textproduct: NWS Flagstaff

SYNOPSIS

The potential for showers and thunderstorms will increase during the afternoon hours of Saturday as tropical moisture lifts northward ahead of what will become the remnants of Tropical Storm Rachel once it makes land fall. The synoptic pattern created by the approaching upper-level trough and the remnants of Rachel will support a cooler, windy pattern across Northern Arizona, with the potential for areas of heavy rainfall and flash flooding this weekend. A Flood Watch begins Saturday, followed by a Wind Advisory for Sunday.

DISCUSSION

The track for Tropical Storm Rachel continues to have minor shifts as it continues to interact with the upper-level trough that is deepening in the Pacific Northwest, but the overall track continues to take the tropical moisture mainly over eastern California and into Nevada, western Arizona and Utah. Shower and thunderstorm coverage will increase Saturday afternoon as tropical moisture ahead of Rachel moves into region and daytime heating supports the development of convection along the higher terrain.

The main forecast challenge is determining if most of the rainfall over the next few days will be associated with scattered showers and thunderstorms with high rain rates or more of a steady rainfall event with more widespread rain but more moderate rain rates over a longer period of time. While both would support flooding, the former would support more localized rapid flash flooding, while the latter would trend toward a more slowly development of flooding that is more related to high water in streams and rivers affecting low water crossings and ponding of water in poor drainage areas and on roads. The latest track of Tropical Storm Rachel has westward compared to yesterday, and hi- res models show some drier mid-level air wrapping around the remnants of Rachel and into Arizona. This would prevent a steady, widespread rainfall and greater cloud cover developing Sunday, but instead continuing to support more of an scattered shower and thunderstorm pattern, potentially leading to more variable rainfall totals and localized flash flooding. This drier air would also support the mixing down of stronger winds aloft, supporting more widespread area of southerly winds around 20 to 30 mph, gusting up to 50 mph across northern Arizona, and increase the uncertainty of how heavy rainfall will be Monday and Tuesday as the main trough moves over Arizona.

Current forecast is taking a blend of the two scenarios, with QPF of 1-3" along the upsloping (south) side of Mogollon Rim, which could see locally higher amounts and up toward the Grand Canyon area and to the Utah border, based on the the path of the greater tropical moisture. A Flood Watch is in effect Saturday afternoon through late Sunday night for Yavapai, western Coconino, northern Gila, and far southern Navajo counties. The strongest winds and gusts are expected Sunday so a Wind Advisory is in effect 8 PM MST Sunday to 8 PM MST Sunday and will be evaluated to see if portions of the advisory need to be extended for some of the higher terrain Monday, depending on how the synoptic pattern evolves.

AVIATION...Saturday 10/06Z through Sunday 11/06Z

Mainly VFR conditions through 18Z Saturday, with ISOLD -SHRA/-TSRA continuing. Shower and thunderstorm activity increasing in coverage from southwest to northeast after 18Z, with localized MVFR conditions in heavier rain. Winds becoming light overnight. Winds S-SW 10-20 kts with gusts up to 30 kts Saturday afternoon.

OUTLOOK...Sunday 11/06Z through Tuesday 13/06Z...Areas of MVFR conditions as SHRA/TSRA continues at times through the period. Periods of IFR possible overnight Saturday through Sunday in embedded +RA and low CIGS. Winds S-SW 20-30 kts with gusts 35-45 kts on Sunday, then S-SW 15-20 kts with gusts 25-35 kts on Monday.

FIRE WEATHER...Saturday and Sunday

Showers and thunderstorms are expected to develop from southwest to northeast across the region through the day Saturday, continuing into Sunday. Embedded periods of heavy rain is possible at times, especially Saturday night into Sunday. South winds 10-20 mph on Saturday, increasing to 20-30 mph with gusts 35-45 mph on Sunday.

Monday through Wednesday...Scattered showers and thunderstorms remain on Monday and Tuesday, with isolated activity expected on Wednesday. South winds 15-25 mph on Monday, becoming south/southwest on Tuesday. Light west/northwest winds expected for Wednesday.

FGZ WATCHES/WARNINGS/ADVISORIES

Flood Watch from Saturday morning through late Sunday night for AZZ004>008-015-016-018-037-038.

Wind Advisory from 8 AM MST /9 AM MDT/ to 8 PM MST /9 PM MDT/ Sunday for AZZ004>018-037>040.