Area Forecast Discussion

SYNOPSIS

Scattered showers and thunderstorms are moving across northern Arizona, mainly along and south of the Mogollon Rim, with lighter showers extending northward toward the Grand Canyon and the Kaibab Plateau. A Flash Flood Watch is in effect through 10 PM MST for Yavapai and Gila counties. Shower chances diminish on Saturday, with a more significant storm system arriving Saturday night through early Monday. This second storm will produce prolific rainfall totals across the region and a Flash Flood Watch will be in effect for all of northern and central Arizona.

DISCUSSION

Radar and satellite imagery show showers and thunderstorm activity is still mostly within Yavapai and northern Gila counties, while stratiform rain, with isolated embedded thunderstorms moving through Coconino and into southern Navajo and Apache counties, mainly along and south of the I-40 corridor. Some lighter showers can be found north of the I-40 corridor, but the focus for heavier rainfall is along and south of the Mogollon Rim, and especially western AZ, where instability and moisture transport ahead of the shortwave trough is the greatest. Widespread rainfall totals of 0.25 to 0.50 inches of rain, with locally heavier amounts of around 1 inch are still possible for Yavapai and northern Gila counties through the rest of the today, especially in areas that experience a heavy thunderstorm or several rounds of showers, keeping the Flash Flood Watch in effect for both counties through 10 PM MST this evening.

As the shortwave trough moves off to the east this evening, the forcing, instability and shear over southern and central AZ will decrease fairly quickly. Forecast trends continue to support that showers will be isolated Saturday morning through early Saturday evening, limiting the risk for flooding, with partly sunny to mostly cloudy skies through the day.

By later Saturday evening, the next Pacific system along the west coast will start to move southeastward as a significant plume of sub-tropical moisture is pulled into the Four Corners region and a shortwave trough passes south of AZ. This air mass will be near records as far as PW is concerned for this time of year and the phasing of the two systems will allow for an extended period of moderate to heavy rainfall.

The threat for severe weather with the second system remains very limit, thanks largely to a fully saturated and somewhat cold column over the state of Arizona. This will limit the redevelopment of instability ahead of the main system. However, later on Sunday and Sunday night when the parent trough moves through, there could be some isolated severe along the cold front.

The period of precipitation for the second event will stretch from Saturday evening through Monday morning, allowing plenty of time for totals to add up. The sweet spot, as of now, looks to be along and south of the Mogollon Rim/east of the I-17 corridor, where 2-4 inches of rainfall and locally higher amounts could fall with the heavier bands. Even within the areas that are not as phased and not seeing as much of an upslope component, as much as 2 inches may accumulate. This is not including the rainfall that fall today, so more than enough to cause serious hydrological issues, especially areas that see rain today. A second FFA is in effect for the entire CWA during this time frame.

Behind the final trough passage Monday morning, we quickly dry out with strong ridging developing for a few days. Long-range ensembles are hinting at another cut-off low forming near the end of next week/weekend, potentially bring another round of showers and thunderstorms.

This is a dangerous situation, especially pertaining to flooding. Both flash flooding and then longer term river flooding will be a problem. Larger stem waterways may be flooding for days after the event, especially the Little Colorado River and the Verde River.

AVIATION...Friday 30/18Z through Saturday 31/18Z

Widespread VFR/MVFR conditions forecast through the TAF period, but isolated IFR conditions possible in heavier storms. -SHRA/-TSRA anticipated through at least 12Z, with heaviest activity along and west of a KGCN-KFLG-KSOW through 00Z, shifting east of a KPGA- KINW-KPAN line afterwards. Gusty/erratic winds likely near/in storms, with southerly winds 5-15 kts otherwise.

OUTLOOK...Saturday 31/18Z through Monday 02/18Z...A brief lull in -SHRA/-TSRA is expected through 00Z Sunday, before heavy rain and isolated thunderstorms return. Significant rainfall forecast for much of Yavapai County and northern Gila County, but heavy rainfall is forecast for much of northern AZ. Thunderstorms are less likely on Sunday, but still possible. Activity will gradually diminish west to east after 12Z Monday. Gusty/erratic winds likely near showers/storms, with southerly winds 10-25 kts Saturday and Sunday, becoming northwesterly winds on Monday.

FIRE WEATHER...Rest of Today through Sunday

Heavy showers and isolated to scattered thunderstorms expected through early morning Saturday. Quieter conditions expected for much of Saturday afternoon/evening, with showers/precipitation quickly returning between 10PM MST - midnight. Heaviest rain looks to be along and south of the Mogollon Rim where localized rainfall amounts could be 3-4.00+ inches. Elsewhere may see 1 to 3 inches. Flash flooding/river flooding are the main threats through the weekend. Gusty/erratic winds possible near/in storms/showers, with south- southwest winds 5-15 mph on Saturday, increasing to 15-25 mph on Sunday.

Monday through Wednesday...Showers and isolated thunderstorms will stick around across far eastern AZ early Monday morning, with drier conditions elsewhere. Although we'll continue to dry out through the week, river flooding may persist into Tuesday potentially even Wednesday. Otherwise, look for minimum RH values between 35-60% each afternoon. Winds will be 10-20 mph out of the west-northwest Monday, shifting to the northeast on Tuesday, and becoming southerly by Wednesday.

FGZ WATCHES / WARNINGS / ADVISORIES

Flood Watch from Saturday evening through late Sunday night for AZZ004>018-037>040.

Flood Watch until 10 PM MST this evening for AZZ008-018-037-038.