textproduct: NWS Flagstaff

SYNOPSIS

A big pattern change will occur starting Sunday afternoon. An incoming trough from the West Coast will interact with tropical moisture and potentially bring heavy rain, strong thunderstorms and possible flash flooding. A Flash Flood Watch has been issued for Monday and Tuesday which is the time period when the heaviest rain is likely to occur. Wednesday afternoon onward things dry out and the region will see dry and warm conditions to start off October.

DISCUSSION

After a pleasant day today and a quiet night tonight things will start to shift to an active pattern starting Sunday afternoon. Moisture from tropical cyclones Odalys and Polo will begin overspreading the area, most likely in the high and mid levels at first which will bring in the clouds and some light rain starting Sunday evening and Sunday night. The lower levels will be dry at first but start moistening up from these initial rain showers.

By Monday a deep West Coast trough approaches with a closed upper level low at its base. This will act upon the tropical moisture already in place and this is when the best rain chances will start. By the afternoon lift from the approaching trough will generate some embedded thunderstorms that could become strong to severe at times with heavy rain, strong winds and some large hail. WPC has upgraded our area into the moderate category for excessive rainfall so to get a jump on messaging to our partners we've already issued a Flood Watch for Monday and Tuesday. This trough looks to take its time moving across the area so Tuesday will be another active day for the region with much of the same risks as Monday although the focus may be more towards the eastern part of the state. Temperatures across the region Monday and Tuesday will be quite cool due to the cloud cover and rainfall, about 10 degrees below average. By Wednesday afternoon precipitation chances should start winding down in the far eastern parts of the state as the trough sweeps east and takes the moisture plume with it.

One thing to note: Despite the fact the tropical moisture is coming from two tropical cyclones to the south, neither one of these circulations will actually make it up into Arizona. Odalys is looping south and will likely die in the open ocean, Polos is expected to make landfall in Baja California, cross the Gulf of California and break apart to our south when it makes landfall again in mainland Mexico. The strong trough and upper level low moving in from the west acting on that tropical moisture drawn north will be more than enough to keep us busy.

By Thursday onward dry air from the west and north will take over and precipitation will be a distant memory for quite awhile as long term models at this time aren't showing anything else coming in. This will be the end of Monsoon 2026, both meteorlogically and calender wise. With some ridging moving in towards the end of the week look for a pleasant start to October as high temperatures recover to a few degrees above average.

AVIATION...Sunday 27/00Z through Monday 28/00Z

VFR conditions. Winds southwest to southeast 5-15 kts through 02Z, then turning light and variable overnight. Look for increasing clouds on Sunday with southwest wind 10-15 kts gusting to 25 kts developing after 18Z. A slight chance of a shower or thunderstorm vicinity of KPRC late Sunday afternoon.

OUTLOOK...Monday 28/00Z through Wednesday 30/00Z...VFR conditions through Sunday evening, with areas of MVFR conditions developing in scattered to numerous SHRA/TSRA thereafter. Daytime winds south 10-20 kts on Monday becoming variable at 5-15 kts on Tuesday. Overnight winds generally light and variable. Gusty and erratic winds near storms.

FIRE WEATHER...Today through Monday

Dry conditions are expected until slight chances for showers and thunderstorms return Sunday afternoon. Precipitation chances continue to ramp up overnight Sunday with widespread wetting rain and embedded thunderstorms on Monday, presenting a moderate flood risk. S-SW winds are 10-20 mph gusting to 30 mph on Sunday, 5-15 mph on Monday.

Tuesday through Thursday...Prolonged widespread precipitation chances continue into Tuesday night as an approaching weather system combines with tropical moisture. Precipitation chances decline from west to east overnight Tuesday into Wednesday morning. There is a threat of heavy rainfall and flash flooding on Tuesday for all of northern AZ. Winds N-NE 10-20 mph on Tuesday, then NW-N 5-15 mph on Wednesday and Thursday.

FGZ WATCHES/WARNINGS/ADVISORIES

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