monday...Monsoonal moisture looks to remain in place through Monday, along with the dynamics of the approaching trough. Given this, a repeat of Saturday looks likely on Sunday and potentially again on Monday. Therefore, severe weather will remain a threat during the afternoon hours, then transitioning to an overnight flash flood risk as a decaying MCS moves northward out of the Lower Deserts. Guidance from the U of A WRF signals towards potentially more widespread nocturnal activity Sunday night into Monday morning. HREF PMMs show an even higher signal for heavy rainfall across central Arizona late Sunday evening into Monday morning. For this reason, an additional Flash Flood Watch has been issued for all of northern and central Arizona.

The limiting favor for all of this each day will be any remnant debris clouds. Given the increased nocturnal activity, daytime heating may be limited during the morning hours. This could potentially result in delayed convective initiation and/or reduced coverage.

Tuesday through Friday...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 Tropical Storm 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...Saturday 29/18Z through Sunday 30/18Z

SHRA/TSRA expected at times through the TAF period. Highest coverage through 06Z, then concentrated mainly along/south of KPRC-KFLG-KRQE 06Z-14Z. MVFR/IFR conditions likely in heavier showers/storms. Outside of gusty storm driven winds that could reach near 40 kts, look for W-SW winds 10-20 kts. Light/variable winds forecast overnight.

OUTLOOK...Sunday 30/18Z through Tuesday 01/18Z...SHRA/TSRA expected at times through the TAF period. Greatest coverage will be between 18Z-02Z, but expect activity to persist overnight as well, especially Sunday night into Monday morning. MVFR/IFR conditions likely in heavier showers/storms. Outside of gusty storm driven winds that could reach near 40 kts, look for south through west winds 10-20 kts. Light/variable winds forecast overnight.

FIRE WEATHER...This afternoon through Monday

Scattered to widespread showers and thunderstorms expected. Greatest coverage forecast for the afternoon hours, but activity expected to persist overnight as well. Localized heavy rain may lead to flash flooding, especially over burn scars. Outside of gusty/erratic winds around/in storms, look for southwest winds 10-20 mph each afternoon, variable 10 mph or less overnight.

Tuesday through Thursday...Shower and thunderstorm activity decreases to isolated to scattered coverage. Outside of gusty/erratic winds around/in storms, look for southwest winds 5-15 mph each afternoon, becoming variable overnight.

FGZ WATCHES/WARNINGS/ADVISORIES

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

Flood Watch from 5 PM MST this afternoon through late tonight for AZZ008-018-037-038.