Area Forecast Discussion

SYNOPSIS

A series of Pacific storm systems will tap into deep tropical moisture for Friday, with widespread showers and scattered thunderstorms moving west to east across the state. A Flash Flood Watch is in effect on Friday 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

Moisture transport is underway, featuring a 20-30kt low-level jet pointed straight up the Gulf of California and into the Lower Colorado River Valley. This advection is due to a deepening trough along most of the west coast, as well as sub-tropical jet and s/wv now located around 29N/120W. This will all come together in the next few hours and will amplify throughout the day, before tapering off to the east later this evening.

PW values are very high for this time of year and the rapid influx of higher dew points across the deserts will quickly destabilize the atmosphere. As the s/wv approaches, we expect large-scale ascent and increased shear, leading to a risk of isolated severe weather. Much of the severe threat will be south of the Mogollon Rim and into the central deserts, closer in proximity to steep, low-level lapse rates and higher boundary layer moisture. Isolated rotating storms may produce large hail and severe wind gusts today in these areas.

As far as overall rainfall, we expect fairly robust totals through this evening, mainly over Yavapai, western Coconino and northern Gila counties. These areas are where some upslope may develop, as well as persistent and training storms. Farther to the north and northeast, dynamics, moisture transport and upslope are not nearly as impressive and totals will drop off quickly heading toward the Four Corners. Within the initial FFA location today, generalized accumulations of 0.50 to 2.00" are likely, with isolated higher amounts. Due to already saturated ground, this will be more than enough to elevate flooding threats by this afternoon.

As the s/wv move east this evening, forcing, instability and shear drops off in a hurry. By daybreak Saturday through early Saturday evening, we do not expect much more than isolated rain showers.

By later Saturday evening, this is when the main event begins. The main trough along the west coast starts to move southeastward as a significant plume of sub-tropical moisture and s/wv is entrained around 28N/122W. 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 have mostly diminished, thanks largely to a fully saturated and somewhat cold column over the state of Arizona. 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 just east of the I-17 corridor, where as much as 4-5 inches of rainfall 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 all on the heels of the rainfall from Friday, so more than enough to cause serious hydrological issues. A second FFA was issued for the entire CWA during this time frame and there are no issues with that.

Behind the final trough passage Monday morning, we quickly dry out with strong ridging developing for a few days. This will be the time to catch your breath, as models are hinting at another cut-off low forming near the end of next week.

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. Don't dismiss the severe threat for Friday either, especially south of the Mogollon Rim. Stay tuned to your favorite weather source and find a way to receive warnings.

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...Friday and Saturday

Rapidly increasing chances for showers and thunderstorms today, starting in the western zones this morning and moving to the eastern zones this evening. Some of the higher rain totals today are likely over Yavapai and northern Gila counties, where flooding is possible. There is also a threat for isolated severe thunderstorms through this afternoon, primarily south of the Mogollon Rim. By Saturday, a brief lull develops with only isolated rain showers in the forecast through early Saturday evening. Winds each day, outside of thunderstorms, are generally light. Expect daytime southerly winds 5 to 15 mph.

Sunday through Tuesday...Widespread moderate to heavy rainfall from Saturday night through Monday morning. Some of the heaviest totals are likely along and south of the Mogollon Rim. Flooding is likely. The threat for thunderstorms has diminished, but a cold front Sunday night may spawn a few stronger storms as it moves from northwest to southeast. Expect southwest winds 10 to 20 mph, with gusts a high as 30 mph Sunday. Winds turn west to northwest at 10 to 20 mph Monday and northeast 5 to 15 mph for Tuesday.

FGZ WATCHES / WARNINGS / ADVISORIES

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

Flood Watch through this evening for AZZ008-018-037-038.