textproduct: NWS Flagstaff

SYNOPSIS

Showers and thunderstorms will increase through the day as tropical moisture is pulled in Arizona from a trough interacting with Tropical Storm Rachel. Cooler, windier, and wetter conditions are expected to continue into at least the first-half of next week.

DISCUSSION...Today through Tuesday

Tropical Storm Rachel, about 500 miles southwest of San Diego as of 8 AM MST, is quickly moving northward as it begins to be pulled northward by an upper-level low off the California coast. As this occurs, tropical moisture is expected to increase across much of Arizona through the day. PHX ACARS profiles have already begun to show around a 0.2 inch increase in PWATs this morning vs yesterday afternoon.

As a result, scattered showers and thunderstorms look to overspread the region from southwest to northeast starting late this morning into the afternoon hours. The most favored area for storms looks to mainly be across western Arizona, as an upper-level jet looks to develop by this afternoon to provide large-scale ascent to support precipitation development.

The remnants of Rachel are expected to make landfall late tonight or early Sunday morning around northern Baja, before moving along the Arizona-California border. Moisture looks to peak across Arizona around this time, with model soundings showing a nearly saturated profile. The upper-level jet looks to become an enhanced over Arizona at the same time, as deepening trough digs into the western US. As a result, more widespread coverage of showers and thunderstorms are expected late tonight into much of Sunday. Gusty southerly winds are also expected, likely enhancing precipitation amounts in the upslope areas. While at the same time, enhanced downsloping will likely lead to rain shadowing of areas north and east of the Mogollon Rim.

By Monday and Tuesday, drier air begins to move in aloft as the main low moves into Arizona. However, model soundings still show plenty of residual shallow moisture, which combined with strong orographic lift will keep scattered showers and thunderstorms in place across the higher terrain.

Overall, the greatest threat from this system will be flash flooding, especially in rain sensitive locations such as burn scars and slot canyons. Their remains some uncertainty with how widespread the risk will be, as much of the area may see a more steady, light to moderate rainfall. However, where the areas of embedded heavy rainfall setup will see a higher flash flood risk. Latest HREF and REFS guidance show the greatest potential for heavy rainfall to be across Yavapai and Gila counties early Sunday morning and peaking by the afternoon hours. A Flash Flood Watch is in effect for much of the Mogollon Rim and south to cover this threat, along with the Grand Canyon and Page areas to cover the burn scar and slot canyon risk.

The threat for severe thunderstorms does increase given the added wind shear, however the amount of instability over the region remains in question and will be the limiting factor. Damaging winds would be the main threat with any severe storm, however, an isolated tornado can not be totally ruled out in any stronger, more organized storm. The greatest risk looks to be across central Arizona on Sunday, however confidence in this threat is low given the potential lack of daytime heating.

Wednesday through Friday...Model solution begin to diverge by the middle of the week. The GFS lifts the trough north and east out of Arizona and into the northern US, with a weak cut-off low developing off northern Baja. The ECMWF on the other hand favors the low breaking off from the jet, and remaining stationary over the Four Corners region. The EPS and GEFS means both seem to favor their operational counterpart, though the run-to-run trend in the GEFS does signal towards a slower departure of the low.

The current NBM forecast through the extended period favors a gradually warming trend, with gusty northeasterly winds developing, which seems reasonable. There remain some precip chances, mainly across eastern Arizona, but these might be slightly on the lower end (especially on Wednesday). However, given the high amount of model disagreement, confidence in the later-half of the week remains very low at this point in time.

AVIATION...Saturday 10/18Z through Sunday 11/18Z

VFR to start with pockets of MVFR near heavier showers and thunderstorms. Eventually becoming MVFR after 06Z-12Z Sunday in the higher terrain terminals. Scattered showers and thunderstorms will become more numerous as the day progresses. Winds S-SW 15-20kts gusts 25-30kts daytime, light and variable until about 09Z-12Z Sunday, then increasing to S-SW 15-20kts, gusts 30kts.

OUTLOOK...Sunday 11/18Z through Tuesday 13/18Z...Mainly MVFR, with IFR possible near heavier showers. Numerous showers and thunderstorms throughout the time period. Winds Sunday S-SW 20-30kts with gusts 40-50kts, winds Monday S-SW 15-25kts with gusts up to 40kts.

FIRE WEATHER...Sunday and Monday

Numerous showers and thunderstorms across the region both days. Periods of heavy rain are possible on Sunday with flooding or flash flooding a threat in some areas, especially burn scars. Wins S-SW 20-35 mph with gusts 35-50 mph on Sunday, S-SW 15-25 mph with gusts 35-40 mph on Monday.

Tuesday through Thursday...Scattered showers and thunderstorms continue Tuesday, ending west to east Tuesday night into Wednesday morning. Dry Wednesday afternoon into Thursday. Winds S-SW 15-25 mph with gusts 30-40 mph on Tuesday, W-NW 5-15 mph Wednesday, N-NE 10-20 mph on Thursday.

FGZ WATCHES/WARNINGS/ADVISORIES

Flood Watch 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.